Experimental and analytical investigations on seismic behavior of reinforced concrete frames and members using high-strength materials
Date of Issue2017-03-31
School of Civil and Environmental Engineering
The increasing use of high-strength concrete (HSC) in the construction industry is due to the numerous benefits offered by HSC compared to the ordinary-strength concrete. The use of HSC is prevalent in the columns of tall reinforced-concrete (RC) structures in cities located at high-seismic zones. The privilege of utilizing HSC in building structures is more significant in those structural members that are dominated by axial loading behavior. For a given load, the columns constructed using HSC require smaller cross-sectional area, which will increase the available space in each building story. The coupling of HSC with high-strength steel (HSS) reinforcement also appears to be an attractive proposition in the design of RC members. The congestion issue of reinforcing bars and hoops in a beam-column joint region can be improved by using HSS bars. However, there are several issues and concerns regarding the extensive use of high-strength materials. Investigation will be done to verify if HSC members and frames reinforced with HSS can be designed to have a predictable strength and to behave in a ductile manner under severe seismic loading. Most of the current building codes and standards do not address design provisions for concrete with a compressive strength of greater than 55 MPa and the reinforcing steel with the yield strength of larger than 550 MPa. The aim of this research program is to investigate the inelastic behavior of HSC columns and beam-column joints reinforced with high-strength steel. Experimental and analytical investigations were carried out on HSC beam-column joints with Grade 700 reinforcing longitudinal bars in beams and columns. Seven interior and five exterior full-scale RC beam-column joints with various reinforcement detailing were designed according to the special seismic provisions of ACI 318-11 code. The variables in test specimens include the concrete compressive strength, the reinforcement yield strength, and the level of axial compression loading. The test specimens were subjected to constant column axial loading as well as the quasi-static lateral loading reversals. The performance of each test assembly was examined in terms of cracking patterns, lateral loading capacity, strain profiles of the reinforcement, secant stiffness, energy dissipation capacity, joint shear strength, and the bond performance. All specimens displayed ductile failure mode and it was concluded that the use of HSC disregarding the presence of axial compression loading could improve the bond condition of the specimens. Parametric studies via finite-element analysis (FEA) were performed to study the influence of various parameters on the strength, bond condition, and energy dissipation capacity of the specimens. In general, the effect of parameters such as material properties, the axial compression loading level, and the effect of the development length of beam bars passing through the joints were investigated. Based on the experimental findings and the analytical parametric studies, several design recommendations were proposed. In addition, a backbone curve model for the shear behavior of joints was proposed. Furthermore, the bond stress distribution along the beam bars in joints was proposed and calibrated based on the experimental result. This bond distribution was incorporated in an existing strut-and-tie model to study the force flow mechanism in beam-column joints. A comparison was made between the observed cracking pattern of test specimens and the predicted cracking pattern based on strut-and-tie modeling. Furthermore, the effectiveness of a column design method that uses longitudinal reinforcement with various steel grades was investigated. Based on analytical investigations, the influence of various design parameters on the ultimate drift capacity of RC columns was studied and new drift-based design provisions and charts were proposed. These models can be used as a design tool for HSC columns reinforced with mixed-grade longitudinal steel bars.