Civil Engineering Reference
In-Depth Information
The value of C
s
should not be taken less than 0.01. For locations where S
1
≥ 0.6g C
s
should not be less than:
0.5S
1
R
I
C
s
≥
⎛
⎞
⎜
⎟
where:
T
L
= long period transition period (ASCE Figure 22-15 to Figure 22-20). It is important to point out that T
L
is the transition between the constant velocity and constant displacement in the response spectra for certain
location. This should have no effect on buildings with low to moderate heights. In effect the above first
equation for C
s
calculations controls the analysis of buildings addressed in this publication.
R = response modification factor depending on the basic seismic-force-resisting system, from ASCE
Table 12.2-1. Table 11-1 lists the values for R for different reinforced concrete seismic force resisting
systems.
I = importance factor depending on the nature of occupancy, from ASCE 11.5.1
T = the fundamental period of the structure in seconds. T can be calculated from the following equation:
T = C
t
(h
n
)
x
where:
C
t
= building period coefficient
C
t
= 0.016 for concrete moment resisting frames
C
t
= 0.02 for other concrete systems
h
n
= building height in feet
x = 0.9 for concrete moment resisting frames
x = 0.75 for other concrete systems
Figure 11-8 shows the fundamental period for concrete building systems for different heights.
For concrete moment resisting frame buildings with less than 12 floors and story height of 10 feet minimum,
T calculations can be further simplified to T = 0.1 N where N is the number of stories
For concrete shearwall structures the fundamental period can be approximated using ASCE Eq. 9.5.5.3 2-2
11.6.2 Vertical Distribution of Seismic Forces
In the equivalent lateral force method the design base shear V is distributed at different floor levels as follows
(ASCE 12.8.3):
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