THE CONCRETE BRIDGE MAGAZINE

SPRING 2010

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Issue link: http://www.aspiremagazinebyengineers.com/i/306855

Contents of this Issue

Navigation

Page 19 of 59

composite steel deck would result in a very abrupt, mechanical looking connection with an integral crown. Equally significant was the high rise of the arch. When studied in either concrete or steel, an integral crown solution looked blocky and massive at the crown, and ran counter to the architectural goal of lightness and openness when viewed from Lake Mead. Cross Section Forms The height of the tallest tapered spandrel columns is almost 92 m (302 ft). Wind studies included considerations of drag and vortex shedding on the main structural sections exposed to the long canyon fetch from over Lake Mead. Studies showed that substantial advantage could be gained both in terms of vibration and drag by chamfering the corners of both the columns and the arch. While this adds somewhat to the complexity of construction, the benefit in terms of reduced demand and material savings were substantial. Construction Methods As with any large bridge structure, the dead load design is dominated by the assumptions of a construction scheme. The typical approach in the United States is to select an erection scheme, but to show it in the plans only schematically, and defer responsibility for both the scheme and the details to the contractor. The design management team decided that this structure was so unique that the typical approach could prove counterproductive in several respects. A substantial length of time for reviewing and approving an erection scheme might delay the project. The management team also believed that more informed bids could be developed if there was a more complete erection scheme shown with the plans. Therefore, the decision was made to show a complete erection scheme for dead load on the plans and allow the contractors to use that scheme or their own. Both precast and cast-in-place concrete methods were permitted for the arch and spandrel columns. The contract was written to allow alternative methods of erection, however the columns across the entire bridge were to be of a single type (precast or cast in place) in order to conform to the time-dependent assumptions inherent in design. Construction The first challenge for the construction team was creating a foothold for foundation construction. Climbing on the side of the cliff 800 ft over the river below was difficult enough, but excavating (and doing so within the loss limits in the specification) was an incredible challenge. The subcontractor who met this challenge was Ladd Construction from Redding, Calif. They not only met the tight schedule for this work, but completed the excavation allowing about half of the rockfall into the river that was permitted. Initial bridge construction began with footing and abutment work, and in the precast yard outside of Boulder City where the contractor set up their own facility to precast the columns. Column segments were trucked to the site as needed for erection, and set into place using both the high-line crane and conventional cranes located at the highway hairpin in Nevada. Four form traveler headings were operated in concert for the cast-in-place concrete arch. After an initial learning curve, the contractor reached a reliable cycle of 2 weeks, and often exceeded that on segments that did not contain a temporary stay for erection. The arch was closed in August 2009 within an impressive ¾-in. tolerance at closure. Spandrel columns were erected using the high-line crane, and superstructure girders continue to be set. The bridge is scheduled for opening in the Fall 2010. ____________ David Goodyear is senior vice president at T.Y. Lin International, Olympia, Wash. Progress on construction and additional background can be viewed online at www.hooverdambypass.org. This is a typical section through the twin-rib arch and roadway showing the piers and integral pier cap. The first precast pier segment, supported by the high-line crane is erected on its footing. The tallest of the tapered spandrel columns is almost 92 m (302 ft) tall. An illustration to show the use of cable stays to construct the arch segments and the position of the high-line crane. 18 | ASPIRE , Spring 2010 Colorado River_spr10-1.indd 18 4/30/14 10:58 AM

Articles in this issue

Links on this page

Archives of this issue

view archives of THE CONCRETE BRIDGE MAGAZINE - SPRING 2010