Building codes have existed in varying forms for many centuries. As early as 1750 B.C., the Babylonian Empire’s code of Hammurabi read, “If a builder has built a house for a man and his work is not strong, and if the house he has built falls in and kills the householder, that builder shall be slain.” During the twentieth century, building codes evolved to become the primary means of ensuring a minimum standard of earthquake design for new structures in California. Because seismic engineering has advanced significantly over the past fifty years, many buildings that were constructed and considered earthquake-resistant according to 1950s and ‘60s standards were soon determined to be deficient. Even though these existing structures are often considered to pose the greatest hazard in earthquakes, building codes in most cases do not apply to them, and responsibility is relegated to building owners like Stanford University. Mitchell Earth Sciences is one of several buildings constructed on the Stanford campus in the 1960s that the University subsequently upgraded to meet contemporary seismic standards.

Building Codes: Early Motivations and First Seismic Provisions

At the time of the 1906 San Francisco earthquake, many California municipalities had building codes, but none considered seismic effects. Not surprisingly, the 1906 earthquake sparked discussion of improving earthquake engineering design and incorporating those improvements in regulatory codes. Professional organizations, particularly the Seismological Society of America, which formed in 1906, and later, the Structural Engineers Association of California, were persistent advocates of code provisions for earthquake-resistant construction.

By requiring that structures be designed to withstand horizontal forces, revisions to the city of Santa Barbara’s building code in 1925 were the first explicit policy and legal consideration of the seismic safety of structures in California. Palo Alto, led by professors at Stanford, also added seismic provisions to its building code in 1926. The 1933 Riley Act required all California local governments to have a building department and inspect new construction, mandating that all structures in the state be designed to withstand a horizontal acceleration of 0.02 times the acceleration due to gravity. These requirements applied only to new structures, and California municipalities could add to the Riley Act requirements at their own discretion. Since the 1960s, California codes have become more uniform across local jurisdictions.

As codes have grown more and more complicated with respect to earthquake design, they have increasingly become the jurisdiction of structural engineers rather than local politicians. Although they are updated frequently, some of the most significant changes have occurred after major earthquakes identified or emphasized structural deficiencies. The 1971 San Fernando and 1994 Northridge earthquakes were two such landmarks in terms of building codes.

Evolving Codes: Ductility Example

Advances in structural dynamics by the late 1960s encouraged structural engineers to consider not only seismic forces, but also the movement or “ductility” a structure must undergo in an earthquake. Stanford-educated John Blume, and other engineers Newmark and Corning recognized this in their 1961 book on reinforced concrete structures: “The modern type building, without any appreciable lateral resistance except in the frame proper, will be subject to possibly large story distortions even in moderate earthquakes.” Buildings without ductility can exhibit brittle failures that rapidly degrade and can lead to collapse. Initially, steel provided the accepted material standard for ductility. Research by Blume and others demonstrated that good detailing—for example, the placement of steel reinforcement and connections—is of prime importance and could provide sufficient ductility in concrete structures. After the 1971 San Fernando earthquake, which damaged many reinforced concrete structures, these recommendations for providing ductility in concrete were adopted on a broader scale.

Mitchell Earth Sciences Retrofit

Prior to retrofit, the design of Mitchell Earth Sciences building, completed in 1970, was representative of good 1960s seismic construction. Since it was designed just before the widespread implementation of important new code recommendations, the structure was later found to be deficient in comparison to modern seismic design. The reinforced concrete beams and columns were sized appropriately to resist expected lateral earthquake forces, but the detailing did not fully recognize the need for ductility. For example, column splices, where reinforcing bars overlap, were located at the base of the column where earthquake forces are the largest. In addition, steel column ties, which wrap around and confine the column reinforcement and concrete, were not anchored as is done now, and could potentially pull out during an earthquake.

Figure 3: Consideration was given to strengthening the building without displacing the occupants or destroying the natural lighting and view from the large windows in the library. Image courtesy of Degenkolb Engineering.

Recognizing these deficiencies, Mitchell was retrofitted in 1997 (Figure 1). The structural engineers for the project recognized the design and construction challenge of working within the context of the existing building, avoiding significant alteration to the exterior of the building, and minimizing interruption to the academic work in that building. The engineers investigated several alternative schemes (Figure 2), and conducted nonlinear analysis of the building based on simulations of dozens of different earthquakes to demonstrate that the retrofit design was adequate. In the final plan, they were able to add a small number of exterior shear walls designed to provide an additional earthquake resisting system to the existing beams and columns (Figure 3). Explicit design for ductility in the structural solution reduced the cost of the project and allowed occupants of the building to continue work uninterrupted during construction (Figure 4). The innovative design was evaluated by peer review prior to construction.

