Page 291 - Forensic Structural Engineering Handbook

9.2 CAUSES OF FAILURES

INTRODUCTION

In any structural evaluation, it is important to recognize and differentiate between funda-
mental categories of actualized or potential failure conditions. These condition categories
include deviations, defects, distress, deterioration, and failure; these terms are defined
below as they will be used in this chapter.
Defects are deleterious nonconformities (deviations from referenced standards or spec-
ified characteristics) of a component or system. Defects can be thought of as flaws that are
introduced through poor design, manufacturing, fabrication, or construction before a struc-
ture begins its service life, and (less frequently) by inappropriate operations and mainte-
nance during its service life. A deviation from a standard or design requirement may be
harmless, but when it has the potential to lead to a failure, the deviation becomes a defect.
Distress is the collective term for the physical manifestation of poor performance as per-
ceivable problems, such as cracks, spalls, staining, corrosion, or decay. Distress can be
thought of as symptoms indicating that defects are present.
Failure is the ultimate manifestation of distress, resulting in an unacceptable difference
between expected and observed performance. Failure can also mean the termination of the
ability of a component or system to perform an intended or required function. Not all failures
are catastrophic; most involve components that simply do not perform as expected. Failures
may result from sudden events (such as fracture) or slow processes (such as corrosion).

Defect Continuum

These conditions may not have distinct boundaries from one another, but rather may fall
along a scale, here termed the defect continuum:
Deviation(s) → Defect(s) → Distress → Failure
Although there is a chronological basis to this continuum, multiple conditions may be present
at the same time in the same system. For example, Fig. 9.1 shows a piece of an exterior sof-
fit that fell from a building, which was a distinct and dramatic failure (an event). During the
ensuing investigation, additional structural distress was discovered that had not yet resulted
in a failure; additional latent defects were discovered that had not yet resulted in distress; and
there were countless other deviations from the design that were probably harmless.

FIGURE 9.1 A piece of a glass fiber–reinforced concrete panel that fell from an exterior soffit—
a sudden failure resulting from defects and deterioration.