Estimating Earthquake Loss in the Midwest
September 2002
In this article, Nathan Gould discusses how
new ground motion maps and the genesis of the damage curves may impact the estimate
of earthquake loss in the Midwest.
by Nathan
C. Gould, D.Sc., P.E., S.E.
ABS Consulting
With new estimates that put the probability of a damaging earthquake in the
Midwest at up to 40 percent in the next 50 years, there is a renewed urgency
in revisiting the validity of current loss estimation models for the Midwest.
The earthquake loss estimates produced by most models are a product of the ground
motion and damage curves, which relate the intensity of the ground motion to
the damage that a building may experience. This article will discuss how new
ground motion maps and the genesis of the damage curves may impact the estimate
of earthquake loss in the Midwest.
Ground Motion
There has been considerable work done in recent years on updating the earthquake
ground motion maps across the United States. In 1996 the U.S. Geological Service
(USGS) completed the development of new earthquake ground motion maps. The 1997
National Earthquake Hazards Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations
for New Buildings and Other Structures was the first major document to
incorporate the revised earthquake ground motion maps. The revised earthquake
ground motion maps are also embedded within the seismic provisions of the 2000
International Building Code (IBC). The 1997 NEHRP provisions and 2000 IBC ground
motion maps represent a hybrid of probabilistic ground motion maps over much
of the Untied States and deterministic ground motion maps near specific faults
on the West Coast.
One of the most significant differences between the new maps found in the
1997 NEHRP Provisions and 2000 IBC and those used in previous building codes
is the return period on which the maps are based. Prior to the 2000 IBC, the
model buildings codes developed ground motion based on a hazard with a 475-year
return period. The 2000 IBC develops earthquake ground motion based on the Maximum
Considered Earthquake (MCE). The MCE is defined as the maximum level of earthquake
ground motion that is considered reasonable to design normal structures to resist.
The intent of the new maps and related ground motion is to present a uniform
seismic hazard for all portions of the United States. The result is significantly
increased ground motion in many portions of the Midwest and eastern United States.
Damage Curves
While earthquake loss estimation for the Midwest and East Coast has recently
focused on the development of the new USGS probabilistic ground motion maps,
it is important to recognize the relationships and assumptions used to develop
damage curves in the hazard models. Understanding the earthquake hazard and
potential loss in the Midwest and East Coast requires insight into local design
practices, current and previous building codes, and how loss estimation factors
used in many hazard models were originally developed for use on the West Coast.
The genesis of many earthquake loss estimation models for most building types
is the 1985 Applied Technology Council document ATC-13, "Earthquake Damage Evaluation
Data for California." The ATC-13 document provides published Probable Maximum
Loss (PML) values for the most common building types. Although the values presented
in the ATC-13 were only intended to be applied on the West Coast, there is not
a similar set of PML values available specifically for the Midwest and East
Coast.
There are several reasons why the loss estimates contained in the ATC-13
document may not be directly applicable to structures in the Midwest and eastern
United States.
Wind Forces versus Earthquake Forces
Until the late 1980s, the vast majority of building and structures in the
Midwest and eastern United States were typically designed only for wind forces.
While wind loads are similar to earthquake loads in that both are considered
lateral loads, the response of structures to wind loads is quite different than
earthquake loads. Wind loads are typically considered to be "static" when applied
to a structure and the loads are proportional to the exposed surface area of
the building.
Earthquake forces are dynamic loads and are proportional to the mass of the
building. So even though an existing structure may have been designed for wind
loads, the resulting lateral load-resisting system may not be appropriate or
adequate for the design earthquake loads.
Building Codes
The building code adopted by most local jurisdictions on the West Coast is
the Uniform Building Code (UBC) which is universally acknowledged as being the
leading building code for seismic design. The UBC began to incorporate seismic
requirements into standard building design in the 1930s while building codes
in the Midwest and eastern United States did not require the consideration of
seismic forces until the 1980s.
Jurisdictions throughout the West Coast not only adopt UBC, but also typically
require some level of independent design review (plan check) and construction
inspection. In the Midwest, the adoption and enforcement of building codes is
much less rigid. Construction inspection related to the structural aspects of
structure may fall to the design engineer if performed at all.
Construction of "Similar" Building Types
Typically buildings can be categorized based on material types and the lateral
force-resisting systems. A concrete frame building with a moderate level of
seismic detailing may fall under the classification of "ordinary concrete moment
frame." The concern is that an ordinary concrete moment frame designed in the
Midwest may not be comparable to an ordinary concrete moment frame designed
on the West Coast. Similar statements can be made for steel moment and braced
frames, concrete tilt-up structures, and many types of masonry and wood structures.
Summary
Loss estimation values developed for the West Coast may not be appropriate
for use in the Midwest and eastern United States without some level of modification.
Combining the effects of the new earthquake ground motion maps with the recognition
that design and construction practices in the Midwest and East Coast differ
significantly from those found in the West Coast can lead to loss estimates
that may be higher than those found using earlier models.
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