Seismic load tests on reinforced concrete beam-column joints with plain round reinforcing bars designed to pre-1970s codes

Technical Abstract

Six as-built full-scale beam-column joints and one retrofitted exterior beam-column joint were tested under simulated seismic loading in this study. Simulated seismic load tests on as-built Units were conducted as part of an investigation of the behaviour of existing reinforced concrete structures designed to pre-1970’s codes when subjected to severe earthquake forces/ simulated seismic load test on retrofitted as-built exterior beam-column joint Unit was conducted to investigate the improvement of the overall seismic performance which can be achieved by jacketing existing damaged reinforced concrete exterior beam-column joint unit using fibre-glass jacketing technique.

All six as-built tests units were full-scale in size, contained plain round reinforcing bars and were replicas of parts of the moment resisting frame of an existing building in Christchurch that was constructed in the 1950’s. Two of the test units were identical as-built interior beam-column joint assemblages. The four exterior beam-column joint subassemblages were fabricated into two groups of two identical Units each. The as-built exterior beam-column joint with the beam bar hooks bent away from the joint core was retrofitted by wrapping the column parts above and below the joint core using fibre-glass jacketing after tested.

1. For the two as-built interior beam-column joint units, the diameter of the longitudinal bars passing through the joint core was relatively larger than the code required values, the joints had no joint shear reinforcement at all, and the beams and columns had low quantities of transverse reinforcement as was typical of pre-1970s construction in New Zealand. One unit was tested with zero axial column load, and the other unit with a constant axial column load of 0.12Agfc‘ to study the seismic behaviour of existing reinforced concrete building frames and the effect of compressive column axial load on the general seismic performance, where fc‘=concrete cylinder compressive strength and Ag = the gross column section area.

Both units when tested displayed low available structural stiffness, low available ductility, and significant degradation of stiffness and strength during testing. The low structural stiffness could be attributed to the slip of the plain round longitudinal bars through the joint. Column bar buckling was found to initiate failure, especially when the compressive axial load was present in the column. The utilization of plain round bars although leading to bond slip was found to improve the joint shear strength, and to suppress the joint shear distortion, shifting the problem area from the joint shear to structural stiffness.

2. For the four exterior beam-column joint units, the joint cores contained only limited shear reinforcement and the columns and beams had only small amount of transverse reinforcement. One of the two identical test Units in each group was tested under simulated seismic loading with zero axial column load while the other unit was tested under simulated seismic loading with a constant axial column load of about 0.25Agfc‘ present.

(a) The as-built exterior beam-column joint Units when tested with zero axial column load demonstrated very poor force strength and stiffness behaviour, and the final failures of the as-built units were dominated by concrete tension cracking along the outer layer of column main bars adjacent to the joint core, which was initiated by the interaction between the column bar buckling and the straightening action of the beam bar hooks, irrespective of the beam bar hook details, should plain round longitudinal reinforcing bars be used. The configuration of the beam bar hooks bent into the joint core was found to result in better seismic response compared to that with the beam bar hooks bent away from the joint core in the case of the zero axial column load and small amount of column transverse reinforcement provided.

(b) The as-built exterior beam-column joint Units when tested with constant compressive axial column load of about 0.25Agfc‘ present demonstrated that the presence of compressive axial column load enhanced the force transfer by bond from the beam tension steel to the surrounding concrete, reduced the beam steel tension force needed to be transferred at the bend, improved the bond condition along the column bars, consequently totally preventing the correct tension cracking which was associated with the interaction between the column bar buc and the straightening action of the beam bar hooks. Consequently, the stiffness, force strength and joint shear behaviour was greatly improved, and the system energy dissipating capacity was improved due to the presence of compressive axial column load. In this case the effects of different beam bar hook details on the seismic performance of the as-built exterior beam-column joint units became very insignificant and the failure trigger became the beam fixed-end rotation. When compared to the test evidence observed with deformed longitudinal reinforcement, although preventing the joint shear failure, enhanced column bar buckling and the straightening action of the beam bars in tension, thus facilitated the failure of concrete tension cracking along the outer layer of the column bars adjacent to the joint core, consequently causing significant reductions in the available shear force strength and stiffness, especially in the available stiffness. Also the adverse effects on the strength and stiffness response of the used steel type was more severe for as-built exterior beam-column joints compared to tas-built interior klingbeam-column joints.

3. Test on the retrofitted as-built exterior beam-column joint unit showed that fibre-glass jacketing in the column areas adjacent to the joint core, when the plain round beam bar hooks are bent away from the joint core and the axial column load is low, was very effective in improving the overall seismic performance of the system. Fibre-glass jacketing in the column areas adjacent to the joint core actuated the postulated alternative joint shear model, greatly improved the structural force strength and stiffness performance. In this case, the low flexural strength attainment resulting from bond degradation along the member longitudinal bars became of concern.

As a whole, the collapse mechanism of similar existing reinforced concrete building structures was demonstrated to be a combination of flexural failure in the beams and columns, rather than any shear failure.