Bending perforated
weathering steel:
Not a pedestrian challenge
Sure Iron Works fabricates
a unique pedestrian bridge
Figure 1 A worker at Sure Iron Works aligns corrugated and
perforated plate with a plate and box member, creating the
girders of a pedestrian bridge.
By Tim Heston, Senior Editor
t’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 (see Figures 1-4).
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.”
I
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.”
