Example 2
Consider the preliminary
design of a 12-story office building shown in Fig. 4.4. The building has a
circular plan with a diameter of 50 ft and a constant story height of 10
ft. Floors 1 to 10 are
supported at the center by a hollow circular reinforced concrete shear wall
with a diameter of 16 ft and at the perimeter by 12 identical hangers
suspended from 12 story-high truss systems located at the twelfth story.
Design the hangers for a dead load of 60 psf and
a live load of 80 psf, choosing the lightest WT section and assuming uniform
section throughout the height of each hanger. Use A36 steel with Fy = 36 ksi.
Solution In order to take into
account the effect of impact for hangers supporting floors, the assumed
live load should be increased by 33 percent (ASD A4.2).
Design
load intensity:
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w = 60 + 1.33(80) =
166.4 psf
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The
load-carrying tributary area for each beam is shaded in the plan of Fig
4.4.
The load distribution
on each radial beam is approximately trapezoidal, as shown in Fig. 4.5,
with inner intensity w2
and outer intensity w1.
The reaction at the outer end of any radial beam is (the
supports condition of the radial beams is assumed simple):
Maximum
tension in a hanger
(neglecting its weight):
Allowable tensile stress:
Required area
of the cross section:
Try WT8 X
22.5
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A = 6.63 in.2
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rx = 2.39 in.
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Ry = 1.57 in.
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Check for the maximum tensile stress, including the weight of the
hanger.
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Weight of the hanger =
10(10)(0.0225) = 2.25 K
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Tmax = 143.17 + 2.25 =
145.42 K
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Maximum
tensile stress:
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O.K.
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Check the slenderness ratio (not mandatory), ASD
B7.
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O.K.
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