Whole lotta shakin' goin' on

Over the last decade, a fundamental shift in the national building codes has changed the landscape of seismic design. Building Teams that work out-side the West Coast could be designing for seismic events, even if they've never had to do so before.

Gone are the days of the general seismic zones (0-4), which dictated how structures are to be de-signed and detailed. The latest national codes, including the International Building Code (IBC) and NFPA 5000, take into account three factors when assessing the need for seismic design: seismic risk, occupancy type, and soil conditions at the site.

These three factors are combined to place a project within one of six seismic design categories (A-F) to determine the seismic detailing requirements. Buildings that fall into the A, B, and C range generally require little or no seismic detailing. Structures in the D, E, and F categories call for moderate to strict seismic design.

What this means for Building Teams that work in traditionally low or moderate seismic zones is that if a project has the right combination of the three criteria, the code will call for a certain level of seismic detailing in order to meet minimal life-safety requirements — in other words, to make sure the building won't collapse. An office complex in Indianapolis built on poor soil, for example, may now require moderate seismic detailing. A hospital built on the same site will require even more stringent design.

"There's no going back to the old days," says S.K. Ghosh. "Many of these changes will bother people, because they represent a change in the way they have always done things. But they are based on science that should not be ignored."

Ghosh, president of his own code consulting firm, S.K. Ghosh Associates, Northbrook, Ill., is also vice chairman of the Building Seismic Safety Council, a government-funded organization.

BSSC publishes the country's most authoritative seismic provisions document: the National Earthquake Hazards Reduction Program Recommended Provisions for Seismic Regulations for New Buildings and Other Structures. The 1997 version of the NEHRP provisions forms the basis for seismic design standards in the American Society of Civil Engineers' structural code — Minimum Design Loads for Buildings and Other Structures, ASCE 7-02 — which, in turn, is referenced in IBC and NFPA 5000.

As more and more states and local governments adopt IBC or NFPA 5000 — 43 states currently subscribe to IBC, either statewide or in certain local jurisdictions — these stricter seismic regulations will come with it.

"It's just now getting to the point where these codes are being adopted and put into practice," says Robert Bachman, a consulting structural engineer in private practice and chair of ASCE 7 Seismic Task Committee. "For California and the West Coast, it's merely going to be a change in nomenclature. For other parts of the country, with the exception of the New Madrid fault area and Charleston, S.C., it's going to be a change in nomenclature and practice. Obviously, some engineers are unhappy."

Soil plays a critical role

The single biggest change in the NEHRP seismic provisions, which first appeared in IBC 2000, is the assessment of soil conditions at the site, says Ghosh. After years of earthquake research, scientists have concluded that poor soil condition can greatly intensify the effect of ground motion.

Under the newer codes, site-specific soil data must be gathered to establish the site classification, rated A-E, with E being the most extreme. If no sample is collected, then the default class is D, which requires some form of seismic design.

"Because of this soil aspect, a building in Atlanta on soft soil may have to be designed like a building in California," says Ghosh. Boston and New York are rated seismically moderate according to the current code, and therefore will require high seismic design on soft soil. (Please see the table on opposite page for examples of how site classification can affect the seismic design category for projects in various locations.)

This soil factor has thrown some Building Teams operating in traditionally nonseismic areas for a loop, says Nathan Gould, general manager of the St. Louis office of Houston-based structural engineer and risk consultant ABS Consulting.

"The owner or architect is often surprised when we come back to them and say, 'Due to the soil, this is a seismic design category D, and steps need to be taken to meet the code,'" he says. Gould has worked on several new projects in Missouri that were designed under the new seismic provisions.

"St. Louis is right at the threshold between C and D," says Gould. "Depending on the soil types — whether it's rock, clay, or different types of deposits — we may or may not get kicked up into the higher seismic design category."

Gould suggests that performing an extensive geo-technical investigation can save in the long run by obtaining a more favorable site classification, and thus a less severe seismic design category.

For a standard low-rise building, soil testing will most likely go down only 25 or 50 ft., "just to get an idea of how to design the foundation," he says. To get an accurate soil type for classification, Gould recommends that testing be done at 100 ft. below the foundation level.

New maps, new landscape

The most recent seismic provisions also take into account the latest seismic maps from the U.S. Geological Survey, which have upgraded seismic risk in some parts of the country. For example, Cincinnati, Nashville, and Charleston, S.C., are now rated moderate or high seismic areas.

"Charleston is now pretty much considered as seismic as coastal California," says Ghosh. "Sacramento has actually been downgraded, so a building on competent soil will only require a moderate level of detailing."

Much greater accuracy

Ronald Reaveley, president of based structural engineer Reaveley Engineers & Associates, Salt Lake City, says the science of seismology now allows engineers to more accurately pinpoint base accelerations at specific sites. "Using the Internet, I can plug in the longitude and latitude and get the seismic acceleration contours for that site," he says. "This allows us to much more finitely predict and calculate the forces."

Reaveley cites Salt Lake City, which used to be rated Zone 3 under the old code. "Now we have a variation of more than 50% going just 15 miles across the city," he says.

What if a project falls into the higher seismic design categories D, E, or F?

For one thing, an owner can expect to pay more for the building, says Charles Carter, chief structural engineer with American Institute of Steel Construction (AISC), Chicago.

"These higher seismic systems cost more across the board," says Carter. "No matter what material you're designing with — concrete, wood, steel, or masonry."

For a steel structure, it may require the Building Team to look at using any of three seismic frame systems: a special moment frame, a concentrically braced frame, or an eccentrically braced frame, says Carter. He suggests referencing AISC's 350-99 Seismic Provisions for Structural Steel Buildings for more on steel seismic solutions. (Also, see table below.)

Concrete structures might require additional reinforcement at the connections, moment frames, and shear walls, says Bachman. (Reference American Concrete Institute's 318-99 Seismic Detailing of Concrete Buildings for more on concrete seismic systems, or see table below.)

For Building Teams dealing with these new seismic provisions for the first time, Gould advises that they define the seismic design category early in the project. "In the past, this wasn't a big deal; it would be picked up later as the structural engineer progressed through the design phase," he says.

Under the new code situation, he suggests that Building Teams pay more attention to the geo-technical investigation and establish seismic design category early on. "If it looks like it's going to be a category D or higher, see what it means to both the structural and nonstructural systems, because it may affect everyone," he says.

COMMON DESIGN TOOLS OF THE SEISMIC ENGINEERING TRADE