Fire safety of concrete structures - e-tell.info fire tests... · performance-based methods for the structural fire engineering design of concrete ... for the fire resistance of precast

Tom LennonFRS, the Fire Division of BRE

Fire safety of concrete structures:Background to BS 8110 fire design

Fire Report 3004 17/6/04 7:49 am Page 1

Tom LennonFRS, the Fire Division of BRE

Fire safety of concrete structures:Background to BS 8110 fire design


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Introduction 4

Description of the project 5

Historical development in national concrete codes 6

Comparison between tabulated values from different codes 14

Experimental background to tabulated values 15

Other relevant research 30

Discussion 33

Conclusions and recommendations 36

References 37

Appendix A – Results from National Building Studies Research Paper No. 12 39

Appendix B – Results from National Building Studies Research Paper No. 18 42

Appendix C - Results of fire resistance tests on elements of building construction 43

Appendix D - Results from Fire Research Note 741 44

Contents

Fire safety of concrete structures: Background to BS 8110 fire design 3

This report has been prepared at the request of The Concrete Centre and the British Cement Association to investigate the backgroundto the methods for establishing the fire resistance of concrete structures specified in the relevant parts of the UK concrete Code BS 81101,2. The work focused on the original research and test results underpinning the tabulated data in BS 8110, which have beenrevisited in order to assess the relevance of the approach to modern forms of concrete construction.

This study is important in that it brings together in one document a body of information covering test results and research carried outover a number of years. There was a danger that much of the important work in support of the development of codes and standardswould be lost. Hence a study was carried out to collate and assess all relevant information to ensure that the important lessons fromthe past are recorded and to help define the strategy for a new generation of codes and standards.

The investigation shows that the experimental results used as the basis for developing the tabulated data in BS 8110 support theprovisions of the Code in relation to assumed periods of fire resistance. In many cases the provisions are very conservative as they arebased on the assumption that structural elements are fully stressed at the fire limit state.

Executive summary


For many years the most common method of ensuring compliance with the requirementsof the Building Regulations in terms of the fire safety of concrete buildings has been torely on tabulated values for minimum dimensions and minimum cover to reinforcement.Historically both reinforced and prestressed concrete have been shown to provide goodresistance to fire. A study commissioned by the Fire Resistance Committee of The Concrete Society3 investigated a large number and variety of fire-damaged concretestructures within the UK. The authors concluded that almost without exception thestructures performed well during and after the fire, and that the majority of structures wererepaired and re-used. However, the report emphasised the need to establish thecircumstances under which spalling would have serious consequences.

The information in the codes is based on the results from standard fire tests on elementsof construction. Such tests generally assume that the structural element is fully stressedat the time of the fire. This is a conservative assumption. The provisions in terms of coverare based on limiting the temperature of the reinforcing or prestressing steel to a singlecritical value. The development of fire engineering methods has questioned the relevanceof standard fire testing in relation to the performance of actual buildings subject to realfires. In recent years concrete construction has become more efficient with the use ofchemical admixtures to improve workability, increase strength and reduce curing times.Modern concrete frames tend to consist of more slender members with all aspects of thedesign process rationalised to improve the speed and economy of construction. There is aneed to assess the performance of modern concrete construction against the provisionsof the Code, and to identify areas where the design can be made more efficient.

Previous research4 identified the lack of up-to-date data on the effect of fire on concretestructures. It pointed out that the industry is in danger of employing the materialinefficiently and that design rules were based on research conducted many years ago. Thereport emphasised the need to conduct research in order to fill in gaps in the industry’sknowledge and to keep abreast with the advances in concrete technology that had takenplace in the preceding 10 to 15 years.

The traditional means of ensuring compliance with the regulatory requirements for firesafety for elements of structure is to adopt the prescribed values set out in tables A1 andA2 of Approved Document B to the Building Regulations. The values relate to a minimumperiod for which the element must survive in the standard fire test measured against therelevant performance criteria of stability, integrity and insulation.

• Stability or loadbearing capacity relates to the period of time a structural element canmaintain the appropriate design load during a fire test.

• Integrity measures the ability of an element (structural or non-structural) to prevent thepassage of flames or hot gases during a test.

• Insulation is a measure of the ability of the material to prevent a prescribed temperaturerise on the unexposed face during the prescribed period.

For elements such as beams or columns the only relevant performance criteria isloadbearing capacity, whereas for loadbearing separating elements, such as compartmentfloors and walls, all three requirements have to be met for the prescribed period of fireresistance.

Introduction

Fire safety of concrete structures: Background to BS 8110 fire design4


This project set out to summarise the background to the design values in the currentconcrete Code, BS 8110-2: 19851 in relation to fire resistance. This report details thebackground to the Code provisions informed by a comprehensive search of the availableinformation from the Fire Research Station (FRS) archives. It provides information on themajor changes since the publication of the 1948 version of CP 1145 as they relate totraditional structural elements (beams, columns, floor slabs and walls). The informationcovers both reinforced and prestressed concrete.

Available test results are provided in the appendices with the information broken down,where possible, in terms of support conditions, period of fire resistance attained, type ofaggregate used, moisture content, overall dimensions, load level and the presence andnature of spalling.

The prescriptive design approach has served the profession well because of its inherentsimplicity. However, in recent years fire engineering design has moved away fromprescriptive design solutions towards a more performance-based approach. It is importantthat the concrete industry develops further guidance for designers and regulators topromote the use of a material with a history of very good performance in fire. There is nowan opportunity to move away from the prescriptive approach in the UK Code and developperformance-based methods for the structural fire engineering design of concretestructures.

Description of the project


The assistance of the following is gratefully acknowledged:Prof Colin Bailey, UMISTDr Pal Chana, BCARohan Rupasinghe, BRE


This section considers the evolution of the concrete design codes in relation to theprovisions for fire resistance.

CP 114: 1948The starting point for this study is the provisions in the 1948 version of CP 1145. Althoughthere was not a great deal of information on fire resistance within this Code it did includetabulated values for the fire resistance of walls and floors to achieve specific fireresistance periods. Table 14 from the Code is reproduced below with the criticaldimensions converted to metric units.

The 1948 version of the Code differentiated between two types of aggregate. Class 1aggregates (foamed slag, pumice, blastfurnace slag, crushed brick and burnt clayproducts, well-burnt clinker and crushed limestone), which provide improved fireperformance, and Class 2 aggregates (flint, gravel, granite and crushed natural stone otherthan limestone), which behave less well in fire situations.

Consequently there were different provisions for thickness of walls to achieve a specifiedfire resistance. Although there were no tabulated values for columns, the Coderecommended the use of Class 1 aggregates for fire resistance periods of two hours andabove for columns with thicknesses in the range 250 – 300 mm. If Class 2 aggregates wereused then a supplementary mesh placed centrally in the concrete cover wasrecommended for fire resistance periods up to 2 hours. For larger columns a 2-hour periodcould be obtained regardless of the aggregate used. Fire resistance periods up to 4 hourscould be achieved by the use of Class 1 aggregates or a light mesh reinforcement.

The tabulated values assumed a minimum cover of 25 mm for a 4-hour period, 19 mm fora 2- or 1-hour period and 13 mm for a half hour fire resistance in relation to hollow tilefloors. No information was provided on the required levels of cover in relation to columns,walls or floors.

The 1948 Code stated that the thicknesses used for structural reasons would normally leadto a sufficient degree of fire resistance.

CP 114: 1957The 1957 version of CP 1146 retained the provisions of the earlier version with respect tothe requirements for walls and floors and provided additional tabulated data for the fireresistance of precast or in-situ inverted U sections where the minimum thickness occurredonly at the crowns, hollow block construction and precast units of box or section,concrete beams and concrete columns. This current report is concerned only with theprovisions in relation to commonly used structural forms.

Tables 2 and 3 give minimum dimensions for columns and beams respectively. It isinteresting to note that the beam provisions are expressed in terms of minimum coverrather than overall depth and are well in excess of the previously quoted values for hollowtile floors. This reflects the particular problems associated with the spalling of concretebeams.

Historical development in national concrete codes


Grade of fire resistance 6 hours 4 hours 2 hours 1 hour 1/2 hour

Thickness of wall to attain grade:With class 1 aggregates 203 152 102 76 76With class 2 aggregates 228 178 102 76 76

Thickness of solid reinforced concrete slab 178 152 127 102 89

Thickness of concrete slab and solidmaterial in tiles of hollow tile floor 152 127 89 76 64

Table 1 Thickness (mm) of walls and floors for fire-resisting purposes (CP 114: 1948)


The Code stated that column thicknesses for 4 hours and 2 hours could be reduced to 305 mm and 229 mm where limestone aggregate is used or where mesh reinforcement isincluded within the concrete cover.

The provisions in the Code are based substantially on an extensive series of fire testscarried out during the period 1936 to 1946 by the Building Research Station at the FireResearch Station test facility at Elstree (Borehamwood). The nature and extent of the testprogramme are documented in National Building Studies Research Paper No. 127. Thisimportant document also includes a summary of the significant results from theprogramme including the results of tests on walls and partitions, floors and roofs, columnsand beams. It is this information that forms the basis for the provisions for concretestructures in fire in the UK.

The information was incorporated into generic fire resistance tables for walls andpartitions, floors and roofs, beams and columns. Table 4 below combines the informationfrom these tables as they relate to reinforced concrete construction.

Notes:1 Walls to be reinforced vertically and horizontally at not more than 152 mm centres and reinforcement to be notless than 0.2% of volume. Walls less than 127 mm thick to have single layer of reinforcement in middle of wall. Wallsmore than 127 mm thick to have 2 layers of reinforcement, not less than 25 mm from each face.2 Increased to 4 hours if light mesh reinforcement placed in cover

3 Increased to 2 hours if light mesh reinforcement placed in cover

There is a clear recognition of the flexibility of reinforced concrete in providing fireresistance through the design and detailing process. Reference is made to the coverassociated with reinforced concrete beams that would suggest the figures in Table 3 areconservative.


