Plastic Hinge and Plastic Zone Seismic Analysis of Frames

Abstract

The most common approach to perform seismic design of structures is the traditional one based on a linear elastic analysis assumption. On the other hand, nonlinear analysis methods due to their higher complexity and computational cost are mainly applied as a verification tool for the assessment of existing structures. The use of nonlinear analysis methods provides a robust and reliable framework for seismic design since they allow the adoption of more elaborate design criteria, based on less simplifying assumptions. For this reason the application of nonlinear analysis approaches for the seismic design of structures as discussed in detail in recent guidelines, e.g., ASCE (2007), is gaining ground among practicing engineers. This development is also attributed to the rapid increase of computing power which makes the implementation of such advance approaches more attractive for real-world applications. Regarding finite element (FE) modeling assumptions, building structures are primarily modeled with one-dimensional beam–column finite elements (e.g., beams or rods). These models are used to represent the linear skeleton of such structures, while more detailed twoand/or three-dimensional FE models such as the ones proposed by Spiliopoulos and Lykidis 2006 are only rarely utilized. The FE structural model should also be able to adequately capture the response of other structural components that may considerably affect the overall capacity, e.g., infill walls, shear walls, and other nonstructural components. A discussion on these modeling issues can be found in the NIST 2010 document. In addition, soil–foundation–structure interaction may also play a significant role on the overall structural response. A detailed discussion on the effect of soil models of different complexity on the structural response can be found in Assimaki et al. 2012. Considering the structural nonlinear response, there are two major sources of nonlinearity: material and geometric nonlinearity. Material nonlinearity is considered the primary source of damage for lowand medium-rise building structures, while geometrical nonlinearities should be taken into account in highrise buildings with small aspect ratios that suffer from large horizontal deflections that introduce P-D effects. For the nonlinear material response, the FE simulation with beam–column members falls into two categories: concentrated (or lumped) and distributed plasticity approach. In concentrated plasticity, also called briefly as plastic hinge approach, the plastic deformations are “lumped” at the ends of a linear elastic element and are based on the moment–rotation relationships of the end sections for a given axial force. In distributed plasticity, beam–column elements allow for the formation of plastic zones along the