September 12, 2006
A good design doesn't guarantee challenge-free fabrication in the bridge industry, as one fabricator found out. Despite material availability obstacles, stringent welding requirements, and massive pipe cutting needs, Stinger Welding and the design team it worked with pulled off a winning pipe bridge design in six months.
|Made from API 5L grades X52 to X60 pipe, the Tempe Town Lake Bridge was fabricated in segments and assembled on-site.|
In an Arizona match-up between concrete and steel, metal edged out the competition.
This contest started in July 2003 when a design team began discussing the Tempe Town Lake Bridge, a critical link in a 20-mile light-rail public transportation system (see "Light rail: How it works") scheduled to start running in December 2008.
Artist Buster Simpson recalled that the meetings were intense, with advantages and disadvantages of the proposed concrete box beam trellis bridge abounding.
"From this series of meetings, there developed a consensus that an innovative triangular steel truss bridge was worth investigating," the Seattle artist said. Although the engineers were obligated to continue developing calculations for the trellis bridge, they also wanted to explore a new approach involving a triangular truss, a design that everyone felt would be a good integration of art and engineering.
Aided by conceptual renderings of the new bridge, the group discussed its ideas with METRO. By August 2003 the decision was final: A steel truss bridge was the better option.
"The team was ecstatic," Simpson said, adding that this design had many advantages: It was a true representation of bridge engineering; it was cost-effective; and it was elegant. All of this was especially important because the bridge would run adjacent to an older, historic bridge (see Figure 1).
A local historic commission played a role in the design committee's decisions regarding what the new bridge would look like. According to artist Buster Simpson, the historic commission saw the steel design as the better choice over concrete because it would pay homage to the adjacent historic railroad bridge (left).
"The team really wanted it [the bridge] to be an honest expression," he said. "It's an honest representation of what bridge engineering means, being able to see the trusses."
Although its design was advantageous, the bridge posed several fabrication challenges.
First, this 1,531-foot-long, 11-span structure connecting central Phoenix to Mesa was to be constructed of steel pipes, which in itself is somewhat unusual. Coolidge, Ariz.-based Stinger Welding, the steel fabrication company for the project, first suggested box tubing instead of round to make the bridge easier to fabricate.
"But round won out because it would offset other nearby square bridge designs," said Gary Gardner, a certified welding inspector (CWI) and quality assurance/quality control manager for Stinger Welding.
And so an interesting pipe fabricating journey began.
Stinger Welding, an 8-year-old company that began as a mom-and-pop repair shop especially for farmers and irrigation professionals, now specializes in expansion joints for bridges, as well as pedestrian, vehicular, and rail bridges since the company attained its American Institute of Steel Construction (AISC) bridge certification. So when bidding started for this project, the company saw it as a good fit.
But once Stinger started working with the rest of the design team, it became apparent that quite a few aspects of the fabrication would be challenging.
"We bit off a very large project," Gardner said. To name a few obstacles:
|Tempe Town Lake Bridge: By the numbers|
|2: Sections per truss that were constructed onshore before being erected by floating barge cranes|
9: Y-shaped concrete piers that support the bridge
11: Tube diaphragms connecting northbound and southbound spans
15: How many feet the bridge sits above the Salt River, also known as Tempe Town Lake
22: Disk bearings floating the bridge on the abutments and piers
42: Steel trusses
43-92: Length range, in feet, of each truss
220: Cross braces between top chords
400: Slip-critical bolted flanges connecting the trusses
880: Diagonal braces between top and bottom chords
1,200: Pounds of metal-cored wire used
2,200: Complete joint penetration (CJP) open-root welds
3,224: High-strength structural bolts
9,500: Pounds of solid wire filler used in SAW
37,000: Pounds of flux-cored filler metal used in FCAW-G
50,000-80,000: Weight range, in pounds, of each truss
175,000: Gross weight, in pounds, of each light-rail car
"It was a big challenge to make sure we could secure the material because of supply—a lot of it was going to China, and the prices were escalating," said Dan Heller, P.E., of T.Y. Lin Intl., Tempe, Ariz., the structural design engineer of record.
By contract, the bridge was to be fabricated in six months—including material receipt. Eventually the Federal Transit Administration approved the use of imported materials for two size combinations, which, before purchase, were subject to on-site inspection by Stinger to verify dimensions, form, and traceability.
Simpson said the design of the triangular truss bridge contributes to a pattern language with the existing three bridges that traverse Tempe Town Lake.
Another aspect of this construction that at first might have been considered a hindrance actually was a benefit: segmented fabrication (see Figure 2).
To optimize structural strength versus weight, meet budget, enable transportation to the work site, and enable a modular erection, Stinger fabricated the bridge in segments. Although this may sound like another challenge, Gardner said actually it was just the opposite.
T.Y. Lin decided on the segmented fabrication method as a way to help ensure the bridge would be fabricated on time.
Invited for an interview and to make comments and recommendations by T.Y. Lin, Stinger Welding eventually helped write the welding specifications. Because Arizona generally uses concrete more than steel, Stinger helped educate others on the team about welding and came up with the majority of the weld design.
Before the welds could be made, however, a lot of pipe cutting was necessary—which created its own test for this project.
The pipes used to fabricate the diagonal members of the bridge were 10-1/2 ft. long with wall thicknesses from 1/2 to 1-1/8 in. These dimensions made the pipe cutting programs for each pipe vastly different.
The cuts had to be made just right for weld fit-up, so tolerances were tight and room for error was even tighter.
