Few modern-day structures match the grandeur or complexity of the Hoover Dam. The American Society of Civil Engineers, Reston, Va., 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 the Hoover Dam will have one more modern marvel to see "the majestic Colorado River Bridge. Located approximately 1600 feet south of the Hoover Dam, the 1900-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 are the twin 1600-foot-long concrete arches" the longest in North America and fourth largest in the world, which will support the bridge deck.
Although tourists will find the bridge visually appealing, its construction is a necessity. The bridge is part of a $240 million Hoover Dam Bypass Project. This 3 1/2-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 Sept. 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 32 miles to the trip. Only trucks authorized through a local permit program for local businesses are allowed to cross the Hoover Dam.
Similar to constructing the Hoover Dam in the early 1930s, the Colorado River Bridge is a massive, multiyear project. Bridge construction began in early 2005 by a joint venture between Obayashi Corp., Los Angeles, and PSM Construction USA Inc., Burlingame, Calif. "On a project of this magnitude, a joint venture works best for bonding and to share the risk," says Terry Pawlowski, engineering manager for Obayashi/PSM JV.
Initially, 48,000 cubic yards 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 also will 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 614 precast column segments and cast-in-place arch segments, which will consume more than 16,000 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 five, 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. Bridge deck construction also has 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 2300 cubic yards of bridge deck concrete was paved, the contractor turned its efforts to constructing temporary pylon 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 6000 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," aYA 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 arch ribs. The north and south arches are being constructed simultaneously in four headings, 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 10,000 psi concrete. In addition to the typical segments, blister segments are being formed to attach the temporary stay 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," says Pawlowski.
In all, 104 individual segments "52 each for the north and south arches" are 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.
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 3/8-inch aggregate, which is delivered to the site at a10-inch slump, in the mix. Originally for concrete consolidation, steel spirals were inserted into the rebar in the walls, so internal vibrators would not get hung up on the surrounding rebar. "From the beginning, we planned on using internal vibrators for consolidation," says Pawlowski.
However, representatives from Wacker Neuson Corp., Menomonee Falls, Wis., worked with Pawlowski and the crews to show them an alternate method for achieving consolidation while saving on labor and reducing congestion on the traveling form.
In early November 2007, Wacker Neuson demonstrated the effectiveness of external vibration on one of the initial arch segments. Six external vibrator motors and two 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 form traveler. "They really helped out and worked with us on this job," says Pawlowski. "In addition to the analysis, they were onsite with setup 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 form travelers," says 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 four form travelers?are being used on the job. The motors are spaced approximately on 4-foot centers to consolidate the 14-inch-thick web walls. More powerful vibrators are spaced 10 feet 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 frequency inverters are being moved from form to form to power the motors. These inverters take high input voltage and convert it to low, safety voltage of 42 V to prevent accidental electrocution. "They also provide speed stability and offer a variable motor vibration speed up to 6000 vibrations per minute," says 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 reach," says Pawlowski. "They also help with surface finish by bringing the cream to the form and working out the air bubbles."
The arch work will conclude with two small 6-foot closure pours to join the north and south segments. Completion of the arches, scheduled for this past September, will pave the way for bridge deck and final bypass completion by the second half of 2010.
Rick Zettler (firstname.lastname@example.org) is president of Z-Comm, a company specializing in construction and aggregate equipment marketing, public relations and freelance writing.