David Goodyear, T.Y. Lin International
The Colorado River Bridge at Hoover Dam crosses the nearly 900-ft (274-m) deep Black Canyon, and sits downstream of the famous Hoover Dam—a monument to engineering in general and concrete technology in particular. Selection of the right structural system for this bridge meant defining the character of a long-span bridge that respected the pioneering work of the great dam builders, and the grandeur of the Black Canyon setting. Concrete was not the only choice, but certainly the most natural in this setting.
Bridge Type
The bridge type selection was guided through two focus groups. The technical issues were presented to a Structural Management Group (SMG) comprised of the state and federal bridge engineers and peer review consultants. The aesthetic issues were presented to a Design Advisory Panel (DAP) comprised of state historic preservation officers, the National Park Service (NPS), Bureau of Reclamation (BOR), Native American representatives, and architectural consultants. Both the SMG and the DAP groups converged quickly on a deck arch as the correct solution to meet both the engineering and aesthetic demands for the project. Once the options for the arches were presented to the executives of the five leading agencies (the Federal Highway Administration Central Federal Lands Highway Division, states of Arizona and Nevada Departments of Transportation, BOR, and NPS), the unanimous selection of a concrete arch set the direction for design.
High Performance Concrete
High performance concrete (HPC) was the designer’s focus from the beginning. There are many characteristics of HPC that provide advantages for a long-span arch, including superior durability, strength, and stiffness. The arch form is an ideal application for concrete owing to the primary compressive strength of a simple concrete box section typically used for the arch rib. In the case of the Colorado River Bridge at Hoover Dam, the 1060-ft (323-m) long arch span required more than just strength. Several aspects of design were controlled by both immediate and time-dependent arch deflections. Here, the stiffness of HPC became an important parameter, surpassing strength in its benefit to design.
The customary concrete strengths for highway design in the region of Hoover Dam are on the order of 4000 to 5000 psi (28 to 34 MPa). The preliminary arch designs showed the great advantages of HPC for such a long-span arch, with even ultra-high performance concrete being reviewed for possible application. As the proposal for high strength HPC was advanced, questions were raised about the ability to produce consistent, high strength concrete and deliver it over the canyon. Additionally, the typical questions about material properties, creep, and shrinkage were highlighted due to the 1060-ft (323-m) long span of the arch. As a result, the project design team retained CTLGroup to develop a demonstration program for HPC using the local materials that would be available to the contractor. This allowed comprehensive testing for the key properties of strength, durability, workability, creep, and shrinkage to better inform the design team, as well as give the prospective bidders a reference point for their own mix design work under the construction contract.
Mix Design Program
The mix design program included a range of approaches, virtually all of which confirmed that the 56-day strength target of 10,000 psi (69 MPa) was achievable. Testing results were consistent with published test results and showed the superior properties of HPC in terms of durability and dimensional stability. The low permeability and low specific creep typical of high strength HPC were confirmed. The testing program also supported the project’s preference to not require job-specific creep testing in the course of construction. Creep tests are time consuming and, in the opinion of the designer, not well suited for the construction phase of a project that starts off with concrete production. The specific creep measured in the testing program was less than half of that typical for conventional concretes. And while the design proceeded on the basis of conventional creep factors, the dimensional stability of HPC was seen as an additional margin warranted for such a significant structure.
Arch Design
The topography of the site required a high rise to the arch. The high rise of the arch ribs, the use of composite deck construction, and the logistics of form traveler construction led to the use of an open spandrel crown as opposed to an integral crown. This meant that arch stability for asymmetric live load would not rely on integral deck framing at the crown. This same geometry affected the earthquake response of the arch ribs, allowing the more flexible framing system with greater deformation along the bridge, and increasing the period of response to limit seismic demands. The latter are most significant at the arch springing, where traditional arch rib design would include increasing the section size to resist higher moments. HPC allowed for a smaller arch cross section and mass, while maintaining requisite strength and stiffness. Arch deflections also controlled spandrel column design and articulation. Secondary moments in the spandrel columns due to long-term arch deflection were a considerable portion of total demand. The superior stiffness of the HPC was key to using the same prismatic section down to the springing and the integral framing of the end spandrel columns.
Summary
It is difficult to imagine a more appropriate application of HPC than a long-span arch such as the new Colorado River Bridge at Hoover Dam. HPC helped make this magnificent span in the shadow of Hoover Dam possible and practical. And the delivery of consistently high quality HPC by the construction contractor showed that even in the harshest of climates, HPC is an excellent choice for long-span bridge construction.
Further Information
Further information about the design of the bridge is provided in ASPIRE™ Spring 2010.
