Influence of nonlinearities in longitudinal bars on overstrength P–M capacity of RC columns and piers

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AbstractThe actual capacity of a reinforced concrete (RC) structure is determined by the overstrength capacity of individual members. RC members can undergo six stages (namely cracking, yielding, spalling, buckling, crushing and rupturing) depending on the cross‐section geometry, reinforcement detailing, and type of loading. For members under compression and flexural actions, buckling of longitudinal bars becomes the ultimate stage in the load‐deformation curve of RC member. Hence, to assess the capacity of RC members, it is essential to have load‐deformation relation of individual bars considering nonlinear effects. This paper presents a simplified analytical procedure of nonlinear analysis to obtain the nonlinear load‐deformation curve of reinforcing bars. The proposed method uses combined geometric and material nonlinearity. The method is compared with the experimental test results from the literature and has shown a good match. Furthermore, the effect of initial imperfection and slenderness of the reinforcing bars on the overall performances have been studied and discussed. This inelastic relation of reinforcements has been used to obtain the P–M interaction diagram of the RC member, demonstrating the effect of nonlinearity in the longitudinal bars. Fiber modeling approach has been used to draw the P–M curve, considering confinement of concrete, bilinear stress–strain of steel under tension and nonlinear under compression. Implications of amount of longitudinal reinforcement, grade of steel, initial imperfection and section size on the P–M curve have been studied. Results show that the nonlinearity of the longitudinal bar can reduce the capacity of the member by 15%. The strength reduction is higher for higher percentage of reinforcements and slender bars. This indicates that the P–M interaction curve exists at three levels, and is discussed in detail in the paper. It is recommended that the effect of bar nonlinearity should be considered in the capacity assessment of RC members.