Incorporating the influence of ground motion duration on structural deformation capacity in seismic design.

Abstract

Many high seismic hazard zones across the world, including New Zealand, are prone to long duration ground motions generated from large magnitude earthquakes. Although the adverse effects of long duration on structural damage and collapse risk have been well recognised recently, current seismic design practice has only implicit considerations of such effects. Since modern seismic design philosophy generally relies on the ability of structures to safely withstand induced deformation demands under earthquake excitation, this study develops a method to explicitly account for ground motion “duration” in the design process by characterising its effect on structural deformation capacity.

Conventional cyclic-symmetric loading protocols used in practice for the performance evaluation of structural components have significantly fewer cycles, especially at low drift levels, compared to long duration protocols representative of seismic demands from large magnitude earthquakes. Experimental tests conducted on identical reinforced concrete columns under (i) a conventional and (ii) a long duration protocol, do not find significant differences in the levels of strength and stiffness deterioration and ultimate deformation capacity. The results are supported by numerical simulations calibrated with experimental data and indicate that seismic demands from conventional loading protocols are more representative of long duration ground motions from megathrust earthquakes than short duration ground motions from crustal earthquakes. Numerical simulations based on a popular semi-empirical prediction algorithm are found to predict the experimental results well up to the point of strength capping. A general need to refine the prediction equations for post-capping response is highlighted.

The effect of duration on structural dynamic deformation capacity is investigated for a range of modern reinforced concrete and steel moment frame buildings and their equivalent single-degree-of-freedom systems. A robust numerical algorithm is developed to estimate the dynamic deformation capacity of a structure by conducting incremental dynamic analysis. Statistical models fit to analyses results indicate that dynamic deformation capacity decreases with ground motion duration. These results highlight the need to explicitly consider duration in the design process to compensate for the lower dependable deformation capacities under longer duration ground motions. The effect of duration on dynamic deformation capacity is not found to be highly sensitive to the following structural parameters: fundamental period of vibration, hysteretic model, and level of ductility. This study also investigates the effect of duration on the distribution of deformation demands along the height of multi-storey steel frames by comparing the response under spectrally equivalent short and long duration ground motions sets. Higher mode effects are found to increase the median drift demands at the upper storeys of medium and high-rise steel frames under long duration records. Thus, ground motion duration is found to affect the collapse mechanisms of such buildings. Providing stronger beam sections at the upper storeys is demonstrated to minimise the impact of these higher mode effects.

Based on the quantified relationship between dynamic deformation capacity and ground motion duration, a method is proposed to explicitly account for duration in the seismic design process for new buildings in New Zealand. This method proposes reducing the design drift limit prescribed in the New Zealand standard NZS 1170.5 in proportion to the median duration of the anticipated ground motions at the site. Hazard-consistent collapse risk assessment of case-study designs for a site in Nelson demonstrate that designs satisfying lower drift limits have higher deformation capacities and lower collapse risk. The range of variation in the mean annual frequency of collapse of structures located at sites with different duration targets is found to be reduced with the proposed method, compared to the current design approach. Therefore, the method proposed in this study is, on average, found to adequately compensate for the increased likelihood of collapse at sites anticipating long duration ground motions. The method is believed to be (i) easy and simple for practical applications and (ii) flexible to be adopted by other modern seismic design codes and guidelines.