Construction and materials Minimum overall size (mm) for period of:

4 hours 2 hours 1 hour 1/2 hour

Aggregates in accordance 457 305 203 153with BS 882

Construction and materials Minimum concrete cover to main reinforcement (mm) for period of:

4 hours 2 hours 1 hour 1/2 hour

Aggregates in accordance 64 51 25 13

with BS 882

Table 2 Fire resistance of reinforced concretecolumns (CP 114: 1957)

Table 3 Fire resistance of reinforced concretebeams (CP 114: 1957)

Type of construction Fire resistance period (hours)

6 hours 4 hours 2 hours 1 hour 1/2 hour

Walls and partitions1

Class 1 aggregates 203 152 102 76 76

Class 2 aggregates 228 178 102 76 76

Solid reinforced concrete slab 178 152 127 102 89

Reinforced concrete columns

Class 1 aggregates 305 – 508 254 – 305

Class 2 aggregates 305 – 5082 254 – 3053

Table 4 Minimum thicknesses (mm) to achieve indicated performance (NBS 12, 1953)


CP 110: 1972The next major change came with the publication of CP 1108 in 1972. Anchor9 hastabulated minimum section sizes and cover from CP 110 and compared the provisions tothe Building Regulations requirements and alternative European specifications. Table 5below summarises the code provisions from CP 110.

Notes* Mesh reinforcement required in concrete cover.

Figures in brackets refer to lightweight aggregate concrete, concrete cover is an average value.

The Code indicated that for beams and slabs (both reinforced and prestressed) theinfluence of restraint could be incorporated into the design by reducing the requirementsfor cover to the next lowest category. For instance, a reinforced concrete beam simplysupported would require a minimum width of 180 mm and an average cover to thereinforcement of 45 mm for a 2-hour fire resistance period. If the beam is built into astructure so as to provide restraint against thermal expansion at both ends then therequirement for cover was reduced to 35 mm, although the minimum width remainsunchanged. The provisions of the Code related specifically to end restraint against thermalexpansion rather than continuity over the supports.

It is interesting to note that there was a distinction in CP 110 between solid and coredslabs, with cored slabs requiring minimum thicknesses in excess of that for solid slabs; thisis presumably based on the insulation criteria. These separate provisions have beenremoved in Part 1 of the present Code2 1997 and are not clearly presented in Part 21 1995.Clause 4.2.5 of Part 2 mentions that an effective thickness should be used for cored slabsdepending on the proportion of solid material per unit width of slab. It would be muchclearer to include a separate table for cored slabs or at least to incorporate an additionalnote to Table 4.4 in the Code.


Table 5 Provisions of CP 110: 1972 Type of Minimum dimensions (mm) for a fire resistance (hours) of:construction

4 3 2 11/2 1 1/2

Reinforced concrete solid slab

Thickness 150 150 125 125 100 100Cover 25 25 20 20 15 15

Reinforced concrete hollow cored slabs

Thickness 190 175 160 140 110 100Cover 25 25 20 20 15 15

Prestressed solid slab

Thickness 150 150 125 125 100 90Cover 65* 50* 40 30 25 15

Prestressed hollow cored slab

Thickness 190 175 160 140 110 100Cover 65* 50* 40 30 25 15

Reinforced concrete simply supported beam

Width 280 (250) 240 (200) 180 (160) 140 (130) 110 (100) 80 (80)Cover 65* (50) 55* (45) 45* (35) 35 (30) 25 (20) 15 (15)

Prestressed simply supported beam

Width 280 (250) 240 (200) 180 (160) 140 (130) 110 (100) 80 (80)Cover 100* (80) 85* (65) 65* (50) 50* (40) 40 (30) 25 (20)

Columns (4-sided exposure)

Minimum dimension 450 (300) 400 (275) 300 (225) 250 (200) 200 (150) 150 (150)


BS 8110: 1985 and 1997The current Code provisions in BS 8110 refer to tabulated data published in Guidelines forthe construction of fire resisting structural elements10. As a document referenced inApproved Document B, the provisions in these BRE guidelines effectively become one wayof achieving the requirements of the regulations in terms of minimum thickness andminimum cover. One of the major changes is the variation in requirements for normal-weight and lightweight concrete and, for beams and floors, the recognition of the impactof continuity on the performance of reinforced concrete elements in fire.

As there are extra provisions to allow for concrete density and continuity, no attempt ismade to tabulate the provisions in a single table. The provisions relating to columns,beams, plain soffit (flat slab) floors and ribbed open soffit (downstand) floors are detailedin Tables 6 to 9. In relation to beams and floors the tables also provide information onprestressed concrete. Values from Part 1 of BS 8110 are included in brackets in Tables 6to 8 where appropriate. It should be noted that the covers in Part 1 relate to cover to allreinforcement (including links) while the values in Part 2 and the BRE guidelines arespecified in terms of cover to the main reinforcement. For consistency the values fromTable 3.4 of Part 1 have therefore been increased by 10 mm for beams and columns toallow for the inclusion of links to the main reinforcement. This approach is consistent withthe guidance in the Code.

NotesThe guidelines allow for a decrease in cover for a corresponding increase in width.

* Reduced to 25 mm where the maximum aggregate size is less than or equal to 15 mm.


Table 6 Minimum dimensions for reinforcedconcrete columns from BRE guidelines (BS 8110)

Type of Minimum dimensions (mm) for a fire resistance (hours) of:

construction4 3 2 11/2 1 1/2

Fully exposed

Dense concreteWidth 450 (450) 400 (400) 300 (300) 250 (250) 200 (200) 150 (150)Cover 35 (35) 35 (35) 35 (35) 30 (30) 25 (30)* 20 (30)*

Lightweight concreteWidth 360 320 240 200 160 150Cover 35 35 35 25 20 20

50% exposed

Dense concreteWidth 350 300 200 200 160 125Cover 35 30 25 25 25 20


1 face exposed






Reinforced simply supported

Dense concreteWidth 280 (280) 240 (240) 200 (200) 150 (200) 120 (200) 80 (200)Cover 80 (80) 70 (70) 50 (50) 40 (30) 30 (30)* 20 (30)*


Reinforced continuously supported

Dense concreteWidth 240 (280) 200 (240) 150 (200) 120 (200) 80 (200) 80 (200)Cover 70 (60) 60 (50) 50 (40) 35 (30)* 20 (30)* 20 (30)*


Prestressed simply supported



Prestressed continuously supported




Table 7 Minimum dimensions for concretebeams from BRE guidelines (BS 8110)

NotesThe guidelines allow for a decrease in cover for a corresponding increase in width.

* Reduced to 25 mm where the maximum aggregate size is less than or equal to 15 mm.






Dense concreteThickness 170 (170) 150 (150) 125 (125) 110 (110) 95 (95) 75 (75)Cover 55 (55) 45 (45) 35 (35) 25 (25) 20 (20) 15 (20)*

Lightweight concreteThickness 150 135 115 105 90 70Cover 45 35 25 20 15 15


Dense concreteThickness 170 150 125 110 95 75Cover 45 (45) 35 (35) 25 (25) 20 (20) 20 (20) 15 (20)*



Dense concreteThickness 170 150 125 110 95 75Cover 65 55 40 30 25 20


Prestressed continuously supported

Dense concreteThickness 170 150 125 110 95 75Cover 55 45 35 25 20 20


Table 8 Minimum dimensions for plain soffitfloors from BRE guidelines (BS 8110)

Note* Reduced to 15 mm where the maximum aggregate size is less than or equal to 15 mm.


NoteWidth refers to the width of the downstand portion of the floor at the level of the lowest reinforcement.

The relationship between the various documents referenced in this report and thecorresponding national regulations and codes is illustrated in the flowchart Figure 1.





Dense concreteThickness 150 135 115 105 90 70Width (and cover) 175 (65) 150 (55) 125 (45) 110 (35) 90 (25) 75 (15)

Lightweight concreteThickness 130 115 100 95 85 70Width (and cover) 150 (55) 125 (45) 100 (35) 85 (30) 75 (25) 60 (15)


Dense concreteThickness 150 135 115 105 90 70Width (and cover) 150 (55) 125 (45) 100 (35) 90 (25) 80 (20) 75 (15)



Dense concreteThickness 150 135 115 105 90 70Width (and cover) 175 (65) 150 (55) 125 (45) 110 (35) 75(25) 70 (20)


Table 9 Minimum dimensions for ribbed open soffit floors (BS 8110)

Table 10 Minimum dimensions for concretewalls with vertical reinforcement (BS 8110)

Type of construction Minimum dimensions thickness/cover (mm) for a fireresistance (hours) of:

4 3 2 11/2 1 1/2

Walls made from denseaggregate with less than 175 150 1500.4% reinforcement

Walls made from denseaggregate with 0.4 – 1% 240/25 200/25 160/25 140/25 120/25 100/25reinforcement

Walls made from 190/25 160/25 130/25 115/25 100/20 100/10lightweight aggregate

Walls made from denseaggregate with over 1% 180/25 150/25 100/25 100/25 75/15 75/15reinforcement



Figure 1 Relationship between documentsreferenced and national regulations and codes

National Building Studies 12 (1953)7 and National Building Studies 18 (1953)11:Investigations on building fires

CP 114: 19576

CP 110: 19728

Design and detailing ofconcrete structures for fire

resistance: Interim guidance bya Joint Committee of

The Institution of StructuralEngineers and

The Concrete Society (1978).14

Tables produced by the Fire Research Station

Guidelines for theconstruction of fireresisting structural

elements:BRE Report (1982)10

Guidelines for the construction of fire

resisting structural elements:

BRE Report (1988)10

BS 8110-2 (1985)1

FIP/CEBRecommendations for

the design of reinforcedand prestressed

concrete structuralmembers for fire

resistance (1975)12

Schedule 8: BuildingRegulations 197615

FIP/CEB Report onmethods of assessmentof the fire resistance of

concrete structuralmembers (1978)13

Approved Document ADB 198516

Approved DocumentADB 199117, ADB 200018

BS 8110-1(1985 and 1997)2


The provisions according to the various standards are summarised in Table 11 withreference to examples of specific structural elements.

Notes1 Class 1 aggregates2 Class 2 aggregates3 Increased to 4 hours with light mesh in cover4 Increased to 2 hours with light mesh in cover5 Dense concrete 6 Lightweight concrete7 Thickness and cover dependent on load ratio at ambient temperature and reinforcement ratio8 Simply supported/dense concrete9 Simply supported/lightweight

10 Continuous/dense concrete11 Continuous/lightweight12 Simply supported one way spanning

The extent of the notes required for use with Table 11 provide some indication of the careto be taken in the use of tabulated data. The table itself illustrates the gradualdevelopment of knowledge related to the performance of concrete structures in fire. Theclose correlation between the UK codified values and the values taken from NationalBuilding Studies Research Paper No. 12 show the importance of this body of work on thedesign of concrete structures in fire. The results from this publication are investigated ingreater detail later in this report. What is also significant is the development of specificprovisions for different types of aggregate, for lightweight concrete and for the effects ofcontinuity.

