Page 202 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch04.qxd 9/29/05 5:09 PM Page 4.30

BUILDING CODES, LOADS, AND FIRE PROTECTION*

4.30 CHAPTER FOUR

TABLE 4.16 Values of Approximate Period Parameters C t and x
Structure type C t * x
Moment resisting frame systems of steel in which the frames resist 100% of the 0.028 (0.068) 0.8
required seismic force and are not enclosed or adjoined by more rigid components
that will prevent the frames from deflecting when subjected to seismic forces
Moment resisting frame systems of reinforced concrete in which the frames 0.016 (0.044) 0.9
resist 100% of the required seismic force and not enclosed or adjoined by more
rigid components that will prevent the frame from deflecting when subjected
to seismic forces
Eccentrically braced steel frames 0.03 (0.07) 0.75
All other structural systems 0.02 (0.055) 0.75
*Metric equivalents are shown in parentheses.
Source: From Minimum Design Loads for Buildings and Other Structures, SEI/ASCE 7-02, American Society of Civil
Engineers, Reston, Va., with permission.

where the vertical distribution factor is given by

C = wh k (4.12b)
xx
vx
∑
n
i=1 wh k
ii
where w x and w i = portion of the total gravity load of the structure (W) at level x or i
h x and h i = height from the base to level x or i
k = 1 for building having period of 0.5 s or less
= 2 for building having period of 2.5 s or more
Use linear interpolation for building periods between 0.5 and 2.5 s.
For horizontal shear distribution, the seismic design story shear in any story, V x , is determined by
x ∑
V = n F i (4.13)
i=1
where F i = the portion of the seismic base shear induced at level i. The seismic design story shear is
to be distributed to the various elements of the force-resisting system in a story based on the relative
lateral stiffness of the vertical resisting elements and the diaphragm.
Provisions also should be made in design of structural framing for horizontal torsion, over-
turning effects, stability, and building drift. More advanced methods of seismic design, using
modal, linear, or nonlinear time-history analyses, may be required for taller or irregular structures.
Irregularities in mass, stiffness, and geometry should be considered.
(Federal Emergency Management Agency, “NEHRP (National Earthquake Hazards Reduction
Program) Recommended Provisions for Seismic Regulations for New Buildings,” 2000,
Washington, D.C.)

4.8 IMPACT LOADS

The live loads specified in building codes and standards include an allowance for ordinary impact
loads. Where structural members will be subjected to unusual vibrations or impact loads, such as
those described in Table 4.17, provision must be made for them in design of the members. Most
building codes specify a percentage increase in live loads to account for impact loads. Impact loads
for cranes are given in Art. 4.9.

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