Michael P. Collins

Simplified Modified Compression Field Theory for Calculating Shear Strength of Reinforced Concrete Elements

In this article, the authors propose a simplified MCFT (modified compression field theory) and demonstrate that this simplified MCFT is capable of predicting the shear strength of a wide range of reinforced concrete (RC) elements with almost the same accuracy as the full theory. The authors summarize the results of over 100 pure shear tests on reinforced concrete panels. The ACI approach for predicting shear strength as the sum of a diagonal cracking load and a 45-degree truss model predicts the strength of these panels poorly, with an average experimental-over-predicted shear strength ratio of 1.40 with a coefficient of variation of 46.7%. The modified compression field theory (MCFT), developed in the 1980s, can predict the shear strength of these panels with an average shear strength ratio of 1.01 and a coefficient of variation (COV) of only 12.2%. The authors contend that their new, simplified method gives an average shear strength ratio of 1.11 with a COV of 13.0%. They demonstrate the application of this new simplified method to panels with numerical examples. They conclude that, on many occasions, a full load-deformation analysis is not needed and this quick calculation of shear strength is appropriate and useful.

How Safe Are Our Large, Lightly Reinforced Concrete Beams, Slabs, and Footings?

The current American Concrete Institute (ACI) shear design procedures can be very unconservative if applied to large, lightly reinforced members because these procedures do not recognize that as the size of such members increases, the shear stress required to cause failure decreases. This paper describes an extensive experimental investigation aimed at evaluating the significant parameters that influence the magnitude of this size effect in shear. It was found that the reduction in shear stress at failure was related more directly to the maximum spacing between the layers of longitudinal reinforcement rather than the overall member depth. High-strength concrete members displayed a more significant size effect in shear than normal strength concrete members. Some simple modifications to the ACI shear design procedures are suggested that will result in a more consistent level of safety across the possible range of concrete strengths and member sizes.

A GENERAL SHEAR DESIGN METHOD

The authors present a simple, unified method for the shear design of both prestressed concrete members and nonprestressed concrete members. The method can treat members subjected to axial tension or axial compression and treats members with and without web reinforcement. The derivation of the method is summarized and the predictions of the method are compared with those of the current American Concrete Institute (ACI) Code.

EFFECT OF CONCRETE STRENGTH AND MINIMUM STIRRUPS ON SHEAR STRENGTH OF LARGE MEMBERS

There is currently a concern that ACI shear design procedures can be unconservative if applied to thick one-way slabs or large beams containing only minimum stirrups. This research discusses the results of 21 large beams tested to look into these concerns. Based on experimental results, this paper concludes that until the present ACI shear provisions are modified, it would be prudent to use the recent shear provisions of the AASHTO LRFD specifications as they provide a more consistent level of safety. A simple spreadsheet is described that allows these provisions to be conveniently applied.

Effect of Aggregate Size on Beam-Shear Strength of Thick Slabs

This study investigates the safety and accuracy of the American Concrete Institute shear design method when applied to thick slabs by focusing on the size effect in shear and the role played by the maximum coarse aggregate size in transferring shear stress across cracks. An experimental program was conducted in which 10 large-scale and 10 geometrically-similar, small-scale, shear-critical reinforced concrete slab-strip specimens were loaded to failure. It was found that the major mechanism of shear transfer in these element types is aggregate interlock, and that the maximum aggregate size plays an important role in beam-shear capacity of reinforced concrete members. The abilities of the ACI design method and a simplified design method based on the modified compression field theory (MCFT) to predict the failure loads are compared. Findings indicate that the ACI design method is unconservative when applied to thick slabs or large wide beams constructed without stirrups, but the simplified MCFT design method is both safe and accurate. The simplified method also can accurately predict the effects of decreasing the maximum aggregate size on the beam-shear behavior of lightly reinforced concrete members.

One-Way Shear Strength of Thick Slabs and Wide Beams

This paper discusses nine recent tests designed to investigate if current provisions for one-way shear in the ACI 318-05 Building Code are unconservative when applied to thick slabs or large, wide beams. The tests addressed the influence of member width and the presence of shrinkage and temperature reinforcement, which are two factors that may influence the shear capacity of slabs. Member width was observed to have no significant effect on the shear stress at failure for one-way slabs and for wide beams. The presence of shrinkage and temperature reinforcement also did not influence the one-way shear capacity. These findings indicate that ACI 318-05s provisions, which dictate different levels of useable shear capacity for slabs, wide beams and narrow beams, can result in inadequate levels of safety for both thick slabs and large wide beams. Because narrow design strips have been shown to behave in shear in a similar manner to wider members, the well-established size effect of decreasing shear stress at failure as the member depth increases also applies to wide beams and thick one-way slabs. The author recommends that ACI 318-05s basic expression for shear strength be reformulated to better predict the shear capacity of members regardless of depth or classification.

Simple model for predicting torsional strength of reinforced and prestressed concrete sections

A noniterative method for calculating the ultimate torsional strength and the corresponding deformations of reinforced and concentrically prestressed concrete sections is presented. This method is based on the truss model. It avoids the need for iterations by making simplifying assumptions about the thickness of the concrete diagonal, the softening of the concrete due to diagonal cracking, and the principal compressive strain at ultimate conditions. A simple check on the spalling of the concrete cover is implemented. The calculated torsional capacities of 86 beams are compared with the experimental results and very good agreement is obtained.

Influence of Axial Tension on the Shear Capacity of Reinforced Concrete Members

The modified compression field theory is used to predict the response of reinforced concrete members subjected to combined shear and axial tension. It is predicted that even members containing only longitudinal reinforcement is capable of preventing excessive widening of the cracks. Tests of 24 such specimens are used to verify these predictions. Based on the analytical study, the experimental program, and a review of previous experimental studies, it is concluded that the current ACI design procedures for members in combined shear and tension are excessively conservative.

Shear Strength of Large Concrete Members with FRP Reinforcement

Increasing interest in the use of fiber-reinforced polymer (FRP) reinforcement for reinforced concrete structures has made it clear that insufficient information about the shear performance of such members is currently available to practicing engineers. This paper summarizes the results of 11 large shear tests of reinforced concrete beams with glass FRP (GFRP) longitudinal reinforcement and with or without GFRP stirrups. Test variables were the member depth, the member flexural reinforcement ratio, and the amount of shear reinforcement provided. Results showed that the equations of the Canadian CSA shear provisions provide conservative estimates of the shear strength of FRP-reinforced members. Recommendations are given along with a worked example on how to apply these provisions including to members with FRP stirrups. It was found that members with multiple layers of longitudinal bars appear to perform better than those with a single layer of longitudinal reinforcing bars. Overall, it was concluded that t...

Strut-and-Tie Models for the Design of Pile Caps : An Experimental Study

The paper describes the test results from 6 large pile caps that failed in 2-way shear. ACI Building Code procedures for the shear design of pile caps are unable to predict the experimental results because the procedures neglect certain important parameters, such as the amount of longitudinal reinforcement, and overemphasize other parameters, such as the effective depth. Strut-and-tie models were found to describe more accurately the behavior of deep pile caps.