Comments on structural fire response and collapse analysis

Dr B Lane

15th September 2005

Some context

• Commercially unacceptable now to build tall without addressing extreme events

• Demand for information and knowledge has been unprecedented

• High-rise building design and expectation from this design has changed - and did so very quickly 

• Current trend in London is “are you compliant with NIST”

• We had to take an interest

Responding to demand

• We had to be in a position to provide practical advice

• Therefore we had to understand the events of 9-11 from the human response, through to emergency response, and onwards to the collapse mechanism 

• Focusing on the collapse or the structural fire response

Designing Structures for fire

• Typical composite steel frames

• Complex geometries + long span systems

• WTC 1 and 2 representations

• WTC 7 (confidential)

• Post-tensioned slabs

• Spalling prevention design

• Madrid Fire – structural response

• High-rise design for extreme events (various)

NIST investigation

• We support the call for specific performance based objectives for structural design for fire

• Within a risk assessment framework

• Not so concerned with local collapse

• Support design for burnout – but not until the profession has a realistic post-flashover fire model

NIST investigation – some comments

• Thermal resistant façades and subdivision of floor plates a real concern

• Commercially we have to ask what greater robustness will this give a client? 

• Trying to prevent a fire from occurring or trying to limit its size 

• Reliance on single lines of resistance

• We would propose instead design with redundancy

• Designing the structural system itself to withstand the effects of fire

Structural response to fire

• Very specific material and geometry changes take place in fire

• Expansion of all the materials and therefore the structural dimensions

• Degradation of the material strength and therefore their ability to carry load

Structural response to fire

• But structures seem to perform very well in fire and various events or real tests on multi-storey buildings have demonstrated this

• Fire proofing is not the sole means of protecting a structure in fire

Recent analysis on code compliant buildings

• 2 buildings

• Same construction form

• Same agreed design basis fires

• One collapses, One does not

Why did Building B fail and not the Tower Building A?

• Fire proofing ratings identical

• Slab depth and temperatures identical

• Slab spans ~4.5m

• Beam spans ~12m

• Beam temperatures similar

• Columns offset in Building B not in Tower Building A

• Columns 10.5m apart in Building B, 9m in Tower Building A

• Yet column sizes the same

Significance?

• If the buildings had been designed with columns that could cope with the long span beams as they expanded in the fire, collapse would not have occurred

• Fire proofing is therefore but one aspect of limiting structural response to fire

• The design of the structure itself we see as the most critical aspect requiring change for the future

• We do not believe this has made a significant entry to NIST’s recommendations

WTC studies

• We have developed complex models of a representation  of WTC1

• This was not a forensic analysis

• Purely to understand if that structural form had any specific collapse mechanisms

• Ongoing work

3-D Modelling Strategy

Quarter FloorSingle Storey

Quarter FloorMulti-Storey

Half FloorSingle Storey

Simple Models

SingleColumn

MultipleColumns

Cross-SectionModel

Fire Temperatures3 Floor Fire Modelled

Analysis of 1 hour (3600s)

Trusses undergo rapid heating to reach 800oC in 

a very short time

Perimeter columns and spandrels heat linearly to 

400oC after 3600s

Slab temperatures evaluated at 5 discrete 

points through the depth

Core assumed to be at ambient

Quarter Floor: Collapse Mechanism

Large inward bowing of columns at 

failure

Spandrels undergo 

significant buckling

Short-span double trusses 

see smaller deflections than longer 

spans

• Similar collapse mechanism as seen in the WTC

• But with no impact included

Comparison with Project 6 work

• This form of quantification of response has not been presented in Project 6

• But is required to reach the highly significant conclusion about the role of fire proofing

• Must be carried out to assess real designs

• It is our view that without including the floors correctly in the global models, collapse, or the lack of collapse, cannot be captured nor quantified

Differences in global model approach• NIST model:• Detailed forensic model of the fire

• Floors represented as membranes only-no trusses, purely elastic

• Thermal expansion on columns only• Not on the floor systems

• Deflected shape not calculated

• Compatibility of deflections across the floor not calculated

• No push out forces modelled

• Pull forces on columns manually added to model

• Material stiffness of floor modelled only

• Arup/Edinburgh model:• Fire incorporated in a parametric way 

to representforensic model (design approach)

• Floors modelled as per the real building

• Including thermal expansion on all elements

• Floor response (deflections and compatibility of deflections across the floor) calculated by model

• Push/pull forces on columns calculated by model

• Support stiffness provided to the columns from the floors modelled accurately

Pull-in force applied by user

Push and pull forces calculated by model

Impact on results• NIST model:

• Forces on columns applied by user – 10 min steps

• Collapse theory based on fitting pull-in forces to match the deflected shape of the columns from video footage

• Forces not necessarily representative of the real case

• NIST have not calculated response of towers with fire protection applied

• Arup/Edinburgh model:

• Behaviour calculated by the global model for the duration of the fire with no user intervention

• Collapse mechanism caused by thermal expansion, compatibility of deflections, pull/push forces on columns resulting in bending failure of the columns – all calculated by the model

• Collapse theory can be tracked and explained 

• Knowledge gained can be used to understand other structural systems in fire

WTC studies

• Based on our observations of collapse without aircraft impact included, and NIST’s representations of the global response of the structure to fire, we do not consider NIST to be in a position yet to conclude had the aircraft not knocked the fire protection off, collapse would not have occurred

Just another modelling “stand off” ?

• Our view is that this main conclusion on fire proofing has serious implications for the future of design:

• A continuing reliance on fire proofing in lieu of structural detailing for fire - a serious concern; 

• It has resulted in the role of structural design being underplayed in NIST’s final recommendations

• Due to the absence of the acknowledgement that fire must become a design load for structural engineers in order to genuinely improve the structural performance in fire

The Future?

• We want to see • design to prevent progressive collapse in fire written into all building 

codes• A move away from total reliance on fire proofing – moving away 

from ignoring real response to fire• An active move by this profession to create fire as a design load for 

structural engineers• This profession to carry out structural fire analysis of buildings in 

order to improve tall building guidance and advice

• All this within a risk-based framework - we do not believe this level of analysis is needed for all buildings, in all scenarios, all locations

To finish

• Significance of the NIST report

• Continue to work as a community with NIST to implement new design tools in the future

- Comments on structural fire response and collapse analysis
- Some context
- Responding to demand
- Designing Structures for fire
- NIST investigation
- NIST investigation – some comments
- Structural response to fire
- Structural response to fire
- Recent analysis on code compliant buildings
- Why did Building B fail and not the Tower Building A?
- Significance?
- WTC studies
- 3-D Modelling Strategy
- Fire Temperatures
- Quarter Floor: Collapse Mechanism
- Comparison with Project 6 work
- Differences in global model approach
- Pull-in force applied by user
- Impact on results
- WTC studies
- Just another modelling “stand off” ?
- The Future?
- To finish