Shear design of reinforced concrete members under axial compression

The failure of the Sleipner offshore oil platform in 1991 indicated that serious deficiencies exist in the current design procedures for shear design of reinforced concrete members subjected to axial compression. The current design requirements of the ACI code are based on a few tests conducted with relatively low strength concrete in the 1960s. To investigate the shear behaviour of reinforced concrete members subjected of high levels of axial compression and shear, twenty four tests were conducted on lightly reinforced members subjected to different levels of axial compression and shear. The main variables in the study were the ratio of axial compression-to-shear, stirrup reinforcement ratio, concrete strength and specimen width.; The results clearly indicate that the current detailed ACI procedure for shear design of members subjected to axial compression can be unconservative for members under moderate and high levels of axial compression. It is recommended that the detailed procedure be eliminated from future editions of the ACI code. The simple alternate method specified by the ACI code was able to predict the experimental results conservatively with more consistency than the detailed method. The AASHTO-LRFD procedure was able to predict the strength as well as the mode of failure consistently. More detailed procedures based on the modified compression field theory were able to predict not only the strength but also the deformation behaviour of the test specimens with good accuracy.; No beneficial influence of increasing the strength of concrete beyond 70 MPa was observed in the tests. Specimens with very high strength concrete of over 80 MPa were found to be weaker than similar members with 60 MPa concrete. The current analysis and design procedures do not account for this anomaly.; It is postulated that the reduced "aggregate interlock" capacity due to smoother crack surfaces for higher strength concrete partly contributes to the reduced shear capacity. Fifteen "push off" type of shear friction tests were conducted in the study to evaluate the influence of fracture of coarse aggregate particles on the crack surface. The results indicate that the shear friction capacity of cracked concrete with fractured aggregates on the surface was substantially lower compared to concretes with no fractured aggregates on the surface. Based on the experimental results a simple model to account for the reduction in shear capacity due to fracture of coarse aggregates is suggested.; Specimens under very high levels of axial compression usually fail at first cracking. Based on well known tests by Kupfer a simple strain based model for reduction of the concrete cracking strength under conditions of combined compression and tension is proposed.; The proposed models were incorporated into a simplified sectional analysis program that is based on the modified compression field theory. The models improved the predicted behaviour for members under higher levels of axial compression.