Revisiting Earthquake Lessons - Unreinforced Masonry Buildings
Tuesday, January 28, 2020
Author: Fred Turner
Building Types: Unreinforced Brick and Stone Masonry, Load-bearing walls, Non-load-bearing walls, Adobe
Earthquakes: 1868 Hayward, 1872 Owens Valley, 1906 San Francisco, 1925 Santa Barbara, 1933 Long Beach, 1945 El Centro, 1971 Sylmar, 1989 Loma Prieta, 1994 Northridge, 2003 San Simeon, 2010/11 Christchurch, 2014 South Napa
Lessons: Unreinforced masonry (URM) buildings can pose significant life loss, social, and economic risks in California even though fewer than one in 600 buildings have this wall system and most have been retrofit or partially retrofitted. Due to California’s comparatively short history, rapid population growth since World War II, and the predominance of wood frame construction, the state has far fewer URM buildings than most other regions of the world. Risks concentrate in commercial areas of older cities. URM buildings without retrofits are typically far more vulnerable to life-threatening damage than other building types. Common retrofit approaches have varied in their effectiveness. Some retrofits have performed poorly particularly in aftershocks affecting previously damaged buildings. Due to high retrofit costs and often limited economic viability, performance objectives for retrofits have traditionally been set much lower than for other building types. With moderate to severe ground motions, both unretrofitted and retrofitted buildings can shed masonry off the tops of walls due to the lack of positive connections, low mortar cohesion, reliance on friction, and low overburden. Walls lacking positive connections to roofs and floors can separate and fall. Risks to life are highest around the perimeters of buildings, with masonry typically falling along sidewalks, streets, alleys, and through the roofs of lower, adjacent buildings. Through-bolts with bearing plates have typically been more effective in resisting forces perpendicular to walls compared to screen anchors. Retrofits have lower initial costs but are generally less cost-effective than replacements. Retrofits remain the best option for historical buildings that can’t be replaced.
Adobe: Unreinforced masonry has been in use for thousands of years. In California, the first URM buildings, several hundred of which still exist, were made of sun-baked clay or adobe, introduced by missionaries and other settlers in the late 1700’s. Many of these have been previously damaged in earthquakes, repaired and retrofitted. The Getty Seismic Adobe Project has publications and proceedings that offer insights into the seismic evaluation, retrofit, and repair of these buildings (Getty, 2020).
Kiln Emergence and Decline: Before brick kilns were established in the state, primary sources of masonry were adobe, riverbeds, stone quarries, and ships’ ballasts, the latter mainly for firebricks. The first buildings with entirely brick walls in California were constructed in 1831 at Fort Ross, and in 1847 in Monterey, San Diego, and San Francisco. Most early brick kilns were small, site-built, and independent operations. By 1854, Sacramento had 500 brick buildings. Older bricks may have been inconsistently fired by small manufacturers resulting in variable strengths and durability. However, by the 1880’s, there were 50 large brick manufacturers in the state producing 120 million bricks annually, and, by 1900, most small, independent manufacturers were forced out of business by large manufacturers. After the 1933 Long Beach Earthquake, brick kilns faced a significant decline in large part because the state’s Riley Act effectively prohibited new bearing wall unreinforced masonry construction. Masonry continues to be produced and used as a nonstructural material (Calbricks, 2020).
Conflagrations: Major fires in New York and Boston (1835), Denver (1863), Chicago (1871), Boston (1872), Seattle (1889), Baltimore (1904), and San Francisco (1906) prompted the building industry to shift to fire-resistive construction, resulting in the rapid growth of brick use for structural systems in California. Throughout the mid- and late-1800’s, California cities boasted about their first brick buildings since they were in great demand after fires reduced many wooden buildings to ashes. Masonry was the preferred alternative for fire-resistive construction even in regions that experienced damaging earthquakes, despite widely available evidence of inadequate seismic resistance.
Pre-1933 California Earthquakes: Many if not most casualties in past California earthquakes have been caused by the partial collapse of URM buildings. The Mw 7.9 Fort Tejon Earthquake in 1857 damaged adobe buildings from Santa Cruz to Ventura killing one (Wikipedia, 2020). The 1868 Mw 6.5 Hayward Earthquake killed 30 and caused many collapses in the Bay Area with damage ranging from Santa Rosa in the north to Gilroy and Santa Cruz in the south (Geschwind, 2001). In 1872, the Mw 8.0 Owens Valley earthquake killed 27, destroying the city of Lone Pine (Wikipedia, 2020).
