A strut-and-tie model for predicting shear strength of squat shear walls reinforced with UHPC under seismic loading

Ultra-high performance concrete (UHPC) locally reinforced the boundary elements can mitigate the issues of concentrated damage and insufficient ductility in traditional reinforced concrete (RC) shear walls under strong earthquakes or high axial loads. However, for squat UHPC-reinforced shear walls, existing standards and theoretical models are difficult to accurately capture the influence of UHPC on the shear transfer mechanism. This study evaluated the applicability of existing models based on experimental results from 13 UHPC-reinforced shear walls tested by the authors, and further developed a three-dimensional refined numerical model for squat shear walls. Through analysis of concrete crack propagation, reinforcement stress and diagonal compressive load transfer paths, it was found that the failure mode of squat UHPC-reinforced shear walls was characterized by crushing of diagonal concrete struts, which is consistent with the strut-and-tie model. Based on this, 60 squat UHPC-reinforced shear walls were simulated to address the limitation of the original strut-and-tie model in representing the contribution of UHPC to the compression zone height. The effects of key parameters, including axial load ratio and UHPC distribution width on the development of the compression zone were investigated, and an equation for the compression zone height considering UHPC was proposed. Finally, a shear strength calculation procedure applicable to squat UHPC-reinforced shear walls was developed. The prediction results indicated that the developed model could accurately estimate the shear strength of walls, with an average predicted-to-actual ratio of 1.03 and a further reduction in prediction variability.