The empirical design method recommended by design Codes for flat slab structures assumes that ribbed and plain panels behave in the same manner under load. However, there are significant differences between the behaviour of ribbed and plain plates, the
increased ratio of flexural to torsional rigidities in a ribbed plate resulting in a substantial reduction in the load that is transmitted by torsion of the plate elements. In the existing design method, the total ultimate load design moment for each panel, which is based on thequilibrium requirement of the panel, is distributed
according to specified coefficients derived from an elastic analysis of square plain panels. A theoretical study of the moment distributions within plain and ribbed flat slab panels has been carried out, and the theoretical moment coefficients for internal,
edge, and corner panels are presented. It is concluded that the recommended coefficients should be modified for the design of ribbed panels. Suitable alternative coefficients for internal and external ribbed panels are presented. The effect of solid regions around columns is considered, and it is shown that solid regions attract higher negative moments in the column strip than are allowed for in existing design coefficients. An additional design rule that allows for these effects is proposed.
I.E. Tebbett and J.E. Harrop