By the 1870’s, anchors intended to connect walls to roofs and iron bars called bond iron in mortared bed joints began to be installed in higher-class URM buildings in the Bay Area. However, bond iron had not been observed to significantly improve seismic resistance, particularly in the 1906 earthquake and so it is not common in 20th century URM buildings (Tobriner, 2006).
The 1906 Mw 7.8 San Francisco earthquake and fire killed on the order of 3,000 people (Hansen, 1989). Class A buildings with steel frames and non-bearing URM walls and Class B load bearing URM buildings with internal steel frames generally performed well. However, the more prevalent Class C load bearing URM buildings with wood floors and roofs experienced numerous partial collapses of parapets and upper walls. While complete collapses were rare, scores of victims were killed by falling masonry around building perimeters. Collapses were observed from Ferndale in the north to Paso Robles in the south. Those remaining inside URM buildings during the shaking generally survived. The conflagration in San Francisco following the earthquake caused far more damage and casualties than collapsing buildings. In the decades following this disaster, the building industry’s focus was on reducing fire risks rather than shaking risks. Many damaged or destroyed URM buildings in Northern California were rebuilt with rubble masonry units. Some higher class URM buildings were designed after 1906 to resist higher wind forces with an intent to provide enhanced earthquake resistance. However, most smaller Class C URM buildings were not explicitly design to resist seismic or wind forces (Tobriner, 2006).
In 1925, the Mw 6.8 Santa Barbara Earthquake killed 13, and 36 blocks of its URM downtown were severely damaged (Sylvester & Mendes, 1987).
Government Anchors: In the 1920’s, construction rules were introduced to require thru-bolts and bearing plates at four to eight feet spacing to connect walls to roofs and reduce the chances that URM walls would fall onto firefighters during fires. Steel rods were pinned to roof joists by “dog-ties”, bent rod ends inserted into holes in joist faces and held by bent “keeper” nails. These so-called “government anchors” have provided inconsistent performance in subsequent earthquakes. However, some jurisdictions allowed pre-existing government anchors in lieu of new retrofit anchors at floors but generally not roofs. (Cities, 1995). Most “government anchors” and their load paths are not generally reliable or verifiable by tests for inclusion in modern retrofits.
Uniform Building Code (UBC): In 1927, the first edition of the UBC contained seismic provisions in its appendix for new construction, but it was not widely adopted. In the middle part of the 20th century, cities typically maintained unique seismic design requirements in local ordinances that were loosely similar to parts of the UBC and local variations in practice resulted in many inconsistencies. By the late 1970’s, most local governments were adopting various editions of the UBC with local amendments, but consistency in adoption dates and editions didn’t emerge until the 1980’s.
1933 Long Beach Earthquake: The Mw 6.3 earthquake caused numerous collapses of URM buildings, killed 115, and resulted in major changes to California’s laws and regulations (Turner, 2004). The poor performance of schools in particular and the resulting coroner’s investigation drew the public’s attention to shoddy construction and the lack of explicit earthquake designs, plan reviews and inspection requirements.
1933 Riley Act and Field Act: Prior to 1933, local governments were responsible for regulating construction of new buildings in their jurisdictions. The state’s Field Act took the extraordinary step of revoking local authority for public schools of grades K-14 and assigning that new regulatory authority to the State Architect. The Riley Act required all local governments to review designs, issue building permits, and inspect construction for other buildings (Turner 2004). Prior to that, only some of the larger cities had building departments. In the following years, all cities and counties eventually established building departments in California, although their regulations and enforcement were inconsistent and often several editions out of date compared to the latest editions of the UBC. Some jurisdictions continued to allow URM construction well into the 1940’s. Non-school buildings were initially designed using base shears of three percent of the building weights, while schools were designed for ten percent of the building weights. These requirements were gradually increased in subsequent editions of the UBC and state regulations.
