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The Getty Conservation Institute

Preserving Safety and History: The Getty Seismic

Adobe Project at Work

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).

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 Rancho Camulos, as it appeared
displacement of the large wall blocks that are formed in 1895.
after cracking. The problem for the project was not how Photo: Adam Clark Vroman.
to prevent cracks from occurring: in a moderate to Courtesy Seaver Center for
large earthquake, adobe walls inevitably crack into Western History Research, Los
large blocks. The task instead was to determine how to Angeles County Museum of
prevent overturning by keeping those blocks in place Natural History.
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.
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.