Computer modelling of the corner compartment fire test on the large-scale Cardington test frame

Journal of Constructional Steel Research 48 (1998) 27–45

Computer modelling of the corner compartmentfire test on the large-scale Cardington test

frame

Colin BaileyThe Steel Construction Institute, Silwood Park, Ascot, Berkshire, SL5 7QN, UK

Received 13 November 1996; received in revised form 26 August 1997; accepted 15 October 1997

Abstract

Recent research into the fire design of steel-framed structures has started to be steered awayfrom considering isolated members towards considering the behaviour of the structure as acomplete entity. In the UK, a recent series of fire tests have been conducted on a full scalecomposite test frame at the Building Research Establishment Laboratories at Cardington. Thispaper presents the computer simulation of one of these tests, carried out in the corner bay ofthe frame by the Building Research Establishment, using a purpose written program.

Direct comparison between test results and computer predictions showed good correlation.Additional computer simulations were conducted to show that the windposts, which were usedon the Cardington frame, had a significant effect on the structural behaviour of the frame.Other examples presented in this paper indicated that simplified idealised temperatures pro-duced sufficiently accurate (although slightly conservative) predictions of structural defor-mations. The bridging action of the composite slab was also investigated using the computersoftware. It was shown that this mode of behaviour considerably reduced the deformations ofthe heated steel beams. 1998 Elsevier Science Ltd. All rights reserved.

Keywords:Computer modelling; Fire tests; Test-frame

1. Introduction

On the 23rd June 1990 a localised fire developed in a partly completed 14 storeyoffice block at the Broadgate development in central London [1]. The fire beganinside a large site hut, of size 40 m× 12 m, situated on the first level of the building.

0143-974X/98/$—See front matter 1998 Elsevier Science Ltd. All rights reserved.PII: S0143 -974X(97)00078-3

28 C. Bailey /Journal of Constructional Steel Research 48 (1998) 27–45

Structural elements covering an area of approximately 40× 20 m above the hut wereaffected by the fire. The subsequent investigation highlighted the significant effectsof restraint on a heated localised area within a steel-framed building. Comparisonbetween back-analysis to BS5950 Part 8 [2], and the metallurgical investigation [1]which estimated the actual maximum steel temperature reached during the fire,showed that deformations of some members occurred at lower temperatures thanexpected. This is due to BS5950 Part 8 being limited to considering isolated membersonly, with no consideration of thermal restraint. However, the overall behaviour ofthe frame showed no signs of collapse. This was attributed to the surroundingrestraining structure providing alternative load-paths which re-directed the load awayfrom the weakening members.

Following the investigation of the Broadgate Fire and other UK fires, such asthose which occurred at Basingstoke [3] and Minster Court [4], it was concludedthat future research into the structural behaviour of steel-framed buildings subjectto fire should consider the structure as a complete entity and not as a collection ofisolated members. Large-scale fire tests conducted at BHP Research LaboratoriesAustralia [5,6] and at the Stuttgart-Vaihingen University Germany [7] have shownthe beneficial inherent resistance of steel-framed buildings subjected to a fire. How-ever, the size of the test frame in both instances was fairly small. Therefore, therestraint provided from the surrounding cold frame was low. Recently, in the UK,a series of localised fire tests [8,9] have been conducted on an eight storey steel-framed test building at the Building Research Establishment (BRE) Laboratory,Cardington. This paper presents the structural computer analysis of one of these tests,which was carried out by BRE [8].

The steel-frame test building was designed and constructed to resemble a typicalmodern city centre office development. On plan, the building covered an area of21 m × 45 m (Fig. 1) with an overall height of 33 m. There were 5 equally spacedbays along the length of the building. Across the width there were 3 bays spaced6 m, 9 m and 6 m. Placed centrally was a 9 m× 2.5 m lift core with two 4 m×4.5 m stairwells placed at either end. The steel members were shot-blasted, but leftunpainted. The structure was designed as a braced frame with lateral restraint pro-vided by standard cross bracing, consisting of Grade 50 flat steel, around the threevertical access shafts. Grade 43 rolled steel angles were used as windposts, positionedat 3.0 m centres from ground to fourth floor and at 2.25 m centres above this level.The beams were designed as simply-supported, acting compositely with the floorslab via 95 mm× 19 mm diameter shear studs, spaced at 200 mm centres. The com-posite flooring system consisted of 0.9 mm thick steel deck (PMF CF70), which wascontinuous over a minimum of two spans, with lightweight concrete and A142 anti-crack mesh. The overall minimum depth of the slab was 130 mm. The imposeddesign load was assumed as 2.5 kN/m2, with a partition load of 1.0 kN/m2 on allfloors, except the roof where a plant loading of 7.5 kN/m2 was used. The steel-to-steel connections consisted of fin-plates for the beam-to-beam and flexible end-platesfor the beam-to-column connections. Throughout the structural design the underlyingphilosophy was to obtain a frame that used the minimum amount of material, wasalso simple to manufacture and at all stages of construction and erection reflected