Comparison between tabulatedvalues from different codes


Table 11 Comparison of provisions forcommon forms of construction

Type of structural Minimum dimensions thickness/cover (mm) for a fireelement resistance period (hours) of:

4 3 2 11/2 1 1/2

Reinforced column

CP 114: 1948 >305 254 - 3051

CP 114: 1957 457 305 203 153

NBS Paper No. 121 305 – 508 254 – 305

NBS Paper No. 122 305 – 5083 254 – 3054

CP 110 (1972) 450 400 300 250 200 150

BS 81105 (1985) 450/35 400/35 300/35 250/30 200/25 150/20

BS 81106 (1985) 360/35 320/35 240/35 200/25 160/20 150/20

EC2-1-27 400 – 600 300 – 600 200 – 550 150 – 500 150 – 300 150 – 200

Solid reinforced slab

CP 1141 (1948) 152 102 76 76

CP 1142 (1948) 178 102 76 76

CP 114 (1957) 152/25 127/13 102/13 89/13

NBS Paper No. 12 152 127 102 89

CP 110 (1972) 150/25 150/25 125/20 125/20 100/15 100/15

BS 81108 (1985) 170/55 150/45 125/35 110/25 95/20 75/15

BS 81109 (1985) 150/45 135/35 115/25 105/20 90/15 70/15

BS 811010 (1985) 170/45 150/35 125/25 110/20 95/20 75/15

BS 811011 (1985) 150/35 135/25 115/20 105/20 90/15 70/15

EC2-1-212 175/65 150/55 120/40 100/30 80/20 60/10


Unless otherwise specified, comparison between measured data and assumed fire periodsare on the basis of both minimum dimension and minimum cover.

National Building Studies research papers It is widely acknowledged that the main source of data for the derivation of the tabulatedvalues in successive revisions of the concrete codes of practice comes from fire resistancetests carried out at the Fire Research Station, Borehamwood between 1936 and 1946. The results from these tests are summarised in National Building Studies Research PaperNo. 127 (NBS 12) for a range of different structural elements and different constructionmaterials. Additional source material related to reinforced concrete columns is containedin National Building Studies Research Paper No. 1811. The test results have been revisitedas part of this project and analysed with respect to the concept of load ratio. This performance-based approach is not included in BS 8110 for columns and walls. The concept of load ratio relates the load applied at the fire limit state to the capacity ofthe element at ambient temperature. This is the basis of the provisions for columns set outin the fire part of the Eurocode for concrete structures19.

Reinforced concrete floorsTable A1 of Appendix A is a summary of test data in relation to reinforced concrete floors.One of the most significant aspects is the extent of spalling to the specimens and theimpact of spalling on the overall fire resistance attained.

The experimental values used as the basis of the Code provisions may be classifiedaccording to the support conditions adopted during the standard fire tests. Figure 2illustrates the results from simply supported slabs in terms of fire resistance periodattained against the load ratio. The load ratio has been calculated using the designformulae from BS 8110: 1985 incorporating the partial safety factors but using the actualconcrete material properties as measured during the experimental programme (Table A1).No information was available for the measured values of the steel yield stress so anassumption that fy = 250 N/mm2 has been made for all cases. It could be argued that thecalculation should be performed without safety factors for the concrete strength. The measured values shown in the figures are therefore worst-case values in terms of theload ratio or degree of utilisation.

Figure 2 indicates the fire resistance period attained or time to failure as well as the modeof failure and the critical parameters of overall thickness and cover to the mainreinforcement compared with the tabulated values in the Code.

Generally the Code provisions are very conservative when compared with the experimentalresults. The specific combinations of thickness and cover make a direct comparisondifficult. In general the mode of failure was cracking through the full depth of the slableading to collapse into the furnace. Spalling was not particularly significant with theexception of one specimen made from limestone aggregate, which collapsed after 35minutes.

Loadbearing failure in a standard test is generally a function of the temperature of thereinforcement. The most effective way to delay such a failure is to increase the cover tothe main steel. The thickness of the specimen will determine the temperature of theunexposed face (insulation criteria). If cover is used as the means of assessment then alltest specimens achieved at least the requirement of the Code and in many cases themeasured performance was well in excess of that assumed using the tabulated values.This is illustrated in Figure 3.

Experimental backgroundto tabulated values



It is clear from the test results that loadbearing failure is the critical mode for all the slabswhere the concrete is directly exposed to the fire. The increased cover will, in the absenceof spalling, lead to increased fire resistance periods. The provisions in relation to simplysupported slabs are therefore conservative due to insufficient variation in the value ofapplied load to determine the sensitivity of the results to the load ratio. However, as thepredominant mode of failure is collapse into the furnace it could be assumed that lowervalues of imposed load (such as are used for the fire limit state) would increase the fireresistance.


Figure 2 Applied load ratio and failure time forsimply supported floor slabs (NBS 12, 1953)

Figure 3 Comparison of measured andassumed design values based on depth

of cover for simply supported floor slabs (NBS 12, 1953)

0

25

50

75

100

125

150

175

200

225

250

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Load ratio

Tim

e (

min

ute

s)

Plaster finish

Plaster finish

140/25

127/13

127/25

127/13

114/13

114/13

114/13

114/13

114/13

125/35

Current

Code

requirements

110/25

95/20

Plaster finish

75/15

0

25

50

75

100

125

150

175

200

225

250

F20 F16 F21 F19 F17 F22 F18 F23 F24

Sample reference

Fir

e e

nd

ura

nce/r

esis

tan

ce (

min

ute

s)

Plaster finish

Plaster finish

Plaster finish

Fire endurance measured

Fire resistance assumed

Did not fail

during test

Insulation

Loadbearing

Failure mode


Figure 4 Comparison of measured andassumed design values based on minimumthickness for simply supported floor slabs

Figure 4 is a comparison of measured values and assumed fire resistance periods from thetabulated data based purely on the minimum thickness of the floor slab. Apart from onerogue value (F20) the results support the Code provisions. Although it is useful toinvestigate the assumptions in terms of loadbearing capacity (minimum cover) andinsulation (minimum thickness) separately they need to be taken together. For simplysupported slabs the measured results, taken together, support the provisions of thecurrent Code.

For continuous slabs the Code provisions maintain similar values for minimum thicknessbased on the insulation criteria and allow for a reduction in the cover to the mainreinforcement. Figure 5 shows the results for continuous (BS 8110) or restrained (NBS 12)slabs. The different terminology is important as the reduction in cover in CP 110 was basedon the influence of restraint to thermal expansion while structural continuity implies arotational restraint such as that found over supports. One of the most significant results isthe noticeable increase in spalling when compared with the results from the simplysupported slabs.



Fire resistance assumed0

25

50

75

100

125

150

175

200

225

250

F20 F16 F21 F19 F17 F22 F18 F23 F24

Sample reference

Fir

e e

nd

ura

nce/r

esis

tan

ce (

min

ute

s)


Apart from one test all specimens had a cover of only 13 mm. In terms of the provisionsof the Code in relation to minimum thickness and minimum cover, all the specimens tested(with the exception of the specimen attaining a fire resistance period of 6 hours) would beexpected to have a fire resistance of only 30 minutes. The comparison between measuredand assumed resistance is shown in Figure 6.


Figure 6 Comparison of measured andassumed design values based on depth of

cover for restrained floor slabs (NBS 12, 1953)

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

F53 F34 F33 F49 F48 F73 F71 F25 F77 F74 F45 F68 F72 F76 E3/S1 F67 F63

Sample reference

Fir

e e

nd

ura

nce/r

esis

tan

ce (

min

ute

s)

Sprayedasbestos



Figure 5 Applied load ratio and failure timefor restrained floor slabs

Insulation

Loadbearing

Did not fail

during test

Integrity

Results for specimensthat spalled are shownthus: 152/13 and othersthus: 114/13

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Load ratio

Tim

e (

min

ute

s)

178/38

152/13

152/13 152/13

152/13

182/13

114/13114/13

114/13

102/1382/13

82/13

82/1382/13

82/13

102/13

102/13

75/15

95/20

110/20

125/25

150/35

170/45Sprayed asbestos

Plaster

covered

Current

Code

requirements

Failure mode


Only one specimen lies below the assumed design value and in many cases the measuredvalues show the tabulated data to be extremely conservative with 4 hour fire resistanceperiods attained for specimens with only 13 mm cover. The load ratios used in the test arevery high. From the data available the performance of the floor slabs does not seem to besignificantly influenced by the value of the imposed load. Of much greater significance isthe degree of spalling observed. Where the specimens failed to attain a fire resistanceperiod of 2 hours the reason for this, and for the subsequent integrity failure, was due tospalling. Figure 6 indicates that spalling has been taken into account in thedevelopment of the tabulated data and accounts in large part for the degree ofconservatism present. The tabulated values have been set according to a mode offailure driven largely by spalling.

Reinforced concrete wallsThe information on reinforced concrete walls from the National Building Studies report isincluded in Table A2 of Appendix A. Although there are only three tests in this categorythe results indicate the improved performance of crushed brick aggregate compared withgravel.

Figure 7 shows the results for reinforced concrete walls in terms of the Code provisionswith respect to medium (0.4 to 1.0%) and high (greater than 1.0%) reinforcement.Although there are obviously not many tests on which to base conclusions, it is clear thatthe results available show that the Code provisions are reasonable and that load ratiodoes not have a significant effect on performance as the failure criteria isgoverned by the insulation properties of the concrete. European research alsosupports this conclusion.


Figure 7 Applied load ratio and failure timefor reinforced concrete walls

Insulation

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requirements

Failure mode


Reinforced concrete columnsAlthough there may be few test results to assess the provisions for walls the same cannotbe said for reinforced columns. A total of 95 individual tests have been analysed and theresults processed according to section size and applied load.

Column sizes: 152, 203 and 229 mm Figure 8 shows the results from tests on 152 mm, 203 mm and 229 mm square columnsin terms of load ratio while Figure 9 is a comparison between measured and assumed fireendurance/resistance periods.

The results generally support the provisions of the Code and indicate a relationshipbetween the amount of load present and the fire endurance period attained.

The results above show the Code provisions to be generally conservative and indicate arelationship between applied load and fire endurance period attained.


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Figure 8 Applied load ratio and failure time forsmall reinforced concrete columns

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Figure 9 Measured and assumed values of fire endurance/resistance for small

reinforced concrete columns



152 mm

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Column size 254 mmFigure 10 shows the results for 254 mm square columns while Figure 11 shows therelationship between the measured values and the assumed fire resistance period takenfrom the tabulated data in the Code.

Again the results indicate that the Code provides a generally conservative approach.Figure 10 also shows some correlation between load ratio and decreasing fireperformance of columns. Although the Code provides a conservative approach it does nottake into account the complex interaction between applied load and fire resistance.


