Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

Downloaded From : www.EasyEngineering.net
Forms for Beams and Floor Slabs 343
The allowable span length of the beam bottom based on bending,
shear, and deflection may be calculated as follows:
For bending, the maximum span length
from Eq. (5-34), l b
= [120F b
S/w] 1/2
= [120(1,875)(6.0)/472] 1/2
= 53.4 in.
For shear, the maximum span length
from Eq. (5-36), l v
= 192F v
bd/15w + 2d
= 192(218)(16)(1.5)/15(472 + 2(1.5)
= 144.8 in.
ww.EasyEngineering.n
For deflection not to exceed l/360, the maximum span length
from Eq. (5-37a), l ∆
= [1,743EI/360w] 1/3
= [1,743(1,700,000)(4.5)/360(472)] 1/3
= 42.8 in.
Deflection not to exceed ¹⁄16 in., the maximum span length
from Eq. (5-37b), l ∆
= [1,743EI/16w] 1/4
= [1,743(1,700,000)(4.5)/16(472)] 1/4
= 36.4 in.
Following is a summary of the allowable span lengths of the 2-in.-
thick beam bottom:
For bending, the maximum span length = 53.4 in.
For shear, the maximum span length = 144.8 in.
For deflection, the maximum span length = 36.4 in.
For this example, deflection governs the maximum span length of
the beam bottom. Therefore, the shores must be spaced at a distance
not to exceed 36.4 in. to prevent excessive deflection of the beam bottoms.
For constructability, the spacing would likely be at 36 in. on
centers. The total load on the shore from the concrete beam will be
(472 lb per lin ft)(36.0/12 ft) or 1,416 lb, plus the dead and live loads
from the concrete slab on either side of the centerline of the concrete
beam to the midpoint of the row of shores perpendicular to the direction
of the concrete beam.
Example 11-5
Determine the safe spacing of shores based on bending, shear, and
deflection of the beam bottom as illustrated in Figure 11-7(b). The beam
is 16 in. wide and the total depth from the top of the concrete slab to
the bottom of the beam is 24 in. Consider a 50 lb per sq ft live load on
Downloaded From : www.EasyEngineering.net