29C. Bailey /Journal of Constructional Steel Research 48 (1998) 27–45

Fig. 1. Layout of the Cardington test frame showing location of the fire test.

normal building practice rather than specialist research procedures. Sand-bags, eachweighing 11 kN, were arranged to simulate the load applied by the raised floor(0.4 kN/m2), the services (0.25 kN/m2), the ceiling system (0.15 kN/m2), the par-titions (1.0 kN/m2) and a third of the imposed load (0.83 kN/m2). This load wasapplied to the test area and surrounding structure and remained constant during thefire tests.

2. Test observations

On the 23rd October 1995 a fire test was carried out by the BRE [8] on the cornerbay, between the second and third floors of the composite steel test frame. The extentof the fire compartment is shown in Fig. 1. The internal boundaries of the compart-ment were constructed using fire resistant board and placed centrally on gridlines Eand 3, as indicated in Fig. 2. A full-height breeze block wall formed the boundaryon gridline F, with a one metre high breeze block dado wall and glazing formingthe boundary on gridline 4. The glazing system consisted of two windposts (150×75 × 10 RSA) positioned at third points along the edge beam. The bottoms of thesewindposts were effectively fixed rigidly to the edge beam, with the tops of the postsbolted through slotted holes of 80 mm length to the underside of the edge beamabove. Therefore, any movement under normal working conditions could be accom-modated by the windposts. The compartment was totally enclosed with all windowsand doors shut. A fire load of 40 kg/m2 was provided by twelve timber cribs placedon the second floor, over an area of 54 m2, giving a total fire load of 2160 kg. Theframe was loaded by sandbags to provide a total load (including the self weight of


Fig. 2. Compartment construction and finite element mesh layout.

the structure) of 5.48 kN/m2, representing the total dead load and a third of theanticipated imposed load. All beams were left unprotected. The columns inside thecompartment were protected up to the underside of the floor, including the beam-to-column connections.

Since the compartment was effectively sealed, the fire was largely influenced bythe lack of oxygen in the early stages of the test. Fig. 3 shows the atmospherictemperatures recorded in the compartment during the test. Measurements where takenat various locations within the compartment. All the atmospheric temperaturesrecorded are shown in Fig. 3 which indicates that the difference in temperaturethroughout the compartment was fairly small for the duration of the test. It can beseen that due to lack of oxygen the intensity of the fire was very low for approxi-mately the first 94 minutes of the test. The glazing was then broken from an externalsource allowing the compartment to be vented. This resulted in an increase in tem-perature, with a maximum atmospheric temperature of 1060°C recorded after 102minutes.

Only a limited description of the test is presented here to enable the structuralmodelling to be addressed, which this publication is mainly concerned with. If moredetail is required the reader is directed towards [8,10].


Fig. 3. Recorded atmospheric test temperatures at various locations within the compartment.

3. Analytical Modelling

3.1. Direct comparison with test results

A finite-element computer model called INSTAF [11,12] was used to model thetest. The software uses 3 types of elements;

O beam-column elements, to represent the steel members,O spring elements, to represent the steel-to-steel connections,O shell elements, to represent the continuous floor slab.

The extent of the structure modelled is indicated in Fig. 1 with the adopted meshlayout shown in Fig. 2. The boundary conditions, which represent the continuationof the structure, are assumed to be fixed in horizontal and rotational displacement,perpendicular to the boundary. This condition effectively creates lines of symmetryat these boundaries. Since the test location is situated in one corner of the frame, itcould be argued that the use of symmetry should not be employed. Nevertheless,without the assumed use of symmetry, the full extent of the floor would requiremodelling. This would drastically increase the computing time, and the additionalaccuracy possibly gained from modelling the whole floor would be difficult to justify.


