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

342 Chapter Eleven
Downloaded From : www.EasyEngineering.net
the bottom of the beam is 24 in. Consider a 50 lb per sq ft live load on
the formwork. The unit weight of the concrete is 150 lb per cu ft and
the permissible deflection is l/360, not to exceed ¹⁄16 in.
The 16-in. wide beam bottom will be fabricated from 2 × 12 S4S
lumber. Because the 16-in beam bottom is wider than an 11¼-in. standard
2 × 12, two pieces of lumber will be used. One piece of lumber
will be a full size 2 × 12 and the second piece will be 4.75-in. cut from
another 2 × 12 to make up the full 16-in. beam bottom. The lumber
will be cleated together to form the beam bottom.
The lumber will be No. 1 grade Southern Pine. Assume a dry condition
of lumber, moisture content less than 19%, and a short loadduration
of less than 7 days. Because the lumber will be laid flat, the
flat use adjustment factor, C fu
, may be used. From Tables 4-2, 4-4, and
4-7, the allowable stresses will be:
ww.EasyEngineering.n
Allowable bending stress, F b
= C D
(C fu
)(reference design bending value)
= 1.25(1.2)(1,250 lb per sq in.)
= 1,875 lb per sq in.
Allowable shear stress, F v
= C D
(reference design shear value)
= 1.25(175 lb per sq in.)
= 218 lb per sq in.
Modulus of elasticity, E = 1,700,000 lb per sq in.
The uniformly distributed load acting vertically against the 16-in.
wide beam bottom between shores may be calculated as follows:
Concrete, 150 lb per cu ft (24/12 ft)(16/12 ft) = 400.0 lb per lin ft
Assumed dead load, 4 lb per sq ft(16/12 ft) = 5.3 lb per lin ft
Live load, 50 lb per sq ft (16/12 ft) = 66.7 lb per lin ft
Total load, w = 472.0 lb per lin ft
The physical properties of the section modulus and the moment
of inertia for a piece of 16-in.-wide and 2-in.-thick dimension lumber
laid flat between shores can be calculated as follows:
From Eq. (5-9), the section modulus can be calculated as follows:
Section modulus, S = bd 2 /6
= 16 in.(1.5 in.) 2 /6
= 6.0 in. 3
From Eq. (5-8), the moment of inertia can be calculated.
Moment of inertia, I = bh 3 /12
= (16 in.)(1.5) 3 /12
= 4.5 in. 4
Downloaded From : www.EasyEngineering.net