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Author: Melvyn Green
More information on constructing simple, seismically safe buildings could go a long way toward reducing fatalities.
Studies of recent earthquakes have confirmed that loss of life occurs principally in single-family dwellings of unreinforced masonry, usually constructed by owners or local masons with site-found materials, such as ashlar or rubble stone, earth (adobe), or manufactured masonry block or brick. The 2005 earthquake in Pakistan resulted in more than 75,000 deaths from building collapses, primarily in rural dwellings and schools constructed mostly of stone and some manufactured masonry, some with a concrete bond beam and columns at corners, most with concrete roofs. The recent earthquake in Iran had similar results, but the structures there were constructed of earth rather than stone.
After disasters, many nations and organizations provide short-term and long-term relief. Donor organizations, such as the World Bank, which often provide or pay for replacement housing, want new structures to be earthquake-resistant both to ensure the safety of the occupants, and possibly to protect their investments. Plans for replacement buildings often call for concrete-masonry unit walls and concrete or wooden roofs. Although this kind of construction may be feasible in urban areas and towns and villages near roads, it may not be feasible in many other places. In the earthquake zone of Pakistan, for example, many villages are accessible only by trail. Thus construction materials for earthquake-resistant buildings would have to be carried long distances by hand or, at best, by mule. As a result, villagers often reconstruct or replace destroyed or damaged buildings using the same methods and materials that were used in the original construction.
One of the problems for people in remote, earthquake-prone areas is a lack of information about how to improve construction. In fact, most engineering research has been focused on the seismic rehabilitation of large, multistory structures rather than low-rise masonry and earthen buildings, and very little information is available on how to construct a simple building with built-in seismic safety.
Earthquake Response in the United States
Earthquakes in the United States have been followed by federally funded research to provide guidance to engineers on seismic strengthening. The loss of life in brick buildings in past earthquakes, particularly the 1971 Sylmar (Los Angeles area) earthquake, led to a research project funded by the National Science Foundation (NSF) carried out by a team with members from Agbabian Associates, S.B. Barnes and Associates, and Kariotis and Associates (ABK). Focused on unreinforced masonry buildings with wood-framed roofs and floors, the results of the project were reviewed by the professional community and later adopted into building codes. This earthquake-resistant construction, initially permitted by the city of Los Angeles as a “special procedure,” has since gained acceptance and is now included in the Uniform Code for Building Conservation and the International Existing Building Code. The “special procedure” has been the basis for strengthening several thousand brick buildings. Although these provisions have not brought buildings up to current building code standards, they have reduced the chances of death and injury in earthquakes.
The 1994 Northridge (Los Angeles area) earthquake caused significant damage to steel-moment-frame building connections. In the aftermath, the Federal Emergency Management Agency (FEMA) funded research by a joint team of the Applied Technology Council (ATC), California Universities for Research in Earthquake Engineering (CUREE), and the Structural Engineers Association of California (SEAOC) to test and evaluate steel-moment connections (see http://www.sacsteel.org/). This research led to a different detailing of steel joints for new buildings—several so-called FEMA connections and proprietary designs. Numerous other post-earthquake studies of concrete buildings have focused on beam-column joints and lightly reinforced buildings. In other parts of the world, however, much less has been done, especially for single-family dwellings and low-rise buildings.
Earthen buildings constructed of a mixture of sand and silt with clay as the binder are found on all continents and in all countries (Figure 1—see PDF version for figures). The most common types of earthen construction are adobe and rammed earth.
Adobe bricks are made in a mold and are usually 16 to 20 inches long and 8 inches or more wide, a size that can be lifted by one person. Adobe buildings are constructed in a running-bond pattern with a mortar of adobe mud between blocks.
In the rammed-earth construction method (Figure 2), earth is packed into forms in a manner similar to the placement of concrete. The side of a formed unit may be as much as 4 feet high by about 6 feet long, depending on the thickness of the wall. Joints between units are packed with mud.
Historically, a bond beam, usually of wood, was used in earthen buildings. In the seismic zones of California, a concrete bond beam, or collar, is constructed at the top of walls, usually at the roof line; in some buildings, a parapet may be constructed above the bond beam. In recent years, engineers in California have attached the bond beam to the wall with vertical connector rods. However, in many places around the world, the bond beam is not connected to the wall at all.
