(TWO-WAY SLABS)
When a slab is supported other than on two opposite sides only, the precise amount and distribution of the load taken by each support, and consequently the magnitude of the bending moments on the slab, are not easily calculated if assumptions resembling real conditions are made. Therefore, approximate analyses are generally used. The method applicable in any particular case depends on the shape of the slab panel, the conditions of restraint at the supports and the type of load. Two basic methods are commonly used to analyse slabs that span in two directions. The theory of plates, which is based on elastic analysis, is particularly appropriate to the behaviour under service loads. Yield-line theory considers the behaviour of the slab as a collapse condition approaches. Hillerborg's strip method is a less well-known alternative to the use of yield-line in this case. In some circumstances, it is convenient to use coefficients derived by an elastic analysis with loads that are factored to represent ULS conditions. This approach is used in BS 8110 for the case of a simply supported slab with corners that are not held down or reinforced for torsion. It is also normal practice to use elastic analysis for both service and ULS conditions in the design of bridge decks and liquid-retaining structures. For elastic analyses, a Poisson's ratio of 0.2 is recommended in BS 8110 and BS 5400: Part 4. In EC 2, the values given are 0.2 for uncracked concrete and 0 for cracked concrete. The analysis must take account of the support conditions, which are often idealised as being free or hinged or fixed, and whether or not the corners of the panels are held down. A free condition refers to an unsupported edge as, for example, the top of a wall of an uncovered rectangular tank. The condition of being freely or simply supported, with the corners not held down, may occur when a slab is not continuous and the edges bear directly on masonry walls or structural steelwork. If the edge of the slab is built into a substantial masonry wall, or is constructed monolithically with a reinforced concrete beam or wall, a condition of partial restraint exists. Such restraint may be allowed for when computing the bending moments on the slab, but the support must be able to resist the torsion and/or bending effects, and the slab must be reinforced to resist the negative bending moment. A slab can be considered as fixed along an edge if there is no change in the slope of the slab at the support irrespective of the incidence of the load. A fixed condition could be assumed if the polar second moment of area of the beam or other support is very large. Continuity over a support generally implies a condition of restraint less rigid than fixity; that is, the slope of the slab at the support depends upon the incidence of load not only on the panel under consideration but also on adjacent panels
#Eng_alzbyr_Rashid .
When a slab is supported other than on two opposite sides only, the precise amount and distribution of the load taken by each support, and consequently the magnitude of the bending moments on the slab, are not easily calculated if assumptions resembling real conditions are made. Therefore, approximate analyses are generally used. The method applicable in any particular case depends on the shape of the slab panel, the conditions of restraint at the supports and the type of load. Two basic methods are commonly used to analyse slabs that span in two directions. The theory of plates, which is based on elastic analysis, is particularly appropriate to the behaviour under service loads. Yield-line theory considers the behaviour of the slab as a collapse condition approaches. Hillerborg's strip method is a less well-known alternative to the use of yield-line in this case. In some circumstances, it is convenient to use coefficients derived by an elastic analysis with loads that are factored to represent ULS conditions. This approach is used in BS 8110 for the case of a simply supported slab with corners that are not held down or reinforced for torsion. It is also normal practice to use elastic analysis for both service and ULS conditions in the design of bridge decks and liquid-retaining structures. For elastic analyses, a Poisson's ratio of 0.2 is recommended in BS 8110 and BS 5400: Part 4. In EC 2, the values given are 0.2 for uncracked concrete and 0 for cracked concrete. The analysis must take account of the support conditions, which are often idealised as being free or hinged or fixed, and whether or not the corners of the panels are held down. A free condition refers to an unsupported edge as, for example, the top of a wall of an uncovered rectangular tank. The condition of being freely or simply supported, with the corners not held down, may occur when a slab is not continuous and the edges bear directly on masonry walls or structural steelwork. If the edge of the slab is built into a substantial masonry wall, or is constructed monolithically with a reinforced concrete beam or wall, a condition of partial restraint exists. Such restraint may be allowed for when computing the bending moments on the slab, but the support must be able to resist the torsion and/or bending effects, and the slab must be reinforced to resist the negative bending moment. A slab can be considered as fixed along an edge if there is no change in the slope of the slab at the support irrespective of the incidence of the load. A fixed condition could be assumed if the polar second moment of area of the beam or other support is very large. Continuity over a support generally implies a condition of restraint less rigid than fixity; that is, the slope of the slab at the support depends upon the incidence of load not only on the panel under consideration but also on adjacent panels
#Eng_alzbyr_Rashid .