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

174 Chapter Six
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The entire weight from the floor slab will be supported by shores,
with no portion of this weight transmitted to the beam forms. The
height of a shore will be:
Distance from top of concrete slab to bottom of shore, 10.5 ft + 5 in.
= 131.0 in.
Depth of slab, decking, joists, stringers: 5.0 + 1.0 + 7.25 + 7.25
= 20.5 in.
Height of slab shores = 110.5 in.
Thus, the height of a slab shore will be 110.5 in., or slightly over 9 ft.
Consider the strength of a 10-ft-high shore. For a 4 × 4 S4S wood shore
with an effective length of 10 ft, Table 5-5 indicates an allowable load
of 3,819 to 4,688 lb. The maximum area that can be supported by a
3,819 lb shore can be calculated as follows:
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[3,819 lb]/[121 lb per sq ft] = 31.5 sq ft.
The shores for the floor slab will be spaced 5 ft apart in rows
which are spaced 3 ft apart (refer to Figures 6-4 and 6-5). This spacing
of shores provides a total area of 3 ft × 5 ft = 15 sq ft, which is less than
the allowable 31.5 sq ft. This spacing of shores provides a margin of
safety of 31.5/15 = 2.1 for the shoring. The decking, joists, and stringers
must have adequate strength to transmit the loads from these components
of the formwork to the slab shores. The methods of determining
the size and spacing of joists and stringers have been discussed in
Chapter 5 and are discussed further in Chapter 11.
Tubular Steel Scaffolding Frames
Tubular steel scaffolding, as illustrated in Figure 6-6, has several
advantages over single-post shores for supporting loads from concrete,
including the following:
1. A two-frame section, with X-braces between frames, will
stand without additional supports.
2. A choice of frame heights, with adjustable screw jacks on the
legs will permit easy and fast adjustment to the desired exact
height.
3. A choice of lengths of diagonal braces will permit variations
in the spacing of frames over a wide range.
4. The use of screw jacks on the bottom of the legs will compensate
for uneven floor conditions.
5. It provides better safety for workers.
6. It can be used for purposes other than shoring formwork.
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