Page 212 - Forensic Structural Engineering Handbook

6.14 ENGINEERING RESPONSE TO FAILURES

Most failure analysis requires more accurate modeling of structural behavior than design
analysis. Nonlinear effects and post-damage capacities need to be accurately evaluated. The
rapidly advancing sophistication of computer analysis programs allows us to analyze complex
structural behavior faster and in more detail than ever before (Fig. 6.10). However, caution is
needed, since the computer can be wrong. It is critical to understand the default assumptions
and underlying analysis techniques used in each program to gauge their effects on the results.
The model must reflect real-world behavior. If the computer model says the structure is fine
and the structure has collapsed, the model is wrong. If the computer model says the structure
should have already collapsed and the structure is still standing, the model is wrong.

FIGURE 6.10 Nonlinear analysis and evaluation of complex structures is possible using
today’s available computer software.
Similarly, identifying any single design code provision that the structure does not meet is
not necessarily a key factor in a failure. Be particularly careful in comparing existing struc-
tures to current design codes (Fig. 6.11). Design codes change with time, physics does not.
Over-reliance on the computer results is often a contributing factor to design errors and
should not be repeated in failure analysis. Basic structural models should be verified by
hand. There is no structural collapse so complicated that the basic global loads, stresses,
and capacities cannot be verified by hand to a first-order degree of accuracy.
One positive advantage of rapidly changing computer technology is that rendering of col-
lapse scenarios is much faster and the results easier to convey to a nontechnical audience.
It is critically important that all models, hand or computer, accurately reflect the
observed behavior of the structure

Laboratory Analysis
Laboratory analysis will be needed to determine actual material properties. These would
include strength tests for steel, concrete, wood, masonry, and other materials as appropri-
ate. The purpose of these tests is to determine actual material properties at the time of col-
lapse as opposed to code-required minimum values. These tests should help identify any
time- or environment-dependent changes in material properties. Some of the more common
laboratory techniques for different materials are included in Table 6.7.
In addition, load testing to failure can supply useful information for post-limit behavior
that is not well captured by typical design specifications and testing.