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Equilibrium Engineers principal Ed Quesenberry is accustomed to accounting for wind and earthquake loads, but the Oregon Zoo’s Elephant Lands project came with another one.

“We had this other load case, which is the elephant load,” he said. “In a lot of cases, it was the elephant load that controlled the design. It was very interesting and very challenging.”

Engineering the 35-foot-tall, 20-inch-thick cast-in-place concrete walls being installed by Lease Crutcher Lewis crews required extensive calculations to ensure strength to withstand the force of an adult bull elephant. Also, two separate wall sections reinforced with rebar sandwich a layer of foam insulation to meet the zoo’s stringent energy efficiency requirements.

“It took a lot of engineering and planning before the first concrete was poured,” said Wayne Starkey, project engineer and owner’s representative.

The 13,000-square-foot barn and adjacent 17,000-square-foot Forest Hall indoor viewing area are part of a $57 million project to expand the zoo’s elephant habitat and enhance visitors’ experiences. When complete, the two attached buildings will be the largest the zoo has ever constructed and account for elephants’ ability to reach up to 24 feet standing on their hind legs.

“We had to meet with zoo staff and with the elephant keepers to determine … the reasonable design criteria for elephants,” Quesenberry said. “The biggest concern was the elephants are very strong. They can push on things.”

Equilibrium Engineers determined the required strength of the indoor habitat walls by accounting for the weight of the zoo’s heaviest elephant, Tusko – about 14,000 pounds. This included ensuring that the walls could withstand the impact an elephant of that size traveling at 5 mph as well as 7,000 pounds of leaning force. As a test, zoo trainers coaxed Tusko to push against a pressure gauge mounted on the wall of the existing elephant barn.

“With very little effort, he was able to impart a load of 4,000 pounds, so we knew the lean load of 7,000 pounds was OK,” Quesenberry said.

Even with the ability to test Tusko’s strength, a lot of the structural engineering had to be based on theory and research of similar projects, he said.

“It’s a very interesting design process because you’re dealing with these very unpredictable elephants,” he said. “A lot of it was based on anecdotal evidence from other zoos.”

Every part of the barn that elephants can touch must be solidly locked down to avoid damage, Quesenberry said. Steel posts in the stalls, for instance, will be filled with concrete and embedded 4 feet into the foundation.

“If they can move something, they’re interested and want to move it, so we tried to design around movement,” he said.

Vertically casting the tall concrete walls is a tricky process, Lease Crutcher Lewis senior project manager Andrew Dykeman said. Each wall pour requires seven to eight hours because crews have to add concrete slowly to avoid a pool at the bottom of the form, he said.

“It’s a very unique type of pour just because of the pour pressures we’re dealing with and the heights of the walls,” he said. “We don’t do pours like that very often.”

So far, crews have worked on walls for the elephant barn; Forest Hall will follow, Dykeman said. The plan is to have structures ready for elephants to move inside in March 2015. The entire project is scheduled to finish in October 2015.

Elephant Lands

Location: 4001 S.W. Canyon Road, Portland

Cost: $57 million

Construction start date: June 2013

Anticipated finish date: October 2015

Owner: Oregon Zoo

Architects: CLR Design, SRG Partnership

Engineer: Equilibrium Engineers

General contractor: Lease Crutcher Lewis