Soaring 275 meters above the Colorado River, the Mike O’Callaghan – Pat Tillman Memorial Bridge or Hoover Dam Bypass Bridge compliments one of the foremost engineering wonders of the world – the Hoover Dam. The 580-meter-long Colorado River crossing is the centerpiece of the $240 million, four-lane Hoover Dam Bypass Project, which included 5.6 kilometers of new approaches on both sides of the river and six other bridges. The program manager for the project was the Central Federal Lands Division of the US Federal Highway Administration. The project design team consisted of HDR, Jacobs and T.Y. Lin International (TYLI). HDR led the design team of the overall bypass, while TYLI led the design of the landmark Colorado River span.
With a main span of 323 meters, the Hoover Dam Bypass Bridge is the fourth-longest, single-span concrete arch bridge in the world. Each arch rib is made up of 52 cast-in-place sections with construction starting from the canyon walls and a closure pour that locks the two halves together. Approximately 6,880 cubic meters of high strength concrete is cast in the arches. The 440 3-meter-tall concrete segments were each precast off site and erected to form the pier columns. At 88 meters, the tallest of the precast columns are the world’s tallest of this type.
The Black Canyon below the dam is a 275-meter gorge with dramatic rock cliffs, steep canyon walls and a vast geological palette. Working in such a setting required rock cuts exceeding 30 meters in height, accounting for winds up to 113 kilometers per hour and setting concrete at night to avoid desert heat reaching more than 50°C.
Original design and innovation
The bridge design satisfied objectives for both architecture and performance. The environmental document required an open structure that would not encroach on the views of the dam or the surrounding terrain of the national historic landmark at Hoover Dam. The engineering performance needed to withstand the demands of wind, earthquake, and service loads well beyond the codified 75-year life.
The approach to design addressed each criterion with a deliberate engineering evaluation and design. The twin arch ribs evolved from the assessment of seismic performance. Twin ribs with ductile struts were a descendent of the shear link tower that the designer had developed for the new Bay Bridge in San Francisco. The twin ribs were also championed as more constructible, being of a size that could accommodate precast segmental construction if the contractor so desired.
The deck framing was a key element in the overall structural system. Due to the extreme height of the concrete columns, a bracing system for column and deck support was an essential consideration for engineering performance and construction cost. The design provided for a deck system that was made integral across the length of the bridge. Integral pier caps were designed using an innovative post-tensioned composite solution, instead of the more typical all steel pier cap solution. Concrete pier caps were selected over steel box cap sections to avoid the fracture-critical design and inspection considerations for a steel cap. The integral deck and cap system were designed as part of a continuous deck/diaphragm system used for lateral and longitudinal bracing of the columns. This deck provided lateral support to the tallest columns through rigid lateral supports at the abutments and integral connection to shorter columns within the column array. This structural system allowed the design to proceed without the need for interior column bracing.
High performance concrete was a must for an arch of this span. High performance concrete was used for stiffness and compressive strength in the arch and columns. The design team conducted concrete material testing to demonstrate the material performance that could be achieved with local production capabilities in order to assure design strengths required for the arch ribs (69 MPa) and for the precast segmental columns (41 MPa).
The presence of a local wind microclimate due to the terrain was obvious and presented a question to be answered in the course of design. The amount of time needed for a thorough assessment of wind conditions was not available to designers. The solution was to set an anemometer on site to record for six months of design, and correlate with the general wind speeds at the Las Vegas airport. The correlation over the six months of local record was then extrapolated over the 25 years of records at the airport to provide a statistical basis for projection of design wind speeds. The result was a 25 percent increase in design wind speeds due to the local terrain effects.
The design team conducted special site studies to address the seismicity of the site. At the time of the design, the U.S. seismic design code for bridges was in a state of flux. The current code was based on a 500-year return period, with a very modest peak ground acceleration (PGA) at the bridge site. The design team performed an extensive geological reconnaissance as part of a probabilistic site hazards analysis. The resulting design was based on a 1,000-year event, and the PGA was set at approximately double the then codified value. The new AASHTO PGA, established four years after the start of design, is approximately the same value as selected for design.
Overall, the bridge system was a careful integration of design and construction considerations. The use of precast segmental construction for the columns and steel box tubs for the superstructure allowed work to progress in parallel with construction of the foundations being carved into the rock of Black Canyon. The use of the steel superstructure reduced the risk of delays and eliminated control issues inherent with a cast-in-place concrete superstructure in the open environment over the gorge. The cast-in-place stayed method for the arch ribs allowed the most flexibility for closing the arch without affecting the geometry of columns and deck (since they were not in place until after closure).
The Mike O’Callaghan – Pat Tillman Memorial Bridge exemplifies innovation at work. The design team overcame formidable obstacles and as a result, a world-class structure was born. It now frames the view of the Black Canyon from Hoover Dam for the coming generations of tourists, and is the cornerstone in a new, efficient highway system funneling commercial traffic between the states of Nevada and Arizona.
Innovative construction method
The construction contract was awarded in September of 2004 to Obayashi-PSM, JV. A limited notice to proceed was issued for November 2004, with full field work beginning in 2005.
The Hoover Dam Bypass Bridge was built using a variety of limited access techniques in order to gain access from the canyon rims. With cables hanging from 101-meter-tall towers, a 12.2-metric-ton trolley and load block assembly was the key to delivery of men and materials to the work front. The concrete segments for the arch were poured using four headings of self-advancing form travelers. Most of the arch segments were placed at night to avoid the triple-digit desert temperatures. Liquid nitrogen was required to control the peak curing temperature of the high performance concrete. Every second arch segment was supported by a temporary stay cable, with tuning required at many stages in order to control geometry. Tuning was also an element of design, in that the designer targeted a limiting bending moment at the springing in order to control demands in the arch ribs. The contractor reached a reliable cycle of two weeks for casting arch segments, and often bested that cycle on segments that did not have a stay cable to install. The arch was closed within an impressive 19-millimeter tolerance at closure.
The Mike O’Callaghan – Pat Tillman Memorial Bridge exemplifies innovation at work. The design team overcame formidable obstacles and as a result, a world-class structure was born. It now frames the view of the Black Canyon from Hoover Dam for the coming generations of tourists, and is the cornerstone in a new, efficient highway system funneling commercial traffic between the states of Nevada and Arizona. The project reflects the skill and determination of the people who built it, all of whom take pride in their accomplishment. A groundbreaking dedication ceremony was held on October 14, 2010, and the bridge opened to traffic on October 19, 2010. The new bridge was named in memory of two heroes from the States of Nevada and Arizona.
David Goodyear, P.E., S.E., PEng is the chief bridge engineer at T.Y. Lin International in San Francisco. He can be contacted at email@example.com.