Earthquake Damage to
Historic Buildings: Assessing Principal Risk Factors
This computer model illustrates the comprehensive methods used to fully
reinforce a building for the future. Oakland City Hall, CA. Computer Model ©Douglas Symes, San Francisco. |
Typical earthquake damage to most older and
historic buildings results from poor ductility--or flexibility--of the building
and, specifically, poor structural connections between walls, floors, and
foundations combined with the very heavy weight and mass of historic materials
that are moved by seismic forces and must be resisted. In buildings that
have not been seismically upgraded, particularly unreinforced masonry buildings,
parapets, chimneys, and gable ends may dislodge and fall to the ground during
a moderate to severe earthquake. Walls, floors, roofs, skylights,
porches, and stairs which rely on tied connections may simply fail. Interior
structural supports may partially or totally collapse. Unreinforced masonry
walls between openings often exhibit shear (or diagonal) cracking. Upper
stories may collapse onto under-reinforced lower floors with large perimeter
openings or atriums. Unbraced infill material between structural or rigid
frame supports may dislodge. Adjacent buildings with separate foundations
may move differently in an earthquake creating damage between them. Poorly
anchored wood frame buildings tend to slide off their foundations. Ruptured
gas and water lines often cause fire and water damage. Many of these vulnerabilities
can be mitigated by understanding how the forces unleashed in an earthquake
affect the building, then planning and implementing appropriate remedial
treatments.
Six principal factors influence how and why
historic buildings are damaged in an earthquake:
(1) depth of the earthquake
and subsequent strength of earthquake waves reaching the surface (2) duration
of the earthquake, including after-shock tremors (3) proximity of the building
to the earthquake epicenter, although distance is not necessarily a direct
relationship (4) geological and soil conditions (5) building construction
details, including materials, structural systems, and plan configuration;
and (6) existing building condition, including maintenance level.
The first three factors--the depth, duration, and proximity to the fault--are beyond human control. Recent earthquakes
have shown the fourth factor, geological soil conditions, to be as important
as any of the other factors because loose, soft soils tend to amplify ground
motion, thereby increasing damage. Further, there is the tendency of soft,
unstable soils to "liquefy" as the ground vibrates, causing the
building foundations to sink unevenly. This fourth factor, geological and
soil conditions, is difficult to address in a retrofit situation, although
it can be planned for in new construction. The last two factors--the building's
construction type and its existing physical condition--are the two factors
over which building owners and managers have control and can ultimately
affect how the historic property performs in an earthquake.
"The Seismic Retrofit of Historic Buildings" an Historic Preservation Brief
