Hi every one i have modelled staad with same number of columns and all the column sizes are same. so as per the staad literature the stiffness of columns calculated as 12EI/L^3. My member sizes are same and the floor heights are same in staad but the story stiffness is not same. It changes from story to story pls help?
There was an approximate methodology available in STAAD.Pro. The program used to compute story stiffness of a story by summing up the lateral stiffness of columns. The stiffness of a column was calculated as 12EI / L3 however this method is approximate one as it fails to capture actual story stiffness including the effect of bracings. Story stiffness should be computed using a displacement based approach. The story stiffness is calculated as the inverse of inter-story drift when a unit load is applied at the particular story. So in your case variation of story stiffness might be entirely justified. One may refer the following section for example calculation:
Home > What's New? > STAAD.Pro CONNECT Edition > CONNECT Edition Update 3 > RR 21.03.00-2 Features Affecting the Analysis and Design Engine > RR 21.03.00-3.5 Soft Story Checks per IS 1893