Alternative Reinforcement Details for RC Moment Resisting Frames with Limited Ductility Demands

Abstract

however, when provided lap splice length was decreased to 40db or 30db, inelastic deformations and energy dissipations of spliced columns were degraded due to early bond failure with severe splitting cracks along splice regions. Further, lap splice details also significantly affected seismic performance: Bottom offset bar splice showed lower flexural strength but greater ductility and energy dissipation with flexural failure mode

on the other hand, top offset bar slice and splice without offset bend showed greater flexural strength but lower ductility and energy dissipation with bond splitting failure. Since tensile stresses significantly vary along splice regions in columns, average bond demands of spliced bars decrease with increasing moment gradient. Thus, a modified lap splice length model for lap-spliced columns considering moment gradient along splice regions was proposed to reduce splice length required in current design codes. The reduced splice length correlated well with the present and existing cyclic test results of columns with lap splices. In low-rise buildings with small sizes of columns and joints, insufficient shear capacity of joints without shear reinforcement has resulted in joint shear failure before adjacent beams and columns are subjected to flexural yielding. Also, since it is difficult to secure anchorage length of 90º hooked beam bars, attention should be paid to the joint reinforcement details. In this dissertation, cyclic load tests of beam-column connections with small effective joint area were conducted to investigate the effect of joint shear reinforcement ratio, anchorage length of beam bars, joint types (interior and exterior), and U shaped bars. The joint shear strength was increased proportionally with the increase of joint shear reinforcement ratio. However, when the anchorage length of beam bars was not secured, the joint shear strength was not proportional to joint shear reinforcement due to bond deterioration of beam bars. Based on present and existing test results of exterior joints with shear failure, shear strength model was empirically presented considering joint shear reinforcement ratio. In order to evaluate the effect of anchorage directions of beam bottom bars on beam-column connection behaviors, exterior connections with only beam bottom bars bent away from the joints were compared to those with both top and bottom beam bars bent inside the joints. Since shear forces in exterior joints without shear reinforcement were transmitted only by a diagonal strut mechanism, the joint behaviors were affected by anchorage directions of beam bottom bars. On the other hand, joint shear forces in exterior joints with shear reinforcement are transferred by both a diagonal strut mechanism and a truss mechanism, so the effect of anchorage directions of beam bottom bars is not significant. Finally, two-story and two-bay frame tests were performed to verify the applicability of reinforcing bar details verified by member tests, and to investigate the overall seismic behavior of low-rise buildings in system level. Seismic behavior and a role of a shear wall in a frame were also examined. In moment frames without a shear wall, plastic hinges were formed at both ends of 1st columns, leading to weak-column and strong-beam behaviors and bond failure with severe splitting cracks along splice regions. Exterior joints with U-bars showed satisfactory seismic performance. In a moment frame with a shear wall, on the other hand, sliding failure was occurred horizontally in the upper part of shear wall at top floor. In order to prevent such sliding failure, dowel reinforcement should be additionally provided between a wall and a beam (or a slab) to secure a sufficient shear friction strength. Various member tests and frame tests were carried out to evaluate seismic performance of alternative bar details for low-rise buildings. Based on experimental results, analytical models for columns and exterior joints were developed. Also, for the newly revised Small Building Code, the design criteria and commentary on the alternative bar details in columns and beam-column joints are presented.

Though low-rise buildings with two stories or less account for more than 80% of the total number of buildings in Korea, non-seismic details are generally used without any verification of seismic performance, leading to large earthquake damages in low-rise buildings. Under a Gyeongju earthquake in the magnitude of 5.8 on the Richter scale in September 12, 2016, the structural damages were mainly concentrated on low-rise buildings because the earthquake loading contained high frequency components. As the interest in seismic design of low-rise buildings increased after the earthquake, the enforcement ordinance of domestic Building Law for buildings subject to seismic design was revised in February 2017. The range of buildings for seismic design was changed from buildings more than three stories or gross area of 1000 m2 to buildings more than two stories or gross area of 500 m2. In additions, either Korean Building Code (KBC 2016) for general buildings or Small Building Code for low-rise buildings can be applied for seismic design of low-rise buildings. The current Small Building Code have been established so that non-structural engineers can design reinforcing bar details without any structural calculation, and the details follow complex reinforcement details specified in KBC 2016. However, such complex bar details are not often observed in low-rise buildings because construction supervision is not carried out properly, and relieved non-seismic details are more preferred due to convenient bar placement. Thus, in this dissertation, in order to improve structural safety and construction efficiency of low-rise buildings, reinforcing bar details reflecting characteristics of low-rise buildings were developed, and seismic performance of various bar details was experimentally verified. The main objective of this dissertation is to evaluate seismic performance and applicability of alternative reinforcing bar details in low-rise buildings. The alternative bar details were developed considering characteristics of low-rise buildings, which can ensure both construction efficiency and structural safety. A number of cyclic loading tests were performed to evaluate seismic performance of columns, beam-column connections, and two-story and two-bay frames. Seismic performance criteria of low-rise buildings were determined as a frame with limited ductility demand (Intermediate moment frame in KBC 2016 and ACI 318). For the comparison with the alternative details, seismic details specified in current seismic codes and non-seismic details used in construction fields without verification were also evaluated. Analytical models for columns and beam-column connections were also discussed. The main findings from experimental and analytical studies are as follows. Since low-rise buildings generally exhibit a weak-column and strong-beam behavior, anchorage details and spacing of column transverse reinforcement are more significant in seismic performance of the buildings. Thus, seismic performance of column transverse bars with seismic, non-seismic, and alternative details was evaluated to conduct square and rectangular columns with various tie spacing (s = d/2, d/3, and d/4). According to test results, immediately after concrete cover spalling off, premature anchorage failure in 90º hooked and lap-spliced ties occurred, leading to relatively less ductility of columns when compared to the column with 135º hooked hoops. Also, since the shear strength of concrete Vc was gradually degraded as shear deformation increased, the shear strength Vn under inelastic deformation was significantly influenced by shear resistance of transverse reinforcement Vs. Based on this result, shear strength degradation model for columns was proposed and verified through a number of present and existing test data. Furthermore, since lap splices of column bars are generally used in the bottom of columns where potential plastic hinges may form in low-rise buildings, adequate splice length and splice details of column bars are determined by considering seismic performance such as load and ductility capacities, and energy dissipations. Thus, reversed cyclic load tests of spliced columns were performed with various splice length (ls = 30db ~ 50db) and details (Bottom offset bar splice, Top offset bar splice, and Splice without offset bend). In spliced column with ls = 50db satisfying ACI 318 requirement, the nominal flexural strength was attained with ductile behavior