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Base Shear in Seismic Design

posted Jul 20, 2019, 7:51 AM by jeffery jim

Earlier this week, I was asked to generate base shear for foundation of a structure in order to determine the critical frame for earthquake proof building.

I was surprised by the need of having such derivation since logically, base shear ratio for symmetrical structure is almost zero with exception of space utilization or function at certain location of the building and the building is only 2-storey high.

Based on EC 0 and EC1 as well as BS6399, most of the function of an apartment or most working space have similar and almost similar loading criteria which gives not much changes when it comes to moment diagram and shear force diagram.

Unless certain rooms are designated as heavy duty storage, forge, or machineries; the rest remain the same. Hence, the shear base ratio will be under 10% difference in different axis or planar. Therefore, swaying under such exposure with the height of 2-storey proves not so much difference which may triggers imbalance and requires critical frame consideration in order to have weaker structural members or master frame to be reinforced. 

The utilization of FEM software have taken all of these into account, including the value of delta H and possible pushover analysis. It depends on what a designer is looking for to ensure enhancement of the structural members.

Unless, seismic design structure is done in an obsolete FEM design software, the need to consider base shear is necessary. The generated value from derivation should be included into the amount of loads to be applied in all axes for the design.