Bond Strength
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Four beam-column joint tests were undertaken to assess the accuracy of New Zealand design rules relating to bond strength in beamcolumn joints when applied to large diameter, high-strength reinforcement. The ratio of longitudinal beam reinforcement diameter to column depth intentionally did not meet the requirements of the New Zealand design standard for any of the units. The units were subjected to cyclic displacements up to interstory drift angles of 5%. Bond failure occurred in two of the four test units, at drift levels exceeding those allowed by international codes. It is believed that the unexpectedly good bond performance of the remaining two units was due to the large excess of vertical joint shear and column moment capacity. Keywords: beam-column joints; bond; high-strength reinforcement; load; reinforced concrete; slippage. INTRODUCTION There have traditionally been two grades of steel reinforcement available in New Zealand. Over time, the yield strength of these materials has increased. Most recently, the yield strength of the higher-strength reinforcement was increased from 430 to 500 MPa (62.4 to 72.5 ksi), known as Grade 500E. This resulted in changes to the way reinforced concrete structures must be designed. The research presented here investigated the impact of the new Grade 500E reinforcement on the occurrence of bond failure in beam-column joints. For reinforced concrete structures to function correctly, it is vital that forces can be transferred between the concrete and reinforcement. One mechanism by which this occurs is the bond that forms at the interface between the reinforcement and the concrete. This bond mechanism is particularly important in interior beam-column joints, where reinforcement must transition from being at yield in tension on one side of the joint to being close to yield in compression on the other side, and where there are few alternate options for anchoring reinforcement. If the maximum bond stress is exceeded and the bond between the beam longitudinal reinforcement and the concrete in the joint is broken, the reinforcement can slip freely through the concrete, reducing the stiffness of the beam-column joint significantly. This is unlikely to cause a catastrophic failure, but will increase the deflection of the building if it is subjected to further loading. In addition, bond failure alters the mechanism by which an interior beamcolumn joint resists shear. The failure of the bond mechanism is dependent on concrete strength and confinement, reinforcement strength and diameter, and the length over which the force transfer can occur (column depth in beam-column joints). Previous research at the University of Auckland1,2 indicated that the New Zealand concrete design standard, NZS 3101:1995,3 did not provide sufficient protection against bond failure when high-strength reinforcement is
description
bond strength
Transcript of Bond Strength
Four beam-column joint tests were undertaken to assess the accuracyof New Zealand design rules relating to bond strength in beamcolumnjoints when applied to large diameter, high-strengthreinforcement. The ratio of longitudinal beam reinforcementdiameter to column depth intentionally did not meet therequirements of the New Zealand design standard for any of theunits. The units were subjected to cyclic displacements up to interstorydrift angles of 5%. Bond failure occurred in two of the fourtest units, at drift levels exceeding those allowed by internationalcodes. It is believed that the unexpectedly good bond performanceof the remaining two units was due to the large excess of verticaljoint shear and column moment capacity.Keywords: beam-column joints; bond; high-strength reinforcement; load;reinforced concrete; slippage.INTRODUCTIONThere have traditionally been two grades of steel reinforcementavailable in New Zealand. Over time, the yield strengthof these materials has increased. Most recently, the yieldstrength of the higher-strength reinforcement was increasedfrom 430 to 500 MPa (62.4 to 72.5 ksi), known as Grade 500E.This resulted in changes to the way reinforced concretestructures must be designed. The research presented hereinvestigated the impact of the new Grade 500E reinforcement onthe occurrence of bond failure in beam-column joints.For reinforced concrete structures to function correctly, itis vital that forces can be transferred between the concreteand reinforcement. One mechanism by which this occurs isthe bond that forms at the interface between the reinforcementand the concrete. This bond mechanism is particularly importantin interior beam-column joints, where reinforcement musttransition from being at yield in tension on one side of thejoint to being close to yield in compression on the other side,and where there are few alternate options for anchoringreinforcement. If the maximum bond stress is exceeded andthe bond between the beam longitudinal reinforcement andthe concrete in the joint is broken, the reinforcement can slipfreely through the concrete, reducing the stiffness of thebeam-column joint significantly. This is unlikely to cause acatastrophic failure, but will increase the deflection of thebuilding if it is subjected to further loading. In addition, bondfailure alters the mechanism by which an interior beamcolumnjoint resists shear. The failure of the bond mechanism isdependent on concrete strength and confinement, reinforcementstrength and diameter, and the length over which the forcetransfer can occur (column depth in beam-column joints).Previous research at the University of Auckland1,2indicated that the New Zealand concrete design standard,NZS 3101:1995,3 did not provide sufficient protectionagainst bond failure when high-strength reinforcement isused in beam-column joints. In order to rectify this situation,a database of interior beam-column joint test results wasassembled and analyzed to determine appropriate designcriteria.4 Based on this research, an amendment to theNew Zealand design rules for structural concrete wasreleased in late 2003. Beam-column joint tests, however, thatused beam longitudinal reinforcement with a yield strength of500 MPa (72.5 ksi) or greater and a bar diameter larger than
20 mm were not available for inclusion in this database. Theexperimental program described herein provided informationon this combination of reinforcement strength and diameter,and gave an opportunity to assess the amended design rules.RESEARCH SIGNIFICANCEAlthough there is a history of interior beam-column jointtesting dating back over 30 years, few test specimens havehad high-strength beam longitudinal reinforcement of largediameters. The use of large diameter bars and high-strengthreinforcement reduces reinforcement congestion compared
