Impact of torsion on the seismic resistance of common buildings with rigid horizontal diaphragms. Application to wooden structures
The earthquakes are natural disasters affecting the earth's crust, which can have major destructive effects in urban areas. We have a set of scientific, technical and conception knowledge which allow to « build earthquake-resistant », but these methods must be adapted to simple buildings. The development of earthquake-resistant regulations (Eurocode 8 and national annexes) and seismic zoning of France evolve significantly to the need for taking into account the seismic action in the building design. In seismic design, it is essential to take into account the torsion effects that can lead to serious consequences in terms of damage to civil engineering structures. This research work presents a process including different levels of approach to take into account the effect of the torsion. The structures particularly targeted by this work are the so-called soft frames and more specifically the timber buildings. After a course of bibliographic elements related to the timber structures and the structures of current buildings subjected to seismic situations, a parametric study is taken with a method for multi-dimensional combination in order to analyze the influence of different bracing configurations on the sensitivity of the structure to the torsion phenomena. This first stage is based on the adaptation for timber structures, in the context of Eurocodes (structural torsion and accidental torsion), of a New Zealand method developed by Paulay and Priestley originally proposed for reinforced concrete buildings. The study of the behavior of timber buildings in seismic situation is part of an approach using the equivalent linearization method by a behavior coefficient. So the seismic action can be modeled by equivalent static forces which are then transmitted to the vertical structural elements by horizontal diaphragms. The objective of this work is to define the distribution of seismic forces on each brace with the inclusion of torsion from cartography of bracing and masses implantation. This original method, taking into account the torsion effect, is implemented and all the results lead to the creation of a rich database, for a seismic situation, in the case of design capacity, with ductile bracing and linearized behavior through the behavior factors and rigid horizontal diaphragms. This database can be used to simplify the approach of the torsional effects of the current timber buildings. It can also be used as a reference for the analysis of the influence of the semi rigid diaphragms or the actual non-linearity of bracing. After implementation of this method, a numerical approach considering the behavior of generic structures is conducted to illustrate the effects of different levels of simplification inherent to the implemented original method. In this step, the calculations use the finite element method (software Cast3M). The dynamic analysis is made on the basis of linear or linearized behavior to evaluate the effects of different calculation methods proposed by Eurocode 8, Part 1. The calculations done in this phase allow mainly to validate the applicability of simplified methods for timber structures, and to predict the torsional effects on these structures with different degrees of precision.