Within the analysis the following assumptions were made:

O The yield stress of steel is 308 N/mm2 for Grade 43 and 390 N/mm2 for Grade50, based on coupon test results provided by the supplier.

O The elastic modulus of steel is 210 kN/mm2.O The ambient-temperature steel:concrete modular ratio is taken as 15, and this is

reduced by 30% over the furnace area. This reduction was based on the data givenin EC4 Part 1.2, which indicates that the elastic modulus of concrete reduces by0.71 at 100°C.

O Only the top 70 mm of the slab is considered, which represents the concrete thick-ness above the top of the trapezoidal steel deck.

O The recorded test temperatures of the steel beams are used in the analysis. Thesewere recorded at discrete thermocouple positions, as shown in Fig. 4. Also shownin this figure is the length of beam over which, in the analyses, these temperaturesare assumed to occur.

O The temperature in the slab is taken as the average recorded test temperature.O The limiting bending stress in the slab in sagging regions is 25 N/mm2, and

2.5 N/mm2 in the hogging regions.O The steel-to-steel connections are assumed pinned, since the central beam is con-

nected using fin plates.O The windposts are represented by rigid supports.

Direct comparison was made between the model and test results for displacement

Fig. 4. Position of thermocouples in the test.


measurements taken at 7 positions, shown in Fig. 5. Comparison was also made withstrains measured during the test on the lower flange of the beam between gridlinesD and E and gridlines 3 and 4. Identification of displacement positions correspondto those given in [10].

The recorded test and computer predicted displacements at the centre of the com-partment (D19) with relation to time, are shown in Fig. 6. It can be seen that forapproximately the first 103 minutes the computer model follows the test results verywell. The maximum temperature of the internal secondary steel beam at this timewas recorded as 780°C. Above this temperature the computer model predicts slightlyhigher displacements up to the maximum recorded steel temperature of 903°C. Thismaximum temperature occurred at 114 minutes, after which the steel began to cool,resulting in extensive strain reversal. At the present time, only the steel membersincorporate strain reversal within the model [13]. It is hoped in the future that theconcrete elements will also incorporate strain reversal, thus making the comparisonbetween test results and computer predictions closer during the cooling phase ofthe fire.

The overall structural deformation is shown in Fig. 7. This clearly indicates thelocalised damage caused by the compartment fire. Strain gauges were placed on thesteel members outside the compartment and a comparison between the model andtest results for the strain of the bottom flange of the beam (Fig. 5) was made. It wasfelt important to compare strains at this position since it is on a ‘boundary’ in theadopted model representation. The beam is also effectively a continuation of theheated central beam in the fire compartment. This comparison is shown in Fig. 8.

Fig. 5. Positions at which comparison is made between computer predictions and test results.


Fig. 6. Comparison between computer predictions and test results at position D19 (mid-span).

During the heating phase of the fire the test results and computer predicted strainscompare very well. When the steel within the compartment reaches its maximumtemperature, the model predicts higher strains than those recorded in the test. Thisis almost certainly due to the boundary conditions assumed in the model, in whichno horizontal movement is allowed.

The comparisons between test and computer predicted displacements for otherlocations (as shown in Fig. 5) are presented in Appendix. The percentage differencebetween computer predicted and test displacements at 114 minutes (maximum steeltemperature) are summarised in Fig. 9.

3.2. Effect of windposts

From Fig. 9 it can be seen that, for the displacements at positions D9 and D22,there is a significant difference between the test results and computer predictions.The latter estimated displacements of approximately 40 mm lower than the test whenthe beams reached their maximum temperature. This is due to these displacementpositions being in the proximity of the windposts, which in the computer model arerepresented by rigid vertical supports. In the structural frame the edge beam wassupported by the windposts above the fire compartment acting in tension. The topsof these windposts were fixed to the higher edge beam by bolts through slotted holesof 80 mm length. The slippage of these bolts during the fire test could account forthe difference between the predicted and recorded displacements at D9 and D22.However, this cannot be confirmed since the positions of the bolts within the slottedholes were not recorded before or after the test.


Fig. 7. Computer predicted structural profile at maximum temperature recorded in the test.

Fig. 8. Comparison between computer predicted strains and test results at the position shown in Fig. 5.


Fig. 9. Summary of the difference between computer predictions and test results.

