By William S. Ginell and E. Leroy Tolles

The destruction wrought by California's periodic and often violent earthquakes is a grim reminder that many historic and culturally significant buildings pose substantial risks to the life and safety of their occupants. In addition, the damage to our Spanish colonial and early American heritage—in the form of irreplaceable historical fabric, architectural details, objects, and decorations—increases with each new seismic event. California's historic adobe structures, which include missions and secular buildings, have been particularly hard hit by devastating earthquakes.

Although we now understand a good deal about the behavior of modern reinforced masonry buildings during quakes, until recently little was known about the factors that determine how adobe buildings respond to seismic forces. In 1990 the Getty Conservation Institute undertook a research project to study methods for retrofitting historic adobe structures—minimally intrusive methods consistent with maintaining the architectural, historic, and cultural values of the buildings. The material most commonly used for retrofitting adobe buildings is steel-reinforced concrete. Its installation is extremely invasive and can result in the destruction of much of a structure's historic fabric in an attempt to save it (see When The Earth Moves).

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he primary objective of the Getty Seismic Adobe Project (GSAP) was to develop relatively inexpensive and less-invasive techniques that could limit the danger to life by preventing structural collapse of adobes. The principal mode of failure of adobe walls is out-of-plane overturning, which can often be prevented when adequate connections to the floor and/or roof systems are assured, since adobe walls are often two to three feet thick. Other damage that may lead to collapse can be reduced or prevented by limiting the relative displacement of the large wall blocks that are formed after cracking. The problem for the project was not how to prevent cracks from occurring: in a moderate to large earthquake, adobe walls inevitably crack into large blocks. The task instead was to determine how to prevent overturning by keeping those blocks in place during continued shaking. Where thin adobe walls are concerned, mid-height failure may also occur, and the means to prevent this type of damage required investigation.

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Work was based on the premise that if significant shifting in cracked portions was prevented—and mid-height failure eliminated—an adobe would remain stable. Earthquake-simulation tests were carried out on model adobe buildings, both retrofitted and unmodified. As part of the project, nine small-scale (1:5) and two large-scale (1:2) model buildings were constructed and tested on computerized earthquake-simulation shaking tables that subjected the models to "quakes" of increasing severity. A wealth of information was accumulated on how adobe buildings respond to simulated earthquakes and how retrofitting can prevent catastrophic damage. Tests showed that the use of nylon straps and thin, flexible steel rods strategically installed in an existing adobe could greatly enhance the stability of the building by preventing walls from overturning.

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The 1994 Northridge earthquake in Los Angeles vividly demonstrated once again the destruction that can be sustained by adobe buildings. One such building, the Del Valle Adobe at Rancho Camulos, located about 18 miles northwest of the epicenter, was damaged extensively. Now this adobe has become the first historic structure to be retrofitted in light of the results of the GCI's work under GSAP.

The Del Valle Adobe, situated near Piru, California, is a rancho of Mission San Fernando and is considered an outstanding stylistic example of California's old ranchos. Established as a nonprofit organization in 1994, the 40-acre site, now called the Rancho Camulos Museum (part of a much larger, functioning 1,400-acre ranch), includes the adobe main residence, a brick winery, a smaller adobe outbuilding, and the original chapel. Many of the historic features of the buildings—such as the cocina (kitchen), the Greek Revival detailing of the fireplaces, chair railings, and corredor posts—remain as exemplars of early California architecture. The main residence is one of the attractions of the rancho because it served as the model for the home of the heroine in the well-known romance novel Ramona by Helen Hunt Jackson; the novel is noted for its portrayal of the idyllic pastoral days of early California.

The earliest portion of the building, constructed in 1841, consists of three rooms that are one-and-one-half stories in height and a one-story, one-room extension. Over the years, the building evolved into a u-shaped complex with a central courtyard. The single-story room, known as Ramona's room, is situated at the southeast corner. During the earthquake, two walls of Ramona's room collapsed. The gable-end wall at the southeast corner was severely damaged but did not collapse; the stone walls at the north end of the west wing suffered severe cracks at the corners.