1939 Garrison Act: The focus of the Riley Act was on new school construction and there was not a clear consensus on how to effectively retrofit existing schools. But in 1939, the state began to retroactively require compliance with the Field Act for existing schools by passing the Garrison Act. Much experience was gathered in early efforts to retrofit URM schools regulated by the State Architect. Typically, new reinforced concrete pilasters were placed in vertical chases cut into existing URM walls at great cost. Many pre-Field Act URM public schools were demolished and replaced, a typically more cost-effective alternative than retrofits.
Post-Long Beach Earthquakes: The 1940 El Centro Earthquake provided the first near-fault ground motion recording. Prior to that, engineers were assuming that earthquakes generated far lower seismic forces. The 1952 Mw 7.3 Kern County Earthquake caused 12 deaths (Wikipedia, 2020). The 1983 Mw 6.5 Coalinga Earthquake, destroying much of its URM downtown, the 1985 Mexico City Earthquake, and the 1987 Mw 5.3 and 6.0 Whittier Narrows earthquakes refocused state and local policymakers’ attention on addressing risks posed by URM buildings.
Parapet Ordinances: Los Angeles (1949), Long Beach (1959), San Francisco (1975) enacted parapet retrofit ordinances. Work associated with these ordinances was completed in the first two cities in the 1960’s.
Other Pioneering Cities: URM ordinances were enacted in Santa Rosa and Sebastopol shortly after the Mw 5.6 and 5.7 earthquakes in 1969. Ordinances required retrofits to meet lateral force levels in the 1955 UBC. The Long Beach URM ordinance was enacted shortly after the 1971 Mw 6.5 San Fernando earthquake. These early ordinances resulted in substantially higher demolition rates than later ordinances, in large part because the retrofit requirements were based on requirements for new construction had much higher retrofit costs than post-1980 ordinances. Santa Monica (1978), Gardena and Huntington Beach (1979), Santa Ana (1980), and the City of Los Angeles (1981) ordinances followed despite political opposition and after much deliberation. SEAOSC’s initial recommendations for the Los Angeles ordinance were initially rejected by policymakers. The City of Los Angeles then asked SEAOSC to lower its recommendations to reduce retrofit costs. The great majority of other jurisdictions followed Los Angeles’ lead and adopted ordinances based on LA’s Division 88, or, in later years, the Uniform Code for Building Conservation (UCBC 1985 – 1997) or the International Existing Building Code (IEBC 2006 – present) Appendix Chapter A1.
California State Laws: In 1979, a law encouraged local governments to enact URM retrofit ordinances that allowed less strength and stiffness than for new construction (19160–65 H&S Code). The law also provided a 15-year grace period to prevent local governments from requiring additional retrofits within that timeframe (19166 H&S Code). A more comprehensive URM Law in 1986 (Ca.Gov 1986) required local governments in regions of high seismicity (old Zone 4) to inventory URM buildings, to establish a risk reduction program that, at minimum, notified owners, and to report information on retrofit progress to the state’s Seismic Safety Commission. Reporting continued until 2006 when budget cuts eliminated the program. The state provides a property tax exclusion for URM seismic retrofits (RTC 2012). It also requires unretrofitted URM buildings to have warning placards at each entrance, but this law is largely unenforced. A 1992 law encourages disclosure of URM vulnerabilities when commercial buildings are sold.
Research for URM Retrofit Provisions: Because of high retrofit costs and questionable practices by engineers attempting to apply requirements for new construction to retrofits of existing URM buildings, ABK, a joint venture in Southern California, applied for a National Science Foundation grant to conduct component tests and develop alternative retrofit provisions (ABK,1984). While this research was not completed, several technical reports were published that provided the basis for the City of Los Angeles’ Rules for General Application (1987) and, later, the Special Procedure in IEBC Appendix Chapter A1, as well as ASCE 31 Seismic Evaluation of Existing Buildings and ASCE 41 Seismic Evaluation and Retrofit of Existing Buildings described below. The Special Procedure was an early attempt to account for rigid wall/flexible diaphragm response and the damping effects of interior wood frame walls acting as so-called crosswalls. It provides generally less onerous criteria than the General Procedure that distributes seismic forces based on ASCE 7. The Special Procedure was intended to limit diaphragm deflection to 6 inches. Other scant relevant research is summarized in ASCE 41’s Commentary.
