Assessment and design of RC frames incorporating effects of restraint to beam hysteresis elongation

In reinforced concrete (RC) frames, beam elongation under cyclic loading is typically restrained by columns, hence inducing axial compression that results in beam overstrength and elevated force demands on adjacent columns and joints. Despite the pronounced influence of these restraint effects, they are not explicitly addressed in current seismic design procedures due to the lack of reliable methods for estimating the beam axial restraint stiffness and the resulting compression. To address this, a thermal analogy approach to evaluate the restraint stiffness is proposed and validated against cyclic test results. A prediction model is also developed for determining the restraint-induced axial compression, with due account for the degradation caused by column yielding. Based on these developments, a seismic design methodology that incorporates the beam axial compression effects is established. The proposed methodology is applied to a multi-story RC frame and compared with detailed numerical results. It is shown that the proportion of restraint-induced force demands increases from the frame center toward the edges. Importantly, despite employing various capacity design amplification factors, design codes are found to grossly underestimate these effects by up to 50%, particularly for shear demands in exterior columns. Considerable beam overstrength are also shown to occur in code-designed frames, with interior beams exhibiting overstrength approaching 40%. In contrast, the methodology proposed in this study effectively mitigates the beam overstrength and captures the restraint-induced force demands on both the columns and joints. By dealing with the restraint effects through beam reinforcement optimization and column-joint strengthening, the proposed methodology enables the mobilization of the intended strong-column/weak-beam and strong-joint mechanisms, hence offering a rational and practical solution for significantly improving the seismic performance of RC frame structures.