3. For periods greater than Ts, the design
but shall not be less than:
spectral response acceleration shall be as given by
the following equation:
C S = 0.044 SDS
(3-9)
S D1
where:
Sa =
(3-12)
T
T =
The fundamental period of the
where the value of Ts shall be as given by the
structure.
The
above
equations
are
shown
following equation:
graphically in Figure 3-1.
S D1
(2) Modal Analysis Procedure. The required
Ts =
(3-13)
S DS
modal periods, mode shapes, and participation
factors shall be calculated by established methods of
structural, analysis assuming a fixed-base condition.
(b) Modal base shear. The portion of the
base shear contributed by the mth mode, Vm, shall be
(a) General response spectrum. Where a
determined from the following equations:
design response spectrum is required in this
document, and where site specific procedures are not
Vm = C sm Wm
(3-14)
used, the design response-spectrum curve shall be
developed as indicated in Figure 3-2, and as follows:
2
n
∑ wi f im
Wm = i=n
1. For periods equal or less than To, the
1
(3-15)
∑ wi f i2m
i=1
given by the following equation:
where:
Sa = 0.4 SDS + 0.6 SDS (T/To)
(3-10)
Csm =
the modal seismic response
Where TO = 0.2TS and TS is defined by Equation 3-
coefficient determined below,
13.
2. For periods greater than To and less
Wm =
the effective modal gravity load
than or equal to Ts, the design spectral response
including portions of the live load as defined in Sec.
5.3.2 of FEMA 302,
equation:
wi = the portion of the total gravity load of
Sa = SDS
(3-11)
the structure at level i, and
3-7
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