Navajo Bridge Project
CEE321 Structural Analysis & Design Extra Credit Paper
The original Navajo Bridge was built by the Arizona Highway Department, and designed by the Kansas City Structural Steel Company in 1929. Governors from Utah, Colorado, New Mexico, and Arizona, along with 5000 other people attended the opening of the new bridge. In 1929, Arizona was not a very developed state, and only dirt roads approached both sides of the bridge. Before the bridge was built, Lees Ferry was the only means of transportation between Arizona and Utah. Lees Ferry was a undependable way to get across the Colorado River, because during storms, or floods, the ferry would not run. The Navajo bridge remained the only link between Utah and Arizona for almost thirty years. The engineering techniques that were used to build this bridge were cutting edge technology in 1929. When the old Navajo bridge was finished it was the highest steel-arch bridge in the country, extending 470 feet above the Colorado River. The bridge currently connects The Grand Canyon National Park and Glen National Park, which are on one side of the bridge, to a Navajo Indian Reservation which is on the other side of the bridge (Thomas 1991,Yozwaik 1995).
When the original bridge was built in 1929, as seen in the below photo, bridge building regulations were different when compared to modern day regulations. The original bridges maximum load capacity (22.5 tons) was not designed to support heavier modern day vehicles. Both the loads and widths of these modern day transportation vehicles are a lot larger than the trucks used in 1929. Engineers, in 1929, had no idea that the size of vehicles would increase so drastically, and 18 feet wide seemed more than sufficient for two cars to pass one another in opposite directions. If the engineers wanted to make the bridge any wider than 18 feet in 1929, it would be necessary to add a third arch to the bridge which would have cost more and seemed unnecessary at the time. In 1990, with the new traffic flow, a width of 18 feet was insufficient. Other factors that played a role in deciding to build a new Navajo bridge were the sharp turns on each side of the bridge that decreased visibility and the load limit of the old bridge was not sufficient enough. The posted load limit was 40 tons where the actual load limit should have been 22.5 tons (Cannon 1994).
Due to the mentioned transportation problems, state officials made the decision that something needed to be done about the old bridge. Cannon and Associates competed for the project along with other firms. They had to summit a cost proposal and a design proposal for the bridge. The Arizona State Department of Transportation looked at all the proposals of the different engineering firms and decided to hire Cannon and Associates.
There were doubts as to whether this project could be undertaken because of the stringent environmental regulations. Cannon and Associates needed to follow the National Environmental Policy Act, which requires organizations to examine the environmental consequences of any project about to be undertaken. In the Navajo Bridge Project, consideration was given to the endangered species, the rock falling into the canyon, disturbance of the water, not damaging the sides of the canyon, plus deciding what was going to be done with the existing bridge which was on the historic register. When an item is listed on the historic register the item is only allowed to be fixed or improved, but it cannot be destroyed (Cannon 2000).
Measures that were taken to build the bridge in accordance to the National Environmental Policy Act were:
1. The new bridge would be built in the same style as the original bridge, which was a steel spandrel structure.
2. The original bridge would remain undisturbed and be used as a pedestrian bridge.
3. Material would be kept from falling into the Canyon.
4. A visitor center would be built to inform visitors about the history of the site, the vistors center which can be seen in the below picture was designed to blend into the environment by using similar rocks on the outside of the structure that would be found in the surrounding environment.
5. Special measures were to be taken to not destroy a endangered species of cactus that was found on the canyon walls (Cannon 1994).
Cannon and Associates also had to deal with opposing interest groups such as, The Sierra Club, The River Rafters, and the Navajo Nation. One of the jobs Cannon and Associates needed to do was meet with these interest groups, listen to their concerns with the project, and take these concerns into consideration. Some of the concerns that were discussed during these meetings were:
1. Traffic flow would be increased, with the new larger bridge.
2. Having two bridges would be an eye sore.
3. Traffic would still be allowed to continue on the old bridge until the new bridge was finished.
4. The river rafters would still be allowed to float past under the bridge during construction (Cannon 2000).
If the concerns of these interest groups were ignored, these groups would have opposed the project. This would have been a problem because 20% of the 15 million dollar project was funded by the Arizona Department of Transportation (ADOT), and 80% by The Federal Highway Administration. These agencies were both concerned with the approval of the general public, so if there were complaints from certain interest groups, red flags would have gone up and the project would have either been delayed, or canceled.
