Experts Discuss Designing for Seismic Conditions and Considerations
March 15, 2011
Approximately 90 percent of the world’s earthquakes occur in what’s known as the Pacific Ring of Fire—a horseshoe-shaped region that includes such areas as Japan, New Zealand, Chile and California. Earthquakes have struck many of these places of late, from Chile last year, to New Zealand just weeks ago and Japan only days ago. If such a disaster were to next strike California, are the state’s earthquake-prone regions, such as San Francisco, ready to withstand the potentials of mass-destruction?
Glenn Heitmann, president and chief executive officer of Heitmann & Associates, a curtainwall consultant in St. Louis, says he thinks so.
“We were heavily involved with the design and construction [of a high-rise tower] there and significant care, detail and attention were paid around the building being in a high seismic area,” Heitmann says. “[We knew] the building must be allowed to move in the case of such an occurrence.”
Designing buildings to sway rather than break helped buildings and structures in Japan.
“The devastation was horrible, but it goes to show that buildings need to be built not just for everyday occurrences, but also for those that might only happen every 100 years,” says Heitmann. “A building façade and structure that are rigid typically don’t fare as well under [seismic] conditions; those that accommodate movement tend to do better.”
When it comes to designing structures for seismic conditions, particularly fenestration systems, there are number of conditions that must be considered.
“When designing for seismic applications, we need to know what movement changes of the building are expected,” says Jim Hicks, director of engineering at Graham Architectural Products. “We design the clearances around the movement of the structure of the building.”
He explains that his company’s seismic system consists of a compensation channel system in which the window basically floats. In the structure, the compensation channel is installed first and then the window into that.
“The channel has to interface with the façade work as well as the structure of the building,” says Hicks. He adds that with a seismic system they don’t typically expect there to be any tension or flexion of the glazing.
“Our windows freely float; they don’t bottom out,” he says.
Product testing is also an important consideration. Scott Warner, executive vice president of Architectural Testing in York, Pa., chaired the American Architectural Manufacturers Association (AAMA) committee that developed two seismic test methods, AMA 501.4 and AAMA 501.6.
“Reconnaissance reports after major earthquakes note architectural glass can be damaged by seismic-induced drifts in building frames. Falling glass fragments during earthquakes represent a life safety hazard to pedestrians and building occupants, yet specific provisions for the seismic design of architectural glass in model building codes have historically been either non-existent or limited to general statements, such as ‘building drifts shall be accommodated,’” Warner explains. “The recent publications by AAMA and the reference in ASCE 7 for the first time establish glass fall design provisions.”
The AAMA publications are AAMA 501.4-2000, Recommended Static Test Method for Evaluating Curtain Wall and Storefront Systems Subjected to Seismic and Wind Induced Interstory Drifts, which is a static racking test method focusing more on the seismic serviceability of curtain and storefront wall systems, and AAMA 501.6-2001, Recommended Dynamic Test Method for Determining the Seismic Drift Causing Glass Fallout from a Wall System Panel, is a dynamic racking test method focusing on the seismic safety of architectural glass components within curtain and storefront wall systems.
“Essentially, the AAMA 501.6 test involves mounting individual, fully glazed wall panel specimens on a dynamic racking test apparatus, which moves back and forth in sinusoidal motions at gradually and progressively higher racking amplitudes,” says Warner.
Warner explains that the current code requires the resistance to glass fallout of an individual glass panel be greater than the relative seismic displacement the component must accommodate as a result of being attached to the primary structural system of the building.
“In the absence of special drift-accommodating connections between the main building frame and the curtainwall framing members, this relative seismic displacement demand is governed by the calculated seismic interstory drifts for the specific building being designed for site-specific earthquake conditions,” he says.
So, given the recent events in Japan and the stringency of its building codes, could this change the way we build buildings here in the United States? Heitmann thinks it could.
“If history repeats itself … usually when something horrific happens we tend to go back, look at our codes and re-evaluate,” he says.