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A Study on Stress Block Parameters

Manish Kumar Roy


The compressive stress distribution in concrete, when mathematically modeled, can be defined by characteristic parameters known as stress block parameters. There are three such parameters, namely, shape factor, k1, depth factor, k2, and maximum stress factor, k3. There are various shapes of stress blocks, like parabolic, equivalent rectangular, equivalent triangular, and rectangular-parabolic. Various standard codes have adopted an equivalent rectangular stress block, whereas the Indian code has adopted rectangular-parabolic stress block. This paper studies the effect of the shape of stress block on stress block parameters and its impact on flexural and axial capacity of the section based on compressive strength of concrete. The four stress blocks considered for the present study include rectangular-parabolic stress block as per IS: 456–2000, rectangular stress block of ACI 318–14, parabolic stress block of ACI and parabolic stress block proposed by Pul, et al. The compressive strength considered for this study is in the range of 25 to 150 MPa. The results indicate that when a parabolic stress block is transformed into any other shape, the depth factor, and area factor are unchanged because the transformed shape is an equivalent stress block. The flexural capacity is greater as per the parabolic stress block of ACI and minimum for stress block of IS: 456–2000. However, when the axial capacity estimated using stress block of IS: 456–2000 and stress block of ACI 318–14 are compared, it is higher for IS: 456–2000 by about 13%.

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