There was a $250,000 study done to decide whether the old bridge needed to be widen, or a new bridge needed to be built. The study also involved deciding where the bridge was going to be placed. One idea was to cut straight across instead of the current elliptical shape that the road has. This seemed like the most logical approach, but when a cost analysis was done it was realized that this option was too expensive, and would disturb sacred land.
Some of the proposals to solving the Navajo Bridge problem that were taken into consideration, were to widen the bridge, to build another bridge and have one bridge for traffic in one direction, and another bridge for traffic in the other direction, or build a whole new bridge for traffic. Whatever was done, the old bridge could not be torn down because it was on the historic register.
The solution that Cannon and Associates proposed to ADOT and the Federal Highway Administration was to widen the existing bridge which would cost around $5.8 million. ADOT turned down the proposal because the bridge could only be widened to 24 feet where the current standard width of a bridge is about 40 feet. An alternate proposal was to have two bridges with traffic going in one direction on one bridge, and traffic on the other bridge going in the other direction. This idea was also rejected because ADOT felt that it would create an unusual traffic situation because the approaching roads would have needed to be split. The idea that was finally used was to build a bridge parallel and adjacent to the existing bridge, and use the old bridge as a pedestrian bridge for tourists. This was the best solution because the old bridge would be put to good use, the traffic problems would be solved with an increased load capacity and a larger width on the new bridge, and the interest groups would be satisfied because an information center would be built to inform travelers of the history of the bridge. The photos below are photos of the new bridge which is the same style as the original bridge.
During the geotechnical evaluation of the bridge, the canyon walls were divided into three different zones: A, B, and C. These zones specified the stability of each particular site being evaluated. Site A was the most stable, and Site C was least stable. The location of the bridge was decided to be 150 feet down river from the original bridge. The bridge was to be supported by "concrete footings on dolomitic limestone (Cannon 1994)."
Since the bridge is 430 feet above the Colorado River, a method called cantilevering was used to build the bridge. Cantilevering is done by using a tieback device to hold up the bridge, and the bridge is built piece by piece from each side of the canyon. When determining the necessary strength of the tieback, wind, temporary decking, debris netting, a mobile crane and a material transport vehicle all needed to be taken into consideration. It was decided that "on each side of the canyon, workers drilled 20 rock anchors 55 feet into the rock for each arch rib and post-tensioned them (Green 1995)." This method was used so machines and people would be able to go out on the bridge when it was still being built; this was a very useful method for this situation considering that the bridge was so high off the ground.
One problem that occurred while erecting the bridge, was a miscalculation of the anchor bolts position. Each bolt in the shoes, which were located at each end of the bridge, were offset by 1 5/8 inch. To fix this problem the steel castings, which have holes in them that are meant to fit the shoes, needed to be returned to have the holes widened to accommodate the mistake. This mistake cost the project an extra $42,000 which was picked up by Arizona Department of Transportation. The mistake was not the Department of Transportations fault, but they agreed to pay for it, calling it a building cost.
In September of 1995, the new bridge was dedicated. A dozen antique cars drove across the bridge, and a bucket of the Colorado River was dumped on the bridge in the dedication of the new bridge. Today, the new bridge is fully operational, and the old bridge is still being utilized as a pedestrian bridge (Shyr 1995).
From the beginning, to the end of this project, Cannon and Associates needed to address many different aspects of engineering. Cannon and Associates were able to focus on each aspect of the project with equal importance. Whether the focus was the actual design of the bridge, the cost analysis of the bridge, or public relations issues, no aspect was considered of any lesser of importance, which was the reason for the tremendous success in the completion of the Navajo Bridge.
Cannon, Jerry A., and Robert D. Turton. "Spanning the Grand Canyon." Civil Engineering November 1994: 38-41.
Cannon, Jerry A., "Navajo Bridge Project. CEE296 Classroom Presentation. Arizona State University, Tempe. 22 Feb. 2000.
Green, Peter. "Grand Canyon Span." Engineering News-Record February 13, 1995: 28-30.
Shyr, Luna I., "New Navajo Bridge Replaces Historic Span." Phoenix Gazette 15 Sep.1995, final ed.: B6.
Thomas, Bob, "Marble Canyon to cross new bridge Modern span to join landmark in linking banks of Colorado." Arizona Republic 7 April 1991.
Yozwiak, Steve, "Marble Canyon Getting a New Navajo Bridge." Arizona Republic 15 April 1995.