Will Adoption of the International Building Code Reduce Seismic Risk?
January 2004
The proposed International Building Code makes
significant changes in both the structural and nonstructural seismic requirements.
Its adoption will raise the level of the design seismic ground motion over much
of the United States. If implemented, it should reduce the seismic risk to both
buildings and nonstructural elements.
by Nathan
C. Gould, D.Sc., P.E., S.E.
ABS Consulting
One of the major differences between the International Building Code (IBC)
and the model building codes (UBC, BOCA, SBC) that it is replacing are significant
changes in both the structural and nonstructural seismic requirements. The seismic
provisions contained in the IBC include the latest ground motion maps, references
to the most current structural design standards, and enhanced quality assurance
provisions.
Given all these “enhancements,” the question is often asked whether the seismic
risk to buildings and the nonstructural support equipment and components will
be reduced as a result of the implementation of the IBC.
IBC Seismic Provisions
The IBC seismic provisions provide a significant departure from the previous
model building codes. The most frequently discussed change, and often the most
contentious aspect of the IBC seismic provisions, especially in regions of the
country where seismic design does not currently play a significant role in new
building projects, is the earthquake ground motion contained in the new maps.
The 2000 IBC seismic provisions are based in large part on the 1997 National
Earthquake Hazards Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures.
The ground motion maps found in the 1997 NEHRP provisions (and the 2000 IBC)
are based on the 1996 U.S. Geological Service (USGS) ground motion maps. The
maps are a significant departure from the ground motion maps used in the previous
model building codes, which were typically based on ground motion maps from
the 1991 NEHRP provisions.
One of the most significant changes in the new maps is the use of the Maximum
Considered Earthquake (MCE) ground motions to develop design ground motions.
The MCE ground motions are typically defined as the maximum level of earthquake
ground shaking that is considered reasonable for typical structures to resist.
The basic approach is to provide an approximately uniform margin against collapse
throughout all regions of the United States.
While many in the property and insurance industry may like the idea of having
a uniform level of risk associated with the seismic design, there is little
doubt that the level of design ground motion has significantly increased in
many regions of the country. A good illustration of the impact of the new ground
motions maps can been seen in Figure 1 below which shows a comparison of regions
of seismicity for the State of Missouri using the 1991 (on the left) and the
1997 (on the right) NEHRP maps.
Figure 1
The state of Missouri is typical of areas of the United States that have
a combination of low, moderate, and high regions. As seen in the figure above,
the areas encompassed by both the moderate and high regions increased using
the 1997 NEHRP maps. Given that these maps are for B-C soils, the area encompassed
by the high and moderate regions only increases as soil amplification is considered.
In addition to the changes in ground motion, other significant changes that
impact both the design force levels and the level of seismic detailing required
include the following.
- The IBC Seismic Design Category. The SDC
(referred to as the Seismic Performance Category in the BOCA and SBC), which
is used to determine the level of seismic detailing required for both structural
and nonstructural elements, is highly dependent on the soil properties at
the site. The site soil characterization in the IBC is based on the top
100 feet of soil below the structure. The inclusion of soils information
in the calculation of the SDC coupled with the use of a 100-foot soil column
to determine the soils classification, places a premium on obtaining detailed
soils information at an early stage of the design.
- Importance Factor. The IBC incorporates
an importance factor (I) for enhanced seismic protection for selected use
groups. While the 1997 UBC included an importance factor, the latest editions
of the BOCA and SBC did not.
- Redundancy Factor. The IBC incorporates
a redundancy factor similar to that found in the 1997 Uniform Building Code.
The redundancy factor reflects the number and distribution of lateral force-resisting
elements. The redundancy factor penalizes structures that lack multiple
lateral elements or do not have an equitable distribution of lateral elements
throughout the structure. The BOCA and SBC model building codes do not include
a redundancy factor.
More Stringent Nonstructural Requirements
In regions of moderate to high ground motion, which would include IBC Seismic
Design Categories C and above, the requirements for the anchorage and/or lateral
restraint for selected nonstructural components have been significantly increased.
In addition to increased forces, the design team also needs to consider the
following changes.
- All architectural components with SDC C or greater are no longer exempt
from seismic design. There is no exclusion for veneer connections, penthouses,
non-fire resistance membrane ceilings, and access floors including equipment,
regardless of the Importance (Ip) Factor.
- All SDC C Mechanical, Electrical, and Plumbing (MEP) components with
Ip > 1.0, and that provide a Life-Safety function, contain hazardous
materials, or storage are no longer exempt from seismic design.
- The IBC contains significantly greater prescriptive design requirements
than BOCA or the SBC for Architectural and MEP Components.
- Electrical equipment, boilers, and pressure vessels with Ip > 1.0 are required to be seismically certified, in addition to having their
anchorage designed for the appropriate seismic forces.
- Construction documents must show sufficient support/anchorage information
to verify design compliance.
- Quality assurance plans are required for special inspections and structural
observation.
As has been noted in previous articles, the relative cost of nonstructural
damage and related business interruption costs, as compared to the costs associated
with structural damage, is often extremely high. The above modifications related
to the design of nonstructural components, if adopted, should help to address
the vulnerabilities typically associated with poorly installed or under-designed
nonstructural components.
Summary
The adoption of the International Building Code will raise the level of the
design seismic ground motion over much of the United States. Given that the
MCE ground motion used in the IBC is intended to present a uniform margin against
collapse for all regions of the United States, the seismic risk to structures
and nonstructural equipment designed in accordance with the IBC should at least
partially reflect this push toward uniformity.
In addition, other structural and nonstructural seismic provisions in the
IBC, if implemented, should further reduce the seismic risk to both buildings
and nonstructural elements.
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