DESIGN GUIDELINES - Leading Construction and Building Group

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Many in situ slabs are designed to span in two directions, with the main reinforcement running in both directions, provided suffi cient lateral support exists to carry such slabs. Typically these types of slab allow greater spans and loads to be accommodated, like parking garages.

Other alternatives to accommodate greater loads and spans include downstand beams and or upstand beams, although not usually used in residential buildings. Mushroom heads on columns reduce potential bearing problems (push through) of fl at slabs where downstand beams need to be avoided.

One way spanning slabs only require lateral support to the walls or columns that carry them, and since they do not bear on walls in the non-span direction, this type of slab design is used more in residential type applications, as it can allow for greater flexibility of layout/design of a typical double storey building.

An alternative form of suspended slab is “a waffle slab”, or more commonly referred to as a “coffer slab”, formed by in situ concrete beams integral with the slab. Coffers are formed on the soffit by formers which are later removed; (reducing self weight), and hence the use of less concrete. Flat slabs in upper floors have been mainly used where ceilings are to be formed directly on the soffit. Coffered soffits of in situ tee beams can be found mainly where suspended ceilings are used. And more rarely where the soffit is left exposed as cast.

Slabs are designed as simply supported ribbed slabs in the conventional manner. However they are more versatile than the pre-stressed slab, since longitudinal top reinforcement can be cast-in for cantilever action.

Slabs are designed as simply supported pre-tensioned ribbed slabs, in accordance with the requirements of SABS 0100-1 or the appropriate National code. The pre-stressing force opposes the tendency to downward deflection and causes an upward camber in the units under no-load conditions. Together with the high-strength concrete employed, larger span/depth ratios can be achieved than with reinforced concrete. In lightly loaded roof slabs, for instance, span/depth ratios around 50 are not uncommon.

Rib and block system slabs are designed as a series of ‘T’ sections with the in situ cast concrete providing the compression flange, and the precast beam the tension reinforcement. The beams and the composite slab are designed for specific spans and loads and are reinforced accordingly, (complying with the relevant National Code.) Two or more beams may be placed together to accommodate concentrated line loads parallel to the span. If necessary blocks may be omitted over the support to increase the shear capacity (called a stiffener rib).


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