Chen, Dongzhu

Cheng, Shiming

Torero, Jose L.

University of Queensland eSpace

Discussion: Fire-induced structural
failure: the World Trade Center, New
York
Jose´ L. Torero PhD, CEng, FREng, FRSE
Director, BRE Centre for Fire Safety Engineering, University of Edinburgh,
Edinburgh, UK
Shiming Chen PhD, PE
Professor, School of Civil Engineering, Tongji University, Shanghai, China
Dongzhu Cheng BSc
Research Student, School of Civil Engineering, Tongji University, Shanghai,
China
1. Contribution by S. Chen and D. Cheng
The collapse of the twin towers of the World Trade Center
(WTC) on 11 September 2001 is an extreme case that any
structure would be subjected to. There are inherent uncertain-
ties in the process of modelling fire, so that complete
reconstruction of a fire event and the temperature evolution
– and therefore a solid determination of causality – is difficult.
The methodology addressed by Torero (2011), based on a
combined analysis using modern structural and fire analysis
tools, certainly helps in the understanding of complex fire-
induced failures and the collapse mechanism of super high-rise
buildings and such research reduces the disaster risks that the
world is still confronted with, whether they are induced by
natural hazards or extreme terrorists.
1.1 Coupling fire dynamics, heat transfer and
structural analysis
Four aspects of the forensic assessment are identified but the
methodology does not address the coupling effects of fire and
structure for a fire-induced structural failure. It follows the
parametric approach of establishing likely temperature evolu-
tions, conducting finite-element structural analysis and then
comparing likely structural responses to the evidence features
of the structure.
In most cases, complex failures or the collapse of buildings is
very difficult to interpret by classic cause and origin analysis
since many dominating factors and combinations of interactive
modes are still not well understood. Spatial and temporal
resolution of stress analysis cannot be chosen arbitrarily once
the requirements for thermal analysis have been decided
(Baum, 2011). Attention must be focused on the coupling of
fire dynamics simulations, heat transfer analyses and structural
analyses of the damaged building. It is only when these
simulations are coupled together in some way that the effect of
fire on structures can be quantitatively assessed. This is also a
challenge in fire engineering research.
1.2 Evidence of fire-induced structural failures
A forensic investigation is structured like a pyramid: there
should be a large foundation of verifiable facts and evidence at
the base, which form the basis for analysis according to proven
scientific principles. It appears that the pieces of evidence used
by a conventional fire investigator are the same as those
adopted in the new methodology, as illustrated in Figure 1 of
Torero’s paper. The approach of ‘fire reconstruction’ requires
understanding of the science underpinning fire and structural
behaviour to develop a unique representation of the scenario.
It seems there is no clear difference between the two
methodologies in a simple case of fire investigation, but fire
reconstruction in a forensic assessment of a building should
match the evidence. But what are the most important pieces of
evidence that will dominate the collapse sequence in the
complex scenario of a fire-induced structural failure?
Fire reconstruction is crucial in illustrating fire-induced structural
failures and possible scenarios a structure would experience. It is
not simply a trial and error approach to reduce the scenarios to a
single one, since the evidence identified in most cases will be a
mixture of patterns of different damage. The National Institute of
Standards and Technology (NIST) report (Sunder et al., 2006)
used photographic evidence to compare with predicted deforma-
tions and to validate the sequence of collapse. It is important to
clarify what essential evidence should be identified in a fire
reconstruction and it is also necessary to determine the initial state
of a structure in a fire reconstruction simulation. For instance,
whether or not the fire reconstruction of the WTC twin towers
would indicate final collapse of the buildings contradicts the
statement of ‘the building structure would still be there’ (Nalder,
1993). It is suggested that, in this case, the WTC fire
reconstruction should also consider the severe damage caused
by aircraft impact and the consequent dynamic effects.
2. Author’s reply
The author agrees with the contributors in that there are
numerous uncertainties when attempting a forensic analysis of
Forensic Engineering
Volume 165 Issue FE2
Discussion: Fire-induced structural failure:
the World Trade Center, New York
Torero, Chen and Cheng
Proceedings of the Institution of Civil Engineers
Forensic Engineering 165 May 2012 Issue FE2
Pages 105–107 http://dx.doi.org/10.1680/feng.11.00027
Paper 1100027
ice | proceedings ICE Publishing: All rights reserved
105
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fire-induced collapse, especially in a scenario of the nature of
the WTC. The contributors also state that, due to these
uncertainties, ‘a solid determination of causality’ is difficult.
This may be the case, but it cannot be interpreted as
impossible. The correctness of this determination will depend
on the evidence available, the tools used in the analysis and, as
indicated by Torero (2011), by the objectives of the investiga-
