Page 204 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch04.qxd 9/29/05 5:09 PM Page 4.32

BUILDING CODES, LOADS, AND FIRE PROTECTION*

4.32 CHAPTER FOUR

or load and resistance factor design (LRFD) is used. Load combinations according to SEI/ASCE 7-02,
which is the principal load standard, are given here. Note that the most critical load combination may
occur when one or more of the loads are not acting.
For ASD, the following load combinations should be investigated:

1. D
2. D + L + T
3. D + (L r or S or R)
4. 0.75[L + (L r or S or R) + T] + D
5. 0.75(W or 0.7E) + D
6. 0.75[L + (W or 0.7E) + (L r or S or R)] + D
7. 0.6D + W
8. 0.6D + 0.7E
where D = dead load
L = floor live load, including impact
L r = roof live load
S = roof snow load
R = rain load (initial rainwater or ice, exclusive of ponding)
W = wind load
E = earthquake load
T = restraint load

Note that because the earthquake load, E, is defined for use with LRFD, it is reduced by a factor
of 0.7 in the ASD load combinations. Also, SEI/ASCE 7-02 states that increases in allowable stress
shall not be used with the load combinations, unless it can be demonstrated that such an increase is
justified by structural behavior caused by rate or duration of load.
For LRFD, the following load combinations should be investigated:
1. 1.4D
2. 1.2(D + T) + 1.6L + 0.5(L r or S or R)
3. 1.2D + 1.6(L r or S or R) + (L or 0.8W)
4. 1.2D + 1.6W + L + 0.5(L r or S or R)
5. 1.2D + 1.0E + (L or 0.2S)
6. 0.9D + 1.6W
7. 0.9D + 1.0E

4.12 FIRE PROTECTION

Building codes play a dominant role in defining the level of structural fire protection that is expected
by society. Typically, fire protection is implemented in design through prescriptive code compliance.
Alternatively, in situations with unusual conditions, unique designs, or narrower project constraints
beyond prescriptive code limits, a more detailed fire engineering or “performance-based design” can
be implemented through a qualified consultant. The results need to demonstrate equivalence to the
fire safety objectives of the applicable code. As a consequence, a working knowledge of building
codes is an important prerequisite for contemporary design.
In the past, keeping abreast of building codes was difficult, even for the largest design offices, since
most major cities and a number of states maintained locally developed codes. Today, this impediment

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