Uniform Code for Building Conservation: In 1985, the UCBC was published, and later the 1992 edition reflected the first statewide SEAOC consensus for URM bearing wall retrofit provisions. It reflects the provisions in the City of Los Angeles’ Division 88 with modifications based on observations of performance in the Whittier and Loma Prieta earthquakes. Provisions are somewhat consistent with California’s Historical Building Code’s requirement to meet 75% of the force levels of the code for new construction, however, the Special Procedure doesn’t require R values that are consistent with ASCE 7.
After the UCBC was no longer published in the late 1990’s the Guidelines for the Seismic Retrofit of Existing Buildings (2003) briefly replaced it until the International Existing Building Code (IEBC) was first adopted for use in California in 2007. Retrofit provisions for URM buildings are contained in IEBC Appendix Chapter A1. However, enforcement of the chapter is discretionary and only triggered by major alternations unless it is accompanied by adopted triggers that require mandatory retrofits.
Provisions in the IEBC are primarily force-based, with incomplete and inconsistent consideration of deformation compatibility and certain load paths. Many aspects of conventional structural design are not required by the provisions in these model codes including chords for diaphragms, continuous ties and subdiaphragm criteria for wall anchorage. As a result, some retrofit designs have not included adequate load paths to fully develop anchorage into diaphragms. They warrant re-evaluation and additional retrofits.
A few local governments in California have enacted ordinances with requirements significantly lower than those in Appendix Chapter A1, notably the “Bolts Plus” program in San Francisco. However, SEAOC does not recommend retrofits lower than specified in IEBC Appendix Chapter A1 or in Chapter 16 of ASCE 41 (See IEBC Commentary 2015).
Performance of Retrofitted URM Buildings in Past Earthquakes: Performance of retrofitted URM buildings has been mixed in past earthquakes. Notably, performance has been markedly worse in aftershocks after initial damage due to degrading strength and stiffness. Life-threatening damage and deaths (five in Loma Prieta, three in Christchurch) have occurred in previously retrofitted URM buildings. In general, retrofits have been effective for one event with a short duration of moderate ground motion. (CERC 2012). Beyond that, anchors should be retested or replaced and sliding or rocking mortar joints should be removed and repointed. Partial retrofits have at times not performed markedly better than nearby unretrofitted buildings (ATC 31, 1992). Through-bolts with bearing plates typically perform better than screen anchors for which effectiveness depends greatly on workmanship and rigorous inspections that were often lacking during construction. The SEAOC consensus is that Appendix Chapter A1 retrofits significantly reduce but do not eliminate risks to life and may not reliably meet prescribed performance objectives, particularly since the criteria were developed long before performance-based provisions (IEBC Commentary 2015). Systematic performance observations were conducted after the 1987 Whittier, 1989 Loma Prieta, 1994 Northridge earthquakes and the 2011 Christchurch Sequence. Anecdotal retrofit performance has also been mixed resulting in considerable life-threatening masonry in the 2003 San Simeon, 2010 El Mayor Cucapah, and 2014 South Napa earthquakes and several other earthquakes since the 1980’s. Generally retrofitted, undamaged URM buildings with complete load paths and without low mortar strengths have significantly reduced risks to life, but have not ensured repairability in past damaging earthquakes.
Alterations of Retrofitted URM Buildings: Engineers should be aware that some retrofits rely on nonstructural partitions as so-called crosswalls to dissipate energy and reduce the response of flexible diaphragms. However, over the past several decades, some owners and occupants may have inadvertently altered or removed partitions thus compromising the retrofits. Engineers should discourage nonstructural alterations without first considering the potential adverse impacts on overall performance.
Performance-Based Engineering Approaches for URM Buildings: ASCE 41 and its predecessor standard ASCE 31, contain evaluation and retrofit provisions for URM buildings. Chapter 16 of ASCE 41 contains simplified retrofit provisions similar to the IEBC Appendix Chapter A1 Special Procedure. ASCE 41’s Commentary suggests that such retrofits will meet a Collapse Prevention performance level in a BSE-1E ground motion with a 20% in 50-year recurrence. However, this is largely based on judgment. The ATC 140 Project, currently underway, is expected to include an attempt to analytically test this assumption.
