What is the significance of the 'P-delta effect' in nonlinear response history analysis and what structural characteristic makes a building more susceptible to it?

The 'P-delta effect' in nonlinear response history analysis (NRHA) refers to the secondary bending moments induced in a structure due to the interaction of axial loads (P) with lateral displacements (delta). These moments are additional to the moments caused by the lateral loads themselves and can significantly amplify the lateral displacements, potentially leading to instability and collapse. The significance of the P-delta effect is that it can reduce the structure's lateral stiffness and load-carrying capacity, especially under large deformations. In NRHA, it is crucial to account for the P-delta effect to accurately assess the structure's stability and predict its response to earthquake ground motions. A building's susceptibility to the P-delta effect is primarily determined by its slenderness, which is typically represented by the height-to-width ratio. Taller, more slender buildings are more susceptible to the P-delta effect because they experience larger lateral displacements under seismic loading. Additionally, buildings with low axial load capacity in their columns are also more vulnerable. If the axial loads are high relative to the column's capacity, even small lateral displacements can induce significant secondary moments, leading to instability. Therefore, the P-delta effect is particularly important to consider in the seismic design and retrofitting of tall, slender buildings and buildings with weak columns.