Introduction to Structural Fire Engineering

Introduction to Structural FireEngineering 1z

2z3z

June 21st, 2016

This webinar is presented on behalf of NCSEA

David Barber, Arup DC

Darlene Rini, Arup San Francisco,

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This presentation is protected by US and International copyright laws. Reproduction, distribution, display and use of the presentation without written permission of the speaker is prohibited.

© Arup North America Ltd, 2016

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Learning Objectives

To learn about fire resistance

To understand prescriptive fire resistance design

‐ Building construction classifications

‐ Prescriptive fire resistance requirements

‐ Standard fire resistance tests of building elements

To get familiar with performance‐based approaches

Fire Resistance

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What is Fire Resistance?

Ability of a building component or assembly to withstand exposure to fire to minimize risk of:

Collapse (Stability) Fire and smoke spread (Integrity) Transfer of excessive heat (Insulation)

Stability

No collapse or excessive deflection

Integrity

No gaps

Insulation

No excessive heat transfer

Compartmentation

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To maintain stability and fire containment throughout the duration of the fire such that:

1. Building occupants can safely escape with minimal fire exposure

2. Fire fighters can safely perform fire fighting activities

3. General public and other buildings in close proximity are safeguarded from fire hazards of the building of fire origin

Code Intent of Fire Resistance

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Fire and Smoke Hazard

Smoke is the greatest threat to occupant life safety

If a fire is not contained or suppressed, it can spread further increasing the risk to life safety, property protection and business continuity ($$$)

In rare occasions, uncontrolled fires can lead to structural collapse (e.g. 9/11, University of Delft, Windsor Tower)

MGM Grand Fire (1980)

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The Great Fire of London (1666)

Historic Fires

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Great Chicago Fire (1871)

Historic Fires

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These “great” fires highlighted the destructive nature (loss of life, assets) of uncontrolled fire events

Codes were developed in response from insurance industry (reactive in nature) to limit losses

e.g. hourly ratings, fire sprinklers, standpipes, egress/life safety systems, fire detection and alarm, special hazard fire suppression, smoke management, fire load/contents control, etc.

Code Development

Designing to Prescriptive Codes

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Prescriptive Approach Use, Height, Floor Area Construction Type Combustible vs. Non‐combustible

+ Hourly Rating

Building Code Requirements

Recall intent of hourly ratings:

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Steel• Applied protection:

- Spray‐applied protection- Intumescent paint (thin or epoxy)

• Encasement:- Blankets- Boards (e.g. gypsum wallboard)- Concrete/Masonry

• Concrete fill• Alternative Solutions

- Increased member sizes- Water

How is Fire Resistance Achieved?

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Concrete• Dimensions• Minimum Rebar Cover

Timber• Light frame

- Gypsum encapsulation• Heavy timber

- Increased dimensions- Sacrificial char layer

How is Fire Resistance Achieved?

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Trial‐by‐error approach until late 1800s to early 1900s when standard fire tests were introduced

Standard Fire Testing (ASTM E119) – Single elements, assemblies or components are placed into a furnace and heated to a standard fire curve

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200

400

600

800

1000

1200

0 50 100 150 200

Tem

pera

rtur

e (C

)

Time (min)

ASTM E 119

How is Fire Resistance Determined?

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Performance Criteria (ASTM E119)

Load bearing‐capacity• the ability of a load bearing element of construction to continue to perform

its function• Failure assumed at ~ 1000 °F (538 °C)

Integrity• prevent passage of flames or gases through holes, cracks, fissures or by

collapse

Insulation• should not allow the temperature rise on the unheated side of the element to

exceed 250°F (140°C) above its initial value

Standard Test for Fire Resistance

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Test Furnace Interior – Floor Assembly

15 ft

12 ft

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Wall Furnace Interior

10 ft

10 ft

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Hose Stream Test A 5‐min hose stream is required

after furnace test to assess integrity of assembly after fire exposure (ASTM E2226).

