Page 171 - Forensic Structural Engineering Handbook

5.4 ENGINEERING RESPONSE TO FAILURES

perishable evidence and its potential value; can recommend action, and may be in a position
to persuade those who are in control of the site. The decisions made will directly affect the
abundance—or scarcity—of evidence, upon which investigations will depend. Sparse evi-
dence leads to tentative conclusions; robust information provides the basis for a persuasive
argument.
A successful investigation can be defined as one that satisfies its stated goals in the most
efficient manner possible. Although investigations may justifiably have other goals, for
purposes of this chapter, it is assumed that the goal is to determine, within a reasonable
degree of engineering certainty, the most probable cause(s) of the failure.
This chapter deals with issues that an engineer may be faced with when he or she first
steps onto a collapse site, and in the ensuing days. It includes issues such as safety, pre-
serving perishable evidence, reserving samples, documentation, interviews, document
gathering, and preliminary evaluation. With most of these issues, speed and accuracy are
of the essence.

SAFETY

The engineer called to the site of a collapse may be requested to assess the safety and sta-
bility of the structure for a variety of possible reasons:
• To assist in identifying the safest routes through the debris, or identifying areas that must
be avoided until stabilized. These routes may be needed by rescue personnel to reach vic-
tims, by safety officials who need to reach utility shut-off valves, or by workers attempt-
ing to stabilize the structure
• To assist in identifying “pockets” within the debris where victims might be sheltered
trapped
• To assist in identifying components that are in imminent danger of further collapse
• To evaluate methods of stabilizing the structure, such as by adding shoring, bracing, or
tiebacks
• To assist in determining whether it is advisable to provide protection for the public, or
whether to restrict public access. Protection may include safety netting, sidewalk bridges,
and other barriers. See Figs. 5.1 to 5.4.
• To assist in evaluating alternative demolition or dismantling sequences. The load paths
after a collapse may differ significantly from the intended load paths, and may not be
readily apparent. It is important to try to identify stressed components, or potentially
stressed components. If a stressed component is cut or removed, it may release its load in
a sudden and uncontrolled manner, possibly causing injury or disturbing other compo-
nents. If it is necessary to remove a potentially stressed component, consideration should
be given to first relieving its load (possibly using cables with come-alongs), removing the
member, then slowly releasing the temporary load in a controlled manner. For complex
collapses, the active load paths may only become apparent as demolition proceeds, so it
is important that the process be constantly monitored. Removal of posttensioned ele-
ments requires special attention to prevent unexpected releases of load.
The evaluation of the Banker’s Trust (Deutsche Bank) building following the
September 11, 2001 terrorist attacks in New York City is a good example of one that
required many of the above-mentioned services. 1