Standing Strong

While the need to incorporate structural resilience into the design of a project can pose engineering challenges, innovative materials and design approaches are critical for developing cost-effective solutions.

This includes incorporating structural resilience into design to address natural disasters like floods, hurricanes and earthquakes. It also involves assessing multiple risks resulting from these extreme weather conditions, identifying strategies that will minimize their impact and designing structures to not only withstand these shocks, but also swiftly resuming operations after each incident.

When it comes to risks in areas with high seismic activity, our engineering professionals are working with clients to develop proven design solutions that provide the capability to withstand the potentially devastating impact of these tremors.

Understanding the need to plan ahead for natural structural threats like earthquakes is at the forefront of every project design for building engineers like Ahmad Rahimian, PhD, PE, SE, F.ASCE, executive vice president and director of building structures at WSP.

In the 1990s, Rahimian and his team designed the 738-foot Torre Mayor skyscraper – then the tallest building in Mexico City – as earthquake resilient as possible. They used steel and concrete, but also techniques like super-diagonal bracing and fluid viscous dampers.

It wasn’t long before the structure was tested for its seismic resiliency. When Mexico City was rocked with a 7.6 magnitude earthquake shortly after the Torre Mayor skyscraper was completed in 2003, the tower’s residents were largely unaffected.

According to Rahimian, a key part of making more earthquake-resilient structures is the specific combination of steel and concrete.

“We know steel has value to make buildings lighter, while at the same time, steel gives strength and stiffness to it,” he said. “For Torre Mayor, the mandate was to use local materials as much as possible, so at the lower level, we have more concrete, less steel. It’s all composite — the steel sections are embedded in concrete.”

Rahimian used similar techniques in the development of WSP’s modified SpeedCore construction system. SpeedCore is a composite steel-concrete system, featuring wall modules that are composed of two steel faceplates, tie bars connecting the faceplates and concrete infill.

This approach offers many advantages for buildings that use structural steel as a primary construction material in areas of higher seismic activity.

Most of the structural engineering concerns regarding resiliency are focused on the risk of larger, but also less frequent, earthquakes. But smaller, more frequent incidents also pose dangers for critical facilities.

“From a design perspective, we tend to focus on the proximity and frequency of earthquakes near our site in conjunction with their magnitudes,” said Tony Ghodsi, PE, SE, senior vice president of building structures at WSP. “The distance is the bigger issue, because while an 8.9-magnitude earthquake in Antarctica won’t be a big concern for Californians, a smaller magnitude on the Hollywood fault line most certainly will.”

Another concern is each design’s non-structural components that require greater resiliency. This includes ceilings, mechanical systems, building enclosures and other non-structural features necessary for the building to function.

“Things like that don’t get the same scrutiny as structural components do, but these non-structural features need to be considered just as critically when evaluating a building’s vulnerability,” Ghodsi said.

In addition to mitigating devices like dampers, engineers are employing performance-based design optimization practices for structures, specifically for buildings that stand 240 feet or taller.

Engineers are carrying out their pursuit for greater earthquake resiliency on bridges and through tunnels.

Our Future Ready^® specialists are using performance-based design practices, advanced geotechnical analysis and three-dimensional structural modeling to predict the behavior and potential damage of bridges during an earthquake.

This professional approach is informing the tunneling industry, as designers are mitigating these seismic risks by analyzing geological risk, creating design redundancies and enhancing each tunnel’s ability to swiftly recover after an event.

“An important consideration for a seismically resilient tunnel is adequate ductility,” said Stephen Klein, PE, GE, senior engineering manager at WSP. “Ductility ensures the tunnel lining can deform without significant damage. This is the key to meeting performance objectives and returning the tunnel to service as soon as possible after a major seismic event.

“Strain limits are often specified to indicate how much inelastic behavior is acceptable, Klein continued. “Some cracking is acceptable, but too much can lead to time-consuming repairs, increasing recovery time. Finding the right balance between the amount of inelastic behavior and the time to repair the tunnel is fundamental to achieving a resilient tunnel structure.”

One example is California’s East Bay Municipal Utility District Mokelumne Aqueducts, which supplies water to more than 1.4 million residents of the East Bay through an aqueduct system.

Approximately 10 miles of these aqueducts cross the Sacramento-San Joaquin River Delta using pile-supported elevated pipelines. Vulnerable to seismic hazards — partly because of the liquefaction of loose surficial soils and excessive structural deflections during strong ground shaking — a devastating earthquake damaging this part of the structure could disrupt the water supply to this region for an extended period of time.

To address this vulnerability, we are designing a 16.5-mile tunnel across the Delta and re-routing the existing aqueducts through this new tunnel.

“The tunnel will be within stable soils at a depth of about 120 to 150 feet, which will allow this lifeline to maintain operations following large earthquakes,” Klein said.

There is no one-size-fits-all answer for designing enhanced earthquake resilience among structures, both above and below the ground. Finding the answer requires a combination of modern innovations and practical, cost-effective applications.

“It requires a lot of creative thinking, because you have to approach these seismic issues a bit differently,” said Matt Fowler, PE, vice president and senior engineering manager at WSP. Since 1984, he has worked on numerous geotechnical and tunneling projects for the firm that address seismic resiliency, including multiple subway projects for Los Angeles Metro and the San Francisco Municipal Transportation Agency.

“There are strong criteria to abide by, but you still need ingenuity and technical excellence,” Fowler said. “You need engineers with a specialized open mindedness, combined with peer review and close collaboration between both structural and geotechnical experts.”

For more information about structural designs and resiliency initiatives, see this article in Structure Magazine.