Civil Engineering Reference
In-Depth Information
Forces along grid line 1 starting at grid line D:
F
1E
=
77218
.
()
=
138 9
.
lb tension
F
1
=
138 9765854
.
−
.
()
=−
3996 96
.
lb compression
G
h
F
1
=−
3996 96
.
+
519 48
. ()
= −
158 2
.
lb
∴
OK
,
closeenoug
H
The 8 percent increase in the second-floor loads to grid lines 1 and 2 is confirmed,
but should be closer to 8.5 percent.
Forces along grid line D starting at grid line 1:
7723272
2
.
+
.
F
2
=−
()
8
=−
161 76
.
lb tension
D
(.
63 26041 0415764
2
−
.
)(.
+
−
.
b
F
4
=−
161 76
.
+
()
58
=
0
l
D
Forces along grid line E starting at grid line 1:
(.
76 58
+
772 1583272
2
.
)(.
+
+
.
pression
F
2
=
()
8
=
674 4
.
lb com
E
(.
63 25158 9265764
2
−
.
)(.
+
−
.
F
4
=
674 4
.
−
()
58
=
0
lb
E
The increase in the shears to the first-floor shear walls is approximately 53 plf due to
the offset in the walls at grid lines 1 and 2. If the walls had been designed using the
conventional tributary width method, the walls and foundations could have been
underdesigned.
▲
12.4 Offset Walls Not in the Same Line of Resistance
There are several options for the layout of longitudinal and transverse shear walls in
apartment structures. These types of structures were briefly discussed in Chap. 6. The
most common approach for the placement of longitudinal shear walls is to provide
walls at the exterior and interior wall lines, as shown on the left of Fig. 12.15. This
provides the greatest stability, decreases the load to the individual shear walls, and
increases redundancy. Either one or both corridor wall lines can be used as lines of lat-
eral resistance for this layout. Prior to the introduction of methods for analyzing portal
frames and shear walls with openings, very narrow segmented shear walls were the
only wood-framed lateral-force-resisting elements available for use. The large number
of hold-down anchors required to make these narrow segmented walls work made
their use undesirable. To alleviate this problem, the method of eliminating the shear
walls at the exterior wall lines by cantilevering the diaphragms off of the corridor walls
became a more attractive option. Choosing this option requires careful consideration by
the engineer because of minimization of the lateral system.
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