Figure 10 Applied load ratio and failure timefor 254 mm square columns

Figure 11 Comparison of failure time withassumed fire resistance period



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Column size 280 mmFigure 12 shows the relationship between applied load and failure time for 280 mm squarecolumns. Here the relationship between applied load and time to failure is not sostraightforward.

The comparison between fire endurance period achieved in the tests and fire resistanceassumed in design is shown in Figure 13 below.

There are some instances where the assumed period of fire resistance is less than thatachieved during the test for this particular size of column. However, the only areas wherethere is a significant shortfall in performance relates to a specimen where the load ratio isin excess of 0.5. This is a higher ratio than is generally seen in practice for the fire limitstate.


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Figure 13 Comparison between failure timeand fire resistance period assumed

(280 mm square columns)




Column size 305 mmThe corresponding values for 305 mm square columns are shown in Figures 14 and 15below. Again this shows the relationship between applied load and fire endurance periodand indicates that the test data generally supports the tabulated values from the Code.

There is an important relationship between minimum dimensions for width and cover. If cover is seen to be the governing factor in terms of slowing the increase in thetemperature rise of the reinforcement then the results should be compared against apresumed performance of 1 hour. However, if the overall size of the column is the maindeterminant then the comparison should more effectively be made against a fireresistance period of 2 hours.


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Figure 15 Comparison between failure timeand fire resistance period assumed (305 mm square columns)




Column size 355 mmThe plot for the 355 mm square columns is shown in Figure 16. This indicates a significantreduction in fire performance with increasing values of load ratio. However, the appliedload is much higher than typically applied in practice and, in some cases, could have ledto a structural collapse even before the start of the fire test. It would only take a smalleccentricity of loading to exceed the ambient temperature capacity at a load ratio of 0.9.

Figure 17 shows the comparison between assumed fire resistance and measured failuretimes. Although these results do not support the Code provisions they need to be viewedin the light of the comments related to load ratio above.

From observations during the testing, the failures were attributed to spalling of the corners ofthe columns. This behaviour is accounted for in the Code through the requirement foradditional measures (such as the use of supplementary mesh within the cover zone) once thecover exceeds 40 mm. As the cover increases the fire performance of the concrete memberwill increase, assuming no spalling takes place. However, increase in cover increases thesusceptibility of the member to spalling. There is therefore a need to either limit the cover (thetabulated values for columns do not exceed 35 mm) or take additional measures. Theincidence of spalling means that there is no direct correlation between increasing cover andincreasing periods of fire resistance. The provisions of the Code take this into account.


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Figure 17 Comparison between failure timeand fire resistance period assumed

(356 mm square columns)




Column size 381 mmThe results for the 381 mm square columns are shown in Figure 18. There is a clearcorrelation between the load ratio and the performance in fire. Here the relationship isalmost linear. The values above a load ratio of about 0.7 need to be viewed in the light of thecomments made above in relation to practical load ratios in operation at the fire limit state.

The comparison with assumed fire resistance period is shown in Figure 19. Again thisneeds to be viewed in the light of the comments about the importance of applied load andalso that the comparison has been made for a cover of 25 mm. The actual column size ismuch greater than the minimum width for the 1 hour category (width 200 mm, cover 25 mm)and it may be that a 11/2 hour fire resistance period (width 250 mm, cover 30 mm) wouldbe a more accurate basis for comparison.

However on the basis of a direct comparison with the provisions of the Code, theexperimental results support the tabulated values. The only member that failed to achievethe prescribed period of fire resistance had a load ratio in excess of 0.9, which isconsidered to be unrealistic for both the fire and ambient temperature limit states.


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Column size 406 mmFigure 20 is the plot for load ratio against failure time for the three tests on 406 mm squarecolumns. What is again quite apparent is the relationship between applied load and fireendurance period. Effectively the failure period of 46 minutes can be discounted from thecomparison because of the excessive load imposed at the time of the test. Again thecomparison with assumed fire resistance periods based on the tabular data will be madefor the 1 hour period because the cover is still only 25 mm. The fire resistance periodattained in the tests for the two columns is therefore twice the assumed value.


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Figure 20 Applied load and failure time for406 mm square columns


Column size 483 mmThe results from the tests on 483 mm square columns are shown in Figure 21 and thecomparison with the assumed values from the tabulated data is shown in Figure 22.

The results from tests on loaded reinforced concrete columns are contained in Table A3of Appendix A and in Appendix B. The comprehensive test programme includes a largerange of sizes, aggregate types and levels of reinforcement. Again the impact of spallingon the performance of reinforced concrete members is highlighted. In many cases thespalling did not lead to collapse of the columns and the commentary often points to cornerspalling or sloughing off rather than explosive spalling. Spalling generally occurred at thecorners of the columns, In many cases the corners were chamfered, but this did notappear to make a great deal of difference to the overall fire resistance attained nor did itprevent significant spalling taking place. Moisture content does not appear to have had asignificant influence. The measured values of moisture content for this size of columnsvaried from 2.3% to 3.48% by weight and there is no variation in the extent or nature of thespalling observed that could be related to the difference in measured moisture content.


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Cover = 32 mm


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Reinforced concrete beamsTable A4 of Appendix A contains the results from tests on reinforced concrete beamstaken from the National Building Studies Research report. From these particular testsmoisture content appears to have had no specific influence on whether the specimenspalled or not. Spalling occurred on one of the specimens with a measured moisturecontent of 4% by weight whereas no spalling was observed for the duration of the test fora similar section with a measured moisture content of 4.7%. However, it should be notedthat this contradicts other published research which shows that moisture content has acritical influence on susceptibly to spalling. The results also indicate that spalling does notnecessarily lead to a reduction in the fire resistance period attained.

Figure 23 shows the results for reinforced concrete beams in terms of applied load andtime to failure. The results are plotted using the BS 8110 design equations both with andwithout the safety factors for material variability. The effect of ignoring the material safetyfactors is to reduce the load ratio. However, the performance seems to be largelyindependent of the applied load. The Code requirements in terms of minimum dimensionsfor width of beams and cover are indicated on the right of the figure.

Two things are immediately apparent from the figure. Firstly, the load ratio is very high and,secondly, the Code provisions are extremely conservative. In the case of beams it isloadbearing that is the critical factor, not integrity. Therefore the critical parameter is theminimum cover to the reinforcement.


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Current Code

requirementsFigure 23 Applied load ratio and failure timefor reinforced concrete beams

Did not fail during test

Loadbearing

250 N/mm2, includes BS 8110 safety factors

250 N/mm2, excludes BS 8110 safety factors

272 N/mm2, excludes BS 8110 safety factors

Failure mode

sdS

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Figure 24 is a comparison between the assumed period of fire resistance and the testresults.


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1 (F42) 2 (F43) 3 (F44) 4 (F51) 5 (F52) 6 (E16/S9)

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Figure 24 Comparison between failure timesand assumed period of fire resistance(reinforced concrete beams)




Although the National Building Studies reports provided a large part of the backgrounddata on which the Code provisions are based, there is insufficient data in the reports toextend the provisions to lightweight concrete, prestressed concrete and ribbed open soffitfloors. The additional provisions for these have arisen as a result of a number of researchprogrammes and commercial tests undertaken over a number of years. Some of the mostsignificant research projects are briefly reviewed here, although it is impossible todetermine the extent to which each project influenced the development of the Codeprovisions.

A report (in two volumes) published by BRE20 details the results from sponsored fire testsundertaken between 1950 and 1972. A summary of the test results is given in Appendix C,covering floors, columns and beams.

What should be immediately clear is the difference between published research resultsand the results from sponsored commercial tests. Commercial tests were designed toprovide a justification for the stated period of fire resistance. Hence little information isprovided on the types of aggregate used, moisture contents, reinforcement details,restraint conditions or whether spalling occurred. In general the mode of failure (stability,integrity and insulation) is not made clear. Where the information provides only the fireresistance period attained it has been assumed that this was the target value and the testwas stopped at this point. However, the results do provide some justification for the statedfire resistance periods quoted by manufacturers and the tabulated provisions relating tolightweight concrete and hollow core slabs.

The results for reinforced columns come largely from work commissioned through theDepartment of the Environment and therefore more details of these research-based testsare available for scrutiny.

Fire Research NotesFire Research Note No. 460, Malhotra, H L. Effect of restraint on fireresistance of concrete floors 21

Malhotra reported the results from tests to determine the effects of restraint on the fireresistance of concrete floors and compared the results with similar tests carried out inHolland. The tests were carried out on precast hollow beams with a concrete topping.Three different levels of restraint were investigated, ranging from simply supported tolongitudinal and angular restraint. Some limited spalling occurred during the third test.The first two tests provided a direct comparison of the effects of longitudinal restraint onthe fire resistance of reinforced concrete floors. The inclusion of longitudinal restraintresulted in an increase of fire resistance of approximately 50% based on a displacementcriteria of span/30. Malhotra pointed out that damage by spalling may, however, be greaterwith increasing restraint.

Fire Research Note No. 38, The fire resistance of prestressed concrete 22

This Note claims that spalling of concrete leading to premature failure only occurs withsmall prestressed beams of slender section directly exposed to the fire. It is estimated thata fire resistance of 2 hours can be obtained with a concrete cover to the steel ofapproximately 63 mm. This is consistent with the Eurocode values and the BS 8110 valuesfor prestressed beams.

Fire Research Note 54, Ashton, L A, Prestressed concrete during andafter fires. Comparative tests on composite floors in prestressed andreinforced concrete 23

This study showed very little difference in performance between reinforced andprestressed construction for short periods of exposure (1/2 hour). The maximum deflectionswere of the same order, with the prestressed floor recovering more quickly indicating noloss of prestress. However for 1 hour exposure the reinforced concrete floor exhibited

Other relevant research



lower deflections and a better rate of recovery. Both types of floor had similar cover to thesteel, based on the requirements of the London County Council Byelaws for 1 hours fireresistance.

Fire Research Note 65, Ashton, L A and Malhotra, H L, The fireresistance of prestressed concrete beams 24

An experimental study was undertaken on scaled-down specimens representative oflarger post-tensioned beams used in buildings of high fire risk. The results indicate thatfull-scale beams of the type tested should give a fire resistance of 2 hours withoutrecourse to special measures, but for 4 hours resistance, extra protection would benecessary for the tendons. This study was initiated due to a sudden brittle failure in astandard fire test of a small prestressed concrete floor unit due to spalling.

