Dimond Engineered to Withstand Earthquakes

After the 7.1 magnitude earthquake on January 24 2016, it became abundantly clear how important structural engineering is.

Back in 2010 when Haiti had a magnitude 7 earthquake, the country had widespread devastation. The same magnitude of earthquake in Alaska caused very little damage.

The reason for the difference is that Alaska has strict building codes that make sure that all buildings are made to stand up to strong earthquakes.

Bruce Hopper, a structural engineer at Stantec, which designed Dimond High (at the time it was USKH Engineering) and who signed off on the final design, said that when an earthquake hits and the ground shakes, the main reason for building damage is inertia.

“Earthquakes are an inertial force,” he said.

When the ground moves, because of Newton’s first law of motion, (which states that an object at rest stays at rest unless acted on by outside force), the upper part of the building stays put while the foundation moves,  causing the building to collapse.

“The building is just stationary in space, and then the ground moves, and then the structural engineer’s job is to make sure that the framing brings the roof along because that roof wants to stay where it is,” said Hopper. “So we do that with a number of different methods.”

At Dimond, one of the main features protecting the building is called a cross brace. Hidden in many of the walls and visible in the commons are two beams in an X shape. They help make sure that the roof moves with the foundation.

The X shape creates triangles, which are very resistant to both lateral movement and collapse because the angle in a triangle cannot change without changing the side length, meaning that the joints in a triangular shape do not have to be as strong.

They are spaced throughout the building so that their combined strength also prevents twisting. “It’s this cross brace frame that we’ve embedded inside of the structure at Dimond High School that’s resisting all these seismic loads,” said Hopper.

Another of the features that is specific to Dimond has to do with the shape of the building.

When a building is laid out, the engineers use a grid system for the walls and cross braces. The primary strength of the building lies along these lines where the cross braces are most effective.

Dimond has two sections, the area with the A and B classroom wings and the area with the auditeria, gym and music rooms.

In the two different sections the primary strength lines are turned 45 degrees to each other, so the sections will not move the same way.

“During an earthquake we’re going to have incompatible movement between the two sections of the building. So what we’ve done is separated the two buildings, it’s actually two structures… Between those two there is a great big what we call a seismic joint, so there’s about 10 inches of separation between the two sections.”

The joint can be seen along the hallway behind the elevators.

There is a similar joint in the Ted Stevens International Airport.

Another feature that all buildings have is redundancy.

Every feature that keeps Dimond standing either has many permutations or is far stronger than the engineers expect to be necessary.

Hopper said, “We don’t rely on just one brace frame to do it all, we put several brace frames in so that we have redundant systems in there. So what we want to do is if we can make two of them work, then we put four in to make sure that we have extras in there, we have a safety factor built in… In case one fails, we want to make sure that we have a backup for that somewhere.”

When designing a building, engineers always have to include a safety factor.

They can predict and calculate how much load the structure is expected to take, and how strong each part should be. They then multiply that by the safety factor, about 125 percent, so that the building will stay up even if the forces are far higher than expected.

This is especially important in high-risk buildings like schools, hospitals and fire stations where there are a lot of people that may not all be able to get out.

The ultimate goal of structural engineers is to protect the people inside the buildings, even at the cost of the building itself.

In some places, the beams supporting the building are designed to bend and absorb destructive energy as friction and heat.

“We’re taking that seismic energy that’s coming in this brace and we’re allowing this plate to bend and we’re also allowing the brace element itself to bend out of plane, and every time it bends it absorbs some of that seismic energy and then we’re converting it into friction or heat inside here. So we’re not trying to resist the seismic force, we’re just trying to absorb that energy and stop the building from moving,” said Hopper.

This damages the beams, but keeps that energy from doing harm elsewhere, and the bent pieces are replaced after the earthquake.

When designing a building, the ground it is on must also be taken into account.

Extensive surveys are done before the engineers start on the design.

When an earthquake strikes, the ground vibrates at a certain frequency, in Anchorage about 2 hertz (cycles per second).

If the building were to shake at the same frequency it would result in harmonic motion and far greater stress on the building. Therefore, the building is designed to shake at a different frequency.

Stephen Jelinek, who worked for Alcan General, the general contractor that constructed Dimond, said that the materials used for Dimond were important to making it safe and efficient.

The foundation is concrete reinforced with rebar (reinforcing bar), steel bars used to create tension.

The frame is structural steel.

Above the ceilings is ribbed metal decking.

These materials were chosen for their strength, resistance to vandalism and efficiency in cost and heat retention.

In contrast, the Anchorage Museum, which is paneled in glass to look cool, is less cost effective because glass lets out heat.

Dimond is resistant to earthquakes because Alaska has stringent building codes and engineers put a lot of effort into making sure that they are strong.cross bracesDimondBuilding1Photo Courtesy of Bruce HopperDimondBuilding2Photo Courtesy of Bruce Hopper