Page 307 - Forensic Structural Engineering Handbook

9.18 CAUSES OF FAILURES

ACCEPTING UNDESIRABLE EXISTING
CONDITIONS

In the design of new structures, prescriptive codes and specifications set minimum stan-
dards for design. When investigating the condition of existing structures, it is not always
feasible (or even technically sound) to apply design equations or design standards directly.
Because building codes generally do not provide guidance on the analysis of existing struc-
tures, it is often necessary to apply engineering judgment to determine whether an existing
condition is acceptable, safe, or serviceable.
When engineers were asked in an informal survey about their comfort level regarding a
calculated overload condition on an existing structure, many considered 5 to 10 percent over-
stress acceptable. They took into account that a factor of safety was built into the original
design; that there are practical limits to the accuracy of such calculations and load estimates;
that typically there are secondary load paths not accounted for in the calculations; that the
actual live and dead loads may be well defined (as opposed to the estimates used before con-
struction); and that being overly conservative could result in substantial economic waste
(unnecessarily repairing structures that are expected to function satisfactorily).
However, building codes do not recognize this distinction between design calculations
for new construction and analysis of existing components. Therefore, an engineer may be
accused of arbitrariness (or even negligence) for accepting such an overstress condition
owing to a lack of published support for the rationale, despite sound engineering judgment.
The International Existing Building Code (IEBC) has been published by the
International Code Council with the intent of becoming the primary code for work in exist-
ing buildings, replacing the provisions regulating repair, alterations, additions, and change
of occupancy in the International Building Code (IBC). It does not require full compliance
with the provisions for new construction that may be overly conservative for evaluating
existing construction. It remains to be seen whether the IEBC will be adopted.
It is even more difficult to standardize acceptance of distressed conditions. Design rules
are based on the behavior of new materials in their original configuration. For example,
while concrete design requires the engineer to consider time-dependent effects such as
creep, the code equations do not take into account reduction in strength or service life due
to deterioration, such as delamination.
There are many questions involved in accepting undesirable existing conditions. Is the
public served by remedying an existing condition to strict compliance with design stan-
dards and building codes? Does it pose an unreasonable financial burden to improve a
structure that may be “good enough” but is not in strict compliance? Does the current con-
dition of the structural element or system fall within the assumptions used to develop the
code equation? In particular, the last question requires that the engineer not only understand
the basis for the code equation, but also use substantial judgment in determining whether
the existing conditions are compatible with those assumptions.

GLOSSARY OF TERMS

Biological degradation factor. A degradation factor directly associated with living
organisms, including microorganisms, fungi, bacteria, and insects. 3
Catastrophic. Description of an unintended event resulting in loss of life, severe personal
injury, or substantial property damage.
Collapse. A structural failure resulting in the total destruction of all or a portion of a struc-
tural system, with consequential damage to the nonstructural systems.