Entire villages around the world are constructed using these methods. Some research has been done on the seismic behavior of adobe construction in several countries, including Peru and the United States. The Getty Conservation Institute, through its Getty Seismic Adobe Program (GSAP), has supported testing of adobe construction and has published the results in several reports (Tolles et al., 2000).1
In mountainous areas, stone has been the traditional construction material for walls. Stone walls are erected as typical masonry lay-up with bond blocks between wythes (Figure 3). In some cases buildings are constructed with single-wythe or unbonded, multi-wythe construction. The roof is constructed of wood trusses with a metal covering. Some later buildings were constructed with concrete bond beams and concrete corner columns (Figure 4). Many also had concrete roofs. A significant number of stone buildings collapsed in the Pakistan earthquake. Inspections after the earthquake revealed that the majority of collapsed buildings were the unbonded, single-wythe construction. These buildings did not have direct connections between bond beams and the stone walls, which might have reduced the number of collapses.
Brick Masonry Buildings
Brick construction, which is widely used in many countries, also has been the cause of many deaths and injuries in earthquakes. Research in the United States has led to many improvements; India has also developed strengthening procedures.
In the United States, strengthening is based on the unreinforced wall acting as a vertical beam between the floor and the roof, or between floors in multistory structures. Connections to the roof and floors keep the walls in place.
In India, instead of a roof or floor diaphragm to brace the walls, the walls are allowed to span horizontally to perpendicular walls (Figure 5). The spacing between walls is limited and is within the traditional building wall-spacing, reflecting cultural preferences. In addition, seismic bands made of wire mesh plastered with a thin layer of concrete are placed at the roof, sill, and window lintel lines, and vertically at corners (Figure 6). This type of construction appears similar to the bond-beam approach, with additional ties at points where failures may occur. Interestingly, the placement of the seismic bands appears to be in line with research results on adobe buildings. It is not clear if this seismic-band type of construction is effective in all seismic zones, however.
Concrete Buildings with Masonry Infill
A common construction type used worldwide, especially for low-rise structures, is a concrete “frame” with unreinforced masonry infill. The “virtual diagonal strut” concept, in which the wall is regarded as a diagonal brace, is one way of evaluating such structures. Another is to consider the building a shear-wall structure. Out-of-plane loads require positive connections, usually epoxy-adhered bolts and metal connectors, between the wall and the bracing diaphragms.
It may not be possible to provide the levels of safety (life safety in the 475-year event and collapse prevention in the 2,500 year event) as envisioned in U.S. building codes for owner-built structures in other parts of the world. Nevertheless, all of the construction types outlined in this paper can be improved.
A number of studies and projects have been carried out over the years around the world, and many countries have assembled, or are assembling, building code provisions for different types of construction. However, these efforts have not been coordinated so that engineers and code authorities can make effective use of them. The Earthquake Engineering Research Institute online World Housing Encyclopedia2 may be a potential resource and repository for such information.
It has been suggested that a deterministic approach be taken in analyzing simple, low-rise buildings. This would involve reviewing how these buildings fail and determining the sequence of failure. For example, we are aware that connections between elements are critical, and research might be directed toward improving connections between bond beams and walls. Another study might determine if a corrugated metal roof could be mobilized to act as a diaphragm with simple connections.
Another study might focus on improving single-wythe masonry with connections or stiffening elements to make structures safer. We also need guidelines for improving connections and the performance of low-rise, concrete frame buildings with masonry infill.
Improving existing owner-built buildings constructed with site-found materials would improve building performance and reduce the number of deaths and amount of damage in earthquakes. Cooperative efforts among nations could provide information to building owners and builders. Activities could be conducted by regional groups working with world bodies such as the United Nations or with individual countries.
Tolles, E.L., E.E. Kimbro, F.A. Webster, and W.S. Ginell. 2000. Seismic Stabilization of Historic Adobe Structures. Final Report of the Getty Seismic Adobe Project. Los Angeles, Calif.: Getty Conservation Institute.
1 More information about the Getty Seismic Adobe Program is available online at: http://www.getty.edu/conservation/publications/newsletters/ 11_1/news1_1.html.
2 Available online at: http://www.world-housing.net/.