Unique to U.S. fire resistance ratings

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Preapproved systems/assemblies • Table 720 of the IBC

Fire Listings • Fire test reports• UL‐Directory• Gyp. Board Association• ICC Evaluation Service• Opinions

Empirical Correlations • Section 721 of the IBC• ASCE/SEI/SFPE 29‐05

How is Fire Resistance Prescribed?

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Architect• Determines Construction Type• Selects potential rated assemblies/systems based on available fire listings• Specifies fire proofing requirements (hourly rating, preferred systems)• Review and approve submittals

Fire Protection Engineer or Fire Code Consultant• Advises architect and may review submittals

Structural Engineer• Commonly not involved in any decision‐making or work

*Note: In most cases, little to no structural or fire engineering (quantified methods)

occurs in practice. Process is primarily driven by the selection of listed assemblies

Current Roles in Practice

Structural Fire Engineering

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Performance‐based approach to determine the fire resistance of an element or structure, in lieu of, adopting prescriptive guidance

Quantify performance of structure to standard fire or credible worst‐case design fire

Identify inherent strengths in the structure that enhance fire performance (e.g. over‐design, structural redundancy, alternate load paths)

Identify weakness of structure in fire conditions and provide mitigating measures to address (e.g. connections not designed to accommodate thermal expansion forces, poor detailing, long spans, offset columns, etc.)

What is Structural Fire Engineering (SFE)?

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1. Standard fire exposure Unrealistic and considered severe

Infinite heating without any cooling

2. Single elements No 2D, 3D system response

Furnace dimensions limited (e.g. 12’x15’ bays)

Elements Often unloaded

Failure temperatures assumed (e.g. ~1000F/538C for steel columns)

Boundary conditions ignored

Thermally induced forces ignored

Secondary load paths ignored

Effects on non‐load bearing fire walls ignored

Connections not tested

Why SFE? Standard Fire Test LimitationsReal fire curves

Standard fire curve (E119)

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Do standard tests accurately translate to modern building design?

What kind of performance will the building experience using prescriptive approaches?

What level of reliability and safety do prescriptive designs provide? Are we over‐designing/under‐designing?

Should a higher level of standard/care be provided for very tall buildings? Life‐line structures, Critical facilities? High‐consequence structures?

What Does This All Mean?

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What Can Happen in Real Fires?

Large deformations/damage can happen if structure is not protected, which one would typically expect. Owners often unaware

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Failure can happen in buildings “fire‐proofed” per prescriptive standards. Consequences are significant for very

tall buildings

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For timber buildings, failure varies dramatically depending on the type of timber

construction (light vs. heavy timber)


Heavy timber

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First Interstate Bank BuildingLos Angeles, 1988

Photo Credit: Boris Yaro

Non-Structural Damage Can Be Significant.

In most cases in the U.S., sprinklers control or

extinguish the fire prior to becoming structurally

severe.

Majority of damage is non‐structural to services

and/or building interiors (smoke, water)

Collapse is rare. (Have we been lucky?)

What Happens in Most Fires?

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What Happens When Fires Aren’t Considered?

I-5 Tunnel Fire, Santa Clarita Image © AP

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What Happens When Fires Aren’t Considered?

MacArthur Maze Fire San Francisco

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Increased Safety (check for robustness in fire, identify weaknesses, less reliance of fire‐proofing, new materials)

Aesthetics (remove or reduce thickness of fire‐proofing, expose steel)

Cost Savings (~1% of construction costs)

Additional Benefits• Assessing performance of existing/historic conditions• Improved health and safety on site• Reduces carbon “footprint”• Limits ongoing maintenance, reduce construction time

Value SFE can bring to a project?

Technical Basis for Structural Fire Engineering

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Fire: ‐ during construction phase on 1st floor ‐ duration 4 hrs, max temp 1000°C‐ large deflections, no collapse

1990 Broadgate Fire (UK)Building: ‐ 14 storey steel structure

‐ partially unprotected (no column protection), no sprinklers

Development of Advanced SFE

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Design Standards do not reflect real fire behavior

British Standards would NOT have predicted this behaviour at temperatures reached

The frame did not act as a single element does in a furnace test

Deformations more severe

Yet Structural stability was significantly improved

This led to the Cardington Tests to determine what really happens to steel structures in fire

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Large Building Test Program at BRE

8 story steel frame building (5 x 3 bays), 21 m x 45m (69’ x 148’), 33m tall (108ft)

Framework of I‐beams (6‐9m spans).

