EFFECT OF COLUMN CAPACITY DESIGN ON EARTHQUAKE RESPONSE OF REINFORCED CONCRETE BUILDINGS

In earthquake resistant design of RC frame buildings, capacity design of columns in flexure is applied to eliminate the possibility of storey sway mechanisms and to spread the inelastic deformation demands and energy dissipation throughout the structure. The paper considers two alternative column capacity designs: the conventional, full capacity design of columns relative to the beams, and the relaxed one allowed by Eurocode 8 depending on how much the seismic action controls the flexural capacity of beams. Twelve RC frame buildings, designed in detail according to the two capacity design alternatives, are nonlinearly analysed under spectrum-compatible motions applied separately in the two horizontal directions and scaled to intensity from once to twice the design ground motion. In both design versions the slab participation to the beam tension flange is considered either as in the design calculations - including those of capacity design - i.e. very little, or as expected in reality, i.e. very significant. In most cases the dynamic response to the design-level motion is found to be nearly elastic, due to the overstrength of materials and members and to the "understress" of the structure due to cracking-induced softening. At higher motion intensities the effect of column capacity design and of the slab participation to the beam flexural capacity is not dramatic: column inelasticity and some light damage cannot be prevented by relaxed or full capacity design, while the large participation of the slab to the beam negative moment capacity does not overly distort the strength balance between beams and columns or the seismic response of the structure. Under any circumstances, column plastic hinging does not lead to a storey sway mechanism.