SiFReport3.doc
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This report describes the results of a brain storming session conducted by the National Institute
of Standards and Technology’s Building and Fire Research Laboratory. The session was held as
part of the Fourth International Workshop Structures in Fire (SiF’06) held on May 10-12, 2006
in Aveiro, Portugal. The goal of the brain storming session was to identify research needs for
predicting fire induced structural collapse of high rise structures.
In view of recent large fires in tall buildings, such as One Meridian Square (USA), East Tower
Central Park (Venezuela), World Trade Center (USA) and Windsor Tower (Spain), there has
been a renewed interest in understanding the structural safety of tall buildings under fire loading.
Researchers have attempted to understand such events by constructing large finite element
models of the entire structural assembly and subjecting the models to gravity and thermal loads.
The resulting models are computationally very expensive and frequently encounter severe
convergence difficulties, even when very small time steps are used near the end of the
computation. Lack of high temperature material property data, robust models for different
physical processes as well as other modeling and numerical issues make it very difficult to solve
for the fire induced structural response and collapse in a reliable and efficient manner.
Questions posed to the panel members and attendees
1.Finite element methods exhibit severe convergence difficulties and large CPU costs for
predicting structural response under extreme fire loads. What is the source of the convergence
difficulties? What is being done currently and what can be done in the future in terms of both
mathematical and numerical analysis that can help overcome some of these issues?
2.The finite element simulations are sometime unable to differentiate local failure of a member
from global collapse of the entire structure. Is it possible to develop new formulations that will
help overcome these difficulties?
3.Are the physical processes (concrete models, spalling, fireproofing damage, creep) in
predicting structural response well understood? What are the most sensitive input parameters in
predicting incipient collapse?
There was some discussion on how the material properties were being used in the different finite
element software. It was noted that finite element models that use different software sometime
predict very different structural response when subjected to extreme fire loading. One of the
reasons that could lead to different results is due to the highly non-linear nature of the material
properties and the manner in which the material models were being utilized in the individual
software. The understanding of the physical properties of building materials exposed to fire was
a key topic because of the uncertainty associated with the inputs that are used in a finite element
analysis. It was suggested that additional physical testing and use of reliability-based analysis,
the state of the art could be advanced in this area.
Modeling Issues :
The discussion classified under the general topic of “Modeling Issues” ranged from having
suitable software to performing large scale experiments and included development of simple
analytical formulations.
It was unanimously agreed to by most participants and all panel members that we need to have
good reliable experimental data for structures subjected to fire loading. It was widely agreed that
numerical models for predicting structural response under fire loading can be developed, but the
models have to be tested and verified using large scale structural experiments. Roadblocks that
were identified in performing large scale experiments were
Large cost.
Safety issues related to collapse of structures
Lack of existing facilities where structures could be tested under fire and gravity loads.
One suggestion was to perform reduced scale experiments to simulate the performance of
structures under fire loads. It was noted that reduced scale experiments are not necessarily less
expensive and that the scaling laws have not developed to a point where such reduced scale
experimental data can be readily applied to real scale structures.
While most participants were convinced of the need for appropriate experimental data to validate
numerical models and to improve our understanding of the general phenomena, there was a split
on what kind of experiments were needed. Some suggested performing experiments on
components to understand the fundamental behavior of columns or beams, while other stressed
the need to perform large scale experiments on full scale composite frames.
There was also some discussion on the lack of a standard software for studying structures under
fire loading. It was suggested that several software were available to perform finite element
analysis of large complex structural assemblies including ANSYS, ABAQUS, LS-DYNA,
SAFIR, VULCAN etc. It was not clear to the community on what were the relative merits of one
software over the other. Was one software better suited to performing a certain type of
calculations. Different finite element models (developed using different software)
sometime predict very different structural response when subjected to
extreme fire loading. Are different finite element software solving different set of mathematical
equations. Is the formulation different? Are the material models different?
The Fire Structure Interface that was developed for coupling
FDS and the ANSYS finite element software. Is there a need to develop an interface for other
finite element software that are used for conducting structural analysis. There was a lot of
interest in developing an interface between FDS and SAFIR. SAFIR is a specialized finite
element software package for simulating structures at high temperatures. It was suggested that
developing an interface where FDS can be coupled with SAFIR would be very beneficial to the
research community.
Sensitivity Analysis : Several participants were interested in issues related to the sensitivity of
the results to the various parameters that exist in any analysis. It was pointed out that simulating
fire induced structural collapse is multi-disciplinary field that involves several disciplines such as
fire dynamics, heat transfer, structural analysis, metallurgy, material science, numerical analysis,
statistical analysis and others. Each of these disciplines introduce several variables / parameters
into the overall analysis. For example, the results of a fire dynamic simulation feed into the
thermal analysis. Parameters such as heat release rate affect not only the results of the fire
simulation but also have an impact on thermal analysis and the computed structural response.
Similarly material properties such as heat capacity and thermal conductivity affect the thermal
analysis as well as the structural response. There was significant interest amongst the participants
of the brain storming session in identifying the most sensitive parameters or physical process for
predicting structural collapse.
Numerical Issues :
It was widely recognized that finite element methods exhibit severe convergence difficulties and
require extremely small time steps when the structure is under distress. There was also general
agreement that these difficulties arose due to:

large structural deformations under fire loading

stability of the numerical techniques used by the software

non-linear material properties and material models
It was suggested that convergence difficulties arise mainly due to the use of implicit time
marching procedures and that these could be avoided if one resorted to the use of explicit or
dynamical analysis.
Explicit vs. Implicit Analysis : In designing buildings to withstand fire, designers have always
had the goal of demonstrating stability and integrity for a specified time period. This can be a
relatively straightforward task using implicit solvers. If convergence was encountered this was
often perceived to be due to buckling of some member(s) that could possibly compromise the
stability of the building. Early FE analyses were conducted on robust short span rectangular bays
and therefore didn't encounter too many convergence issues. As long span systems began to be
modelled such as truss systems and cellular beams, the issues of buckling at early stages within
the fire while still maintaining integrity were encountered. This led to the use of explicit solvers.
In addition, up until 9/11, it had always been the goal to show that a building 'stayed up' in a fire.
When attempting to model the WTC towers, the intention now became to demonstrate 'global
collapse'. As you are aware, the small diameter diagonals of the trusses buckled very quickly and
this caused (implicit) convergence issues at very early stages of the fire. Following extensive
modelling of different portions of the building, it became apparent that an explicit solution would
be needed. Having little experience of this technique, we were forced to consider many analyses
in order to understand the principles and effects that individual parameters could have on the
results. Such parameters include time scaling and mass scaling. To structural engineers and fire
engineers, fully understanding these aspects are a challenge as they are really mathematical
programming and physics issues. Guidance on the use of explicit modelling is best sought from
the developers of the FE code and also experienced users in other areas such as car crash
simulations
As a rule of thumb I always initially run a model using an implicit solver (for which we have
validated in the Cardington tests). If that shows failure/convergence issues, then it can be
modified to an explicit analysis. Output from the explicit analysis can then be compared to the
implicit model to demonstrate the accuracy of the underlying parameters employed within the
code. In terms of model size, if the intention is demonstrate that a building will collapse, then the
full force redistribution within the structural frame needs to be accounted for. Often it can be
difficult to assess just how much is needed. I often recreate the entire 'footprint' of the building
and incorporate several floors. The number of floors is important to capture column behavior
correctly - this is of extreme importance when fires are spread over several floors. Preferably,
there should be as few 'user-defined' physical boundary conditions imposed. Ideally there should
only be boundary conditions at the very base of columns in the building to represent ground
level. It is also important to realise that large FE analysis can not be split into sub-models and
resembled in terms of applied forces - this can induce fundamentally incorrect physical
responses. Other source for convergence instabilities include a) local instabilities, which do not
affect global stability of the structure and b) local large deformations, which cause single
elements to fail. Steps that can be taken to improve performance include:

a) conducting dynamic analysis that allows for local failure and

b) allowance for the numerical analysis to locally exceed the analysis limits

c) increase multi-processor capabilities.
One suggestion that was put forward to overcome the convergence problem associated with
implicit methods was to delete the elements that are undergoing large deformations. The finite
element simulations are sometime unable to differentiate local failure of a member from global
collapse of the entire structure. Is it possible to develop new formulations that will overcome
these difficulties? One approach to differentiating local failure from global failure is to use
dynamic analysis. Use of dynamic analysis however can create other problems. It would be
useful to develop other approaches based on static principles.
The role of element or node density and resolution in the models on the predicted behavior is not
well understood.
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