Composite metal deck floors

Stability = Braced frame at cores

Only the columns were protected

Exposed to several realistic fire scenarios (6 initially)

Cardington Fire Tests

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Cardington Fire Tests

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Temperatures reached 900C in first 10min

Peak gas temperature = 1213C

Peak steel temperature = 1150C

HRR = 58MW

Equivalent time (E119) = 74min

Large displacements/ deformation observed for unprotected steel beams (L/15), but no collapse

Bottom flange buckling near supports

Significant cracking of composite slab around internal column

Office Demonstration Test – Results

Deflected floor slab after the fire test (Office Test 6)

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Better understanding of performance of steel structures in fire Redundancies available in steel buildings

• Alternate load paths• Tensile membrane action• Secondary beams have less of a role in overall stability

Composite floor slab – important for overall stability Need to think about connections in fire

Impact on Design?

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Cardington fire tests led to significant amount of research Additional fire tests and real fire events have furthered our understanding

(e.g. 9/11, WTC analyses, FRACOF tests, Czech tests, etc…) Design guides, international standards, and references are available (e.g.

NFPA Handbook, SFPE Handbook, ASCE/SEI/SFPE 29, App. 4 Steel Manual, CASE, Eurocodes, ASCE 7 Appendix in progress)

Design References and Guidance Documents

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AISC• Specification for Structural Steel Buildings (2010) Appendix 4• Fire Resistance of Structural Steel Framing, DG No. 19 (2003)

ASCE• Standard Calculation Methods for Structural Fire Protection –

ASCE/SEI/SFPE 29‐05 • Minimum Design Loads for Buildings and Other Structures – ASCE 7‐10• Performance‐Based Design of Structural Steel for Fire Conditions

ACI • ACI 216 – Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies

American Wood Council • National Design Specification for Wood Construction

U.S. Codes, Standards and References

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ASTM• Standard Test Methods for Fire Tests of Building Construction

and Materials (2013)

ICC • International Building Code (2015)

- Alternative design, materials and methods of construction• ICC Performance Code for Buildings and Facilities (2006)• ICC Performance‐Based Building Design Concepts: A Companion

Document to the ICC PC (2003)

NFPA• Building Construction and Safety Code, NFPA 5000 (2006)• NFPA Fire Protection Handbook 20th Edition


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NIST• Best Practice Guidelines for Structural Fire Resistance Design of Concrete and Steel Buildings (Tech. Note 1681) –2010

SFPE • Engineering Guide: Performance‐Based Fire Protection• SFPE Handbook of Fire Protection Engineering• Engineering Guide: Fire Exposures to Structural Elements • Engineering Guide: Fire Safety for Very Tall Buildings


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Structural Eurocodes include the following sections:• Basis of design

- Fire exposure - Verification methods- Methods of structural analysis

• Material properties- Mechanical properties- Thermal properties

• Design procedures- Tabulated data- Simple calculation methods- Advanced calculation methods

• Construction details

Eurocodes

General Approach

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Utilizing IBC “Alternative Materials and Design Methods” approach

Identify credible worst‐case fire scenario(s) for the specific building details- Compartment sizes/geometry- Fuel loads- Ventilation- Localised fire, flashover fire, travelling fire

Quantify structural response to realistic fire scenario(s) - Single element analysis- Advanced whole frame analysis

General Approach to SFE Design

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Design fire

Heat transfer

Check Stability

Load

Capacity

Check Stability

3 Stage process. Similar to ambient structural design

Design Process for Structural Fire Analysis

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Fully‐developed ExteriorLocalized

Step 1 – Determine Realistic Design Fire

Fully developed, post‐flashover fire (entire compartment/floor)

Localised fires (atrium, large volume spaces, high ceilings)

