Figure 11-1: Interaction diagram fora reinforced concrete column

153 Fora condition where bending moment is increased and axial load decreased, as represented by point C, part of the cross section is subjected to tension, which is taken by steel reinforcement if, for simplicity, it is assumed that the concrete is a material that cannot resist tension. This is a stage when sufficient tension is not developed to cause yielding of the steel, and the failure is still by crushing in the concrete. Proceeding to the state represented by point D, the failure combination of axial load and bending moment is such that the ultimate strain cu in the concrete and tensile yield strain yin the steel are simultaneously reached. This stage is known as the balanced condition, and Mb and Pb are the moment and axial load resistances of the section at the balanced condition. At any failure combination between points A and Don the curve, failure is caused by crushing in the concrete before the steel yields. Tensile yielding in the steel can occur with a lesser bending moment than that at the balanced condition if the compression is removed by decreasing the axial load. This stage is represented by the lower portion,
DF, of the curve. Since the axial load is less, the steel yields before the ultimate concrete strain, cu is reached. With further bending, the concrete compressive strain reaches cu and failure occurs. At point F, the section is subjected to bending moment only (Mo, and failure occurs well after the steel yields. Because the resistance of across section with given properties of steel and concrete depends upon the percentage of reinforcement and the position of the steel with respect to the centroidal axis, a set of interaction diagrams needs to be drawn for each drilled shaft cross section that is analyzed. The nominal resistance interaction diagram, shown as the solid line in Figure 11-1 and Figure 11-2, should be obtained for all critical sections of the drilled shaft. Computer programs for lateral analyses typically include options for generating this interaction diagram for specified cross-sections. The factored resistance interaction diagram, illustrated as a dashed line in Figure 11-2, identifies the boundary in which factored force effects should reside. The method to determine the boundary is described herein. The factored resistance interaction diagram (shown as the dashed line in Figure 11-2) is determined by multiplying the nominal moment and nominal axial resistances by the resistance factor φ (AASHTO
2014).
𝑃𝑃
𝑐𝑐
= 𝜑𝜑𝑃𝑃
𝑚𝑚
𝑀𝑀
𝑐𝑐
= Equation 11-10) Equation 11-11) Where Pr Factored axial resistance.
P
n
= Nominal axial resistance. Mr Factored moment resistance. Mn Nominal moment resistance.
φ = Resistance factor (see below.

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