Page 403 - Forensic Structural Engineering Handbook

11.34 MATERIAL-SPECIFIC FORENSIC ANALYSES

One of the most serious errors that is made as a result of such modifications, and which
sometimes occurs even without modification, is to use a structure in a type of service which
the original designer never intended. Examples of such modifications are reinforcement by
welding of storage tanks to take higher loads than originally contemplated, increasing live
loading in a structure without verification of fatigue life, or changing the density or height
of a fluid in a storage tank without recalculation of resulting stress increases. All these types
of errors have been documented to result in fractures in structures, some of them disastrous.

Failure Resulting from Deterioration
Loss of section and in some cases fatigue cracking and fracture are examples of corrosion-
induced deterioration that can ultimately damage or destroy structures. Fortunately, since
deterioration is common, few instances of corrosion have caused loss of life, but the col-
lapse two decades ago of a major span in the Mianus River Bridge due to corrosion
processes is one example that did. During the past 35 years, corrosion conditions have also
developed in a number of other bridge structures that have resulted in loss of service, costly
repairs, and concern about the safety of these structures. Corrosion by itself may lead to
failure conditions, but it can also promote, through section loss or pitting and grooving,
fatigue crack initiation and growth that can cause failures.
Because most corrosion conditions are time-dependent, this is one failure mode that can
usually be controlled by inspection and correction as it occurs. However, good inspection
of structures requires that there be access to critical locations, that these locations be
known, and that inspections be performed at appropriate intervals. Since corrosion is an
electrochemical process of a complex nature, it is difficult to describe briefly all the possi-
ble causes of corrosion in structures. However, the major types of corrosion can be high-
lighted, and some potentially critical situations are identified in the following paragraph.
For many structures, the primary corrosion mode is general atmospheric corrosion, and for
steel products this occurs by rusting. Rust is the product of reactions of iron and steel with oxy-
gen and moisture, and typically proceeds relatively slowly unless additional chemical contin-
uants, such as ions from sodium chloride or other salts, are present to accelerate the process.
The classical solution to this type of corrosion in structures is to create a barrier layer between
the corroding solution and the steel surface, usually by painting. When the paint layer fails or
is otherwise removed, corrosion occurs. Since the integrity of this barrier layer can usually be
assessed by visual inspection, it is a situation that can be monitored and controlled.
There are some conditions under which corrosion in structures cannot be easily assessed,
typically where corrosion occurs in crevices and less accessible locations. Areas with crevices,
under debris and dirt, and in confined spaces can also undergo accelerated and more aggres-
sive corrosion processes. These locations are often kept moist, and electrochemical cells form
which change the water chemistry and promote corrosion. For some years, this fact was not
understood by users of “weathering” (ASTM A588) steels that were used in the unpainted con-
dition with the expectation that normal atmospheric corrosion processes would lead to the for-
mation of a protective dark patina which would preclude further significant corrosion. The
formation of the patina is dependent on wet and dry cycles of service. In the presence of dirt
and debris, or in confined spaces, the surfaces of these steels do not dry, and thus the effec-
tiveness of the corrosion protection system is greatly reduced. The same principles also apply
to painted structures if the paint is allowed to deteriorate. Sometimes corrosion products that
form in these confined spaces can fill the space and can exert considerable mechanical force,
separating members, cracking fasteners, and inducing mechanical failures. The latter was the
case in the Mianus River Bridge failure and LaGuardia Parking Garage bolt failures.
As indicated earlier, another form of corrosion that occurs in structures is stress corro-
sion, and it is characterized by cracking phenomena rather than by general or even localized
section loss. This type of corrosion usually occurs in higher-strength alloy steels, typically