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

172 Chapter Six
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Neglecting the thickness of the formwork between the bottom of
the concrete beam and the top of the shore, the maximum height of a
shore can be calculated as follows:
Distance from top of concrete beam to bottom of shore, 10.5 ft = 126 in.
Subtracting the distance from top of slab to bottom of beam = 18 in.
Height of beam shore = 108 in.
Thus the height of a beam shore will be 108 in., or 9 ft. Table 5-5
indicates that a 4 × 4 S4S single-post wood shore with an effective
length of 9 ft will have a capacity of 4,655 to 5,656 lb. Many patented
shores with an effective length of 9 ft will also have a load capacity in
this range. The safe spacing of shores can be determined by dividing
the capacity of a shore by the weight per linear foot of the load that is
applied to the beam as follows:
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[4,655 lb]/[583 lb per ft] = 7.9 ft
The distance of 7.9 ft is the maximum safe spacing based on the
capacity of a shore with an allowable load of 4,655 lb. According to
the foregoing calculations, one shore should be placed about every
8.0 ft along the beam. However, this spacing disregards the strength
of the beam forms between the shores. Usually the beam bottom is
limited to a deflection not to exceed l/360. If the beam bottom is made
of 2-in. nominal thickness lumber and if any restraining support provided
by nailing the beam sides to the beam bottom is neglected, the
maximum safe spacing of the shores may be determined from the
stress and deflection equations given in Chapter 5. Because the beam
bottoms will be installed over multiple supports, the equations for
beam 9 in Table 5-2 will apply.
The 15-in. wide beam bottom will be fabricated from 12-in. wide
No. 2 grade Southern Pine. Assume a dry condition of lumber and
no short-duration load. From Table 4-2, the reference design value is
975 lb per sq in. Because the lumber will be laid flat, the reference
bending stress can be increased by 1.2 (refer to Table 4-7). The following
allowable stresses for the beam bottom lumber:
Allowable bending stress, F b
= 1.2 (975) = 1,170 lb per sq in.
Allowable shear stress, F v
= 175 lb per sq in.
Modulus of elasticity, E = 1,600,000 lb per sq in.
The width of the beam bottom is 15 in. and the actual thickness is
1.5 in. for 2-in. nominal S4S lumber. The physical properties of the
beam bottom will be as follows.
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