Steel material

Bending perforated weathering steel: Not a pedestrian challenge

It’s not surprising that Steve Horn, a New Yorker to the core, uses a pizza crust to describe one of the most unusual jobs to pass through his Brooklyn fab shop.

The president of Sure Iron Works took on the job of fabricating two pedestrian bridges—not out of the ordinary, until you consider the specs: 0.25-inch-thick HPS 70W, high-strength weathering steel that’s perforated with diamond-shaped holes in a lattice pattern and corrugated with positive and negative bends of various radii. And, oh yeah, the length of the 62.75-foot bridge is cambered slightly, hence that pizza crust analogy.

The panels are for two pedestrian bridges at Yale University that will span the historic Farmington Canal, today a bike path. The bridges’ overall radius is 928 ft., making for an extraordinarily slight camber. Looking at the bridge head-on would show a curve so slight that, if extended 360 degrees, would form a circle (or pizza) with a diameter of more than a third of a mile. 

The bridge design calls for a bottom box member, a web section consisting of that corrugated high-strength plate, and a top plate. The corrugated plate consists of 4-ft.-long sections, welded together to meet the requirements of the AWS D1.5 “Bridge Welding Code.” The structure has to be held to tolerances spelled out in guidelines for architecturally exposed structural steel (AESS) from the American Institute of Steel Construction (AISC). This basically reduces all tolerances by half; if something normally has a tolerance of ±0.25 in., for this job it is ±0.125 in.

All this creates some major challenges for Horn’s 17-employee shop. “This is a unique job,” Horn said. “These types of bridges have never been built before. I don’t think I’ll see this type of job ever again in my life.”

Unique Design, Unique Requirements

The unique design came about from a joint venture between architectural firm Pelli Clarke Pelli and structural engineering firm Guy Nordenson and Associates. They needed a fabricator with AISC bridge certification and experience with AESS. 

Some fabricators declined even to bid on the project, many claiming that it just couldn’t be done. Perforated material? Positive and negative bump-bends of various radii, on an ever-so-slight camber, so panels aren’t even perfectly square? What kind of backgauge programming would this require? How could you weld it together and keep the tolerances, considering the effects of weld distortion and shrinkage? What about hardness and consistency issues within the metal? And thanks to the perforation, the amount of material under the punch actually changes with every bend. 

Also, what about the bending stresses? Even though these are pedestrian bridges, they are being built as a road bridge to meet the standards of AASHTO, or the American Association of State Highway and Transportation Officials. The perforated plate makes up a large portion of the girder assembly. What if that plate were to buckle and fail under load?

Then project organizers called on Horn at Sure Iron Works, which had worked with the structural engineering firm on past jobs involving AESS. Instead of saying it couldn’t be done, Horn and his team went down to the shop to see for themselves. The skilled operators, through trial and error, found they could bump the profile on an old, massive mechanical brake—with no backgauge. “We did it the old-fashioned way, and it worked,” Horn said. “So I knew we could match those complex profiles.”

Sure Iron isn’t bridge-certified through AISC, though, so to fulfill the requirements, which are under increased scrutiny since the collapse of the I-35W bridge in Minnesota, the company partnered with MC Ironworks in Pennsylvania. MC Ironworks is a long-standing certified bridge fabricator and as such was hired to handle the fabrication of the bridge box members and manage quality control. Currently in the works, the bridge project is set for completion during the first half of 2010.

Backgauge Issues

The job requirements call for a sophisticated brake backgauge for good reason. Sure, operators can bump one profile through trial and error on that old mechanical brake, but remember that camber—and the pizza crust. “Looking at it, you would think you were bending 90 degrees to the face,” Horn said, “but you’re not. That right there, in a nutshell, is what makes this job so difficult.”

Because each panel isn’t perfectly square, the backgauge has to be programmed precisely. So to handle the job, the company purchased a Cincinnati Maxform 135-ton press brake with a 14-in. stroke (to avoid interference with the large, corrugated forms) and, most important, a five-axis, programmable backgauge.

The Fabrication Process

Welding perhaps is the most challenging part of the whole job. The more welding, the greater potential for shrinkage and distortion; welds that are a little off add up to some big discrepancies over the span of the bridge. 

For this reason, Horn initially wanted to bend longer sections, which would have required less welding. But in the end it just wasn’t practical; the brake’s backgauge could extend only so far, and working with long pieces brought up handling and interference issues. Also, bumping with air bending, with the bending force going just beyond the material yield strength, was the only practical option. Bottoming would have required a truly massive press brake, and could easily have distorted the perforated metal.

The process starts with a detailed 3-D model derived from the architect’s original model. The plate is sent out to a shop that laser-cuts the lattice pattern, processed so that the operators at Sure Iron can bend against the grain. At either edge of each 4-ft. panel is a column of half-diamond shapes with their interiors tabbed in place (for reasons described later), and with two laser-etched lines representing the centerpoint of the radius cut of the diamond, which allows workers to align one panel to another. 

When the plate arrives at the press brake, technicians face several challenges on top of the complex backgauge programming. First, material hardness varies, so sometimes operators encounter unexpected degrees of springback. Second, the material is perforated, meaning that the exact amount of material under the punch changes slightly with every bump.

Sure Iron technicians made initial tonnage calculations as if they were bending solid, high-strength plate, then reduced the tonnage to suit during test runs. Still, the tonnage calculation isn’t precise, because of the variables mentioned, so operators tweak parameters as needed after a series of bumps. 

To get it just right, they mocked up a flange with the bend profiles cut into it, made with coordinates generated from the 3-D model. That flange mockup acts as a feeler gauge. Technicians place the plate inside to ensure the final bends fall within tolerances.

The brake has American-style, precision-ground, nitride-coated tooling from Wilson Tool Intl., including a 5.75-in. block punch with a 0.125-in. tip radius and a die with a 2.5-in. V opening, a tool set that holds up to significant wear as it bends the high-tensile-strength bridge steel. The tool’s 70 Rockwell C nitride coating permeates 0.20 in. into the tool steel.

The nitride coating allows “a very low coefficient of friction between the material, punch tip, and the die shoulders,” explained Bill Mosca, senior sales engineer at Wilson. “Also, this is bump-bending, and the radius is not severe. He isn’t close to air bending a 90-degree at all.”

Figure 3The plate essentially serves as the web section of the bridge girder. Its wave pattern changes throughout the bridge structure. Although this enhances structural stability, it adds complexity to bend programming. 

With precise backgauge movements programmed into the control, brake operators bump a series of nine hits for each bend using laser-etched marks on the plate as a guide. They also must flip the plate several times to complete the positive and negative bends.

The bent panels then are welded, and this is where those columns of tabbed half-diamonds on either end of the plate come into play. Excess weld distortion and shrinkage can throw the entire bridge assembly off. To reduce distortion and meet the demands of X-ray inspection, welders start and stop their welds within those tabbed interiors of the diamond—runoff tabs, essentially. When they finish, they center-punch the location of the cut to re-establish the diamond shape, then bore out the diamond to match the pattern. This leaves clean, undistorted welds connecting the latticed panel sections.

A Fabricating Challenge

All this begs the question: Why build such complicated bridges? The designers made them to match the look of surrounding buildings and allow more sunlight onto the bike path below, running through the heart of the Yale University campus in New Haven, Conn. The street adjacent the new pedestrian bridges, Hillhouse Avenue, was once described by Charles Dickens as “the most beautiful street in America.”

That said, aesthetics count for a lot in this case, but form has to follow function, especially with safety at stake. Project organizers were lucky enough to partner with a company that didn’t turn down a unique fabricating challenge.

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