Seismic Matters

Designs using precast concrete in seismic regions protect lives and livelihoods.

The first responsibility of any building or structure is to protect the lives of the occupants inside. That is where the entire concept of resilience begins. Buildings will face any number of challenges during their usable life, and those stressors will vary depending on where they are built. Some may have to endure hurricanes or strong storms; others may have wildfires or other threats to contend with.

In many parts of the United States, earthquakes can be a real threat to the integrity of lives, communities, and buildings, so it is important to consider that when designing for life safety and resilience. There are many seismic zones in the western mountain states, and precast concrete producers in that part of the country have demonstrated many times why precast, prestressed concrete can be a great choice for projects being built in seismic zones.

The primary goal of seismic design is to protect the lives inside by preventing the complete failure of the structure. Damage to the building is acceptable as long as it does the work of shielding occupants from harm.

Structural Integrity
“The intention of the code is to provide for the integrity of the structure for people to get out and be safe through a seismic event,” explains Jim Linskens, design engineer with Wells Concrete in Colorado.

According to Designing with Precast and Prestressed Concrete, a PCI reference document, “the key reason designers have gravitated toward precast concrete components [for earthquake resistance] is because they can span long distances between attachments to the main structure. Design methods and details have been developed to accommodate these applications in seismic areas.”

In short, precast concrete is a material that is both incredibly durable and surprisingly fl exible. The right confi guration, with the right connections between panels, can create a structure that is very strong and provides suffi cient ductility to move with seismic energy and maintain integrity. The primary goal is to stand strong enough for long enough to protect occupants and allow them to exit to safety. Precast concrete has that capability.

“The current philosophy for the design of earthquake-resistant structures permits minor damage for moderate earthquakes and major damage for severe earthquakes, provided collapse is prevented,” the guide continues. “The design details often require large, inelastic deformations to occur to dissipate energy and shed inertial forces. This is achieved by providing member and connection ductility.”

Quick and Strong
The Brigham Young University Auditorium in Rexburg, Idaho, is designed with the idea of keeping occupants safe from earthquakes. It is located in a Category D seismic zone, and with an interior designed to seat 15,000 attendees, the importance of ensuring the integrity of the structure during a potential seismic event was top of mind.

The building’s primary shear wall is clad with an insulated carbon cast panel system, which also helps meet thermal high-performance requirements. The lightweight, 53-lb/ft2 precast concrete cladding supplied by EnCon Utah decreases the overall mass of the building and reduces the seismic base shear and seismic lateral force resistance system. Because of the thickness of the panels, a voided precast concrete panel system with prestressed super columns was used. The wall was constructed in just 10 weeks.

Schedule was also a driver for Palazzo Verdi in Colorado, another project that needed to take seismic safety into consideration. In that case, a total–precast concrete lateral system from Wells Concrete (formerly Rocky Mountain Prestress) was selected.

The building mass and lateral force distribution, vertically and horizontally through the available lateral system, was evaluated in specifi c detail and reevaluated throughout the project, Linskens says. “In Colorado, for heavier massed buildings like those built from precast concrete, seismic forces are predisposed to be the governing criteria over wind. We chose to design a central core to have enough strength and ductility for the entire magnitude of forces as prescribed by the code, but then also have the fi rst-level perimeter walls provide strength and resistance to the eccentric torsional forces they would attract to address the displacement, translation, and rotation of this perceived soft story diaphragm shift.”

Throughout the western United States, a region often impacted by earthquakes, precast concrete is frequently used in designs intended to resist seismic forces.

“Precast is an exceptional product for seismic design because you know exactly what you are getting,” explains Adam Bauer, owner and sales and marketing director with Missoula Concrete in Missoula, Mont. “It’s a lot of upfront work in engineering, but it’s proven before the panels go into place. Instead of working on a different building type where you are constantly adding and shoring up along the way, with precast we know for a fact what its output is prior to arrival.”

Missoula Concrete recently provided panels to a project in nearby Kalispell, Mont. The Flathead Valley Community College Performance Center and Gymnasium required a great deal of seismic resistance in its design. The design and engineering team on that project decided precast concrete was the answer. The project design required massive panels: 56 ft tall for the performance center and 32 ft high for the gymnasium.

“It’s a high-seismic zone because it’s right by Glacier National Park,” Bauer says. “The panel-to-panel connections were tremendous. There is a lot of hardware in those panels—about 650 pounds of hardware per panel, on average. There had to be a lot of coordination between the engineer of record, the precast concrete engineer and production to see what was possible.”

The performance hall uses noncomposite, 7-3-3 insulated sandwich walls panels, and 3½-4-3½ composite wall panels were used for the gymnasium. “When you use noncomposite panels, you have to beef up the concrete on the structural side,” Bauer says. “The loading capacity on the performance hall was the main driver for going with that seven-inch interior wythe.”

As the project nears completion, Bauer notes that success in seismic design with precast concrete comes from getting all the players together as early as possible. “It is critical for an engineer to be in contact with the precaster as early as possible to make sure you don’t over-engineer and that you come up with a solution that works efficiently.”

Live Demonstration
On March 18, 2020, a precast concrete producer in Utah had a real-life stress test of their seismic design. An earthquake measuring 5.7 on the Richter scale struck near the Forterra structural precast concrete plant in Salt Lake City. The strongest seismic event to hit that area since 1992, peak ground accelerations measured across the street from the plant were very close to the design-level accelerations required by the code for seismic design of buildings.

“Immediately after the initial shock, our engineering team mobilized to assess plant infrastructure and investigate any potential damage to our facilities, which are largely comprised of precast concrete wall panels supporting double-tee roofs,” recalls Lee Wegner, sales manager with Forterra in Salt Lake City. “We observed only a few instances of minor, nonstructural concrete spalling around the base of the wall panel. Otherwise, buildings withstood the seismic event unscathed.”

The facility was built almost 20 years ago and did its job when put to the seismic test. In a demonstration of functional resilience, the building kept occupants safe and then was able to return to usable function with little repair. Precast concrete buildings like this all across the West stand ready to stay strong, preserve communities, and protect life.