Continuous ribbed slab example
This 300 mm deep ribbed slab is required for an office to support a variable action of 5 kN / m2 It is supported on wide beams that are the same depth as the slab, as shown in Figure 3.9.
Long section through slab
One hour fire resistance required: internal environment. Ribs are 150 mm wide @ 900 mm cc. Links are required in span to facilitate prefabrication of reinforcement. Assume that partitions are liable to be damaged by excessive deflections. In order to reduce deformations yet maintain a shallow profile use /ck = 35 MPa and /yk = 500 MPa.
Notes on ribbed slab design
There are various established methods for analysing ribbed slabs and dealing with the solid areas:
■ Using UDLs simplifies the analysis and remains popular. One method is to ignore the weight of the solid part of the slab in the analysis of the ribbed slab (and the weight of the solid area added to the supporting beam actions). This ignores the effect the solid areas have on bending in the ribbed slab.
■ Alternatively the weight of the solid part of the slab is spread as a UDL over the whole span. This is conservative both in terms of moment and shears at solid/shear interfaces but underestimates hogging in internal spans.
■ The advent of computer analysis has made analysis using patch loads more viable and the resulting analysis more accurate.
■ The ribbed part of the slab may be designed to span between solid areas. (The ribs span d / 2 into the solid areas which are assumed to act as beams in the orthogonal direction.) However, having to accommodate torsions induced in supporting beams and columns usually makes it simpler to design from centreline of support to centreline of support.
■ Analysis programs can cope with the change of section and therefore change of stiffness along the length of the slab. Moments would be attracted to the stiffer, solid parts at supports. However, the difference in stiffnesses between the ribbed and the solid parts is generally ignored.
In line with good practice analysis, this example is carried out using centreline of support to centreline of support and patch loads55. Constant stiffness along the length of the slab has been assumed.
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