A shrinkage value of 0.030 to 0.035 per cent is commonly assumed and may be acceptable for members of ordinary size in normal environments. For very dry conditions or where an accurate value of the total defection is essential, a closer study, possibly with tests, should be made to determine an appropriate value for shrinkage. Differential strains producing curvature or deflection will occur in concrete flexural members reinforced at one face only, because shrinkage will be resisted by the steel but not at the other face. Deflections from this mechanism can be prevented by provision of symmetrical reinforcement at both faces of the slab. Wood species commonly used for floor and roof systems shrink 5 to 7 per cent across the grain when dried from green to oven-dry.
Similar expansions result from a corresponding rise in moisture content.
The shrinkage and expansion across the grain caused by changes in moisture content normally experienced in the wood in buildings is about 3 per cent. Most building materials other than metals are affected dimensionally by changes in moisture content, which varies with relative humidity. A temperature gradient across a concrete or wood member can produce a moisture content gradient, and this will cause differential shrinkage, resulting in warpage or deflections. As a drop in temperature causes a rise in relative humidity and moisture content, and produces an expansion that may partially compensate for a temperature gradient warpage, the direction of warpage should be observed. Temperature A temperature differential across a member causes a thermal expansion differential that produces warpage or deflection.