Implicit framework for developing design recommendations on shear strength of RC beam-column joints

Abstract

Shear strength of reinforced concrete (RC) beam-column joint depends on several parameters including the strut width, which in turn is a function of the compression depth in associated beams and columns framing into the same joint. Compression depths accounts for the possible effect of axial force and moment acting at the beam- and column-ends at the time of failure. Therefore, joint shear strength appears to be a function of the member force resultants acting at the members framing into it in addition to the properties of the constitutive material. However, most seismic codes prefer to recommending a simple expression for the joint shear strength considering the contribution from all parameters. This paper aims to develop an implicit approach for the recommendation on joint shear strength, which is consistent with the design recommendations of the seismic standards and involves the following steps: i) Assume a column section with reinforcement and axial load ratio; ii) Estimate the column flexural design strength and compression depth; iii) Estimate beam capacity using strong-column-weak beam principle; iv) Estimate beam flexural design strength using appropriate overstrength model followed by the computation of compression depth; v) Estimate strut width and orientation using a strut model; vi) Compute joint shear strength without the contribution from shear stirrups; and vi) Repeat the entire process for a large number of column sections with varied range of parameters and identify the dominant contributors for making recommendation in the form ACI 318/IS 13920. Outcome is the plot of joint shear strength coefficient against the dominant contributors. Contribution from the shear stirrups is accounted for separately. Proposed implicit framework applies to all seismic standards with a little change contingent on the respective design philosophies. Proposed framework is validated against the experimental results reported in the prior art for a set of 69 specimens.