Accounting for bolt slippage would not necessarily cause higher displacements atthe central position of the compartment, since bolt slippage would also influence thebridging behaviour of the slab between gridlines 3 and 4. If the qualitative represen-tation of the slab deformation (shown in Fig. 10) is considered, it can be seen that

Fig. 10. Qualitative representation of the behaviour of the concrete floor slab.


as the edge beam deflects vertically, the amount of concrete in tension reduces. Thiscauses less cracking, and thus a stiffer slab in the bridging direction. To investigatethis behaviour the model was re-run, but this time with the rigid supports representingthe windposts removed.

The structural profile when the steel members reached their maximum temperature,with the windposts removed, is shown in Fig. 11. The predicted displacement of theedge beam, which reached a maximum temperature of 690°C, was 260 mm. Thustensile concrete cracking in the proximity of the edge beam, in the bridging directionof the slab, was very small. The comparison between computer predictions and testresults for the displacement of the central compartment position is shown in Fig. 12.When comparing the predicted central displacements, (Figs. 6 and 12) it can be seenthat removal of the windposts results in lower central displacements. This reinforcesthe theory presented qualitatively in Fig. 10 in relation to tensile concrete crackingin the slab. If, taking into account bolt slippage, the actual support from the windpostswas considered, then the predicted central slab displacement would fall between thetwo extreme cases presented.

Therefore, it can be seen that the windposts are beneficial to the behaviour of theedge beam but cause more tensile cracking in the concrete slab.

Fig. 11. Computer predicted structural profile (with the windposts removed) at the maximum temperaturerecorded in the test.


Fig. 12. Comparison between computer predictions and test results at position D19 (mid-span), with thewindposts removed.

3.3. Effect of temperature distribution on structural behaviour

To investigate the effects of temperature distribution on the steel members, themodel was re-analysed with windposts included, assuming an idealised temperaturepattern. It was assumed that the bottom flange and web were heated at the samerate, with the top flange heated at 80% of this value. All beams within the compart-ment were subjected to the same temperature pattern. Fig. 13 shows the comparisonbetween the computer predictions for the idealised temperature pattern, the testresults and the predictions using the recorded temperatures.

Adopting the idealised temperatures results in slightly larger vertical displace-ments. From these results it can be concluded that idealised temperatures can con-servatively be used in further parametric studies investigating the influence of largercompartments and different structural configurations.

3.4. Slab behaviour

To investigate the influence of the bridging action of the slab, the analysis whichused ideal temperature patterns was compared against an analysis carried out on anisolated composite beam, with an effective width of span/4, as shown in Fig. 14.

The comparison between the two analyses is shown in Fig. 15. From these predic-tions it can be seen that after 200°C, the bridging action of the continuous floor slabhas a significant effect. Just above 800°C the isolated composite beam would beclassed as ‘failed’ if the BS476 Part 20 [14] criterion was adopted. In the analysiswhere the continuous floor was included, the displacements do not exceed span/30.


Fig. 13. Comparison between computer predictions using idealised temperatures and test results.

Fig. 14. Isolated composite beam.


Fig. 15. Comparison between vertical displacements predicted using an isolated composite beam andthe sub-frame shown in fig. 2.

4. Conclusions

The following conclusions are obtained from the computer simulations presentedin this paper.

1. Direct comparison between the test results and computer simulation showed verygood correlation for the central (maximum) displacement and the strain valuesoutside the fire compartment area. In both the test and computer simulation itwas shown that the damage to the structure was localised to the compartmentarea. The good agreement between the test result and computer simulation alsojustifies the use of a limited sub-structure for the computer analysis, for this com-partment fire. Additional studies need to be conducted to identify if this is thecase for compartment fires in other areas within the ‘footprint’ of the building,in particular compartment fires positioned in the centre of the building.

2. For the vertical displacement in the proximity of the windpost positions, a notice-able difference between the test results and computer predictions was found. Thelatter estimated displacements of approximately 40 mm lower than the test whenthe maximum temperature was reached. It was shown that this was caused bythe computer simulation representing the windposts as rigid vertical supports. Inthe test frame the windposts were connected via bolts through slotted holes of80 mm length. Bolt slippage could account for the discrepancy between the com-puter predictions and tests results. Unfortunately, this cannot be confirmed sincethe position of the bolts within the slotted holes were not measured before or afterthe test. In addition the movement of the bolts during the test were not measured.