Fire Research Note 741, Malhotra, H L, Fire resistance of structuralconcrete beams 25

Malhotra pointed out the scarcity of data in relation to the performance of reinforcedconcrete beams when compared with prestressed beams (see above). The programmeinvolved tests on 24 beams including 13 simply supported beams with a clear span of 7.3 m and an overall length of 7.6 m. Additional specimens were cast with an overall lengthof 11.3 m supported over the same span as the smaller beams and with loaded cantileverends to produce negative bending moments at the supports to simulate continuousconstruction. The overall test programme consisted of 24 tests including prestressed andencased steel beams as well as reinforced concrete. The factors studied were:

• Type of beam – reinforced, prestressed, steel encased

• Type of aggregate – normal (gravel), lightweight (expanded clay and foam slag)

• Type of steel – mild steel, cold worked steel, hot rolled alloy steel

• Thickness of cover – varied between 25 and 63 mm

• Use of supplementary reinforcement

• End conditions – simply supported, simply supported with continuity

Leaving aside the encased steel beams, Tables 12 and 13 describe the main testparameters for prestressed and reinforced beams respectively.


No. Type of Type of beam Shape of Cover Supplementary aggregate x-section (mm) reinforcement (Y/N)

7.6 m long specimens

1 Gravel Post-tensioned with tendons Rectangular 100 Y

2 Gravel Pre-tensioned with tendons Rectangular 100 Y

3 Gravel Pre-tensioned with strands I section 50 N

4 Gravel Pre-tensioned with strands I section 50 Y

5 Gravel Pre-tensioned with strands I section 50 N*


6 Gravel Post-tensioned with strands Rectangular 100 Y

7 Gravel Post-tensioned with strands Rectangular 100 Y

Table 12 Test parameters for prestressedconcrete beams

Note* Encasement of 13 mm plaster


The results from these tests are summarised in Appendix D. The specimens were storedfor periods up to three years to ensure a stable moisture content. The tests were generallyterminated prior to collapse by carefully monitoring deflections. However, in three cases(3, 6 and 7) collapse occurred before the test could be terminated.

For the reinforced concrete beams made with gravel aggregate the occurrence of spallinggreatly reduced the fire resistance achieved. Additional specimens were therefore madewhich incorporated supplementary reinforcement in cases where cover thicknesses werelarge.

The tests confirmed the improved performance in fire of beams made using lightweightaggregates. The specimens made from lightweight concrete withstood heating for 6 hours without showing any signs of spalling or reduction in concrete cover, whereas thesiliceous aggregates showed signs of damage by spalling within the first 30 minutes of thetest, with the extent of the damage varying from specimen to specimen. The report wasquite specific about the effect of spalling on fire resistance of concretes made fromsiliceous aggregates, stating that premature failure was due to spalling. The inclusion of asupplementary mesh 25 mm below the exposed surface gave at least the expectedperformance and in some cases better than expected. The report emphasised the need toadopt mitigating measures when dealing with concretes made with siliceous aggregates.

The report recommended the use of supplementary reinforcement for covers in excess of40 mm. This is consistent with the provisions of CP 110: 1972 and included in BS 8110:(clause 4.3.4). The current requirements of 40 mm for dense concrete and 50 mm forlightweight concrete are conservative in relation to lightweight aggregate where it issuggested supplementary reinforcement is not required for covers up to 63 mm. The timetaken for the reinforcement to reach a temperature of 550ºC was 360 minutes for thelightweight aggregate and 260 minutes for the gravel aggregate. The thickness of concretecover to limit the rise in temperature for a given size of beam is inversely proportional tothe square root of thermal diffusivity. It is concluded that, for lightweight aggregates areduction in cover of about 20% is possible for similar performance in fire. This isconsistent with the provisions in BS 8110.


No. Type of Type of beam Cover Supplementary aggregate (mm) reinforcement (Y/N)


8 Gravel Mild steel 63 N

9 Gravel Cold worked deformed 63 N

10 Gravel Cold worked twisted 63 N

11 Gravel Hot rolled alloy 63 N

12 Expanded clay Mild steel 63 N

13 Foamed slag Mild steel 63 N

14 Gravel Mild steel 63 Y

15 Gravel Hot rolled alloy 63 Y

16 Gravel Cold worked twisted 63 Y

17 Gravel Hot rolled alloy 38 Y




20 Gravel Mild steel 63 N

21 Gravel Cold worked deformed 63 N

Table 13 Test parameters for reinforced concrete beams


What is of primary interest is how the experimental evidence discussed above relates tothe provisions of the national standard. There is a direct link between the groundbreakingstudies reported in the National Building Studies report and the tabulated values in BS8110. One useful means of looking at the various inter-relationships between theprovisions of the Codes and the available research results is to start with a standardsolution from the 1948 tabulated values and see how this changes with time. If we takethe simple examples of a solid reinforced concrete slab and a reinforced concrete columnsubjected to a four-sided exposure then in some instances very little has changed over theyears. Where no restrictions on the type of aggregate apply and the effect of continuity isnot allowed for the situation is summarised in Table 14 in terms of specified minimumdimensions and (where appropriate) minimum cover for a fire resistance period of 2 hours.

Notes* Axis distance rather than cover to main steel** Axis distance rather than cover, based on a load ratio of 0.5

Over the years the codes have been extended to provide more information on the effectsof continuity, the inclusion of prestressed concrete, the use of lightweight concrete, thechoice of aggregate and the depth of cover.

From the data analysis undertaken in this project a number of general conclusions can bedrawn:

• For reinforced concrete walls, the test data supports the provisions of the Code.

• On the basis of the limited data available, load ratio does not have a significant impacton the performance in fire of reinforced concrete walls.

• For reinforced concrete columns, the test data has highlighted the important influence ofapplied load on their performance in fire. This is discussed below in relation to Figure 25.

• For reinforced concrete beams, the test data indicates that the provisions for their fireresistance are extremely conservative. In some cases measured performance is morethan three times greater than the assumed design value based on the tabulatedapproach.

• For restrained (continuous) floor slabs the test data supports the provisions of the Codein relation to their fire performance

• For simply supported reinforced concrete floor slabs the test data supports theprovisions of the Code in relation to their fire performance.

• For all floor slabs, load ratio does not have a significant influence on the slabs’performance in fire.

• The provisions of the Code in terms of restrained floor slabs take spalling into accountthrough a reduction in the minimum value for cover to the main reinforcement.However, there is some confusion over the use of the terms “restrained” and“continuous”. There is evidence from standard tests to support the CP 110 approachbased on restraint to thermal expansion (lateral restraint). However, the limitedexperimental evidence available19 does not support the view that the same benefits canbe achieved through structural continuity (rotational restraint).

Discussion


Table 14 Code provisionsSource document Solid reinforced concrete Reinforced concrete column slab – minimum (4 sided exposure) – minimumdimension/cover (mm) dimension/cover (mm)

CP 114: 1948 127 305

CP 114: 1957 127/13 305

National Building Studies 127 305

CP 110: 1972 125/20 300

BRE guidelines 125/35 300/35(BS 8110: 1985/1997)

EN 1992-1-2 120/40* 300/45**


• Spalling severely limits the fire resistance periods for reinforced concrete restrained floorslabs. Where spalling can be prevented, either through protection to the soffit or asuitable choice of dimensions, fire resistance periods of up to 6 hours can be achievedfor flat slabs. The prescribed values include the effect of spalling in that they have beenbased on the results of specimens where significant spalling took place.

• The provisions for spalling account for a large part of the conservatism inherent in theCode provisions.

Figure 25 shows the relationship between load ratio and fire endurance period for a rangeof different section sizes and a small variation in cover. It is important to consider theresults in light of typical values of imposed load on columns in buildings and the reducedpartial factors to be adopted for design for the fire limit state. There are no discrepanciesbetween the provisions of the Code and the test results at load ratios below approximately0.5. For column sizes of 254, 279, 305 and 406 mm only one specimen failed beforeachieving the required fire resistance period and all these specimens were loaded tovalues higher than would be typically in place in a building during a fire. At first sight theperformance of the 381 mm column (29 mm cover) looks of particular concern with sixspecimens out of a total of 13 failing to achieve the prescribed level of fire resistance.However, the values of load ratio for these specimens range from 0.77 to 0.96 (i.e. they fallwithin the shaded area). It is highly unlikely that this level of applied load would beimposed for columns in buildings.

There is therefore no concern over the performance of existing structures based on thetest data used to develop the provisions of the Code. However, this project has highlightedthe need for levels of applied load to be incorporated into the design procedures forreinforced concrete columns subject to fire. Such an approach is already included in thefire part of EC2 and will form the basis of fire design procedures in the years to come.

The simplicity of the BS 8110 tabular data, in particular figure 3.2 of BS 8110 Part 12, is ofgreat benefit to designers. However, an attempt to cover provisions for such a wide rangeof concrete members in a single table may lead to unnecessary conservatism. Forhollowcore slabs the results from standard fire tests20 suggest that dimensions


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Figure 25Test data for

reinforcedconcretecolumns

Section size and cover


significantly below the tabulated values would provide adequate levels of fire resistance.As the fire resistance of floor slabs with adequate cover is related to insulation values onemight expect the same to be true of solid slabs.

If the units are sized according to the ambient temperature loading requirements andsimply checked against the provisions of the Code this is a reasonable approach. However,if they are sized initially on the requirements for fire resistance then they are likely to beconservative and the potential exists for a reduction in the dimensions to fulfil normaltemperature structural requirements.

This applies not only to prestressed hollowcore units but also to reinforced units cast fromlightweight aggregate. Solid floor units tests20 have shown that 2 hours fire resistance canbe obtained for unprotected floor slabs made using lightweight aggregate with an overallthickness of 102 mm. This could not be achieved from a simple reliance on tabular data,which would require a minimum dimension of 115 mm to achieve the 2 hour requirement.This would suggest that the tabular data is generally conservative. The work by Malhotra21

suggests that a reduction in overall thickness could be justified, based on the effects ofstructural continuity and restraint to thermal expansion. Again this is not allowed for in theCode, which does take into account the increased likelihood of spalling through areduction in the minimum value for cover. For downstand beams a fire resistance periodof 2 hours has been achieved with a slab thickness of just 89 mm using lightweightaggregate.

For floors the critical criteria in terms of minimum dimensions relates to the insulationproperties of the material. In general this will be the critical factor in the choice of theminimum dimension for floor slabs provided spalling does not take place.

A number of obstacles remain to the development of a more rational approach to the fireengineering design of concrete structures. There is a wealth of information on theperformance of concrete elements subject to standard fire tests. However, little researchinformation is available on the performance of concrete structures subject to realistic(natural) fire exposures. The prescriptive approach adopted in BS 8110 has proved veryeffective over a number of years as indicated by the performance of real buildings in realfires. However, the design methods available in the structural Eurocodes will enable aperformance-based approach to be taken to the design of concrete structures in fire,leading to a more efficient construction.