External flaming (structure is located outside internal space)

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Travelling Fires

Multi‐story Fires

Step 1 – Determine Realistic Design Fire

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Heat transfer to the structural element but also through and along the element

1‐D Analysis

Layered – Thru‐thickness gradient (e.g. thick plate),

Finite element, lookup tables

Lumped mass – one temp only (e.g. steel)

2‐D Analysis Gradient across one planar cross‐section (e.g.

composite beam)

3‐D Analysis

Contours along member axis and at any section

Step 2 – Perform Heat Transfer Analysis

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Single element analysis methods Tabulated data

Simple calculations methods

Advanced calculation methods Slab analysis (spreadsheet, computer modeling)

Frame analysis (computer modeling)

Step 3 – Examine Mechanical Response

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i

iin QR

Demand (Load)

Applied Loads during fire

(1.2DL + 0.5LL) per ASCE 7 or AISC 360

Thermal expansion

Thermal bowing

Capacity (Resistance)

Reduction in fy, fp, E, fu

Reduction in f’c

Spalling, Charring, Loss of

Section

Step 3 – Examine Mechanical Response (cont…)

How does fire affect the basic governing equation?

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Early discussions with Building and Fire Department

Intro concepts, approach and possible outcomes

Investigate viability of PBD

Recommend 3rd party peer review (list of experts can be provided)

Agree on Key Modeling Assumptions and Acceptance Criteria

Perform and Document Analysis/Results/Recommendation

Obtain final sign‐off of Performance Based Solution

Typical Time Frame – several months

Design Approvals Process

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Need to demonstrate code compliance • Education • Uncertainty of approval• Additional work/documentation• Need for Peer Review

Perceptions of risk/threats by AHJ, Owner

Coordination within Design Team – A/E, FPE and SE • Design responsibility• Schedule, Fee• Standard or design basis fire• Design loads, analysis, drawings, etc. • Iterations & trade‐offs on desired vs. possible goals

Practical Issues

Steel

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Steel Properties at High Temperatures

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Unrestrained thermal expansion:

Average beam temperature increasing

TLL

TLL

t

Thermal Expansion

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0 mTt

Tm

TEAEAEAP Tm

Restrained thermal expansion:

Restrained Thermal Expansion

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2

2

lEIPcr

2

2

lEITEA

22

lrTcr

l

Buckling or Yielding due to restrained thermal expansion:

TEAP

E

T yy /

Buckling : Yielding :

Restrained Thermal Expansion…

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yTEIM ,

Beam trying to rotate as lower half heated and trying to expand

Restraint stops this and so a new moment develops

Thermal Bowing

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Unrestrained beam failure commences

840 F (448 C)

Restrained beam failure commences

1470 F (798 C)

Buckle event

How Does End-Restraint Affect Response?

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Thermal contraction forces can be high (10s ‐100kips)

Can connections cope (force or ductility)?

Potential Issues During Cooling?

Timber

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Timber - Fire Fundamentals

Image © Moshe Safdie & Associates

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Inherent protection against heat Charring behaviour Well‐understood and researched 1.5 in/hr [0.65mm/min]


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Wood performance in fire is predictable

Load carrying under fire is reliable

Design for fire resistance, through increasing wood cover


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History of fire tests: Standard furnace tests Tests in real fires Carried out internationally Correlations based on density,

moisture, grain direction, sawn or engineered wood

Figure 2-2 from TR-10

Determination of Charring Rate

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One dimensional char rate = 1.5 in/hr

Design char rate increased by 20%:• Corner rounding

• Fissures

• Zero strength layer behind the char

For 1hr exposure = 1.8in/hr

For 2hrs = 1.58in/hrFrom APA

Determination of Charring Rate

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90 minute FRR fire test on 270mm x 415mm glulam beam (from APA)

Engineering Design

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What is the area (b x d) needed ?

