Page 200 - Forensic Structural Engineering Handbook

6.2 ENGINEERING RESPONSE TO FAILURES

serviceability issues, since they do not affect the load-carrying capacity of the structure.
A structure has essentially failed when it cannot be used to carry the intended loads with a
reasonable safety factor.

Why Investigate Failures
There are varying reasons to investigate a structural failure: to determine the cause(s) of the
failure; to understand the key failure mechanisms in order to avoid them in the future; to
remediate the structure and return it to service; as part of litigation or insurance claims; and
to assure the public that remaining structures are stable and can be used for their designed
purpose (Fig. 6.1).

FIGURE 6.1 The partial collapse of this parking structure was attributed to failure of cast-in-place
concrete corbels which occurred only in one exterior bay, rendering the balance of the structure safe.

Why Structures Collapse
Structures collapse because something has disturbed the equilibrium they were in prior to
collapse; this can be caused by increased loads, by reduction in structural capacity, or some
combination of both. Loads on structures include environmental loads (snow, ice, wind,
temperature, impact, and earthquake), construction loads, service loads, and other unusual
or extreme loads (blast, fire), any one or more of which may increase by reasons not con-
sidered in the design (Fig. 6.2). Decrease in structural capacity can be the result of many
factors, including corrosion, degradation, damage, fatigue, and inadequate maintenance or
repairs, and design or construction errors (Fig. 6.3). The ultimate purpose of a collapse
investigation should be to determine the likely contribution of each of these factors. Ideally,
the sequence of collapse can be traced from the first overload mechanism through to com-
plete collapse.