Older Hazardous Concrete Buildings
Concrete is a popular building material in California. For the most part, it serves its purpose well. To perform well during earthquakes, however, reinforced concrete buildings and bridges need to be properly reinforced with steel. This lesson has been learned and relearned in past California earthquakes including the 1971 San Fernando earthquake, 1979 Imperial Valley earthquake, 1989 Loma Prieta earthquake, and 1994 Northridge earthquake. Many hazardous concrete buildings, including many government buildings, also have high and important occupancies, posing a significant life safety risk to California residents.
Despite longstanding awareness of the problem of some older existing concrete buildings, they continue to exist throughout the state. One reason has been the challenge of deciding whether a building is indeed at high risk: earthquake engineers need better evaluation tools to sort out the good buildings from the bad ones. Another reason is the high cost of building retrofitting — better tools are needed for cost-effective, non-intrusive retrofitting.
PEER has recognized the need and the challenge of mitigating the high seismic risk posed by some of our older existing building stock, and has devoted a significant portion of its research activity toward that problem. Some important accomplishments of PEER are summarized below.
Failure-Critical Concrete Columns
The failure of load-bearing columns is a leading cause of building collapse during earthquakes, and is a primary focus of earthquake engineers charged with the safety evaluation of an existing building. Existing guidelines for seismic rehabilitation (FEMA 273) provide limited and overly conservative recommendations for columns, leading to prohibitively high retrofit costs. More effective evaluation procedures are needed if California engineers are to accomplish the task of mitigating the collapse risk of these existing buildings.
PEER answered the challenge by channeling research funds to study collapse-critical concrete columns. This research has led to improved guidance on the earthquake resistance of older existing columns, including groundbreaking new work to help engineers assess the levels of shaking that would actually lead to total collapse of a column. The results of this work have been adopted by FEMA 356, which is the consensus document on seismic assessment and rehabilitation of existing hazardous buildings.
A popular form of concrete building construction uses a flat concrete slab (without beams) as the floor system. This system is very simple to construct, and is efficient in that it requires the minimum building height for a given number of stories. Unfortunately, earthquake experience has proved that this form of construction is vulnerable to failure in which the thin concrete slab fractures around the supporting columns and drops downward, leading potentially to a complete progressive collapse of a building as one floor cascades down onto the floors below.
PEER has studied the failure of flat-slab structures and has identified critical conditions that can lead to collapse. These studies have been applied in the case study of an existing reinforced concrete building to demonstrate how damage and failure can be assessed for existing buildings. The research also has led to improved understanding of how to better design new buildings, leading to the first-ever design recommendations for this system, which were adopted into the 2003 IBC and are currently under consideration by the ACI Building Code Committee.
A final area where better information is needed for concrete building performance is the connection between floor beams and columns. Beam-column connections make the building rigid against the side sway of the building under earthquake loading. This aspect of buildings was little studied during the heyday of concrete building construction in the 1960s, but since then earthquakes have shown how important these connections are to holding the building together.
PEER has directed two major projects at this important problem, one studying corner joints and another studying side joints. Both projects have led to better information on strength and deformability of these critical components. In addition, research was conducted to demonstrate the capabilities of advanced composite materials for economic and effective retrofit of these connections.
Putting It All Together...
Engineers responsible for assessment of existing buildings have been hampered by limitations of available tools. Popular computer programs for earthquake analysis of concrete buildings are based on tools developed in the 1970s and 1980s, restricting engineers to relatively simplified, approximate analyses. Engineering guidelines for building assessment had been written with these computer programs in mind, and were likewise relatively simple and approximate. For example, if one column in a building was calculated to have a crack that required repair, the whole building was assigned to the “needs repair” category — obviously, this approach grossly overestimates the damage that the building has sustained and the cost that will be required to fix it.
PEER has been developing a new software technology that takes advantage of the latest information technologies to break the barriers of existing engineering tools. Combined with PEER’s performance-based earthquake engineering methodology, PEER has been able to demonstrate the complete performance assessment of older hazardous concrete buildings. For example, in a case study conducted with practicing earthquake engineers, PEER has been able to calculate expected losses for a building as a function of earthquake shaking intensity. Although the technologies are not quite ready for complete implementation in engineering practice, they show great promise as the leading technology for the next generation of assessment procedures.