The current Code provisions are broadly based on standard fire tests carried out by theJoint Fire Research Organisation (later FRS) at Borehamwood between 1936 and 1946.The results from research programmes have been augmented by the results from testssponsored by the concrete industry on specific products. When any departure is requiredfrom the tabulated data assessments have to be made from data that has not beenupdated or from data obtained from international sources. Whilst in many cases a reliableassessment can be made, there are a number of areas where the lack of knowledge maybe restricting the economic use of concrete.

Calcareous aggregates such as crushed brick, which give the best performance in theNational Building Studies research, are rarely used nowadays. The majority of concrete ismade from siliceous aggregates such as river gravel or granite. It is therefore appropriatefor the tabulated data to relate to siliceous aggregates as the standard case.

There is evidence that the use of lightweight aggregates gives a much improvedperformance in terms of insulation and the incidence of spalling. This is adequatelyreflected in the current Code. Evidence of the effect of limestone aggregates comparedwith siliceous aggregates is not very clear. Although the Code states that “concretes madefrom limestone aggregates are less susceptible to spalling”, this is not borne out from theresults of the National Building Studies report. Unlike previous versions, the current Codevalues do not allow for any reduction in requirements based on the use of limestoneaggregates. The BS 8110 approach is therefore valid in this respect.

There is clear evidence from performance in real fires over a number of years that thetabular approach has proved effective. This study has highlighted a number of designissues that need to be looked at in a little more detail. In general the provisions of the Codein terms of minimum cover to the reinforcement are based on limiting the temperature rise,and therefore the reduction in strength, to values of 550°C and 450°C for reinforcing barsand prestressing tendons respectively for the prescribed period of fire resistance. Theassumption is that the elements are supporting the full design load at the fire limit state.This is a conservative assumption and may lead to the inefficient use of concrete inbuildings.

The relationship between load ratio and fire resistance should be explored further. Thisconcept is already used in European standards for columns and walls through theintroduction of a factor (µfi) to take account of the load level in the fire situation. Theresults from the National Building Studies research reports highlight the importantinfluence of applied load on the fire endurance of columns.

This study of the background information underpinning the provisions of BS 8110 hasshown that the experimental results support the tabulated data in the Code in relation toassumed periods of fire resistance. In many cases the provisions are very conservative,largely due to the requirements related to the prevention of spalling and to the assumptionthat concrete elements are fully stressed at the time of the fire.

The provisions of the Code, although drawing extensively on the experimental resultsdiscussed in this report, are also informed by engineering judgement based largely onobserved performance of concrete structures in real fires. The beneficial aspects ofstructural continuity provide an enhanced level of safety above that derived from theresults of standard fire tests. The importance of adequate detailing, particularly atconnections between structural members, has been identified as a crucial aspect of thefire engineering design of concrete structures.

The development of the structural Eurocodes has provided an opportunity for UKdesigners to adopt a performance-based approach to designing concrete structures forthe effects of real fires, taking into account the beneficial aspects of whole buildingbehaviour and the inherent continuity and robustness of properly detailed concretebuildings.

Conclusions and recommendations



1. BS 8110: Part 2: 1985. Structural use of concrete, Part 2, Code of practice for specialcircumstances. British Standards Institution, London, 1985.

2. BS 8110: Part 1: 1997. Structural use of concrete. Part 1, Code of practice for design andconstruction. British Standards Institution, London, 1997.

3. Tovey, A K and Crook, R N. Experience of fires in concrete structures, in ACISymposium on evaluation and repair of fire damage to concrete. San Francisco, 16 -21 March, 1986. American Concrete Institute, Detroit, Ref SP-92

4. Hopkinson, J S. Concrete research and fire. Building Research Establishment NoteN14/81 BRE, Garston, 1981.

5. CP 114. The structural use of reinforced concrete in buildings. British StandardsInstitution, 1948.

6. CP 114. The structural use of concrete in buildings. British Standards Institution, 1957.

7. Davey, N and Ashton, L A. Investigations on building fires, Part V. Fire tests on structuralelements, National Building Studies Research Paper No. 12, Department of Scientificand Industrial Research (Building Research Station), HMSO, London, 1953.

8. CP 110. Code of practice for the structural use of concrete, British StandardsInstitution, 1972.

9. Anchor, R D. The design of concrete structures for resistance to fire. Building ResearchEstablishment Note N7/77. BRE, Garston. 1977.

10. Morris, W A, Read, R E H and Cooke, G M E. Guidelines for the construction of fire-resisting structural elements. Building Research Establishment Report, BR128 BRE,Garston, 1988 (revised).

11. Thomas, F G and Webster, C T. National Building Studies Research Paper No. 18,Investigations on building fires, Part VI. The fire resistance of reinforced concretecolumns. Department of Scientific and Industrial Research (Building ResearchStation), HMSO, 1953.

12. FIP/CEB. Recommendations for the design of reinforced and prestressed concretestructural members for fire resistance. FIP Commission on Fire Resistance. Slough,Cement and Concrete Association for FIP, 1975, FIP Guide to good practice. 19 pp.

13. FIP / CEB. Report on methods of assessment of the fire resistance of concrete structuralmembers. FIP Commission on Fire Resistance of Prestressed Concrete Structures.Slough, Cement and Concrete Association for FIP, 1978, 91 pp.

14. Design and detailing of concrete structures for fire resistance: interim guidance by ajoint committee of the Institution of Structural Engineers and The Concrete Society.London, The Institution, 1978, 59 pp.

15. Building Regulations, 1976. Schedule 8.

16. Building Regulations, 1985. Approved Document B, Fire Safety.



19. prEN 1992-1-2 Eurocode 2: Design of concrete structures – Part 1.2: General rules –Structural fire design, CEN, Brussels, July 2003.

References




20. Fisher, R W and Smart, P M T. Results of fire resistance tests on elements of buildingconstruction, Volumes 1 and 2. Department of the Environment and Fire Office’sCommittee, Joint Fire Research Organisation, Fire Research Station, BuildingResearch Establishment, HMSO, 1975 and 1977.

21. Malhotra, H L. Effect of restraint on fire resistance of concrete floors. Fire ResearchNote 460, Department of Scientific and Industrial Research and Fire Officer’sCommittee, Joint Fire Research Organisation, Borehamwood 1962.

22. The fire resistance of prestressed concrete, Fire Research Note 38, Department ofScientific and Industrial Research and Fire Officer’s Committee, Joint Fire ResearchOrganisation, Borehamwood 1952.

23. Ashton, L A. Prestressed concrete during and after fires. Comparative tests oncomposite floors in prestressed and reinforced concrete, Fire Research Note 54,Department of Scientific and Industrial Research and Fire Officer’s Committee, JointFire Research Organisation, Borehamwood 1953.

24. Ashton, L A and Malhotra, H L. The fire resistance of prestressed concrete beams. FireResearch Note 65, Department of Scientific and Industrial Research and Fire Officer’sCommittee, Joint Fire Research Organisation, Borehamwood 1953.

25. Malhotra, H L. Fire resistance of structural concrete beams, Fire Research Note 741,Ministry of Technology and Fire Offices’ Committee, Joint Fire Research Organisation,Borehamwood 1969.



Appendix AResults from National Building Studies Research Paper No. 12

Table A1 Test results for reinforced concrete floors - from investigations on building fires

Table A2 Test results for reinforced concrete walls - from investigations on building fires

No. Fire resistance Failure Moisture Thickness Aggregate Cover Spalling Comments Support Span Test load Ref.period attained criteria content (%) (mm) (mm) (Y/N) (m) (kN/m2)

(minutes)1

1 3600 n/a 7.3 178 Crushed brick 38 N Restraint 3.99 8.1 F632 240 n/a 4.4 152 Limestone 13 Y Explosive spalling Restraint 3.99 9.3 F763 120 Insulation n/k 152 Limestone 13 Y Explosive spalling Restraint 3.99 9.3 F754 180 Insulation n/k 152 Limestone 13 Y Explosive spalling Restraint 3.99 9.3 F725 240 n/a 5.2 152 River gravel 13 Y Restraint 3.99 7.2 F676 120 Insulation 5.1 140 River gravel 25 N Plaster finish Simple 3.66 2.9 F247 120 Insulation 5.1 127 River gravel 13 N Plaster finish Simple 3.66 0 F238 120 n/a 4.6 127 River gravel 13 N Plaster finish Simple 3.66 3.3 F229 120 Stability 4.3 127 River gravel 25 N Simple 3.66 3.1 F17

10 60 Stability 6.7 114 River gravel 13 N Simple 3.66 3.6 F2111 30 Stability 6.3 114 Limestone 13 Y Explosive spalling Simple 3.66 3.6 F2012 60 Stability 12.4 114 Crushed brick 13 Y Simple 3.66 4.1 F1913 120 Stability 4.1 114 River gravel 13 N Simple 3.66 3.6 F1814 60 Stability 4.1 114 River gravel 13 Y Simple 3.66 3.6 F1615 30 Integrity 5.7 114 River gravel 13 Y Restraint 3.99 7.2 F3316 90 Stability 5.3 114 River gravel 13 N Simple 3.66 3.6 F2517 60 Integrity n/k 102 River gravel 13 Y Explosive spalling Restraint 3.99 6.7 F7718 120 n/a n/k 114 Limestone 13 Y Restraint 3.99 6.7 F7419 60 Insulation 4.6 102 Limestone 13 Y Explosive spalling Restraint 3.99 6.7 F7320 60 Insulation 4.8 102 Limestone 13 Y Explosive spalling Restraint 3.99 4.8 F7121 120 n/a 3.8 102 River gravel 13 Y Restraint 3.99 7.2 F6822 240 n/a 4.1 83 River gravel 13 N Asbestos covering Restraint 3.99 9.3 E3/S123 0 Integrity 6.0 83 River gravel 13 Y Plaster finish Restraint 3.99 9.3 F5324 30 Integrity 3.8 83 Crushed whinstone 13 Y Explosive spalling Restraint 3.99 9.3 F4925 60 Insulation 4.2 83 Torphin whinstone 13 Y Restraint 3.99 9.3 F4826 120 n/a 10.7 83 Crushed brick 13 N Restraint 3.99 9.3 F4527 30 Integrity 5.3 83 River gravel 13 Y Explosive spalling Restraint 3.99 9.3 F3428 60 Insulation 3.9 190/63 River gravel 25 N Downstand beams Restraint 3.99 9.3 F54

