The depth of the horizontal support nib is determined from the loadbearing and fixing requirements. The most important consideration is that there should be a minimum bearing of 100 mm on the structural slab plus an allowance of 25 mm for any inaccuracies in the edge of the slab and any danger of spalling. Figure 1.20 shows the minimum dimensions of the horizontal support nib and its bearing upon the type of fixings to be used. Allow for I 75 mm with dowel fixings and I 25 mm using cleat fixings.
In addition to providing space for reinforcement, the height of the nib is usually affected by the type of fixing used and whether or not it is necessary to provide cast-in fixing sockets. Where an angle cleat fixing is used, the suggested height of nibs is 150 mm. For a dowel fixing, this can be reduced to 125 mm. Concrete corbels and stainless steel bearing angles
Note possibility of concrete spalling
125 mm min.
Min. bearing 150 mm allowing 25 mm for inaccuracy of edge of slab
150 mm miri.
Min bearing 100 mm allowing 25 mm for inaccuracy of edge of slab
1.20 Minimum dimensions of horizontal support nibs: (a) dowel fixing; (b) cleat fixing.
are extensively used to restrict the height of the gravity support device to fit under raised floors.
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