Page 201 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch04.qxd 9/29/05 5:09 PM Page 4.29

BUILDING CODES, LOADS, AND FIRE PROTECTION*

BUILDING CODES, LOADS, AND FIRE PROTECTION 4.29

However, C s need not be greater than

C = S D1 (4.9a)
s
TR I )
(/
s
and shall not be less than
(4.9b)
C s = 0.044SS DS I s
or, for buildings in seismic design categories E and F,

C = S 1
s (4.9c)
RI / s
where S D1 = design spectral acceleration at a period of 1.0 s
T = fundamental period of the structure (s)
S 1 = mapped MCE spectral response acceleration
The two design spectral accelerations, S DS and S D1 , are based on the MCE spectral response for
short (S MS ) and 1-s (S M1 ) periods, respectively, as determined by the following:
2
S DS = / 3S MS (4.l0a)
2
S D1 = / 3S M1 (4.l0b)
(4.10c)
S MS = F a S s
(4.10d)
S M1 = F v S 1
where S 1 = mapped MCE, 5% of critical damping, spectral response acceleration at a period of 1 s
from Fig. 4.4
S s = mapped MCE, 5% of critical damping, spectral response acceleration at short periods
(g), from Fig. 4.5
F a = site coefficient for short-period earthquake (Table 4.14)
F v = site coefficient for 1-s-period earthquake (Table 4.15)
S M1 = MCE, 5% of critical damping, spectral response acceleration at a period of 1 s, adjusted
for site class effects
S MS = MCE, 5% of critical damping, spectral response acceleration at short periods, adjust-
ed for site class effects

A rigorous evaluation of the fundamental elastic period T requires consideration of the intensity
of loading and the response of the structure to the loading. To expedite design computations, T may
be determined by the following approximation, or by other equations given in SEI/ASCE 7-02:
x
T a = C t h n (4.11)
where T a = approximate fundamental period, sec
C t = period parameter from Table 4.16
x = period parameter from Table 4.16
h n = height above the base to the highest level of the building, ft
For vertical distribution of seismic forces, the lateral base shear V should be distributed over the
height of the structure as concentrated loads at each floor level or story. The lateral seismic force F
at any floor level is determined by the following equation:
F x = C vx V (4.12a)

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