After the January 17, 1994, Northridge (Los Angeles) earthquake, The Masonry Society (TMS) sent a reconnaissance team to assess the performance of masonry structures. That team carried out basic visual evaluations of about 140 structures. The findings showed considerable progress since the San Fernando earthquake of 1971. Modern reinforced masonry, on the whole, performed quite well. Many older unreinforced masonry structures, as expected, were badly damaged. But older unreinforced masonry structures, retrofitted to comply with Los Angeles' Division 88 requirements, did much better. This article presents the highlights of a report issued by TMS describing the results of the reconnaissance team's investigation.
About the Northridge Earthquake
At 4:31 a.m., in the pre-dawn hours of Monday January 17, 1994, the San Fernando Valley (about 30 miles north-west of central Los Angeles) was shaken by its most devastating earthquake in 60 years. The so-called Northridge Earthquake had a magnitude of 6.7 and peak ground accelerations of about 1.0 g, as strong as the acceleration of gravity. According to news accounts, the earthquake was responsible for more than 50 deaths (of which 22 were attributed to earthquake-induced heart attacks) and at least 5,000 injuries. According to the City of Los Angeles, more than 10,000 buildings were red-tagged (prohibited entry) or yellow-tagged (restricted entry), and more than 25,000 dwelling units were vacated. Some areas flooded from broken water mains. Small areas were heavily damaged by fire. Damage estimates ranged from $15 to $30 billion.
The Masonry Industry's Response
The masonry industry was ready. In 1993, The Masonry Society (TMS), with support from the Council for Masonry Research (CMR), had set up an Investigating Disasters Program. Action plans were in place. Available masonry specialists had been identified, and had undergone training in post-disaster reconnaissance. Within 12 hours of the earthquake, the TMS reconnaissance team was selected, notified, and in Los Angeles beginning their investigation. Other segments of the masonry industry sent teams as well.
In four days, the TMS team carried out visual surveys of about 140 masonry structures, undamaged as well as damaged. Based on those surveys, the team published its report, with the following objectives:
- To identify masonry design and construction practices that give good earthquake performance.
- To propose changes to design and construction practices that would probably improve the performance of masonry in future earthquakes.
- To indicate areas where not enough was known, and where research may be needed.
Types of Masonry Structures
The greater Los Angeles area contains tens of thousands of masonry structures, of many different types including reinforced masonry and unreinforced masonry (URM) structures. This article presents a few typical examples to illustrate how different types of masonry structures withstood the Northridge Earthquake.
Modern Masonry Structures
As a whole, modern reinforced masonry structures, whether single-story (Figure 1) or multi-story (Figure 2), apparently performed well. They showed little or no structural damage and usually resumed normal functioning soon after the earthquake.
Unretrofitted URM Buildings
Because the City of Los Angeles' "Division 88" retrofitting ordinance had been in effect since 1981, few unretrofitted URM (unreinforced masonry) buildings were found in Los Angeles itself. Many, however, were found outside of the city. The overwhelming majority of unretrofitted URM buildings suffered significant damage, such as complete collapse, collapsed walls, fallen parapets, or severe diagonal cracking of piers. Such damage has also been observed in previous earthquakes. Examples of poor performance are noted here to emphasize the need for seismic retrofitting of URM buildings, and also to provide a yardstick for comparison with the performance of retrofitted URM buildings. A typical example of poor performance by an unretrofitted URM building is shown in Figure 3.
Retrofitted URM Structures.
Los Angeles' retrofitting ordinances, among them the so-called "Division 88" ordinance, require that all URM parapets be removed or braced, that mortar joints be tested and strengthened to meet minimum values for shear strength, and the unreinforced masonry be anchored to floor and roof diaphragms. Most URM buildings in Los Angeles had been retrofitted in this manner.
URM buildings provided with basis seismic retrofitting, like that required by Division 88, appeared to perform much better than unretrofitted buildings. They often did show damage, but this damage was not as severe as in unretrofitted URM buildings. Figure 4 shows a URM building, outside of Los Angeles, that had been partially retrofitted. Anchors had been used to attach the walls to the second floor; however, the wall was not anchored to the roof, and the parapet was not braced. The figure clearly shows how the building was severely damaged above the second floor, and how the damage stopped at the second floor, where retrofitting measures had been taken.
Masonry Cantilever Walls
Tremendous contrasts were observed between the performance of masonry highway noise barrier walls (engineered, reinforced and inspected), and masonry site walls (rarely engineered, generally improperly reinforced or grouted, and rarely inspected). Noise barrier walls behaved very well in the earthquake. Site walls, in contrast, frequently overturned or failed in other ways.
This article and the TMS earthquake reconnaissance report from which it was derived are based on the field observations, photographs, and written contributions of the following individuals: Daniel P. Abranms, Samy Adham, James E. Amrhein, John Chrysler, Robert H. Hatch, John C. Kariotis, Richard E. Klingner, Mark A. Pickett, Phillip J. Samblanet, Matthew J. Scolforo, Narendra Taly, John G. Tawresey, and Joseph E. Zlab.
The TMS Investigating Disasters Program was developed and is maintained by The Masonry Society. TMS acknowledges the financial contributions of the Council for Masonry Research (CMR) in helping to initiate the TMS Investigating Disasters Progrm, in supporting the TMS Northridge Earthquake Reconnaisance Team, and in helping to produce the TMS report.
Dr. Klingner was the leader of the TMS Northridge Earthquake Reconnaissance Team. This article presents the highlights of the TMS reconnaissance team's findings, as published in the following report: Performance of Masonry Structures in the Northridge, California Earthquake of January 17, 1994, Richard E. Klingner, technical editor, The Masonry Society, Boulder, Colorado, June 1994. (ISBN 0-9626074-7-9).
Copies of this report may be purchased from The Masonry Society, 2619 Spruce Street, Suite B, Boulder, Colorado 80302, telephone 303.939.9700, facsimile 303.444.3239. Our commendations to Dr. Klingner and other members of the TMS Northridge Earthquake Reconnaissance Team for documenting their observations in such a timely manner.