Mike O’Callaghan – Pat Tillman Memorial Bridge

By Micah Jacobson


You have probably seen or heard about the new bridge that has been constructed as a bypass to the Hoover dam. When I saw this bridge I was very impressed by its structure, construction and magnitude. The bridge is made up of over 1,000 ft twin concrete arches. They are concrete piers that connects the arch to the concrete piers cap that holds up the steel box girders.

The arches are 14’ x 20’ and spaced 45’ on center. They are connected by concrete panels containing steel struts for bracing. The longest span of the bridge (the span under the arch) is 1,060 ft. The total length is 1,900 ft. This is the longest concrete arch in the western hemisphere and the first concrete-steel arch composite bridge in the US.


The bridge uses 10 ksi of concrete. Typical structural concrete used for pre-stressed highway bridge is around 7 ksi of concrete. The concrete is high strength, but not as high strength as can be made. There is methods today that a produce concrete that exceeds 21 ksi. Half of a decade ago high strength concrete was considered to be 5 ksi. The problem when you use high strength or ultra-high strength concrete is that there is an increase in fragility. Concrete itself is fragile, unlike steel that can yield, warp and bend before failure. Concrete has a low strain tolerance before it fails. There is no yield plateau or strain hardening like you see in steel. As the strength of the concrete increases, the stress strain relationship increases (young’s modules). This is important for structures when it comes to seismic events. When an earthquake happens there is a lot of contorting by the structure. This causes a lot of strain on the structural elements. The area under the stress strain curves its material and of toughness. This is a measure of the energy the material can absorb before failing. High strength concrete starts diminishing under the curve as the strength increases and the stress strain relationship becomes steeper. As you gain strength, you lose toughness or the ability to absorb energy into the structure (such as shaking).

This bridge is certainly a marvel and fascinating piece of engineering to study, as is the design and consideration behind it.

Pictures from: http://faculty.wiu.edu/JR-Olsen/wiu/