Design of ductile reinforced concrete moment resisting frames using Grade 500E reinforcement

Abstract

This report describes a series of four interior beam-column joint tests conducted at the University of Auckland. These tests were funded by the Earthquake Commission research foundation. The purpose of the tests was to provide data on the seismic performance of beam-column joints with grade 500E beam longitudinal reinforcement of large diameters. In particular the tests examined the occurrence of bond failure and reinforcement slip in such joints.

The test units were representative of an interior beam-column joint from a typical, monolithic moment resisting frame. The units were approximately two-thirds of full size, and of similar size to five beam-column joints previously tested at the University of Auckland that used smaller diameter beam longitudinal reinforcement. The test units were designed using the capacity design procedures of NZS 3101:1995. However, to promote bond failure the units did not follow the requirements limiting the ratio of beam longitudinal reinforcement diameter to column depth.

The four beam-column joint sub-assemblies were subjected to a cyclic load history. Load steps of increasing storey drift were applied. Despite none of the units meeting New Zealand design requirements for the prevention of bond failure, at drift levels equivalent to those allowed in New Zealand the performance of all four units was good, with no strength loss, and no significant reinforcement slip. At greater drift angles bond failure occurred in only two of the four tests, despite testing reaching storey drift angles that were approximately twice those allowed in New Zealand and it is believed that the reason for the exceptionally good performance of two of the units was the large excess vertical joint shear and column moment capacity. The tests described re-emphasised the high yield drift angles, and hence low appropriate design ductility levels, of moment resisting frames including grade 500E beam longitudinal reinforcement.

This report also shows that the use of grade 500E longitudinal reinforcement in the beams of moment resisting frames is not an effective design solution. A comparison was made of beam-column joints designed for equal forces using grade 300E and grade 500E beam longitudinal reinforcement. It was found that in most cases the use of grade 500E reinforcement requires at least twice as many bars, resulting in worse reinforcement congestion in the joints, where congestion has always been a problem. This situation would be worsened if realistic allowance was made for the higher forces that the frame reinforced with grade 500E longitudinal reinforcement would have to sustain, due to the lower ductility inherent in the design.

It was concluded that current New Zealand design rules for the prevention of bond failure in frames reinforced with grade 500E beam longitudinal reinforcement are currently mildly conservative. This was considered a satisfactory situation. However, it is recommended that designers do not use grade 500E longitudinal reinforcement in the beams of moment resisting frames.