Seismic Behavior of Nonseismically Detailed Interior Beam-Wide Column and Beam-Wall Connections

ACI Structural Journal, Sep/Oct 2009 by Li, Bing, Pan, Tso-Chien, Tran, Cao Thanh Ngoc

Six full-scale nonseismically detailed reinforced concrete (RC) interior beam-wide column and beam-wall joints with zero to high axial compression loads were tested to investigate the seismic behavior of the joints. Quasi-static cyclic loading, simulating earthquake actions, was applied. The overall performance of each test assembly was examined in terms of lateral load capacity, drift, stiffness, energy dissipation capacity, and nominal joint shear stress. Three levels of axial compressive column loads were investigated to determine how this variable would influence the performance of the joints. All the specimens failed adjacent to the joint panel with gradual strength deterioration and low attainment of structural stiffness. The low attainment of stiffness and strength was attributed to the slip of the longitudinal bars through the joint core. The test results showed that RC interior beam-wide column joints and beam-wall joints with nonseismic design and detailing attained a drift ratio of 2.0% without significant strength degradation. It was thus concluded that such joints could also possess inherent ductility for adequate response to unexpected moderate earthquakes.

Keywords: axial compression load; beam-column joints; reinforced concrete; seismic.

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INTRODUCTION

Extensive experimental research1 on ductile beam-column joint conducted in different countries throughout past decades has given a better understanding of ductile joint behavior. A relatively limited database is available in literature for nonseismically detailed joints, however, with respect to ductile detailed joints. In addition, nonseismically detailed joints with wall-like wide columns have rarely been studied. The only available experimental study of such joints was conducted by Li et al.2-3 The experiments carried out by Li et al.2 involved two full-scale nonseismically detailed interior beam-wide column joints to investigate the seismic behavior of the joints. The two variables studied in the test specimens were the amount of joint transverse reinforcement and the lap splice details for column and beam reinforcements. The column to beam width ratio of the specimens was approximately 3. The maximum nominal horizontal shear stress in the joint core was determined to be 0.15f'c based on these experimental results. The joint without joint horizontal transverse reinforcement failed at a displacement ductility factor of 2, which correlates well with the model proposed by Hakuto et al.4 This suggests that for reinforced concrete (RC) interior joints without joint transverse reinforcements, joint shear failure occurs around a displacement ductility factor of 2, where the joint shear stress is between 0.11f'c and 0.17f'c . Due to the presence of joint transverse reinforcements, the reinforced joint specimen achieved a better ductility factor of 3.

Reinforced concrete structures consisting of wall-like wide column elements are very common in regions of lowto- moderate seismicity and are the predominant structural system in Singapore. The BS 81105 code used in Singapore does not specify any provision for seismic design or detailing of RC structures. Therefore, it is of great concern that the strength, ductility, and energy dissipation capacity of these structures may be inadequate to sustain earthquakeinduced loads in regions of low-to-moderate seismicity. The need for evaluating and improving detailing of existing structures is obvious. An experiment consisting of six fullscale RC interior beam-wide column joints and beam-wall joints typically found in framed structures designed with nonseismic detailing in Singapore has been undertaken at Nanyang Technological University, Singapore (NTU) to better understand the seismic behavior of such joints. The level of axial compression load exerted on the columns or walls was also studied in this paper.

RESEARCH SIGNIFICANCE

The seismic behavior of nonseismically detailed beamwide column and beam-wall joints and the effects of column axial loads on the performance of such joints have rarely been studied. This study provides test observations and related analyses of such joints. Knowledge gained from the test results can be used to develop theoretical models for seismic evaluation.

EXPERIMENTAL STUDIES

Specimens and test setup

Six full-scale nonseismically detailed interior beam-wide column and beam-wall joints designed based on the BS 81105 code were constructed and tested. These specimens were typical as-built joints abstracted from the existing buildings in Singapore. Figure 1(a) illustrates the schematic dimensions of Specimens C1A, C1B, and C1C. These interior beam-wide column joints have a column-to-beam width ratio of approximately 3.56. The wall-to-beam width ratio of Specimens C2A, C2B, and C2C is approximately 7, as shown in Fig. 1(b). The C2 series specimens had a wall cross-sectional dimension of 1600 x 300 mm (63.0 x 11.8 in.) and a beam cross-section dimension of 230 x 600 mm (9.1 x 23.6 in.), whereas the C1 series specimens had a column cross-section dimension of 820 x 280 mm (32.3 x 11.0 in.) and a beam cross-section dimension of 230 x 300 mm (9.1 x 11.8 in.). All specimens met the criterion of strong column-weak beam. Three levels of axial loading.0.0, 0.1, and 0.35 Ag f'c. were investigated for both beam-wide column joints and beam-wall joints.