STUDY ON EFFECT OF STIFFNESS MODIFIERS IN THE ANALYSIS OF IRREGULAR BUILDING USING ETABS

The natural time period increases with a decrease in stiffness of structural elements, for both design and service load combinations (36.5% and 11.1% increase respectively). 2.  Support moments in both the X and Y directions reduce for the stiffness-modified structure compared to the general structure, by about 2.5%–10% across seismic zones. 3.  Reduction of stiffness in structural members increases lateral displacement, by 23.1%–40.2% in the X-direction and 12.1%–17.9% in the Y-direction. 4.  Storey drift increases for the stiffness-modified structure, by 20.9%–31.5% in the X-direction and 12.1%–18.5% in the Y-direction. 5.  Shear force and bending moment in beams reduce for the stiffness-modified model, by 10.2%–17.2% and 15.6%–23.6% respectively. 6.  Axial force in columns increases for the stiffness-modified column, by 17.8%–24.8%, compared with the general column. 7.  Shear force and bending moment in columns reduce for the stiffness-modified model, by 22.3%–23.6% and 17.7%–22.1% respectively. Overall, neglecting cracked-section stiffness modifiers leads to an under-estimation of lateral displacement and drift and an over-estimation of support moments and beam/column shear and bending forces, which can result in an unconservative estimate of serviceability response and an unrealistic distribution of design forces in tall, plan-irregular reinforced concrete buildings. The study can be extended to other plan shapes and vertical irregularities, and to non-linear dynamic (time-history) analysis to further validate the trends observed using the linear Response Spectrum Method.

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