Figure 4: The solution to the Mitchell retrofit problem was to add shear walls at the corners, seen here as the flat concrete wall at the corner of the building.

The Challenge of Existing Buildings: Legislation Regarding Retrofit of Existing Buildings and Stanford’s Program of Retrofit

Existing buildings—like the un-retrofitted Mitchell—are often considered to pose the greatest hazard in earthquakes. Unlike the regulation of new construction with building codes, there is a limited statewide approach to the retrofitting and upgrading of structures. With the notable exception of public schools, hospitals, and unreinforced masonry structures, this responsibility has been left primarily to owners, like Stanford University.

Figure 5: Damaged schools from the 1933 Long Beach Earthquake, resulted in the Field Act which regulated the construction of school buildings. Forty million dollars property damage resulted; 115 lives were lost.

Seismic Safety of Public Schools in California | The 1933 Long Beach earthquake, which damaged 75% of public school buildings in the city, provided the impetus for the first policies to mandate safety of existing buildings in California. The California Joint Technical Committee on Earthquake Protection responded: “Strengthening of public buildings, however, is subject to the will of the people and there should be no delay in making these buildings—particularly school buildings—safe.” The Safety of Design and Construction of Public School Buildings Act of 1933, which became known as the Field Act regulated the construction or reconstruction of school buildings, and the inspection of existing school buildings. The 1939 Garrison Act made the requirements for existing school buildings more stringent by legislating that if a structural engineer found a pre-1933 school building to be unsafe the structure must be updated to the California Building Code. The seismic safety of schools has continued to be a topic of conversation, and these types of provisions are constantly changing. The Garrison Act was updated in 1968, for example, and legislation in 1999 instituted plans to evaluate and rehabilitate non-wood frame school buildings that do not meet the requirements of the 1976 UBC.

Figure 6: The stairwells on Olive View Medical Center pulled away from the main building, and the first floor buckled during the 1971 San Fernando Earthquake, killing four people.

Seismic Safety in Hospitals | The lack of safety of another class of existing structures became a prominent policy consideration following the 1971 San Fernando Earthquake. Several hospitals, including the Veteran Administration and the Olive View Hospitals, collapsed in the earthquake; 44 people died at the VA hospital alone. As a result, the 1973 Alquist Hospital Safety Act mandated that new hospital structures have higher seismic safety standards. Patterned after the Field Act, the Hospital Safety Act did not originally apply to existing structures, but pressure mounted to deal with the 90% of California’s hospitals that predated the Hospital Safety Act, particularly after the 1994 Northridge Earthquake in which newer buildings generally had better seismic performance. Senate Bill 1953, passed in 1994, required that acute care facilities built before 1973 (including approximately 474 buildings) be upgraded to certain nonstructural and structural standards. According to the legislation, by 2008, these structures should not pose a significant threat to life; by 2030, hospitals are to be retrofitted to a level capable of providing services to the public after disasters. A hospital’s license can be revoked if it is not in compliance by these dates.

Unreinforced Masonry Construction | The state of California’s principal retrofit legislation regulates all unreinforced masonry (URM) construction. Like the Field Act, the 1933 Long Beach earthquake served as a trigger, demonstrating the vulnerabilities of masonry structures. The Coroner’s Jury in Long Beach concluded, “Masonry buildings were the principal sufferers and their failure occasioned the principal loss of life.” Despite consensus regarding the threat posed by masonry structures, it took the City of Long Beach nearly forty years to enact the first ordinance regarding unreinforced masonry structures. The California State 1986 law required all local governments in Seismic Zone 4 to inventory hazardous unreinforced masonry buildings and establish a risk reduction program by 1990. The law recommends, but does not require, that local governments adopt mandatory strengthening programs, establish seismic retrofit standards, and enact measures to reduce the number of occupants in unreinforced masonry buildings. Stanford University is under the jurisdiction of Santa Clara County, which enacted an ordinance mandating that unreinforced masonry (URM) buildings be seismically strengthened or vacated by 2000.

Seismic Upgrading at Stanford | Stanford’s program of retrofitting structures, as exemplified by the Mitchell Earth Sciences building, is motivated not by law, but according to Stanford’s institutional goals. Stanford’s retrofitting program has three overall performance goals: to protect life safety for the Stanford Community, to secure critical infrastructure and facilities, and to reduce interruption in the teaching and research program. Stanford has one of the most proactive retrofitting programs in California.