Civil Engineering Reference
In-Depth Information
(6)
Reinforcement details shown in Fig. 3-18 are determined directly from Fig. 8-5*. Note that
one of the No.5 bars at the bottom must be continuous or be spliced over the support with a
Class B tension splice, and be terminated with a standard hook at the non-continuous supports
for structural integrity (ACI 7.13.2.1).
3.8.2 Example: Design of Wide-Module Joists for Alternate (2) Floor System
(Building #1)
(1)
Data:
› = 4000 psi (normal weight concrete, carbonate aggregate)
f
y
= 60,000 psi
Floors: LL = 60 psf
DL = 130 psf (assumed total for joists and beams + partitions + ceiling & misc.)
Fire resistance rating = assume 2 hours
Floor system - Alternate (2): Assume joists on 6 ft - 3 in. centers (53” standard forms** )
width of spandrel beams = 20 in.
width of interior beams = 36 in.
Note: The provisions for standard joist construction in ACI 8.13 do not apply for the wide-module joist
system (clear spacing between joists > 30 in.). Wide-module joists are designed as beams and one-way
slabs (ACI 8.13.4).
(2)
Determine factored shears and moments using the approximate coefficients of ACI 8.3.3
(see Figs. 2-3, 2-4, and 2-7).
w
u
= 1.2(130) + 1.6(60)*** = 252 psf
6.25 = 1575 plf, say 1600 plf
Note: All shear and negative moment values are at face of supporting beams.
V
u
@ spandrel beams
= 1.6(27.5)/2 = 22.0 kips
V
u
@ first interior beams
= 1.15(22.0) = 25.3 kips
V
u
@ interior beams
= 1.6(27.0)/2 = 21.6 kips
-M
u
@ spandrel beams
= 1.6(27.5)2/24 = 50.4 ft-kips
+M
u
@ end spans
= 1.6(27.5)2/14 = 86.4 ft-kips
-M
u
@ first interior beams
= 1.6(27.25)2/10 = 118.8 ft-kips
+M
u
@ interior spans
= 1.6(27)2/16 = 72.9 ft-kips
-M
u
@ interior beams
= 1.6(27)2/11 = 106 ft-kips
* The bar cut-off points shown in Fig. 8-5 are recommended for one-way joist construction. The reader may consider
determining actual bar lengths using the provisions in ACI 12.10.
** See Table 9-3 for standard form dimensions.
***No live load reduction permitted: since the influence area = 12.5
30 = 375 sq ft < 400 sq ft.
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