Page 268 - Forensic Structural Engineering Handbook

8.8 CAUSES OF FAILURES

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The list of errors in this case abound: (a) the original design did not meet the code
resulting in a zero factor of safety design, (b) the design was not a particularly robust con-
cept (e.g., the use of back-to-back channels or bearing plates for the nuts would have provided
a stronger design), (c) the contractor’s substitution was approved without a proper review,
(d) the contractor introduced a design change in the shop drawings rather than through a writ-
ten request for substitution. The ensuing legal proceedings did highlight the lack of proper
communication between the design engineer and the contractor, thus missing the opportu-
nity to catch the error before it was too late.

Problem Winds
As far as loads go, wind has been a particularly formidable foe, humbling some of the most
accomplished structural engineers. The destruction of the brand new Tacoma Narrows sus-
pension bridge (nicknamed “Galloping Gertie”), in 1940, by wind-induced torsional oscil-
lations or aeroelastic flutter is but one of these humbling lessons. For its designer, Leon
Moisseiff, it overshadowed his many other famous designs and became a symbol of arro-
gance in design. In reality, engineers at that time did not have the tools to analyze the aero-
dynamic effects of wind on suspension bridges, nor did they do it. Hence, Moissieff could
not be found legally liable for a design error or in breach of the contemporaneous standard
of care. However, notorious storm-induced collapses of suspension bridges in the preceding
century were known (e.g., Niagara-Clifton bridge in 1889), and successful bridges had been
designed since, such as the George Washington and the Golden Gate bridges on which
Moisseiff was a consulting engineer. Moisseiff’s design was substantially more flexible and
slender than his other successful bridges, and he did not have the adequate tools to analyze
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the aerodynamic effects on a structure that departed from the established. The collapse of
the Tacoma Narrows Bridge led to a boost in research in the field of bridge aeroelastics and
aerodynamics, which have influenced the design of long-span bridges since then.
The potential damage to New York’s 59-story Citicorp Center in 1978 illustrates
another wind-induced crisis resulting from our evolving wind design practices, issues aris-
ing from redesign during construction, and mostly is a lesson in exemplary ethics. The
Citicorp tower was an engineering challenge from the beginning since it had to accommo-
date a church underneath one of its corners, with no physical connection to it. Boston-based
structural engineer William LeMessurier set the tower on four massive nine-story high
columns, positioned at the center of each of the faces of the tower. The floor loads were
transferred to the base columns through a stacked system of inverted chevron braces. This
daring scheme allowed the tower to cantilever two of its corners by 72 ft, over the church
on one side, and over a public plaza on the other. One year after the completion of the tower,
and prompted by a question from a Princeton engineering student, LeMessurier reviewed
his design for “quartering” winds (i.e., 45 degree wind incidence) as opposed to the per-
pendicular winds that the New York City Building Code required. He was surprised to dis-
cover that the stress levels in some of the chevron braces were greater by 40 percent
compared to those induced by perpendicular winds. Still, his design had ample margins, or
so he thought. Unfortunately, a design change introduced during construction, which
replaced the laborious field welding of the brace joints with bolted connections, had been
approved by his New York office without his knowledge. Furthermore, the bolted connec-
tions were designed for perpendicular winds only and the braces had been treated as truss
members rather than columns for the purpose of connection design; and finally P-δ effects
had not been considered. The resulting connections were therefore too weak to withstand the
70-mph quartering winds just as the hurricane season was approaching. Although the design
was code-compliant, LeMessurier did the right thing. He informed his client and convinced
Citicorp to undertake emergency strengthening of the structure by welding 2-in-thick plates