Beginning in June, tourists visiting Hoover Dam will enjoy the view and benefits of another modern marvel the Colorado River Bridge. Located approximately
Beginning in June, tourists visiting Hoover Dam will enjoy the view and benefits of another modern marvel Û the Colorado River Bridge. Located approximately 1,600 feet south of Hoover Dam, the 1,900-ft.-long structure will connect Nevada to Arizona at Sugar Loaf Mountain, nearly 900 feet above the Colorado River. Twin 1,600-ft.-long concrete arches Û the longest in North America and fourth largest in the world Û will support the bridge deck, providing also the span’s focal point.
The Colorado River Bridge is part of a $240 million Hoover Dam Bypass Project, comprising a 3.5-mile bypass corridor for U.S. Highway 93 that begins at mile post 2.2 in Clark County, Nev., and ends in County, Ariz., near mile post 1.7. The new route is intended to provide relief for motorists confronting the hairpin turns, steep inclines, and downhill slopes of U.S. 93, as well as heightened, post-9/11 security restrictions for Hoover Dam traffic. Since only trucks authorized through a local permit program are allowed to traverse Hoover Dam, which typically accommodates 14,000 vehicles a day, most commercial drivers are forced to travel an additional 32 miles to cross the river via an alternate course.
Like Hoover Dam, built in the early 1930s, the Colorado River Bridge is a massive, multi-year project. Under a joint venture between Obayashi Corp. and PSM Construction, USA, Inc., bridge construction began in early 2005. On a project of this magnitude, notes Terry Pawlowski, engineering manager for Obayashi/PSM JV, a joint venture works best for bonding and to share the risk.
Groundbreaking for the project entailed blasting and excavating 48,000 cu. yd. of earth to make way for bridge piers, abutments, and arch skewbacks. Access issues were resolved by erecting crane towers on both Nevada and Arizona sides of the job site to support a 50-ton cable crane and cable trolley for conveyance of workers, tools and equipment. The crane was vital to placing 614 precast column segments and cast-in-place arch components, which consumed more than 16,000 yards of concrete.
Early 2005 through Spring 2008 saw construction efforts targeting piers, abutments, approaches, and portions of the deck in preparation for the project’s most intricate and challenging phase Û erection of the twin rib arches. From the river’s Nevada bank, crews built out to Pier 5, which serves as an arch base; similarly, work proceeded from the mountain approach to Pier 14 on the Arizona side. Bridge deck construction also progressed to Piers 5 and 14.
Beginning in April 2008, Obayashi/PSM JV undertook the first of seven concrete pours to pave the 88-ft.-wide deck, which upon completion will accommodate four traffic lanes and a pedestrian walkway. When the last of 2,300 yards of bridge-deck concrete was placed, the contractor tackled erection of temporary pylon tower cranes atop Piers 5 and 14 in preparation for arch construction.
Serving as anchors during arch work, the towers comprised a shaft consisting of 13 precast segments, produced with 6,000-psi concrete, and a head formed with four, 10,000-psi concrete components. Temporary cable-stays supporting the arches would be attached to the tower heads; and, upon completion of arch construction, the cables and towers removed.
COMPLEX RIB ARCHES
After tower construction was completed in July 2008, erection of the twin arch ribs became a priority. Accordingly, simultaneous construction of north and south arches was staged in four headings Û two each emanating from the Nevada and Arizona sides to join in the center.
All three of the project’s arch-segment types required 10,000-psi concrete. In addition to typical segments, blister segments were formed to attach temporary stay-cables during arch construction. And, spandrel segments will accommodate piers 6 through 13, designed to support the bridge deck from the arches. In all, 104 individual segments Û 52 each for north and south arches Û were cast 10 to 26 feet at a time using a traveling form system. The arches jut out from Piers 5 and 14 at a 48 degree angle. Pawlowski reports that temporary strand equivalent in length to 350 miles was required to support the arches during construction.
The three arch-segment types had one thing in common: significant rebar congestion. To ensure concrete flow to all areas of the form, Obayashi/PSM JV used a ?-in. aggregate in the mix, delivered to the site at a 10-in. slump. Original plans for concrete consolidation dictated insertion of steel spirals in the walls to prevent internal vibrators getting hung up on surrounding rebar. However, Wacker Neuson Corp. representatives demonstrated an alternate method for achieving consolidation that offered labor savings and reduced congestion on the traveling form.
In early November 2007, the effectiveness of external vibration on one of the initial arch segments became evident as Wacker Neuson applied six AR36/6/042 external vibrator motors and two FUE 75 frequency inverters. The six vibrators were placed on the form and leapfrogged over each other, following concrete up the form. It was a long, 12-hour night, says Wacker Neuson Sales Engineering Manager Fred Paul, but, it gave us a chance to see how many guys were working on the form and where we could help improve efficiency.
Aiming to reduce the number of laborers, ease work load, and increase productivity, Wacker Neuson engineers mapped out scenarios using external vibrators and inverters for each form traveler. They faced the considerable challenge of finding the right vibrator pattern given the angle of arches, since vibrator motor positioning had to compensate for the fact that, as Paul explains, concrete finds true level and true plumb, which is not in line with the angle of the form travelers. Moreover, the cycle in which crews turned motor sets off and on had to be determined accordingly. Since external vibrators do not run continuously throughout the pour, different sets of vibrators were cycled for approximately one minute as concrete filled the form.
A total of 104 Wacker Neuson external vibrators Û 26 on each of four form travelers Û accomplished the job. Spaced approximately on 4-ft. centers, AR36/6/042 motors consolidated the 14-in.-thick web walls. More powerful AR54/6/042 units were spaced 10 feet apart to vibrate the 18-in.-thick top and bottoms. In addition to providing consolidation, Paul affirms, external vibration reduces internal friction of concrete to ensure the material flows into all crevices to encase the steel.
Three portable FUE 75 frequency inverters, moved from form to form, powered the AR36/6/042 and AR54/6/042 motors. The inverters take high input voltage and convert it to a lower, safer 42 volts to prevent accidental electrocution. They also provide speed stability and offer a variable motor vibration speed up to 6,000 vibrations per minute, adds Paul.
As arch work progressed, the external vibrators helped Obayashi/PSM JV deliver a high-quality segment finish, despite reduced labor and vibration time. The crews typically poured and consolidated an arch segment in about five hours, while the overall cycle to complete a segment ranged from 10 to 14 days. The externals helped achieve consolidation where the internals could not reach, Pawlowski asserts. They also contributed to surface finish by bringing the cream to the form and working out air bubbles.
The arch work concluded with two small, 6-ft. closure pours to join north and south segments, followed by removal of all temporary cable stays, requiring an additional three weeks. Thus, Fall 2009 completion of the arches paved the way for bridge-deck and final bypass wrap-up by the second half of 2010. Û www.wackergroup.com