"The configuration of the bridge made it a challenge. It's unusual to have a bridge made out of pipe," said Doaa Aboul-Hosn, assistant resident engineer for METRO. "We had these diagonal pipes that were all different lengths with different angles, and each beveled cut had many different angles in that cut. Normally, fit-up is done by hand in the shop with shop drawings. That would have taken many, many hours to do each one. It would have been a challenge for someone in the shop to figure out all of the angles."
Although frequent calibration was necessary, the CNC thermal cutting equipment Stinger Welding used, in conjunction with specialized CAD software, was the only way to cut the volume of pipe needed in the time it was needed for this bridge fabrication, according to CWI Gary Gardner..
Instead of producing shop drawings by hand, Stinger Welding opted for a more automated approach: combining CAD with an automated pipe cutting machine. The trick was to find the right pairing.
After researching and interviewing several pipe cutting machine manufacturers and sending the company's president and maintenance personnel out to confirm the machine's capabilities, Stinger decided on a Vernon Tool model 0224MPM-5 CNC thermal cutting system. This machine was designed to accommodate pipe from 2 in. to 24 in. in diameter by 250 lbs./foot to a maximum load of 5,000 lbs. It offered five axes of simultaneous computer control.
Even this choice, however, wasn't an easy plug-and-play option. Although Stinger chose to work with Vernon Tool because it was the only company that they felt could get them a machine when they needed it, Stinger had a tight schedule to stick to.
So for the three weeks it took for the pipe cutting machine to be built, Stinger sent blanks and operators to Vernon Tool so they could cut the pipes on the demonstration machine.
As the operators began to use their new pipe cutting machine, however, they realized that their original CAD program was not compatible with it. Quickly Stinger and Vernon Tool worked together to create a specialized 3-D program that would handle the various angles necessary in each cut and communicate them to the machine.
"They spent a lot of resources trying to get the program working with the machine. And then they had to calibrate it frequently," Aboul-Hosn said.
According to Gardner, the integrated system consisting of specialized CAD software, a firmware interface, and CNC thermal cutting equipment cut and rotationally aligned the pipes, beveled the included welding angle, and cut each diagonal to length, allowing for root openings within 0.05-in. tolerance and end-to-end clocking angles within 1/2 degree of rotation (see Figure 3).
"Fabricating this structure within the time frame and budget would not have been possible without such a system," Gardner said.
Stinger Welding used gas-shielded flux-cored arc welding (FCAW-G), gas metal arc welding (GMAW) with metal-cored filler metal, and submerged arc welding (SAW) with solid wire filler metal; stud anchors; and bolted connections to fabricate and join the subassemblies before they were zinc-primed and transported to the work site (see Figure 4).
A variety of welding processes—gas-shielded FCAW, GMAW, and SAW—were necessary for the many joints that had to be made on the Tempe Town Lake Bridge.
Finally, quality control was critical.
"Fatigue is always a big issue, especially around the welds," T.Y. Lin's Heller said. In some areas, this meant adding extra plates. But in all instances, nondestructive testing was mandatory.
"We had a lot of problems with nondestructive testing," Gardner said. "The bridge components were hard to inspect. The diagonal members had to be joined with open-root complete joint penetration welds in accordance with D1.1 code and the design fatigue category. Ultrasonic testing was the examination method of choice because X-ray could not be applied to these joint configurations. It was a challenge because we had to positively ensure that we could always detect very small indications while at the same time be able to gate out false indications, plus ensure that we were effectively differentiating between the two."
NDT technicians with significant experience in inspecting tubular offshore oil structures assisted Stinger's third-party inspection team in the ultrasonic testing and training of other inspectors without this experience. METRO contracted another NDT and inspection source to audit the fabrication, inspection, and material traceability.
All of the joints had fairly straightforward inspection requirements—except for the open-root welds on the diagonal braces.
"A pattern in the location of the ultrasonic indications was quickly noticed, and the validity of the ultrasonic procedure became suspect," Gardner said. "Nearly every joint exhibited large defects—peculiarly, at its most easily welded area. During careful carbon-arc gouging, no inclusions of any kind were found. One ultrasonic technician applied some by-the-book theory, performed some calculations, drew some sketches, determined the cause of the nonexistent indications—geometric reflections—and trained the other ultrasonic techs in interpreting and eliminating this nonproblem."
At press time the new light-rail bridge is being painted as the rest of the stations in the light-rail system are constructed. Simpson has been on-site for this process and, despite the fabrication challenges, sees that the artistic vision for the bridge has been accomplished.
"They both have their own presence—one doesn't overwhelm the other," Simpson said of the light-rail bridge and its historic neighbor. "They complement each other—one is black, one is white. They have a yin-yang kind of look."
|Light rail: How it works|
|Light rail is a form of public transportation that operates along a set pathway on steel rails. The light-rail system in the Phoenix metropolitan area will operate at street level in its own lane separated from automobile traffic and have a certain level of priority at traffic signals. Light rail will travel at posted speed limits on city streets and can reach 55 MPH in future freeway corridors.|
Light rail is powered by electricity from overhead wires. It will operate on two sets of tracks, with trains of up to three cars traveling in each direction. Light-rail trains will operate 18 to 20 hours per day, every day of the week, stopping at stations about every 10 minutes during peak hours and about every 20 minutes off-peak.
Light rail can carry up to 450 passengers in a single three-car train. Initially the system will carry 3,000 to 5,000 passengers per hour during peak hours, the equivalent of an arterial street. With additional vehicles, the system ultimately will be able to carry the equivalent number of people as a six-lane freeway—12,000 to 15,000 people per hour.
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