Page 109 - Forensic Structural Engineering Handbook

2.14 DESIGN AND CONSTRUCTION PRACTICES

The AISC Specification for Structural Steel Buildings adopts the loads and load com-
binations in the applicable building code. In the absence of a specific local, regional, or
national code, the 2005 AISC specification adopts the load and load combinations pre-
scribed by ASCE 7-02. For LRFD, the load combinations are as follows:
1.4 (D + F)
1.2 (D + F + T) + 1.6 (L + H) + 0.5 (L or S or R)
r
1.2D + 1.6 (L or S or R) + (L or 0.8W)
r
1.2D + 1.6W + L + 0.5 (L or S or R)
r
1.2D + 1.0E + L + 0.2S
0.9D + 1.6W + 1.6H
0.9D + 1.0E + 1.6H
where D = dead load
E = earthquake load
F = load due to fluids with well-defined pressures
H = load due to lateral earth pressure, ground water
L = live load
L = roof live load
r
R = rain load
S = snow load
T = self straining force
W = wind load
The nominal live, wind, and snow loads historically have been associated with mean
return periods of fifty years, while the nominal earthquake is associated with a mean return
period of approximately twenty-five-hundred years.
The load factors and load combinations recognize that when several loads act in com-
bination with the dead load, e.g., dead plus live plus wind loads, then only one of these takes
on its maximum lifetime value, while the other load is at its arbitrary point-in-time value,
i.e., at a value that can be expected to be on the structure at any time. For example, under
dead, live, and wind loads, the following combinations are appropriate:
g D + g L
D
L
g D + g L + g W
D
W
a
L
a
g D + g L + g W a
W
L
D
a
where g is the appropriate load factor as designated by the subscript symbol. Subscript a
refers to an arbitrary point-in-time value.
National Design Specification for Wood Construction. The first edition of the National
Design Specification for Stress-Grade Lumber and Its Fastenings was published by the
32
National Lumber Manufacturers Association in 1944. The specification included allow-
able unit stresses for stress-graded lumber, design formulas, and design loads and provi-
sions for timber connector, bolted, lag screw, nail, and wood screw joints. Also included
were guidelines for the design of glued laminated structural members.
The 1944 edition of the specification was revised five times between 1948 and 1953,
and thirteen new editions were published between 1957 and 2005. The 1968 edition was
the first published under the association’s new name, the National Forest Products