URM buildings in California typically have weak mortar compared to stronger masonry units. Mortars with large proportions of lime compared to cement as well as mortars made with salty water or unclean sand with salt contents tend to be the weakest. For such buildings, mortar test values vto can fall below 30 psi warranting pointing and retesting to achieve minimally acceptable mortar strengths and higher cohesion. Typical failure modes in walls with high unit strengths and comparatively low mortar strengths are bed joint sliding, rocking in piers with high height-to-length ratios and low axial stress, and stair-step x cracking in diagonal tension. Less common failure modes are toe crushing and diagonal cracking through units.
ASCE 41’s Chapters 7 and 11 provisions can facilitate more consistent considerations of deformation-based performance and deformation compatibility, but the provisions have rarely been applied to URM retrofits. Very few bearing or non-bearing URM buildings have been evaluated and retrofitted using nonlinear procedures. ASCE 41’s Linear Procedures in Chapter 11 have been found to be quite conservative compared to Chapter 16 for bearing walls. Engineers in Europe and New Zealand have considerably more experience in attempting to apply nonlinear procedures to this building type.
Summary: California will continue to experience life loss, injuries and property loss from unreinforced masonry buildings for many decades to come. Structural Engineers should make efforts to warn owners, occupants and stakeholders in property adjacent to URM buildings about their risks. It may be appropriate to re-evaluate previously retrofitted URM buildings, especially with screen anchors, which should be re-evaluated, retested and, if found deficient, supplemented or replaced. Communities with interests in improving disaster resilience should target URM buildings for replacements or re-retrofits. However, most traditional retrofit approaches with reasonable costs will not be capable of reliably meeting post-earthquake re-occupancy or repair expectations that are commonly expressed by the public. Higher-cost retrofits that rely on seismic isolation and/or energy dissipation have been rarely used but may pose viable alternatives that more effectively protect historical buildings, improve older retrofits, and address buildings without retrofits.
ABK. Methodology for Mitigation of Seismic Hazards in Existing URM Buildings, Agbabian Barnes Kariotis Joint Venture, 1984.
ATC 31. Evaluation of the Performance of Seismically Retrofitted Buildings, Applied Technology Council, Onder Kustu, 1992.
Calbricks. California Bricks, History of Brickmaking in California, 2020, calbricks.netfirms.com/brickhistory.html
CERC. Volume 4, Earthquake Prone Buildings, Canterbury Earthquakes Royal Commission, New Zealand, 2012.
Cities. Reports to the Seismic Safety Commission on the Cities’ Status of Implementation of the URM Law, files in SSC office, 1995.
Geschwind, Carl-Henry. California Earthquakes: Science, Risk, & the Politics of Hazard Mitigation, 2001.
Getty Museum. Getty Seismic Adobe Project, Seismic Stabilization of Historic Structures, 1990-1996, www.getty.edu/conservation/our_projects/science/seismic/index.html 2020.
Ca.Gov. The Unreinforced Masonry Building Law, State of California Statutes, Government Code Section 8875, 1986.
Hansen, Gladys & Condon, Emmet. Denial of Disaster: The Untold Story and Photographs of the San Francisco Earthquake and Fire of 1906, Cameron Books, 1989.
H&S Code. Earthquake Hazardous Building Reconstruction, Sections 19160 to 19166, State of California Health and Safety Code, Chapter 510, Statutes of 1979.
IEBC Commentary. Appendix A: Guidelines for the Seismic Retrofit of Existing Buildings Chapter A1, Seismic Strengthening Provisions for Unreinforced Masonry Bearing Wall Buildings, International Code Council 2015.
RTC. Exclusions from Property Taxes, Seismic Retrofitting Improvements, State of California Revenue and Tax Code Section 74.5, last amended 2012.
Sylvester, Arthur G. & Mendes, Stanley H. Field Guide to the Earthquake History of Santa Barbara, SSA Annual Meeting Field Trip Notes, 1987.
Tobriner, Steve. “An EERI Reconnaissance Report: Damage to San Francisco in the 1906 Earthquake = A Centennial Retrospective,” EERI Spectra, Special Issue II, Volume 22, April 2006.
Turner, Fred. “Seventy Years of the Riley Act and its Effect on California’s Building Stock,” 13th World Conference on Earthquake Engineering, Vancouver, CA, August 2004.
Wikipedia. “1857 Fort Tejon Earthquake,” “1872 Owens Valley Earthquake,” “1952 Kern County Earthquake,” 2020.