Influence of Dynamic Load Rates on the Seismic Response of Reinforced Concrete Walls

Abstract

Field observations following the 2010-2011 Canterbury earthquakes in New Zealand indicated that a number of unexpected failure modes occurred for lightly reinforced concrete walls, such as a lack of cracking in the wall plastic hinge region and premature bar buckling/fracture. In response to these observations, several amendments were made to the New Zealand Concrete Structures Standard, including revised minimum vertical reinforcement requirements for lightly reinforced concrete walls. However, the loading in prior research did not consider load rate effects, and the performance of walls subjected to earthquake-induced dynamic loading was uncertain. To investigate the performance of lightly reinforced concrete walls subjected to dynamic loading, an experimental program was undertaken on fifteen RC prisms representing the end regions of RC walls designed in accordance with various minimum vertical reinforcement provisions. The prism tests verified that dynamic loading rates had a limited effect on the crack pattern but led to increased strengths and deformation capacity. The results of this study were used in conjunction with available wall test data to develop a finite element model to simulate the response of flexure-dominant lightly reinforced concrete walls. Numerical analyses were conducted using the model developed to simulate fifteen rectangular RC walls containing end regions equivalent to the test RC prisms subjected to various loading rates. Similar to the test prisms, it was found that the wall crack pattern was primarily a function of vertical reinforcement content and shear span ratio, and the loading rate was not a significant factor. Dynamic loading rates led to modest strength increases and more pronounced increases in the wall deformation capacity. A numerical parametric study of non-rectangular I-shaped RC walls with various flange lengths and vertical reinforcement contents was conducted for a more comprehensive investigation of load rate effects in RC walls. The modelling results consistently showed that increasing the loading rate had a limited impact on cracking behaviour; the deformation capacity also followed this trend. Instead, walls with increased flange length had increased overstrength capacity and decreased deformation capacity. Other parameters influencing wall cracking behaviour and drift capacity, including concrete strength and reinforcing steel strain hardening ratio, were numerically investigated. Increased concrete strength resulted in reduced cracking and drift capacity. However, the detrimental effect of higher concrete strength was mitigated by an increased steel strain hardening ratio. The results of this research confirmed that load rate effects did not cause any major concerns, and there is no need to revise design standards.