Page 97 - Forensic Structural Engineering Handbook

DESIGN CODES AND STANDARDS 2.3

DESIGN STANDARDS AND THEIR RELATIONSHIP
TO STRUCTURAL PERFORMANCE

If a failure occurs, a comparison of the structure as it was actually built, with the require-
ments of applicable building code, usually will be required. However, the adherence or
nonadherence to building code requirements may or may not have a significant bearing on
the assessment of the actual performance of the structure. Factors of safety, load factors, and
assumptions regarding interaction between structural components used by designers may or
may not reflect actual structural performance, but rather may reflect intended performance.
A fundamental understanding of these engineering principles is a necessary prerequisite to
any forensic investigation. The interaction of structural elements as they existed, rather
than as they were conceived, must be considered. The properties of materials as they
existed, rather than as they were assumed to exist in design, must be determined.

Structural Safety

There are three main reasons that some sort of safety factors, such as load and resistance
factors, are necessary in structural design:
Variability in resistance: The actual strengths (resistances) of structural elements will almost
always differ from their estimated values. The main causes of these differences are the vari-
ability of the material strengths; differences between the as-built and as-designed dimensions;
and the effects of simplifying assumptions made in deriving the member resistance.
Variability in loading: All loads are variable, especially live loads and environmental
loads due to snow, wind, or earthquakes. In addition to actual variations in the loads
themselves, the assumptions and approximations made during the course of analysis
may result in differences between the actual and estimated forces and moments. Due to
the variabilities of resistance and load effects, there is a definite chance that a weaker-
than-average structure may be subjected to a higher-than-average load. In extreme
cases, failure may occur.
Consequences of failure: A number of factors must be considered in determining an
acceptable level of safety for a particular class of structure. These considerations
include such things as potential loss of life; lost time, lost revenue, or indirect loss of
life or property due to a failure; and type of failure, warning of failure, and/or existence
of alternative load paths.
For example, the factor of safety for a public auditorium should be greater than the factor
of safety for a storage building. Similarly, if the failure of a member is preceded by excessive
deflections, as in the case of a flexural failure of a beam, then the persons endangered by the
impending collapse will be warned and will have a chance to leave the building before fail-
ure. This may not be possible if a member fails suddenly without warning, as may be the case
with a reinforced concrete column. Thus, the required margin of safety need not be as high
for a beam as for a column. In some structures, the yielding or failure of one member causes
a redistribution of load to adjacent members; in other structures, the failure of one member
causes complete collapse. If no redistribution is possible, a higher margin of safety is required.

Probabilistic Calculation of Safety Factors
Structural design should provide for adequate safety no matter what philosophy of design
is used. Provision must be made for both overload and understrength. The study of what
constitutes the proper formulation of structural safety has evolved over the past thirty-five