The first all-precast state office building in California is high on design, low on budget and abundant with “green” features.
By Deborah Huso
You wouldn’t know it to look at it or to walk inside, but the CalTrans District 3 Office Building in Marysville, Calif., was a structure on a budget. Built at a cost of $65.7 million, tight by the standards of creating a 208,000-square-foot office space designed to accommodate some 700 employees comfortably, this striking structure built for the California Department of Transportation (CalTrans) was recently granted a LEED Silver rating by the U.S. Green Building Council. Several green features earned the five-story structure that coveted rating, but not least among them is the building’s all-precast construction.
The building is the first all-in-one total precast state office building in California. It was designed with architecturally finished precast columns, beams and floor planks, all part of a hybrid moment frame system designed for the ultimate in seismic stability.
The design-build team that came to work on the project, A.C. Martin Partners architects and Turner Construction Co., were awarded the design and construction contract after competing against two other plans that called for a steel frame structure. “I’ve used precast as an architectural cladding,” says project architect and A.C. Martin principal Carey McLeod, “but this was the first project our company had using a hybrid moment frame. We felt precast would be time and cost effective, and we could integrate the cladding with the frame.”
McLeod and his design-build team were right on target, as they were awarded the CalTrans building contract in late 2006, and precast was the major differentiator for the team. “The other two teams couldn’t do the project within the called-for budget, because they were using a steel frame,” McLeod says.
Precast builds energy efficiency
Precast concrete comprises the building’s structure and cladding as well as its plank flooring. “All the exterior columns and beams are architectural as well as structural,” says Don Clark, president of Clark Pacific, which provided the precast concrete for the project.
The interior of the building is centered around its four-story “canyon”, an open, atrium-like space, which most of the interior office areas overlook. “The open canyon purges hot air and lets cold air come in,” explains Clark. “That cold air is absorbed into the concrete and radiates coolness out during the day.” This reduces the load on the building’s HVAC system.
The CalTrans District 3 building was designed to be 27 percent more efficient than code requires, and precast helped meet that design goal. Apart from precast concrete’s naturally green features, like the thermal mass it provides, the building’s structure also earned points toward LEED certification, because the concrete was manufactured and cast locally.
Simple and smart design
What makes the CalTrans building especially noteworthy, however, is how so many budget-friendly pieces came together to create a structure that is at once simple and striking. The building was designed to look like a studio; appropriately enough, since it houses landscape architects and transportation designers. The office space is open and centered around the canyon, which was created to mimic the landscape of the surrounding Sierra Nevadas. The major cost-saving difference with the canyon, however, is that it allowed for the use of fire shutters to separate one level of the building from another in the event of a fire.
“All-precast construction provided for a really open structure,” says Clark. “They left the precast exposed on the interior to create a loft look.” Clark says he likes how the interior of the building provides a sense of “high space.” Steve Schultz, manager of Design-Build Services with Turner Construction, says the precast construction simplifies interior detailing. “You don’t have to box in steel columns,” he points out. “The loft look of the building really decreases cost.”
Every workspace in the building is within 37 feet of natural light, due in large part to the structure’s long east-west axis and its multitude of windows. The building uses light-reflecting louvers at the canyon’s ceiling to illuminate the workspaces below, reducing dependency on electric lighting.
Seismic stability that outperforms steel
While the CalTrans building design is a standout to the casual observer, one doesn’t readily see the safety provided by its hybrid moment frame. The building is designed to “self-right” in the event of an earthquake. “The code in California requires that after an earthquake your building should not collapse,” Clark says, “but it’s not required to self-right.” In the case of the CalTrans structure, it goes beyond code. Not only will the structure not collapse, but it will also return to vertical with very little structural damage. “It’s probably superior in seismic performance to steel,” adds McLeod.
“This is a very well-tested system being used on a lot of precast buildings,” says project engineer Lawrence Ho, a principal at Englekirk Engineers. The hybrid moment frame consists of the connection of precast columns and beams using standard reinforcing steel and high-strength, post-tensioning cables. This setup offers both the ability of the frame to absorb energy through the movement of the joints and the ability to provide the shear and moment resistance that holds the joints together. Thus, in the case of a seismic event, the building will flex like a rubber band,” says Ho.
“The yielding of the reinforcing steel and post-tensioning system is elastic,” Ho explains. “It has the ability to go through a lot of yielding, but it can also restore balance afterward.”
Ho says that after an earthquake, repairs on a hybrid moment frame structure would be minimal. “Normally, reinforcing steel is imbedded in concrete, so movement would cause spalling,” he points out. “But in a hybrid frame system, in the yielding zone, we debond the rebar with the concrete.” Ho says the debond zone is about 12 inches long.
Meeting a tight budget
McLeod says the major challenge in designing the CalTrans building was keeping the project within budget. “At the time, the rising cost of construction made it hard to do,” he says. The building’s prefab construction saved substantially on building costs, keeping the construction budget at $286 per square foot. The reduced construction cost was the major reason why the design-build team of A.C. Martin and Turner Construction was awarded the project. They competed against teams that proposed steel frame structures with architectural precast panels for the exterior.
The CalTrans building also has precast flat plank floors. Schultz says he prefers the flat plank system over more typical double-tee construction, because it’s harder for subcontractors to work around the stud walls in the latter. “The trade-off is that the spans aren’t as long with flat plank.” But the hollow core of flat planks can also serve as a chase for electrical and mechanical utilities.
The structure also came in on budget because of the time savings afforded by precast construction. The precast components took about three months to manufacture in the factory and then just under three months to erect on site, and erection began while some pieces were still under construction in the factory.
“The key to the success of this project was that we worked very closely with the architect up front,” Clark explains. “He learned our system, and we looked for opportunities for repetition.” Even with the repetition of panels, however, the building still has a unique look from the exterior, thanks in part to the different colors of the concrete and the use of brick cast into the concrete. “The goal on precast is to have fewer pieces and fewer different types of pieces,” Clark adds. That’s where cost savings come in, he says.
“The more precast you can expose, the more cost efficient the project becomes,” Clark explains, pointing out how, in a steel structure, the builder would have had to spend the extra money and time in cladding the structure both on the exterior and around interior columns. The largest single piece on the CalTrans building, the moment frame columns, weighed in at about 80,000 pounds, according to Clark.
The longest span in the building was about 42 feet, according to Ho. “People think precast can’t handle a heavy load, but it can,” he says. “It’s lighter than a cast-in-place system, too.” The advantage is that there are no shear panels in the building. “The shear is handled in the moment frame,” says Schultz. “That’s the biggest design-build benefit.”
The repetitive scheme of the building’s structure and facade also helped keep costs low. According to McLeod, the building featured only two long span dimensions. But the addition of different concrete colors and brick on the exterior keep the facade from looking monotonous despite its simplicity.
McLeod believes early collaboration makes the difference when working with precast. “Have the key players on board when the project is conceived,” he advises.
“The speed of construction and the seismic stability — what we’ve done with this building is an example of why you should consider precast,” adds Ho.
Deborah Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.