For both the LRFD and the STND Specifications, the following procedure is used to compute the required area of steel at three critical sections of each member (left/bottom end, midpoint, and right top end). At the ends of the member, the critical section for the following flexure calculations may be taken at the face of the intersecting members, within a haunch (LRFD C5.7.3.2.1 /C5.6.3.2 and STND 8.8.2), and at the toe of a 45 degree haunch (LRFD 12.11.4.2 and STND 16.7.4.5).
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3.) No compression steel is considered and all tension steel is assumed to be in 1 layer
4.) Assume As
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5.) Compute Mu (pure flexure)
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(note: under LRFD, phi is excluded in the above equation for Mu, as it is present in the P-M diagram)
6.) Compute Po Po (pure compression)
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(note: under LRFD, phi is excluded in the above equation for Po, as it is present in the P-M diagram)
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Standard Specification:
7.) Compute Pb Pb and Mb Mb (balanced condition)
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For slabs, phi = 0.85, for exterior walls, phi = 0.8, and for interior walls, phi = 0.7.
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7.) Compute Pc and Mc (limit of compression controlled section, and Pt and Mt (limit of tension controlled section). Here phi is variable, depending where on the P-M diagram you fall. For the compression controlled area, phi is 0.75. For the tension controlled area, phi is 0.9. For the transition zone, phi will vary between 0.75 and 0.9 (see LRFD Article 5.5.4.2.1/5.5.4.2).
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8.) Under LRFD, there are three parts to the P-M curve, that between pure compression and the compression controlled limit, that between pure flexure and the tension controlled limit, and a third line that transitions between the two limits. As with STND, a straight line is assumed between major points on the curve.
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