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An Efficient Change of Direction

Few modern-day structures match the grandeur or complexity of Hoover Dam. The American Society of Civil Engineers rates it as one of America’s seven modern civil engineering wonders. Standing 724 feet tall and 660 feet at its widest point, the dam draws nearly 1 million tourists annually.

Beginning in June, 2010, tourists visiting Hoover Dam will have one more modern marvel to see, the majestic Colorado River Bridge. Located approximately 1,600 feet south of Hoover Dam, the 1,900-foot-long bridge will connect Nevada to Arizona at Sugar Loaf Mountain, nearly 900 feet above the Colorado River. The focal point of the bridge, twin 1,600-foot-long concrete arches – the longest in North America and fourth largest in the world – will support the bridge deck. 

While tourists will find the bridge visually appealing, it is being built out of necessity. The bridge is part of a $240 million Hoover Dam Bypass Project. This 3.5-mile bypass corridor for U.S. Highway 93 begins at mile post 2.2 in Clark County, Nev., and ends in County, Ariz., near milepost 1.7.

Motorists and commercial traffic will see the completed bypass as a welcome relief to the hairpin turns and steep inclines and downhill slopes for this section of U.S. 93. With the heightened security after the September 11 attacks on the Twin Towers, the 14,000 vehicles crossing Hoover Dam each day are subject to inspection. Most of the commercial traffic is forced to take an alternate route to crossing the river, which adds 17 miles to the trip. “Only those trucks authorized through a local permit program for local businesses are allowed to cross Hoover Dam,” says Dave Zanetell, project manager for the Federal Highway Administration (FHWA).

Massive Undertaking

Similar to constructing Hoover Dam in the early 1930s, the Colorado River Bridge is a massive, multi-year project. Bridge construction began in early 2005 by a joint venture between Obayashi Corporation and PSM Construction, USA, Inc. “On a project of this magnitude, a joint venture works best for bonding and to share the risk,” says Terry Pawlowski, project manager for Obayashi/PSM JV.

Initially, 48,000 yd3 of earth was blasted and excavated to make way for bridge piers and abutments and arch skewbacks. The bridge requires massive amounts of steel and raw material. Nearly 7 million pounds of structural and 7.7 million pounds of reinforcing steel will be used. A combination of more than 2.3 million pounds of temporary and permanent post-tensioning steel will also be required.

With access being a major challenge for building the concrete bridge, crane towers were erected on both the Nevada and Arizona sides of the project. They support a 50-ton cable crane and cable trolley that transports workers, tools and equipment to where they are needed. This crane is the major key for building and placing the 986 precast column segments and cast-in-place arch segments, which will consume more than 25,000 cubic yards of concrete.

From early 2005 through the spring of 2008, bridge work centered on the piers, abutments, approaches and part of the deck in preparation for the most intricate and challenging portion of construction, the twin-rib arches. On the Nevada side of the river, crews built out to pier 5, which serves as an arch base. Similarly, work progressed from the mountain approach to pier 14 on the Arizona side.

At 88 feet wide, the Colorado River Bridge deck will accommodate four traffic lanes and a pedestrian walkway. Bride deck construction has also progressed to piers 5 and 14. Beginning in April 2008, Obayashi/PSM JV made the first of seven concrete pours to pave the deck. When the last of the 2,300 yd3 of bridge deck concrete was paved, the contractor turned its efforts to constructing temporary tower cranes atop piers 5 and 14 in preparation for arch construction.

Complex Rib Arches 

These towers serve as the anchors while arch construction progresses. Thirteen precast tower segments constructed with 6,000 PSI concrete make up the shaft of each tower, while four, 10,000-PSI concrete segments form the tower head. “The temporary cable stays that support the arches attach to the tower heads,” mentions Pawlowski. “Once the arch work is completed, the cables and towers will be removed.” 

With tower construction completed in July, 2008, work proceeded on the twin-rib arches. The north and south arches are being constructed simultaneously in four parts, two each emanating from the Nevada and Arizona sides, that will join in the center. 

Three different arch segment types are formed during construction, all requiring a 10,000 PSI concrete. In addition to the standard segments, blister segments are being formed to attach the temporary supporting cables during arch construction and spandrel segments will accommodate piers 6 through 13, which support the bridge deck from the arches. “We will be using the equivalent of 350 miles of temporary strand to support the arches during construction,” mentions Pawlowski. 

In all, 106 individual segments – 53 each for the north and south arches – are cast 22 to 27 feet at a time using a traveler form system. The arches jut out from Piers 5 and 14 at a 48˚ angle.

The three different types of segments have one thing in common: significant rebar congestion. To ensure concrete flows to all areas of the form, Obayashi/PSM JV is using a 3/8-inch aggregate in the mix, which is delivered to the site at a10-inch slump. Originally for concrete consolidation, steel spirals were designed into the rebar in the walls, so internal vibrators would not get caught. “From the beginning, we planned on using internal vibrators for consolidation,” says Pawlowski. 

However, representatives from Wacker Neuson Corporation worked with Pawlowski and the crews to show them an alternate method for achieving consolidation while saving on labor and reducing congestion on the traveler form.

External Vibration 

In early November, 2007, Wacker Neuson demonstrated the effectiveness of external vibration on one of the initial arch segments. Six AR36/6/042 external vibrator motors and two FUE 75 frequency inverters were used in this first application. The six vibrators were placed on the form and “leapfrogged” over each other, following the concrete up the form. “It was a long, 12-hour night, but it gave us a chance to see how many guys were working on the form and where we could help improve efficiency,” says Fred Paul, sales engineering manager for Wacker Neuson.

With the goals of reducing the number of laborers and workload and increasing productivity, Wacker Neuson engineers mapped out some scenarios with their external vibrators and inverters for each traveler form. “They really helped out and worked with us on this job,” comments Pawlowski. “In addition to the analysis, they were on site with set up and for a number of the pours.”

A challenge for Wacker Neuson engineers was finding the right vibrator pattern given the angle of the arches. “Concrete finds true level and true plumb, which is not in line with the angle of the traveler forms,” explains Paul. Vibrator motor positioning had to compensate for this, and the cycle in which crews turned the motor sets off and on had to be determined accordingly. “External vibrators do not run continuously throughout the pour. In this instance, different sets of vibrators are cycled for approximately one minute as the concrete fills the form.”

In total, 104 external vibrators from Wacker Neuson – 26 on each of the 4 traveler forms – are being used on the job. The AR36/6/042 motors are spaced approximately on 4-foot centers to consolidate the 14-inch thick web walls. More powerful AR54/6/042 vibrators are spaced 10-foot apart to consolidate the thicker, 18-inch thick top and bottoms. “In addition to providing consolidation, external vibration reduces the internal friction of concrete to help the concrete flow better,” says Paul. “This ensures the concrete flows into all the crevices to encase the steel.”

Three portable FUE 75 frequency inverters are being moved from form to form to power the AR36/6/042 and AR54/6/042 motors. These inverters take the high voltage motors and convert it to low, safety voltage of 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.

With arch work nearing completion, the external vibrators are helping Obayashi/PSM JV to reduce labor and vibration time, while delivering a high quality segment finish. The crews can pour and consolidate an arch segment in about five hours, while the overall cycle to complete a segment ranges from 10 to 14 days. “The externals have helped to get consolidation where the internals cannot get to,” comments Pawlowski. “They also help with surface finish by bringing the cream to the form and working out the bubbles.”

The arch work will conclude with two small, 6-foot, pours to join the north and south segments. Completion of the arches, scheduled for September, will pave the way for bridge deck and final bypass completion by the second half of 2010.