Civil Engineering Reference
In-Depth Information
Story Drift and P-
Effect
To check that the maximum allowable limits for story drift are not exceeded (ASCE Table 12-12.1) the
displacement at each floor level
δ
xe
need to be calculated and amplified (See Section 11.8). Then the stability index
θ
should be calculated and checked against 0.1 where the P-
effects are not required to be considered.
Distribution of the Seismic Forces to the Frames in N-S Direction
For north south direction, the lateral forces are carried by the six frames shown in figure 1-3. For the symmet-
rical floor layouts the center of rigidity coincides with the center of mass at the geometric center of the 150' by 90'
floor area.
The seismic force transmitted to each frame must be increased due to assumed displacement of the center of
application of the applied lateral force (5 percent of the building plan dimension perpendicular to the force
direction). For the N-S direction the displacement considered = 0.05
150ft = 7.5ft. To calculate the force in
(
y
k
i
(
y
J
r
k
i
x
i
k
i
()
=
F
i
F
x
+
F
x
e
x
ca
each frame the equation
can be used.
∑
()
Assuming equal stiffness for the six frames the equation simplifies to:
1
6
F
x
+
1
J
r
(
y
=
F
i
F
x
e
x
simplifies to
for this case.
where F
x
indicates the force for floor x
∑
()
2
()
()
( )
( )
( )
2
2
2
2
2
J
r
=
x
2
=
15
+
45
+
75
+−
15
+−
45
+−
75
=
15750
The seismic forces at each floor level are shown in Table 11-11:
Table 11-11 Shear Force at each Frame (N-S)
Lateral
Force
F
x
(kips)
Height
h
x
(ft)
Frame
1
Frame
2
Frame
3
Frame
4
Frame
5
Frame
6
Level
F
x
e
x
3
39
124
932.8
16.3
18.1
19.8
21.6
23.4
25.2
2
26
105
784.3
30.0
15.2
16.7
18.2
19.7
21.2
1
13
52
392.2
36.8
7.6
8.3
9.1
9.8
10.6
The axial, forces, shear forces, and bending moment for interior frame are calculated using the portal frame
method (see Chapter 2). The results of the calculations are shown in Figure 11-13
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