Page 100 - Forensic Structural Engineering Handbook

2.6 DESIGN AND CONSTRUCTION PRACTICES

to the nominal strength or resistance R , multiplied by a resistance factor in LRFD and
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strength reduction factor in USD. Values of f and R are provided by the specific LRFD and
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USD specification.
The left side of this equation represents the required resistance, which is computed by
structural analysis based upon assumed loads, and the right side represents a limited struc-
tural capacity provided by the selected members. In LRFD and USD, the designer com-
pares the effect of factored loads to the strength actually provided. The term design strength
refers to the resistance or strength R that must be provided by the selected member. The
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load factor g and the resistance factor reflect the fact that loads, load effects (the computed
forces and moments in the structural elements), and resistances can be determined only to
imperfect degrees of accuracy. The resistance factor is equal to or less than 1.0 because
there is always a chance for the actual resistance to be less than the nominal value R .
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Similarly, the load factor reflects the fact that the actual load effects may deviate from the
nominal values of Q computed from the specified nominal loads. These factors account for
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unavoidable inaccuracies in the theory, variations in the material properties and dimen-
sions, and uncertainties in the determination of loads.
It has been noted by Ellingwood et al. that the main advantages of load and resistance
factor design are as follows: 7
• More consistent reliability is attained for different design situations because the different
variabilities of various strengths and loads are considered explicitly and independently.
• The reliability level can be chosen to reflect the consequences of failure.
• The designer has a better understanding of the fundamental structural requirements and
of the behavior of the structure in meeting those requirements.
• The design process is simplified by encouraging the same design philosophy and proce-
dures to be adopted for all materials of construction.
• It is a tool for exercising judgment in nonroutine situations.
• It provides a tool for updating standards in a rational manner.

EVOLUTION OF DESIGN STANDARDS AND
MODEL BUILDING CODES

ASCE 7 (Formerly ANSI A58.1)
A report of the Department of Commerce Building Code Committee, entitled “Minimum
Live Loads Allowable for Use in Design of Buildings,” was published by the National
Bureau of Standards in 1924. The recommendations contained in that document were
widely used in the revision of local building codes. These recommendations, based upon
the engineering data available at that time, represented the collective experience and judg-
ment of the committee members responsible for drafting this document.
The ASA Committee on Building Code Requirements for Minimum Design Loads in
Buildings subsequently issued a report in 1945 that represented a continuation of work in
this field. This committee took into consideration the work of the previous committee and
expanded on it to reflect current knowledge and experience. The end result was the
American Standard Building Code Requirements for Minimum Design Loads in Buildings
and Other Structures, A58.1-1945. 9
The A58.1 standard has been revised eight times since 1945, the latest revision corre-
sponding to ASCE 7-05, Minimum Design Loads for Buildings and Other Structures. 10