tion. The contributors’ interpretation of the objective of the
investigation is directly assumed to be the precise recreation of
the ‘timeline’ of the events. My paper is much broader than
that.
The paper clearly indicates that ‘A different approach is to
attempt to model the different components of the event (i.e.
fires, heat transfer, structural behaviour, etc.)’. A proper heat
transfer analysis will of course include the coupling effects of
fire and structures when necessary and when the objectives,
uncertainty of the conditions and input data merit this
inclusion.
The methodology presented espouses a general analysis that
will use all available tools not necessarily to recreate the
timeline of the event but to understand the sequence of key
events in a manner such that the specific objectives of the
forensic investigation are attained. Thus, the specific objec-
tives of the investigation condition the method and techniques
used.
The uncertainties associated with the fire environment, the
amount of ventilation, the damage to the structure and the
state of fireproofing in the WTC scenario were very high. In
this particular case, it is thus impossible to model the fire and
heat transfer to a level of detail that will allow a precise
reconstruction of the timeline. As clearly indicated in the
paper, the NIST report (Sunder et al., 2006) attempts to do
this with no small controversy. Furthermore, the paper
indicates in the summary section that the NIST forensic
investigation had multiple objectives (reconstructing the
timeline, improving building design, improving building
codes, etc.), thus negatively conditioning the outcome of the
investigation.
In contrast, the study by Usmani et al. (2003) opted for a
different objective, which was to understand the modes of
failure; that is, to understand what triggered progressive
collapse not to recreate the timeline. Without having to
predict a timeline, a parametric treatment of the con-
sequences of a fire on a structure (i.e. temperature evolution
of the structure) for a set of scenarios that will adequately
cover the range of conditions possible is not only acceptable
but is the correct approach. This methodology would not
have been acceptable to recreate the timeline nor would a
detailed heat transfer analysis of the specific conditions
have mastered the uncertainty and potential error to
provide the understanding desired by Usmani et al.
(2003). Thus, in the case of Usmani et al. (2003), the
objectives were aligned with the method and the conclu-
sions are clear.
The detail of the coupling of fire dynamics simulations is
dependent on the objectives of the study. For Usmani et al.
(2003), parametric analysis is an adequate form of coupling
that allows a survey of possible structural behaviour; for the
NIST analysis (Sunder et al., 2006), the form of coupling
required to attempt a reproduction of the timeline is much
more complex and probably requires an analysis consistent
with the work presented by Baum (2011). Even then, the
uncertainty associated with the inputs (i.e. fuel, ventilation,
structural damage and displacement of the fireproofing)
forces many to conclude that including that level of detail in
the analysis is an unjustified approach.
The paper describes forensic investigations where facts and
engineering tools have to be used in a manner such that
potential paths are discarded until the objectives are met (see
Figure 1 of Torero (2011)).
It is only when the objective is ‘cause and origin’ that the
evidence forms the basis for the analysis while the engineering
tools are only used for interpretation. The interrelation
between evidence and tools, as defined loosely in Figure 1, is
conditioned by the objective. This is indicated in numerous
sections of the paper.
When describing Figure 1, l indicate that fire dynamics
techniques need to be used to reduce the different hypotheses
to one. Everything that follows emphasises that the gaps left by
the discrete points of evidence need to be filled with the
application of engineering tools. The author believes very
strongly that rigorous use of engineering tools is not a ‘trial
and error process’.
REFERENCES
Baum HR (2011) Simulating fire effects on complex building
structures. Mechanics Research Communications 38(1):
1–11.
Nalder E (1993) Twin Towers engineered to withstand jet
collision. Seattle Times, 27 February.
Sunder SS, Gann RG, Grosshandler WL et al. (2006) Federal
Building and Fire Safety Investigation of the World Trade
Center Disaster: Final Report of the National Construction
Safety Team on the Collapses of the World Trade Center
Towers. National Institute of Standards and Technology,
Gaithersburg, MD, USA.
Forensic Engineering
Volume 165 Issue FE2
Discussion: Fire-induced
structural failure: the World
Trade Center, New York
Torero, Chen and Cheng
106
Downloaded by [ University of Queensland - Central Library] on [23/12/15]. Copyright © ICE Publishing, all rights reserved.
Torero JL (2011) Fire-induced structural failure: the World
Trade Center, New York. Forensic Engineering 164(2):
69–77.
Usmani AS, Chung YC and Torero JL (2003) How did the World
Trade Center collapse? A new theory. Fire Safety Journal
38(6): 501–591.
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Forensic Engineering
Volume 165 Issue FE2
Discussion: Fire-induced
structural failure: the World
Trade Center, New York
Torero, Chen and Cheng
107
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