Conceptual assessment of seismic performance of nonstructural walls in conventional medium rise buildings according to the experiences of past earthquakes

Authors
Azadeh Noorifard
Fatemeh Mehdizadeh saradj
Mohsen Vafamehr
Abstract
Nowadays most of structural engineers consider masonry infill walls as non-structural elements and only their mass is calculated during structural analysis, on the other hand, architects determine the specifications of walls without considering any of their seismic performance. In other words, during the seismic design of conventional medium rise buildings, most focuses are on structural elements and seismic performance of walls is rarely considered. While masonry infill walls are non-structural elements have the most potential to facilitate the entire collapse of buildings and damage them even in mild or moderate earthquakes. Experiences of past earthquakes show infill walls may have positive or negative effects on building›s seismic response. In recent earthquakes, numerous buildings designed by engineers were severely damaged or
even collapsed as a result of anomalies in the basic structural system induced by non-structural masonry partitions. Whereas there were weak structures without any lateral force resistant elements constructed by non-specialist people which remained stable as a result of the contribution of masonry infill walls. Therefore the research process has been defined in a way to answer the followings three main questions:
1. Which faults in design and construction will cause damage to walls in earthquakes?
2. How can non-structural walls lead to the collapse of seismic resistant buildings in earthquakes?
3. What are the effects of non-structural walls in seismic resistance of buildings?
Since there are various definitions of walls, it should be noted that this study included non-structural walls such as infill walls and partitions and excluded shear walls and load bearing walls. The main purpose of this paper is to identify weaknesses of walls and also investigate the positive and negative effects of infill walls on seismic performance of buildings in a conceptual approach for architects. Numerous studies have been carried out about the effects of infill walls on structural behavior in earthquakes by researchers from structure and earthquakes engineering with an intensive approach, but there are a few researches with a comprehensive conceptual approach considering all efficient factors on the seismic performance of walls with perceptible approach to be employed by architects. So this paper is trying to study the damage of walls in past earthquakes and evaluate their positive and negative effects on seismic performance of buildings in three main levels to determine main causes of damage. The results can be used as awareness for architects about the consequences of their decisions in design process. Considering required measures in the design phase and providing more favorable context for the seismic performance of buildings can result in the better performance of structures.
The main approach of this research is analytical and the applied method is experimental. In fact, the observed damage in past earthquakes is similar to a performed experiment in laboratory with a real scale. In this method there is no limitation in terms of scale, material properties and quality of construction in comparison with computer modeling or laboratory methods. According to the extensive approach of this paper, this method can be very helpful in understanding all efficient factors in seismic performance of walls. The main source of data in this paper is based on the damage of buildings in 1990 Manjil-Rudbar, Iran, 2002 Changureh (Avaj), Iran, 2003 Bam, Iran, 1968 Tokachi-oki, Japan, 1985 Mexico City, 1925 Santa
Barbara, California, 1971 San Fernando, California, 1994 Northridge, California, 1995 Kobe, Japan, 1998 Adana-Ceyhan, Turkey, 1999 Kocaeli (Izmit), Turkey, 2001 Arequipa, Peru, 2002 Molise, Italy, 2007 Sumatra, Indonesia, 2008 Wenchuan, Chian, 2009 Abruzzo, Italy, 2011 Tohoku, Japan, 2011 Van, Turkey, 2011 Lyttelton, New Zealand earthquakes.
Based on the experiences of past earthquakes, the seismic behavior of walls can be presented in a unique graph by three levels and analyzing each of them in both positive and negative aspects. In the first level of poor performance, only wall is damaged, which is known as in-plane failure that can happen for both separate and infill walls. The weak components and connections, configuration and the size of openingsare the two main efficient factors for the occurrence of this kind of failures. In the second level, wall is damaged and there would be possibility of damage to other non-structural elements and humans› injury too. These kinds of failure are known as out-of-plane failure and can be occurred in separate and infill walls.
The three factors of weak components and connections, non-proper aspect ratio and weak connections to structural elements are effective on this level of damage. In the third level, infill walls cause damage to the structure of buildings. In this level, wall failure especially in-plane failures may occur first and then followed by structural failure. In other cases wall could remain stable but due to its form, material, connection and position causes the structural damage. Following the structural damage, the damage to non-structural elements such as walls and also human causalities are expected.
There could be imagined, three levels for satisfactory wall performance. In the first level, walls remain stable against in-plane forces. In the second level, walls also remain stable against out-of-plane forces and do not cause damage to other elements. In the third level, walls which remain stable in both two previous levels and have appropriate material, adequate connections between components, proper aspect ratio, durable connections to the structure, proper position of the openings and their arrangement in plan and elevations are desirable, can help in strengthening non-seismic resistant buildings or can provide extra potential for seismic resistant buildings.
As a final conclusion it is necessary to notice the characteristics of walls in order to achieve an ultimate seismic resistant building, besides the seismic design of structural system. A little care to the seismic performance of walls in various stages of architectural and structural design can prevent the adverse effects of walls during earthquakes and exploit their favorable performance. By this method, structural costs can be reduced and also can provide extra potential for seismic resistant of buildings with low quality construction without any increase in cost.

Keywords

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