Chance for tremor is slim, but bridges built to endure

Chance for tremor is slim, but bridges built to endure

September 14, 2011

Earthquake-proofing measures are part of the plan for the new Mescal Road/J-Six Ranch Bridge, near Benson.

The recent quake near Washington, D.C. served as a reminder that the earth occasionally moves in U.S. locales outside of California.

At ADOT, we know that Arizona is not immune to seismic activity, either. While earthquakes are far from commonplace in the Grand Canyon, our engineers take nothing for granted.

Across the state, highway and interstate bridges built since the 1970s have been designed to meet or exceed standards established by the American Association of State Highway and Transportation Officials. Older bridges have been retrofitted to help them hold up better against earthquakes.

Most of Arizona is believed to have little risk of earthquake, but certain areas – Coconino, Yuma and Cochise counties – are known to be vulnerable to seismic movement.

So when the ADOT Bridge Group set about designing the new Mescal Road/J-Six Ranch Bridge near Benson, earthquake-proofing measures were part of the plan.

That starts with enhanced reinforcement of the structure, including columns and foundation.

Each bridge is individually designed to hold up in the conditions unique to its surroundings. For instance, a location with loose sand or silt gets a deeper foundation including drilled shafts or piles.

Each bridge is built in a footprint wide enough to allow horizontal movement of the bridge. Those in potentially vulnerable areas – such as Mescal/J-Six are constructed with anchor bolts incorporated into the girders that allow controlled movement.

In the photo above, you can see the anchor bolts protruding out of the pier cap closest to the front of the picture. There is a hole at the end of each girder that goes through the anchor bolts. These bolts will allow for some movement of the girders during a seismic event, but not very much (a couple of inches laterally and less than that vertically).

You can also see the vertical fixed restrainers on the next pier cap in the photo. Those are basically cable ties that will fix the girders to that pier cap. The girders are placed on either side of the line of fixed restrainers and then a reinforced concrete diaphragm is poured in the gap, which completely ties the girders and the pier cap together. These fixed restrainers allow for no movement of the girders. The pier is permanently anchored and the rest are on the earthquake restrainers. The pier that doesn't move anchors the bridge ... the rest move and disperse any seismic activity.

Allowing for slight, limited movement during an earthquake (even a little one), means the bridges stay sound.

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