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Tuesday | 14 July, 2015 | 9:14 am

Patterns of prevention

By Corinna Petry

Above: The professor and students Steven Florig, Patrick O’Brien and Abhilasha Maurya put geometric cutout plates through tests.

Engineer designs steel frames and structures to resist failure during earthquakes

July 2015 - Matthew R. Eatherton is, above all, a practical man. When he saw California architects design window walls with narrow strips of shear wall supporting them, so the ocean view could be unobstructed, he knew such designs would perform poorly during an earthquake.

After several years of working with architects, his fellow structural engineers and the people who write building codes, he was determined to focus his career on systems that save lives and limit damage.

Eatherton’s work recently won his department at Virginia Polytechnic Institute and State University (Virginia Tech) a National Science Foundation grant to develop geometric patterns in steel plates that will also be designed, fabricated and rigorously tested to withstand seismic events. 

Assistant professor of civil and environmental engineering at the Blacksburg, Virginia-based institution, Eatherton was a practicing structural engineer in Kansas City, Missouri, for two years and in San Francisco for five years.

“The engineering work was so different in California. It was earthquake engineering. I got into high-end residential projects where the designs looked like commercial construction because they were so large and complex. We couldn’t use timber because of architectural, construction and performance requirements so the houses ended up being steel or concrete.

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“There were a few projects with intense earthquake performance requirements, which worked against the architects’ vision”—to provide wide spans of window for premium views of ocean or estate—because such spans provide “little solid wall,” recalls Eatherton, adding, “It’s difficult to get great earthquake performance with so little space for structure.

“We had to explain to architects and owners that building code requirements were not designed to limit damage, but to save the lives of the inhabitants. So we got into designing buildings with higher-than-code-level performance objectives.” From there, he says, “I started thinking about high-performance earthquake resisting systems.”

Eatherton left San Francisco to pursue a doctorate at the University of Illinois at Urbana-Champaign , where he helped develop a rocking-braced steel frame that would “self center” after an earthquake, that is, “bring the building back to plumb and have any structural damage contained to replaceable pieces,” he explains. To date, the professor says, the rocking-braced frame system has been used in a handful of buildings in California and Christchurch, New Zealand. The best path for adoption of this system is to write the design requirements into the building code, but that is a slow process.

Eatherton completed his Ph.D. in 2010 and was hired to teach and conduct research at Virginia Tech, where he continues working on the self-centering steel frame as well as on developing new designs. For example, Eatherton and his team of  engineering students are studying shear panels with geometric cutouts.

‘It’s like a tin can’

Going back to the problematic window walls, Eatherton says that in the case of steel plate shear walls, builders rely on a “very thin plate bounded by two columns and beams at the floor and ceiling. But it buckles at really small shear loads, possibly even during high wind loads, and there are no design checks to prevent that. 

“Some of these plates are 16-gauge, so it’s like a tin can,” he continues. “I’ve walked in there with architects who knocked on it and asked, ‘Is this what will save our owner from an earthquake?’” 

The shear wall, he says, is hard to erect, handle and weld. “And then there are the performance issues. Thin steel plates develop tension fields. Diagonal buckles form, and when the load reverses, there is no lateral load resistance until the tension field forms in the opposite direction. There is low energy dissipation and low stiffness during load reversals.”

Which, of course, can mean excessive drifts, which are likely to shatter the window wall.

“So the team sought to convert global shear deformation into a local yielding mechanism with geometric cutouts in steel plate.” Eatherton likens it to the voids and weakened points in automobile structures that make the vehicle crashworthy, limiting injury to occupants. “There, we see material [designed] with holes that absorb energy. But we have not seen it for building lateral force resisting systems.”

Pattern testing

Eatherton’s team has tested two patterns that work “really well so far and we are looking for more patterns.”

One pattern consists of rings linked together along a diagonal. “The circle works because it deforms and elongates into an ellipse. The diagonal tension pulls the ring into an ellipse and the amount that the ring is elongating is about equal to the amount that it is shortening in the compression diagonal direction.” The deformed shape therefore takes up the slack in the material and prevents the steel from buckling.

At the same time the shear wall faces extreme loading, it is also yielding, which enables the steel to dissipate a great portion of the seismic energy coming from the ground.

The second pattern consists of removing material in a series of shapes around the perimeter of the steel plate. This makes for a solid center in the plate, which remains stiff and resistant to buckling, while the remaining strips around the perimeter will yield to dissipate energy.

“We are using 3⁄8- to 5⁄8-inch-thick plate for the full-scale trials. Our initial experiments are at two-thirds of that scale.”

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Simulations

The engineers use computer modeling to help them pursue more seismic event-resistant patterns for structural steel plate, followed by physical experiments. In the school’s structural engineering laboratory, “we have tested ring-shaped specimens in 3- by 3-foot panels. It has really proved the concept well. We got them to deform to validate that it does not buckle and dissipates a huge amount of energy,” the professor says.

Some two-thirds-scale tests will be performed this summer. “These panels, at 10 feet by 8 feet, would represent a shear wall in a high-rise residential or commercial building. We fully expect similar results to what we experienced with the 3- by 3-foot panels.”

The university is just starting work under a five-year grant from the National Science Foundation which will spur study on shear walls but also with “all sorts of structural systems and where else to apply the concepts, including on steel beams.”

The Virginia Tech engineers will look at things like blast loading, and wall and floor elements. For example, says Eatherton, metal buildings have girts and purlins that buckle and which don’t provide ductility or energy dissipation. So can purlins be redesigned to withstand blast-related shocks?

To date, the university has outsourced the waterjet cutting of patterns in the plates it tests. “We think you might be able to do this with plasma cutting. But we must have a smooth finish to avoid fractures or cracks and waterjet gives you an amazingly smooth surface,” Eatherton says.

The school sources its steel from BMG Metals Inc., based in Richmond, Virginia. It sends the plate to Waterjet Cutting Inc. and Banker Steel, a heavy structural fabricator, both in Lynchburg, Virginia. They fabricate the cutouts according to the engineers’ designs.

Next steps

While teaching classes and studying earthquake engineering, Eatherton attends multiple conferences each year to meet with structural engineers from around the world. At one steel conference, he was encouraged to hear a prediction that within 20 years, “every new building will be a self-centering building.”

He continues reaching out to fabricators that might want to cut seismic patterns to save both buildings and their occupants. “The geometric patterns cannot be patented so the goal is just to give it to the engineering community to use.”

Regarding the potential to revise building codes to accommodate geometric cutouts, he may approach building code committees for the 2022 edition of the AISC seismic provisions.

“There is room for innovation to bring new technology into real-world applications,” says the forward-looking but ever practical Eatherton. MM

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