Versatile” can often be an overused term to describe road- and bridge-building equipment available on the market. But for bridge-building contractors, a versatile paver is critical for allowing companies to pave a wide variety of complex bridge designs. These pavers must have the flexibility to pave narrow bridge spans in the 28-ft range as well as the more complex bridges ranging up to 170 ft wide and beyond, sometimes in the same project.
Today’s contractor is facing a new era of bridge construction and paving. A growing population, more vehicles on the roadways and unprecedented congestion are leading to mounting pressure for the Federal Highway Administration and state departments of transportation to add road-lane miles, which also is increasing the width of bridges. Compounding the issue, “land is at a premium in many cities, resulting in elevated roadways as a design option,” said Ken Drake, senior structural engineer for Volkert & Associates Inc., an engineering, architectural, planning and environmental consulting company based in Tampa, Fla.
Drake and Volkert are currently working on the designs for an ambitious I-4 and Lee Roy Selmon Expressway Connector project in Tampa. A cooperative venture of the Florida Department of Transportation, Florida’s Turnpike Enterprise and the Tampa-Hillsborough Expressway Authority, this aggressive project includes an elevated roadway to improve traffic flow between downtown Tampa, the Port of Tampa and Ybor City.
Although the project is still in the design stage, Drake mentioned that the north half of the expressway will consist of approximately eight bridges, while the south half should include 31 bridges. These structures will consist of narrow, single-lane ramps measuring 28 ft in width as well as bridges in excess of 150 ft wide.
“There is a 380-ft section of elevated roadway that is 161 ft wide, consisting of six traffic lanes with three different ramps diverging from the road,” Drake explained. “There is a second bridge that is 156 ft wide with six to seven traffic lanes and converging lanes.”
The width of the bridges prompted a call to Bid-Well to ask a simple but critical question.
“I wanted to find out how wide a deck the bridge pavers could handle,” said Drake.
The call did not take the manufacturer by surprise.
“We have had inquiries for pavers in the 180-ft-plus range,” said Rob Drew, general manager for Bid-Well, a Terex company. Skewed bridge designs and monolithic pours are pushing the paving requirements of bridge pavers to unprecedented widths. “The design economics of bridge paving are driving the average widths well beyond 100 ft,” Drew added, “with some specs requiring pavers reaching 200 ft wide.”
These unique widths pose a bit of a challenge for bridge paver manufacturers. Constructing a 200-ft-wide paver is not as easy as adding to the frame more truss made of conventional steel. Additional truss increases the paver’s weight, and the tensile strength of conventional steel truss and connectors may not be strong enough to prevent deflection at extended paving widths, which can affect surface smoothness.
A slimmer steel
Nearly five years ago, Bid-Well engineers faced one of their toughest paving challenges by supplying a 4800 paver to the Palmetto Bridge Co. to pave the Cooper River Bridge. The 4800 was built to a 174-ft width, which made it the widest Bid-Well paver of the time. The successes achieved and lessons learned at the Cooper River Bridge combined with multiple inquiries from design consultants for even wider bridge pavers motivated Bid-Well engineers to look at widths exceeding the 200-ft barrier.
Additional weight aside, a primary concern for the engineers was to maintain structural integrity of the paver, so surface smoothness would not suffer. “With conventional steel, expanding bridge pavers to these unprecedented widths can result in upper-chord tubing deflection,” said Daryl Hardy, engineering manager for Bid-Well.
The design team considered incorporating heavier-gauge tubing to the structure, but thicker tubing would add weight to the paver, not a preferred option for the engineers. “We needed steel that increased the structure’s rigidity while keeping the weight of the paver relatively the same,” Hardy explained.
Engineers conducted an extensive international search for a steel product that would deliver the right balance of strength, rigidity and weight. Their inquiries led the team to proprietary steel not available in the U.S. until recently. This ultra-high-strength steel is nearly 120% stronger than the conventional steel used in Bid-Well 4800 pavers.
“The new steel gives us the rigidity we want, and it weighs slightly less than the conventional steel we are using,” Hardy mentioned. It now gives the design team the ability to configure the 4800 paver in a variety of widths that can exceed 200 ft if necessary, which is 25% wider than the practical maximum paving widths of bridge pavers currently available on the market.
As the engineers began working with the new steel, they learned that the standard, constant truss depth of 4800 pavers less than 150 ft wide was not the best configuration for high-quality paving at extended widths. For paving at widths in excess of 150 ft, the new 4800 truss transitions from a 48-in. depth at the legs to a rigid 66-in. depth in the center. The truss consists of 3- x 4-in. tubing. “There is not much stress at the legs, so the 48-in. truss depth works best,” Hardy explained. “Expanding to a 66-in. depth in the center helps to prevent deflection.”
Hot spot remover
As with any design project, strengthening one component on a piece of equipment can lead to weak spots in other areas. Knowing this rule of thumb, the design team for the new 4800 paver employed the latest in finite-element analysis to avoid these weak spots and ensure the new wider paver would deliver the same reliable results as the conventional 4800 paver.
“Finite-element analysis shows us the hot spots of components that get close to their fatigue points,” Hardy elaborated. “The software allows us to eliminate these hot spots prior to field testing.”
As a result, the design team upgraded several areas of the paver to ensure dependable operation. First, a new gusset design utilizes the same ultra-high-strength steel to connect the truss sections. Additionally, larger 1.5-in. crown bolts are now featured in these connections. Together the gussets, crown bolts and ultra-high-strength steel tubing help to prevent machine deflection, even at paving widths of 200 ft and beyond.
Another area of focus for the design team was the paver’s legs. With the bulk of the machine’s weight riding the rails, team members gave the new 4800 stronger legs and mounting systems. Larger lift screws and heavier thrust bearings, mounting plate and coupler motor all work in concert to offer dependable operation.
The first new Bid-Well 4800 paver constructed of the proprietary steel and 66-in. truss depth was recently sold and will be put to work in Las Vegas. It will be paving a bridge deck in excess of 160 ft wide.