Structural Engineering

Avoid soft storey? A soft storey is a storey with lateral stiffness less than the storey just above. So technically, you cannot avoid soft storey just by calculation. Right?

Well, according to the new code for seismic design, IS 1893 Part I (2016), infill masonry walls in soft storeys of RC buildings can be modeled as diagonal struts to contribute to lateral stiffness.Though this doesn’t guarantee that the resulting storey stiffness with the contribution of infill walls, will be greater than that of the storey above.

Method :

• In buildings with RCC moment resisting frames with a soft storey (stiffness irregularity), when the structural plan density (SPD) of masonry infills exceeds 20%, the effect of unreinforced masonry infills (URM infills) has to be considered explicitly using structural analysis especially for buildings in earthquake zones III, IV and V.
• The compressive strength, fª of URM infill prism is calculated as per IS 1905 or according to the formula in IS 1893 (2016) clause 7.9.2.1.
• The URM infill walls are modelled as diagonal struts with ends pin jointed to the RC frames. The width of the struts are calculated as per clause 7.9.2.1 of IS 1893 (2016). Thickness, t is taken as equal to the thickness of the URM walls themselves provided h/t < 12 and l/t < 12 where h and t are the clear height and clear length of the diagonal struts.
• The stiffnesses of the diagonal struts are added to the earlier floor stiffness and the new storey stiffness is calculated. If soft storey condition ceases, we can proceed with further analyses.

[Note : Even after reading the answer please do refer to table 6, fig. 4 and section 7.9 of IS 1893 Part I (2016) for two formulae mentioned above and a clearer understanding of the method in detail.]