Crack damage occurred throughout the building, especially at corners and, because of pounding, at wall intersections. Spalling of interior and exterior plaster was extensive, as was the collapse of adobe in areas that had been weakened by previous repeated exposure to water. In many locations, the walls had pulled away from the ceiling joists, and damage to the walls further reduced their ability to support the joists. The severe damage to the building probably resulted from a combination of factors: the lack of structural elements either tying the walls together or tying the roof-ceiling system to the walls, the presence of pre-existing earthquake-related cracks, and water damage that weakened the lower sections of the adobe walls and foundation.

As part of GSAP, a team consisting of E. Leroy Tolles, Anthony Crosby, Edna Kimbro, and Frederick Webster surveyed the extent of Northridge earthquake damage to historic adobe structures, including Rancho Camulos, immediately after the earthquake (the survey findings were later published by the GCI). At the request of the Rancho Camulos owners, a damage assessment was made; emergency shoring and bracing plans were formulated; and a strategy for obtaining repair financing was developed.

Ultimately, federal funding of $500,000 was obtained through a program administered by the Historic Preservation Partners for Earthquake Response, a collaborative project of the National Park Service, the National Trust for Historic Preservation, the California Office of Historic Preservation, the Los Angeles Conservancy, the California Preservation Foundation, and the GCI. Additional funding of $250,000 was obtained from the County of Ventura.

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The major part of the funding provided for the installation of a complete seismic retrofit system using the technology developed under GSAP. A portion of the funds was used for repair of the main residence and for stabilization of the winery and the small adobe outbuilding. Tolles, who was also principal investigator for GSAP, led the private design team for the project. He was joined by Crosby, a historical architect, and Kimbro, a historian and architectural conservator. The design team worked with Steade Craigo of the California State Office of Historic Preservation and with the Ventura County Department of Building and Safety to ensure that the design conformed to the U.S. Secretary of Interior Standards for restoration of historic properties, and was in compliance with the safety requirements of existing building codes.

The design of the retrofit project was based largely upon the results of the GSAP research. Indeed, this effort involving an existing earthquake-damaged adobe building was the initial application of the principles and techniques that were studied and experimentally validated at the GCI. Because the techniques and technology were innovative and had not been previously implemented, a careful review of the proposed retrofit measures was carried out.

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These measures included horizontal cables around perimeter walls which, in some areas, were anchored to ceiling joists; vertical cables or straps on both sides of adobe walls that were either too thin or particularly vulnerable due to damage from past earthquakes; vertical center-core rods that were placed in newly constructed walls; and anchorage at the floor levels. To our knowledge, this was the first time that pretensioned, vertical stainless steel cables recessed into walls had been used on an adobe structure (they had been previously used to reinforce stone walls for which the height-to-thickness ratio was greater than eight).

This first implementation of the GSAP research results required some redesign of laboratory-tested details for application to real-world conditions. It also required acceptance by building officials and by the California Office of Historic Preservation, as well as input and review by the building owners, who were particularly concerned about safety in and around their building. The seismic retrofit and repair of the main building have been completed, and it is anticipated that repair of the winery will be carried out in the near future.

Detailed information on the Institute's research into seismic strengthening—and on the retrofitting recommendations growing out of that research—will be available in two forthcoming publications from the GCI. The first, GSAP Final Report, will provide a comprehensive description of six small-scale and two large-scale tests conducted to determine the effectiveness of several retrofitting techniques. The second, Planning and Engineering Guidelines for Seismic Retrofitting of Adobe Buildings, will offer specific recommendations on how to fortify historic adobes against seismic destruction in a manner that preserves the integrity and authenticity of this important part of our heritage.

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William S. Ginell is a senior conservation scientist with the GCI, and project director of GSAP. E. Leroy Tolles served as the principal investigator of GSAP.