P-Delta Effects on Tall RC Frame-Wall Buildings

Abstract eng:
Current international design codes impose limits on the P-delta ratio, which appear to have been set to ensure a minimum reloading stiffness during cyclic response and with due consideration for the likely ductility demands imposed on structures. Whilst the current code limits may be reasonable for normal height structures, it is argued that the code limits should be reconsidered for tall buildings owing to limited displacements that real earthquake ground motions impose on such buildings. In this paper, the design of a 45-storey reinforced concrete frame-wall case study structure is used to highlight the significance of the p-delta limit within the modal response spectrum analysis procedure of the Eurocode 8. It is found that the strength of the structure is dictated by the P-delta limit for seismic actions, despite anticipated storey drifts and ductility demands being relatively low. A series of non-linear time-history analyses using a suite of spectrum-compatible real and artificial accelerograms, indicate that P-delta effects do not have a significant influence on displacements or storey drifts of the tall building. The likely causes of this behaviour are identified, making reference to earlier investigations into P-delta behaviour and with consideration of substitute structure concepts. To investigate the significance of the P-delta ratio further, a series of SDOF studies are undertaken for systems designed with P-delta ratios of up to 0.85. The results demonstrate that the p-delta ratio has little influence on the behaviour of long-period systems subject to real earthquake records and therefore it does not appear appropriate to impose strict limits on the P-delta ratio. Instead, it is recommended that the P-delta effects be evaluated for tall-building systems as part of an overall assessment of their response, using advanced non-linear time-history analyses with real records and within a large-displacement analysis regime.

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