3. The computer simulation conducted with the support from the windposts removed,indicted that these had a significant effect on the behaviour of the edge beam.Without the windposts, the predicted displacements of the edge beam reached amaximum value of 260 mm, compared to nominal displacement predicted whenthe windposts were included in the analysis. It was also shown that the supportfrom the windposts affected the cracking behaviour of the slab in the bridgingdirection, from gridlines 3 to 4. With the windposts removed computer analysisrevealed that less tensile cracking of the concrete slab occurred in the vicinity ofthe edge beam. Generally it was found that the windposts had a considerablebeneficial effect on the structural behaviour. However, it should be noted thatwindposts are not necessarily present in all multi-storey buildings.

4. Comparison was made between predicted structural behaviour using the recordedtest temperatures and a constant idealised temperature gradient. The differencebetween the structural response using the accurate measured or idealised tempera-tures was not critical. Therefore, using the idealised temperatures, additional para-metric studies could be conducted assuming different compartment sizes or differ-ent structural layouts.

5. The importance of the bridging action of the slab was shown via comparison withpredictions of structural displacement of an isolated composite beam, with aneffective width of span/4. From temperatures of 200°C and above, the bridgingaction of the slab had a significant beneficial influence on the behaviour of thesteel frame. This reinforces the need for future research to be steered away fromfocussing on isolated members (on which the present fire design codes are based)towards considering the behaviour of the structure as a whole.

At the present time it is not the intention that the developed numerical model shouldbe used for individual future construction projects, to identify if the building hasadequate inherent fire resistance without the need for additional passive fire protec-tion. Instead, the aim is to use the model to conduct extensive parametric studies toprovide definitive design guidance and improved regulations for fire resistant design.These parametric studies will, in the first instance, focus on framed buildings similarto the Cardington Frame. The effect of varying the spans of the beams and slabs,the applied load, slab thickness and material strength, together with varying the firecompartment size and fire intensity needs to be studied. Clearly an extensive task,but hopefully once complete it should be possible to develop simple design tableswhich can be used efficiently by practising designers.

Acknowledgements

The author wishes to thank Dr David Moore and Mr Tom Lennon of the BuildingResearch Establishment for allowing the results from the test to be included withinthis publication.


References

[1] Structural Fire Engineering,Investigation of Broadgate Phase 8 Fire. The Steel Construction Insti-tute, 1991.

[2] BS5950: 1990,Structural use of steelwork in building. Part 8: Code of Practice for Fire ResistantDesign,BSI, London, 1990.

[3] Fire Damage Structural Survey Report to Churchill Plaza, Churchill Way, Basingstoke.AmosBroome Associates PLC. Confidential report. June 1991.

[4] Fire Proved,New Builder.12 September 1991.[5] Thomas R., Bennetts I., Dayawansa P., Proe D. & Lewins R.Fire Tests of the 140 William Street

Office Building.BHP Research, Feb. 1992.[6] Proe D. & Bennetts I.,Real Fire Test in 380 Collins Street Office Enclosure.BHP Research,. Sep-

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construction). Verlag TUV Rheinland, 1986.[8] Lennon, T., Large Compartment Fire Tests.Second Cardington Conference.12–14 March 1996.[9] Kirby, B. R., British Steel Technical European Fire Test Programme- Design, Construction and

Results.Second Cardington Conference.12–14 March 1996.[10] Lennon, T., Cardington Fire Tests: Instrumentation Locations for Corner Fire Test. Building

Research Establishment Report Ni52/95. Confidential report November 1995.[11] Bailey, C. G.Simulation of the Structural Behaviour of steel-Framed Buildings in Fire.University

of Sheffield Ph.D. Thesis July 1995.[12] Bailey CG, Burgess IW, Plank RJ. Computer Simulation of a Full-Scale Structural Fire Test. Struct.

Eng. 1996;74(6):93–100.[13] Bailey CG, Burgess IW, Plank RJ. Analyses of the Effects of Cooling and Fire Spread on Steel-

framed Buildings. Fire Safety Journal 1996;26:273–93.[14] BS476: 1987,Fire tests on building materials and structures, Part 20: Method of determination of

the fire resistance of elements of construction (general principles).BSI, London 1987.

APPENDIXComparisons between test and computer predicted displacements


Fig. A1. Comparison between test results and computer predictions for displacements at position D6(refer to Fig. 2 for location).








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Computer modelling of the corner compartment fire test on the large-scale Cardington test frame