Signed Column Reactions for Combined Footing Design in Response Spectrum Analysis

The loss of sign in the output results of Response Spectrum Method has been a subject of discussion in literature, highlighting the issues caused, manifesting primarily in: (i) the loss of relative signs of forces acting on the structural members and (ii) the loss of relative signs of the support reactions. The method of Dominant Mode Signage (DMS) had been presented as a remedy in some of the structural analysis packages to tackle the issue. The intention of this paper is to elaborate an alternate manual method of applying signs to the support reactions for those using structural analysis packages not yet equipped with the DMS facility, presenting an easy step-by-step process fit for utilising on a day-to-day basis. It also demonstrates why a plot of the envelope of maximum footing pressures on soil on the footing is not symmetric in case of the Response Spectrum Method approach, even for a symmetric building with symmetric loading and supported on a symmetric combined footing, while it is rendered symmetric for the Seismic Coefficient Method approach (ie., the Static method), and as demonstrated, for the proposed method too.