Note1 Rounded down to nearest full period

Fire resistance Failure Moisture Thickness Aggregate Cover Spalling Comments Support Failure Test load Ref. Size (m)period attained criteria content (mm) (mm) (Y/N) time (kN)

(minutes)1 (%) (minutes)120 Insulation 3.9 102 Gravel 25 Y Explosive spalling Not known 130 339 W19 3.05 x 3.0590 Insulation 4.6 102 Gravel 48 Y Surface spalling Restraint 100 - W33 3.05 x 3.05

360 n/a 16.2 203 Crushed brick 25 N No spalling Not known - 1106 W16 3.05 x 3.05




Table A3 Test results for reinforced concrete columns - from investigations on building fires


No. Fire resistance period Failure Moisture Size Aggregate Cover Spalling Comments Loaded attained (minutes)1 criteria content (%) (mm) (mm) (Y/N) (Y/N)

1 60 Not known 4.3 152 x 152 River gravel 25 N Cracking Y2 60 Stability 4.6 152 x 152 River gravel 25 N Y3 120 Stability 5.5 254 x 254 River gravel 25 N Y4 120 Not known 4.8 254 x 254 Torphin whinstone 25 N Y5 120 Stability 3.1 254 x 254 Hillhouse whinstone 25 N Y6 90 Stability 5.5 254 x 254 River gravel 29 Y Steel exposed Y7 90 Stability 4.7 254 x 254 River gravel 25 Y Steel exposed Y8 90 Stability 3.4 254 x 254 River gravel 25 N Cracking Y9 120 Stability 4.9 254 x 254 River gravel 25 N Cracking Y

10 120 n/a 4.0 254 x 254 River gravel 25 Y Y11 120 n/a 3.8 254 x 254 River gravel 25 Y Y12 90 Stability 4.3 254 x 254 River gravel 25 N Y13 60 Stability 4.9 254 x 254 River gravel 32 Y Y14 30 Stability 4.3 279 x 279 River gravel 38 Y Y15 90 Stability 5.2 279 x 279 River gravel 38 Y Y16 60 Stability 4.7 279 x 279 River gravel 38 Y Y17 120 n/a 5.3 279 x 279 Hillhouse whinstone 38 Y Y18 120 n/a 5.7 279 x 279 Torphin whinstone 38 Y Y19 60 Stability 5.1 279 x 279 River gravel 38 Y Restrained Y20 120 n/a 4.7 279 x 279 River gravel 38 N Plastered Y21 120 n/a 5.5 279 x 279 River gravel 38 Y Y22 120 n/a n/k 279 x 279 Cheddar limestone 38 N Y23 180 Stability 4.0 279 x 279 Matlock limestone 38 Y Y24 180 Stability 3.6 279 x 279 Clitheroe limestone 38 N Y25 120 n/a 4.3 279 x 279 Blast furnace slag 38 N Y26 180 Stability n/k 279 x 279 River gravel 38 Y Steel mesh Y27 60 Stability 4.2 279 x 279 River gravel 38 Y Y28 60 Stability 5.1 279 x 279 River gravel 38 Y Y29 30 Stability 5.1 305 x 305 River gravel 51 N Cover removed Y30 60 Not known 3.9 356 x 356 Gravel 38 Y Y31 120 n/a 5.9 406 x 406 River gravel 25 Y Y32 120 n/a 6.3 406 x 406 River gravel 35 Y Y33 60 n/a 5.7 406 x 406 River gravel 35 Y Y34 120 n/a 5.6 508 x 508 River gravel 44 Y Y35 120 n/a 5.2 508 x 508 River gravel 44 Y Y36 120 Stability 5.7 514 x 514 River gravel 29 Y Y37 60 n/a 5.9 305 x 305 River gravel 13 N Hexagonal Y38 30 Stability 3.4 305 x 305 River gravel 13 Y Hexagonal Y39 120 n/a 5.2 356 x 356 River gravel 13 Y Hexagonal Y40 120 n/a 5.0 406 x 406 River gravel 13 Y Hexagonal Y41 90 Stability 5.0 406 x 406 River gravel 13 N Hexagonal Y42 120 n/a 5.1 508 x 508 River gravel 13 Y Hexagonal Y43 120 Stability 4.8 508 x 508 River gravel 13 Y Hexagonal Y



Table A4 Test results for reinforced concrete beams - from investigations on building fires

No. Fire resistance period Failure Moisture Size Aggregate Cover Spalling Comments Loaded attained (minutes)1 criteria content (%) (mm) (mm) (Y/N) (Y/N)

1 180 Stability 4.3 102/140 River gravel 51 N Downstand Restraint2 180 Stability 4.3 102/140 River gravel 38 N Downstand Restraint3 120 Stability 4.0 102/140 River gravel 25 Y Downstand Restraint4 90 Stability 4.5 102/140 River gravel 13 N Downstand Restraint5 240 n/a 6.4 102/140 Torphin whinstone 38 Y Downstand Restraint6 180 Stability 5.4 102/140 Brick/gravel 38 N Downstand Restraint7 120 n/a 4.7 305 River gravel 25 Y Rectangular Restraint




Appendix BResults from National Building Studies Research Paper No. 18

Table B1 Test results for reinforced concrete columns - from investigations on building fires

Notes 1 Rounded down to nearest full period 2 Where measured * Cube strength at 28 days

No. Fire endurance period Failure Moisture Thickness Cube strength at Age Cover Reinforcement Comments Load attained (minutes)1 criteria content (%)2 (mm) time of test (N/mm2) (days) (mm) ratio (kN)

1 56 Spalling 305 x 305 22 295 22 1.6 Sloughing of corners 7472 83 Cracking & spalling 280 x 280 28 238 25 1.46 Sloughing of corners 7473 44 Cracking & spalling 254 x 254 32 238 25 1.23 Sloughing of corners 7474 63 Cracking & spalling 203 x 203 46 238 25 2.76 Sloughing of corners 7475 125 Cracking & spalling 381 x 381 25 292 29 1.77 Sloughing of corners 9976 71 Cracking & spalling 4.2 356 x 356 29 315 25 1.6 Sloughing of corners 14957 52 Cracking & spalling 305 x 305 44 357 25 1.7 Sloughing of corners 14958 420 Cracking & spalling 3.7 483 x 483 27 238 25 1.3 Sloughing of corners 7479 75 Cracking & spalling 3.4 356 x 356 43 364 35 3.0 Sloughing of corners 2243

10 91 Cracking & spalling 2.6 483 x 483 29 250 29 1.9 Sloughing of corners 299011 145 Cracking & spalling 3.9 406 x 406 35 257 29 3.1 Sloughing of corners 299012 58 Cracking & spalling 1.83 229 x 229 26 399 29 4.9 Sloughing of corners 74713 71 Cracking & spalling 2.5 279 x 279 24 406 32 8.1 Sloughing of corners 149514 85 Cracking & spalling 3.9 356 x 356 28 420 38 7.2 Sloughing of corners 224315 77 Cracking & spalling 1.9 406 x 406 23 294 44 7.5 Sloughing of corners 299016 47 Cracking & spalling 2.4 381 x 381 25 434 29 1.8 Sloughing of corners 149517 78 Cracking & spalling 3.2 381 x 381 33 434 29 1.8 Sloughing of corners 149518 161 Cracking & spalling 3.2 381 x 381 41 434 29 1.8 Sloughing of corners 149519 198 Cracking & spalling 381 x 381 23 307 29 1.8 Sloughing of corners 74720 248 Cracking & spalling 381 x 381 29 313 29 1.8 Sloughing of corners 49821 70 Cracking & spalling 381 x 381 25 292 29 1.8 Sloughing of corners 149522 120 Cracking & spalling 381 x 381 26 352 29 1.8 Sloughing of corners 94723 74 Cracking & spalling 381 x 381 27 282 29 1.8 Sloughing of corners 149524 71 Cracking & spalling 381 x 381 28 323 29 1.8 Sloughing of corners 149525 358 Cracking & spalling 4.75 381 x 381 28 314 29 1.8 Sloughing of corners 29926 65 Cracking & spalling 2.4 381 x 381 24 598 29 1.8 Sloughing of corners 149527 123 Cracking & spalling 2.0 381 x 381 27 584 29 1.8 Sloughing of corners 99728 214 Cracking & spalling 305 x 305 17 293 25 1.7 Sloughing of corners 24930 175 Cracking & spalling 254 x 254 31 237 25 1.2 Sloughing of corners 24931 120 Cracking & spalling 254 x 254 35 244 25 1.2 Sloughing of corners 24932 119 Cracking & spalling 2.7 483 x 483 18 217 25 1.3 Sloughing of corners 224333 120 Cracking & spalling 3.48 483 x 483 21 215 25 1.3 Sloughing of corners 179434 351 cracking & spalling 2.3 483 x 483 27 251 29 1.9 Sloughing of corners 99735 120 Cracking & spalling 2.7 483 x 483 22 273 29 1.9 Sloughing of corners 219336 46 Cracking & spalling 1.93 406 x 406 19 329 25 0.9 Sloughing of corners 149537 98 Cracking & spalling 4.7 254 x 254 38* 32 25 0.7 Sloughing of corners 62338 103 Cracking 3.4 254 x 254 36* 44 25 0.7 Chamfered edges 65839 120 Explosive spalling 5.9 406 x 406 27* 40 25 0.9 Chamfered edges 123140 120 Cracking 4.9 254 x 254 39* 43 25 0.7 Chamfered edges 46541 120 Cracking & spalling 4.0 254 x 254 36* 251 25 0.7 Chamfered edges 46342 120 Cracking & spalling 3.8 254 x 254 37* 233 25 0.7 Chamfered edges 46343 119 Cracking & spalling 5.2 279 x 279 33* 42 38 1.0 Chamfered edges 58644 80 Cracking & spalling 4.7 279 x 279 34* 38 38 1.0 Chamfered edges 85845 112 Cracking 4.3 254 x 254 16* 44 25 0.7 Chamfered edges 46546 62 Cracking & spalling 4.9 254 x 254 29* 38 32 4.9 Chamfered edges 91947 120 Did not fail 279 x 279 36* 167 32 4.0 Chamfered edges 91248 227 Cracking & spalling 4.0 279 x 279 38* 175 32 4.0 Chamfered edges 91249 221 Cracking 3.6 279 x 279 36* 187 32 4.0 Chamfered edges 91250 83 Cracking & spalling 5.1 279 x 279 29 36 32 4.0 Chamfered edges 60851 189 Cracking & spalling 279 x 279 29* 45 32 4.0 Chamfered edges 60852 86 Cracking & spalling 4.2 279 x 279 30* 61 32 4.0 Chamfered edges 60853 120 Did not fail 4.3 279 x 279 25* 57 32 4.0 Chamfered edges 907



