Page 298 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch06.qxd 9/29/05 5:15 PM Page 6.20

DESIGN OF BUILDING MEMBERS

6.20 CHAPTER SIX

The beam should be cambered to offset the deflection due to a dead load of 0.50 + 0.022 =
0.522 kip/ft.

×
.
Camber = 5 × 0 522 30 4 ×12 3 = 16 . in
,
384 × 29 000 ×199
Camber should be specified on the drawings as 1.5 in.
Strength of Fully Composite Section. Next, the composite steel section is designed to support the
total loads. The live load may be reduced in accordance with area supported (Art. 6.9). The reduc-
tion factor is R = 0.0008(300 − 150) = 0.12. Hence the reduced live load is 0.5(1 – 0.12) = 0.44 kip/ft.
The factored load is the larger of the following:

1.2(0.50 + 0.25 + 0.022) + 1.6 × 0.44 = 1.63 kips/ft
1.4(0.5 + 0.25 + 0.022) = 1.081 kips/ft

Hence the factored moment is

⋅
M = 163 × 30 2 = 183 4 . kip ft
.
u
8
12
The concrete flange width is the smaller of b = 10 × 12 = 120 in or b = 2(30 × / 8) = 90 in (governs).
The compressive force in the concrete C is the smaller of the values computed from Eqs. (6.24)
and (6.25).
C c = 0.85f c ′A c = 0.85 × 3 × 90 × 3.25 = 745.9 kips
C t = A s F y = 6.49 × 50 = 324.5 kips (governs)

The depth of the concrete compressive-stress block (Fig. 6.5) is
.
a = C = 324 5 = 1 414 in
.
×
.
.
.
085 fb ′ c 085 × 30 90
Since C c > C t , the plastic neutral axis will lie in the concrete slab (case 3, Art. 6.12). The distance
between the compression and tension forces on the W14 × 22 (Fig. 6.5d) is
e = 0.5d + 5.25 − 0.5a
= 0.5 × 13.7 + 5.25 − 0.5 × 1.414 = 11.393 in

The design strength of the W14 × 22 is

.
×
φM = 09 . C e = 09 3245 . × 11 393 = 277 3. kip ⋅ >183 9.ft kip ⋅ft OK
.
n t
12
Partial Composite Design. Since the capacity of the full composite section is more than required,
a partial composite section may be satisfactory. Seven values of the composite section (Fig. 6.6) are
2
calculated as follows, with the flange area A f = 5 × 0.335 = 1.675 in .

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