Page 153 - Handbook of Civil Engineering Calculations, Second Edition

1.136 STRUCTURAL STEEL ENGINEERING AND DESIGN

States with the publication of the Load and Resistance Factor Design Specification for
Structural Steel Buildings. Load and resistance factor design, or LRFD, has joined the old
allowable stress design (ASD) method as a recognized means for the design of structural
steel frameworks for buildings.
“Although ASD has enjoyed a long history of successful usage and is familiar to engi-
neers and architects, the author and most experts prefer LRFD because it is a truer repre-
sentation of the actual behavior of structural steel and unlike ASD, it can provide equiva-
lent margins of safety for all structures under all loading conditions. . . . For these reasons
it is anticipated that LRFD will replace ASD as the standard method of structural steel
design.”
The following selected procedures in this handbook cover structural steel design for
buildings using the load and resistance factor design (LRFD) method drawn from the ex-
cellent Rokach book listed above. And competent authorities on the LRFD method, listed
below, are cited frequently in the Rokach book, and in this handbook, usually in abbrevi-
ated form:
AISC: American Institute of Steel Construction, Inc., Chicago, IL.
AISC LRFD Specification: Load and Resistance Factor Design Specification for
Structural Steel Buildings, published by AISC.
AISC LRFD Manual: Load and Resistance Factor Design Manual of Steel Construc-
tion, also published by AISC.

Equations in the following calculation procedures in this handbook are numbered as
follows. Those equations appearing in the AlSC LRFD Specification are accompanied by
their AISC numbers in parentheses, thus ( ); other equations are numbered in brackets,
thus [ ].
It is recommended that the designer have copies of both the AlSC LRFD Specification
and the AISC Manual on his or her desk when preparing any structural steel design using
the LRFD method. Both are available from the AISC at 1 E Wacker Dr, Suite 3100,
Chicago IL 60601.
Abraham J. Rokach writes, further, in his book cited above, “The ASD method is
characterized by the use of one judgemental factor of safety. A limiting stress (usually F y )
is divided by a factor of safety (FS, determined by the authors of the Specification) to ar-
rive at an allowable stress
Allowable stress F y /FS

Actual stresses in a steel member are calculated by dividing forces or moments by the
appropriate section property (e.g. area or section modulus). The actual stresses are then
compared with the allowable stresses to ascertain that
Actual stress allowable stress

No distinction is made among the various kinds of loads. Because of the greater vari-
ability and uncertainty of the live load and other loads in comparison with the dead load, a
uniform reliability for all structures is not possible.
“. . . Briefly, LRFD uses a different factor for each type of load and another factor for
the strength or resistance. Each factor is the result of a statistical study of the variability of
the subject quantity. Because the different factors reflect the degrees of uncertainty in the
various loads and the resistance, a uniform reliability is possible.”