Multi-story buildings with a ground floor consisting only of columns. Most of these buildings were 3 to 4 stories tall with a parking garage or a living area with many large windows on the ground level. The columns may have been strong enough to hold up the structure, but did not provide an adequate amount of racking resistance during a seismic event. When the earthquake shook the building side-to-side, the upper stories sometimes tipped over to one side. Whether built of wood, steel, or concrete—they all suffered damage.
Wood-frame houses with weak connections between the walls and foundation. Wood-framed buildings are inherently ductile (flexible), which is an attribute during an earthquake. However, the shaking sent some of these houses sliding to one side. Frequently, the shear walls were strong enough, but the connection to the foundation was a weak point that gave way.
Un-reinforced masonry or concrete buildings. Masonry or concrete walls not reinforced with steel bars were not ductile enough to be effective shear walls. And if there is no steel connecting them to their foundation, the joint between walls and foundation can be a weak point.
WHY REINFORCED CONCRETE IS SAFEReinforced concrete walls are a composite system: Concrete resists compression forces, and reinforcing steel resists tensile forces produced by an earthquake. The concrete is cast around the bars, locking them into place. The exceptional ductility of the steel to resist tensile forces, coupled with the rock-like ability of concrete to resist compression, results in an excellent combination of the three most important earthquake resistance properties: stiffness, strength, and ductility. A study at CTL Group revealed that even a lightly reinforced concrete shear wall has over six times the racking load resistance as framed wall construction.
It's no wonder that modern reinforced concrete buildings were found to survive these recent earthquakes with rarely any significant damage.
WHAT'S THE BOTTOM LINE?Studies have shown that properly designed reinforced concrete walls offer greater earthquake resistance than other types of residential construction. When building a house in areas of high seismic risk, always follow good design practice. Make sure the exterior walls are properly designed and constructed—relatively continuous, unbroken walls of stout construction that includes reinforcing steel. Install strong, durable connections of these walls to the foundation. Adhering to these principles will help to ensure concrete homes that are strong earthquake-resistant structures.
ADDITIONAL RELATED RESOURCESComerio, Housing Repair and Reconstruction After Loma Prieta. University of California, Berkeley.
Miranda, Aslani, and Blume, Brief Report on the September 3, 2000 Yountville/Napa, California Earthquake. John A. Blume Earthquake Engineering Center, Stanford University.
National Association of Home Builders Research Center, Assessment of Damage to Residential Buildings Caused by the North-ridge Earthquake. The United States Department of Housing and Urban Development, Office of Policy Development and Research, Washington, DC, July 20, 1994.
Mehrabi, In-Plane Lateral Load Resistance of Wall Panels in Residential Buildings (SN 2403), Construction Technology Laboratories, Inc., (CTL Group) 1999.
Portland Cement Association (PCA)
5420 Old Orchard Road, Skokie, Illinois 60077-1083
Phone: 847.966.6200 Fax: 847.966.9281
Web: www.cement.org
More information? Helpline
1.888.333.4840
www.concretehomes.com