Appendix CResults of fire resistance tests on elements of building construction

Table C1 Test results for concrete floors - from results of fire resistance tests on elements of construction

No. Fire resistance Failure Thickness Comments Supportperiod attained criteria (mm)

(minutes)1

1 240 Not applicable 51/254 Prestressed concrete T beam incorporating 76 mm structural topping Restraint2 30 Not known 102 Hollow prestressed Aglite beams incorporating 38 mm structural topping Not known3 30 Insulation 51/254 Reinforced concrete T form floor Not known4 30 Insulation 56/254 Reinforced concrete T form floor Not known5 60 Not known 127 Hollow prestressed floor units incorporating 32 mm structural topping Not known6 60 Not known 159 Hollow prestressed floor units incorporating 32 mm structural topping Not known7 60 Not known 152 Reinforced aerated concrete slabs Not known8 60 Not known 152 Prestressed concrete hollow units Not known9 60 Not known 102 Prestressed concrete hollow units with 51 mm structural topping Not known

10 60 Not known 89 Concrete rib floor (rib depth not known) Not known11 60 Not known 305 Prestressed concrete jointed double T beam incorporating 51 mm structural topping Not known12 120 Not known 140 Prestressed concrete plank floor including structural topping and screed Not known13 120 Not known 152 Aerated concrete slab floor incorporating 38 mm screed Not known14 120 Not known 229 Reinforced concrete floor slab and beam Not known15 120 Not known 178 Reinforced aerated concrete floor slabs Not known16 120 Not known 102 Aglite concrete floor slab Not known17 120 Not known 159 Prestressed hollow floor slab incorporating 51 mm structural topping Not known18 120 Not known 152 Prestressed hollow floor slab Not known19 120 Not known 178 Reinforced hollow concrete slabs Not known20 120 Not known 150 Reinforced concrete units Not known21 240 Not known 159 Prestressed hollow units with suspended ceiling Not known22 60 Not known 89 Reinforced concrete floor slab Not known

Table C2 Test results for concrete columns - from results of fire resistance tests on elements of construction

No. Fire resistance period Failure Thickness Comments Supportattained (minutes)1 criteria (mm)

1 Failed reload test Reload 229 x 229 Achieved 120 minutes stability but failed reload test Not known2 Reload test could not be applied Stability 180 x 180 Achieved 90 minutes stability prior to failure Not known3 Reload test could not be applied Stability 180 x 180 Achieved 30 minutes stability prior to failure Not known4 Failed reload test Reload 180 x 180 Achieved 90 minutes stability but failed reload test Not known5 Reload test could not be applied Stability 180 x 180 Achieved 60 minutes stability prior to failure Not known6 Failed reload test Reload 280 x 180 Achieved 60 minutes stability but failed reload test Not known7 120 Stability 229 x 229 ‘Lytag’ aggregate Not known8 120 Stability 292 x 292 Prestressed concrete Not known9 120 Stability 203 x 203 Capstone column Not known

Table C3 Test results for reinforced concrete beams - from investigations on building fires

No. Fire resistance Failure Thickness Comments Supportperiod attained criteria (mm)

(minutes)1

1 Failed reload test Reload 51/254 Prestressed concrete double T beam - achieved 90 minutes stability but failed reload test Not known




Appendix DResults from Fire Research Note 741

Table D1 Summary of test results for prestressed concrete beams

No. Time (minutes) Beam type Age Cover Design fire Load Spalling Mean Time toto reach critical (months) (mm) resistance (kN) (Y/N) reinforcement max tempdeflection l/30 (minutes) temperature (deg C) (minutes)

1 256 R/PO/W/SR 28 100 240 187 Y 395 2552 251 R/PO/W/SR 29 100 240 187 Y 400 2503 – /PR/W/X 35 50 90 97 Y 80 324 98 /PR/S/SR 26 50 90 97 Y 410 955 195 /PR/S/X(VG) 33 50 150 97 N 350 1956 264 R/PO/S/SR 37 100 240 210 (52.5) Y 445 1057 – R/PO/S/SR 36 100 240 185 (46) Y 440 280

NoteAll 7.6 m span, except for 6 and 7, which are 11.3 m

KeyR Rectangular PR Pre-tensioned

section W Wire tendonsS Strands VG Vermiculite-gypsum plasterSR Supplementary reinforcement – CollapseX No supplementary reinforcementPO Post-tensioned Figures in brackets are end loads

Table D2 Summary of test results for reinforced concrete beams

No. Time (minutes) Beam type Age Cover Design fire Load Spalling Meanto reach critical (months) (mm) resistance (kN) (Y/N) reinforcementdeflection l/30 (minutes) temperature (deg C)

8 – DG/MS/X 34 63 240 165 Y 4929 – DG/CD/X 36 63 240 151 Y 45510 – DG/CT/X 26 63 240 151 Y 52011 – DG/HR/X 26 63 240 151 Y 50012 400 LC/MS/X 34 63 240 165 N 54513 416 LS/MS/X 32 63 240 165 N 49014 162 DG/MS/SR 18 63 240 165 Y 49515 288 DG/HR/SR 18 63 240 151 Y 63516 270 DG/CT/SR 18 63 240 151 Y 56017 256 DG/HR/SR 18 38 90 165 Y 64018 160 DG/HR/X 18 38 90 165 Y 60019 158 DG/HR/X 18 25 60 169 Y 60020 – DG/MS/X 40 63 240 333 (83) Y 49521 – DG/CD/X 38 63 240 318 (79) Y 500

NoteAll 7.6 m spans, except for 20 and 21, which are 11.3 m

KeyDG Dense gravel HR Hot rolled alloy steelLC Lightweight expanded clay SR Supplementary reinforcementLS Lightweight foamed slag X No supplementary reinforcementMS Mild steel – CollapseCD Cold worked deformed steelCT Cold worked twisted steel Figures in brackets are end loads


Approaches to the design of reinforced concrete flat slabs R M Moss. BRE Report BR 422, 2001, 47 pages.The choice of design method should be based on what is appropriate for the structure, on the designer’s experience, and on what islikely to benefit the client most. This report gives pointers as to how existing design guidance and methods could be developed andmade more user-friendly, particularly with the advent of Eurocode 2. It points out issues for the Permanent Works Designer to consideras a result of the desire to strike slabs earlier and speed up the construction process.

Backprop forces and deflections in flat slabs: construction at St George Wharf R Vollum. BRE Report BR 463, 2004, 35 pages.Analysis of backprop forces measured at the European Concrete Building Project’s in-situ concrete frame building at Cardingtonshowed that the upper floor carried a greater proportion of the load from casting the slab above than that conventionally assumed. Thiswas investigated at St George Wharf by measuring backprop forces during construction. The work confirms that most of the conclusionsfrom research into construction loading and deflection at Cardington are valid for practical purposes.

Best practice in concrete frame construction: practical application at St George Wharf R M Moss. BRE Report 462, 2003, 20 pages. This report demonstrates the practical benefits of adopting many of the innovative features and techniques used in the design andconstruction of the in-situ concrete frame building at Cardington, for which a series of Best Practice guides and companion reports isavailable. Details are given of the application of innovations to a live project involving the construction of a large residential and mixed-use development.

Effect of polypropylene fibres on performance in fire of high grade concrete N Clayton and T Lennon. BRE Report BR 395, 2000, 32 pages.Based on fire tests of concrete columns, this report gives information and recommendations on the use of polypropylene fibres toimprove the performance of high grade concrete in fire. Incorporating polypropylene fibres into the mix prevented spalling but led to nodifference in the ability of the columns to survive the fire test. Separate tests showed that addition of fibres to the concrete led to areduced cube compressive strength, which needs to be allowed for in design.

Fire safety engineering: a reference guide R Chitty and J Fraser-Mitchell. BRE Report 459, 2003, 109 pages. This guide provides basic descriptions for key topics in fire safety engineering and aspects that should be considered by designers,enforcers and other responsible persons. It is a reference for those who only occasionally encounter fire safety engineering or are newto the subject. For experienced practitioners it provides a useful short cut to information in detailed references. The text is formatted sothat users can annotate their copy to build a personalised reference on fire safety engineering.

Performance of high grade concrete containing polypropylene fibres for fire resistance: the effect on strength N Clayton. BRE Report BR 384, 2000, 16 pages.This study accompanies research into the possible enhancement of fire resistance of high grade concrete by polypropylene fibres.Addition of fibres caused a small reduction of cube strength, which was related to the reduction in density due to the addition of thefibres. It may affect the design of structural elements, but there is no significant loss of flexural or cylinder splitting strength from theaddition of polypropylene fibres.

Precast hollowcore slabs in fire T Lennon. Information Paper 5/03, 2003, 4 pages.Two full-scale fire tests were carried out at BRE’s Large Building Test Facility at Cardington. The objectives were to assess the adequacyof precast hollowcore slabs in terms of the functional requirements of Approved Document B of the Building Regulations. This paperexplains the background, describes the test parameters in detail and summarises the results and conclusions.

For more information, visit www.brebookshop.com or email [email protected].

Related titles on concrete and fire


Fire safety of concrete structures:Background to BS 8110 fire designTom LennonFRS, the Fire Division of BRE

The background to methods for establishing the fire resistance ofconcrete structures specified in BS 8110, the British Standard code ofpractice for structural concrete, has been investigated. In particular,research and test results dating back some 60 years, which haveunderpinned the tabulated data in all the codes of practice since 1948,have been examined and revisited

This publication brings together information derived from testing andresearch carried out over a number of years. There was a danger thatthe work supporting the development of codes and standards couldhave been lost. Collating and assessing the relevant information meansthat the lessons from the past are recorded and used to help define thestrategy for the next generation of codes and standards.

The investigations underpinning this publication found that theexperimental results used as data for developing the tabulatedapproach in BS 8110 fully supported the provisions of the code inrelation to assumed periods of fire resistance. Furthermore in manycases these provisions were found to be conservative as they werebased on the assumption that structural elements were fully stressedat the fire limit state and take into account the spalling characteristicsof concrete.

Evidence from performance in real fires over a number of yearsdemonstrates that the tabular approach to determining fire resistanceof concrete elements has been effective. It is suggested that furtherresearch could result in greater economies in construction and cost forconcrete structures.

BRE BookshopBuilding Research EstablismentWatford WD25 9XXTel: 01923 664761Fax: 01923 662477email: [email protected]

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Fire safety of concrete structures - e-tell.info fire tests... · performance-based methods for the structural fire engineering design of concrete ... for the fire resistance of precast