What additional wood cover is required, for an FRR (B x D)

The difference (B – b) is the sacrificial char layer

TR-10 Fig 1-2

Engineering Design - Calculating an FRR

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IBC 722.1 references NDS

FRR of wood – NDS Chapter 16

Method explained in detail in TR‐10

TR‐10 is a compliant methodology for providing exposed wood FRR

All freely available online from AWC

Engineering Design

Concrete

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In general, concrete buildings are considered to be inherently fire resistant (non‐combustible, low thermal conductivity)

Sao Paolo, 1974

Introduction to Concrete and Fire

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While rare, failure can still occur if response of structure to fire is not well

understood or evaluated

Concrete Structures in Fire

Windsor Tower, Madrid, 2005 University of Delft, Netherlands, 2008

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Non‐combustible

Low thermal conductivityo Improved heat transfer relative to steel

(i.e. majority of section will remain cool and have full strength)

o Large thermal gradient will develop through the member x‐section (leads to thermal bowing/curvature)

Homogenous material o Concrete + steel (both materials

required for stability), since concrete has little/no tensile capacity

o Need material temperature history

Potential for spalling

Key Unique Features of Concrete in Fire

Non-linear thermal

expansion

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Heterogeneous mixture (Steel + Concrete)o Capacity of element relies on the concrete for compression and the steel for tension (beams) or confinement (columns)

o If steel gets too hot, concrete element will fail

Rebar (500C)

Concrete (60-177C)

Role of Reinforcement

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Description: loss of surface concrete, caused by local development of high stresses o Explosive ‐ violent and can occur at an early stage

o Surface ‐ local remove of surface material including, pitting and blistering

o Aggregate splitting ‐ failure of aggregate near the surface

o Corner separation ‐ removal of external corners

o Sloughing off ‐ gradual progressive process

Spalling

100°C300°C1200°C 600°C800°C

Concrete Melts

Water of hydration lost

Creep increases

Gravel breaks up

Free water lost- spallingstarts

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Normal weight concrete

High strength concrete

Impact: o Reduction of cover ‐ reinforcing steel heated more quicklyo Reduction of overall thickness ‐ unexposed surface temperature increases more quickly

Examples of Spalling

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Spalling In Practice

Not addressed in U.S. codes It is generally accepted that:

o Ordinary strength concrete in an internal environment has a low moisture content and will not likely spall

o Typically, building fires in office, assembly, residential spaces are not rapid – spalling less likely

o Prescriptive rules in the codes have served us well for buildings

o Performance of concrete in fire is a critical issue in tunnels (rate of heat rise is high) and in high strength concrete applications

Concluding Remarks

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Concluding Remarks

Performance based fire resistance design or Structural Fire Engineering (SFE) is still new in the U.S. and is an evolving field, but is gaining momentum as engineering is moving more towards PBD.

SFE appears to be driven by problems, code limitations, architectural, operational, or safety needs. Cost is secondary

Analysis typically limited to parts of the building (isolated elements, lateral system, secondary steel), and not necessarily the entire structural fire design

The principles are applicable to all types of structures – new and old. Most work is undertaken in buildings, but increased attention in infrastructure (tunnels and bridges)

SFE is considered independently of active fire safety features in the building. This is necessary for redundancy and limiting risk.

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Concluding Remarks

There is limited domestic expertise available for consulting applications

More explicit US code criteria design tools are lacking, with much knowledge imported from international sources. However, this is changing quickly (e.g. ASCE/SEI/SFPE 29, Appendix 4 of Steel Manual, CASE, ASCE 7 in the works, NFPA Handbook, SFPE Handbook, NDS)

More research is necessary , code/standards progress, education & training necessary to broaden applications in the US

Gain user confidence thru successful examples (and recognition of prescriptive design constraints)

SFE primarily applied to steel structures/buildings. Some limited applications in concrete for tunnels and bridges. Heavy timber applications on the rise (USDA support of tall timber)

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Office Demonstration Test (Test #6)

Thank You!This concludes the NCSEA Continuing Education Webinar

Arup North America Ltd.www.arup.com

David Barber, P.E. Darlene Rini, P.E.tel: 202-729-8216 tel: 415- 946-1682

([email protected]) ([email protected])