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
|System type||Description||How it works||Applications||Energy dissipation||Relative cost||Advantages||Disadvantages|
|Base isolation||Two basic types: elastomeric and friction. Elastomeric bearings are multi-layers of rubber and thin steel plates vulcanized together to form a cylindrical bearing unit. Friction isolators dissipate energy through controlled frictional resistance between surfaces fixed to the foundation and isolated from the superstructure above. Friction pendulum is the more prevalent type.||During a seismic event, isolators undergo large horizontal displacements, thus decoupling the building, supported by the isolators, from the ground.||Used principally for high performance buildings with essential functions, such as hospitals, and for retrofitting historic buildings with brittle materials.||Very high||1.4||Very low post-earthquake reduction to building function (continuous occupancy); best seismic performance, protecting both primary structure and contents; protects buildings with minimal intrusive retrofit elements; good life cycle cost.||High first cost; newer technology, so there's conservative code requirements for QC testing and superstructure design.|
|Passive dampers — Fluid-viscous||Like a shock absorber, this device consists of a piston that moves in and out of a fluid-filled cylinder.||Dissipates energy through a response to a building's seismic velocity.||Normally applied in conjunction with a primary lateral force resistant system, like a steel moment frame||Very high||1.6||Similar to base isolation; high energy dissipation; post-earthquake operability; low to zero repair cost.||High first cost; spatial disruption of diagonal brace mounting.|
|Passive dampers — Friction||A hinge-like device composed of several layers of steel that links two or more diagonal braces.||Dissipates energy through controlled friction due to seismic-induced building displacement||Same as fluid-viscous damper||High||1.6||Similar to base isolation; high energy dissipation post-earthquake operability; low to zero repair cost.||High first cost; spatial disruption of diagonal brace mounting.|
|Passive dampers — Visco-elastic||Rubber-like visco-elastic material bonded rigidly between steel plates.||Dissipates energy in response to displacement and velocity.||Same as fluid-viscous damper||Medium||1.6||Similar to base isolation; high energy dissipation post-earthquake operability; low to zero repair cost.||High unit cost for amount of energy dissipated.|
|Passive dampers — Metallic||Metal-plate assembly made of mild steel or lead — metal that is weaker than the structure surrounding it.||Bending or shearing of plate assembly in response to building displacement.||Normally in conjunction with moment frame building.||Medium||1.4||Lowest first cost of the passive damper family.||Prone to material (metal) fatigue; non-robust performance.|
|Eccentric-braced frame||Steel-braced frame adapted from concentric-braced frame; bracing is set apart with an eccentricity and connected by special yielding link elements.||As with large metallic dampers, shear deformation in link element is activated by seismic drift or rotation of building columns.||Low- to mid-rise steel-frame buildings.||Medium to high||1.2||Low first cost||High damage repair costs after a seismic event; difficult to repair damaged link beams; beams are prone to metal fatigue.|
|Buckling-restrained braced frame||Recently introduced seismic energy-dissipating brace configuration similar to concentric-braced frame; braces are concrete-filled steel tubes that encase a steel-core plate.||An assembly of axially yielding core plate surrounded by tubular sleeve, which is designed to prevent buckling of the core plate.||Low- to mid-rise steel-frame buildings.||Medium to high||1.3||Reliable and robust; low first cost; not prone to metal fatigue; easy replacement.||Spatial disruption of braced frame elements; current patent restrictions (it is a proprietary system from Japan-based Nippon Steel).|
|Ductile concrete shear wall||Reinforced concrete walls highly confined through dense steel reinforcement.||In-plane bending of wall mobilized by seismic displacement of building. Energy dissipation through tensile yielding of vertical steel bars.||Low- to high-rise buildings||Medium to high||1.2||Low first cost; applicable for a wide range of building types.||Extensive and expensive repair required following major earthquake.|
|Special moment resisting frame — Steel||Assemblage of interconnecting beams and columns; connections typically welded, but can be bolted; 1994 Northridge earthquake revamped design and construction of these systems.||Connections dissipate energy through flexural yielding of beams.||Low- to high-rise buildings||Medium to high||2.0||Spatial flexibility (no diagonal braces).||High first cost; prone to damage due to high drift; difficult repair; demands high quality control; low reliability.|
|Special moment resisting frame — Concrete||Assembly of highly reinforced concrete beams and columns.||Same as steel moment-resisting frame, with the flexural yielding of beams.||Low- to mid-rise buildings||Medium to high||1.6||Spatial flexibility (no diagonal braces).||Difficult to construct; form and place large, massive columns.|
|Concentric-braced frame||Steel beam-column-diagonal brace assembly between beam-column joints.||Axial deformation of diagonal braces due to horizontal displacement.||Low- to mid-rise steel-frame buildings.||Low||1.0||Inexpensive||Prone to buckling and fatigue (early failure); least reliable; spatial limitations of brakes|