Application of SMA fiber composite as seismic reinforcement for concrete moment resisting frames

For many years, steel has been used as the primary reinforcing material for concrete structures. Despite requisite stiffness, strength, ductility and desired deflection properties, steel reinforcing bars have tendency to incur permanent plastic deformations under excessive loading. Recently, fiber reinforced polymer (FRP) reinforcing bars have also been used in concrete structures to address corrosion issues typically associated with conventional steel bars. However, due to their linear elastic behavior, they are not considered in structures which require ductility and damping characteristics. The use of shape memory alloys (SMAs) with their nonlinear-elastic behavior in the composite could potentially provide solution for this problem. Small diameter super-elastic Nickel-Titanium (NiTi) SMA wires, coupled with polymer matrix is sought in this research as reinforcing bars in reinforced concrete (RC) moment resisting frames (MRFs) to improve the performance of the frames in terms of reducing residual inter-story drifts and damage under earthquake loading, while still maintaining the elastic characteristics associated with FRP. SMA fibers, conventional polymeric fibers and resin are infused together to manufacture the new composite under high pressure and temperature. Uni-axial cyclic tensile tests are carried out to characterize the mechanical hysteretic behavior of the new composite. Analytical constitutive models are developed for the SMA-based composite materials and calibrated based on experimental test results. These material models are then extended for use in structural models to capture the hysteretic behavior of the composite. The new SMA composite reinforcement is placed at the plastic hinge region of the MRFs, where most of the damage is expected. RC MRF prototype structures reinforced with steel, SMA-FRP and conventional glass-FRP (GFRP) composite reinforcement are designed using two different approaches involving equivalent static force procedure and capacity spectrum method. Multiple non-linear time history analyses are conducted using incremental dynamic technique for assessing the performance of the MRFs under suite of ground motions. Main shock-aftershock ground motion sequences are utilized to examine the efficacy of using SMA-FRP composite in plastic hinge zones of MRFs. Damage assessment is performed based on residual inter-story drift and drift performance levels. Efficacy of proposed SMA-FRP composite reinforcement is further explored by embedding it in a small scale beam tested under 3-point bending.