Meeting challenges unique to fabricating tubes into light posts
October 10, 2006
Hapco Aluminum Pole Products, Abingdon, Va., fabricates aluminum light posts that must be beautiful while standing up to wind, and the forces of nature.
If you're a Hapco light post on New York City's Triborough Bridge 100 feet above the tidal water of Hell Gate, you have to withstand fierce, 90-MPH winds; hard-driving rain; salty sea mist; and traffic and bridge vibration. Even though you are only a few years old, you need to appear indistinguishable from your 1936 predecessor. You have to be strong enough to support luminaires and other adornments—and you'd better look beautiful doing it (see lead image).
Hapco Aluminum Pole Products, Abingdon, Va., has been manufacturing decorative and utilitarian aluminum poles like the Triborough Bridge light posts for more than 50 years. The thousands of light posts, traffic lights, and flag poles that its 200 employees fabricate, engineer, and assemble from extruded aluminum tube, aluminum plate, and aluminum castings grace the streetscapes and bridges of more than 27 countries.
The poles can be up to about 80 feet tall. Shaft diameters vary from 3 to 12 inches and wall thicknesses are from 0.095 to 0.500 in. Many of the decorative bases are cast aluminum, as are the luminaire bases and other nontubular components needed to support the posts and arms. The tubular beacons may materialize as vintage "gas light" or postmodern, sculpturelike squared tubular posts (see Figures 1and 2).
"As of late a lot of communities are using our poles for streetscape projects, where they put a lot of banners, plant hangers, sometimes even Christmas lighting on the poles," Hapco President Bob Gunn said. "In terms of the strength requirements, we have to take all that into consideration, as well as the luminaires and the height of the pole when we design and fabricate the pole."
These unique light posts were customized for a city in Puerto Rico, to complement the area streetscape. They were fabricated with a central shaft and several smaller shafts connected with unique castings. Photo courtesy of Hapco Aluminum Pole Products, Abingdon, Va.
Because all of the posts must be made for outdoor use, aluminum's corrosion resistance is of key importance, Gunn said. Many of the company's aluminum light poles have been in use for more than 50 years.
Aluminum bases, installed under poles that are lighter than steel, offer better breakaway performance than other materials, Gunn said. This is important for Department of Transporation (DOT) requirements that utilitarian poles installed within a certain proximity to a highway, and not protected by guardrail, must break away if a car hits it. "It saves a lot of lives by allowing the pole to break away rather than stopping the car too suddenly," Gunn said.
Aluminum's light weight, relative to steel's, enhances its ease and reduces the expense of installation, he added.
To weld the aluminum tubes, or shafts, to the bases, the company uses gas metal arc welding with 4043 filler wire, said Joe Bowman, senior design engineer.
One of the key issues is that welding aluminum causes it to lose strength, Bowman said, so the company welds its poles in T4 temper and heat-treats them to T6 temper. "Artificially aging aluminum after welding helps it regain most of the strength lost during welding," Bowman said. "Our ovens are certified to MIL- and ASTM specs."
In addition, the tubes are T4-tempered for extruded aluminum, Bowman said. "It enables us to form them and then heat-treat them to T6 temper."
One of the challenges the company must compensate for with its design and fabrication is wind vibration, according to Bowman. "With the oscillation from wind comes fatigue—it fatigues the shaft and the connections and shortens the life of the pole. It'll actually cause cracks eventually.
"You also get vibrations from the traffic on the bridge, which moves the foundation—the supporting structure for the poles—so you get some loading from the bridge itself," Bowman said. "One of the problems they have on bridges is that the wind shakes the luminaire so violently it destroys the filaments and they can't keep the lights working."
Vibration Dampers. To address vibration, Hapco developed and patented a vibration impact damper in the '70s. "Also, sometimes we use isolation pads and we have a special vibration arm that we hold a patent on for bridge application that dampens the pole and reduces the loading on the luminaires," Bowman said.
"When the pole tries to move in an oscillating manner, the damper impacts the pole," Bowman said. "It has an off-frequency force that dampens the vibration—that keeps it from increasing in amplitude from the wind loads."
Tapered Shaft. Many of the extruded aluminum tubes Hapco uses to taper shafts are fabricated by the company's supplier, Hydro Aluminum North America, Linthicum, Md. "A tapered pole will fight the wind load much better than a straight product," Gunn said. The industry taper rate is 0.14 in. diameter of change per foot, he added.
Achieving the desired historical or thematic look is as challenging as achieving the mechanical aspects of designing and fabricating for wind load, strength requirements, and durability, Gunn said. "The biggest challenge is meeting what the architect and the end user really want the look to be."
One way Hapco achieves a central thematic look for a streetscape is by offering a broad selection of cast aluminum bases to support the shafts. "There really are two ways to do it," Bowman said. "We have what we call a structural base, and that's actually part of the pole and includes the attachment and the shaft. And at times we do use multiple castings that we can vary and change the look. You may be looking at a situation where the bottom is one casting, the next part is another casting, and then we weld the shaft to that. The larger poles, like traffic poles, very often will be a clamshell type that has a very utilitarian base flange covered by the architectural base that ties it in with the other poles."
In addition, the company offers several different tube cross sections, or profiles (see Figure 3).
Straight, tapered, and fluted tube profiles, or cross sections, enable the light posts to help define a streetscape's character.Diagram courtesy of Hapco Aluminum Pole Products, Abingdon, Va.
"They [municipalities] are starting to tie the whole look together, where they'll do a streetscape—it'll have the small poles, maybe 15 or 20 feet tall, with banners and plant hangers and various luminaires—and then tie it all together. So it makes the whole texture of the street and city," Bowman said.
"Very often, for some of the traffic signal poles that handle longer spans, customers use bases that are in the same family as the light posts so that they have the same fabric and look as their light poles," Bowman said.
Another aesthetics challenge is making the poles appear identical in the field. "If you're going to make 100 poles on an order, they all have to look alike when installed, so the alignment is very critical," Gunn said.
"Your luminaire arm has to be level," Bowman said. "When you walk down the street and you've got 15 in a row,
it's pretty easy to tell if one of them is not straight or plumb because you've got curves and street centerlines and sidewalks and buildings. Banners have to be level and oriented correctly perpendicular to the street, so you have to be very careful about the alignment of banner arms."
To give the illusion that all the light poles are the same height even though they must be mounted on various planes, the pole shafts often must actually be fabricated shorter or longer than the others.
Luminaires are designed by lighting engineers to be a very specific height above the roadway on the DOT projects, because the photometrics dictate a certain light pattern coverage, Bowman said. "But some of the mounting configurations are behind a guardrail, some are on banks that slope up or down from the roadway so the pole heights have to be adjusted depending on the mounting height of the base."
"Going down the same highway, you'll see 50 poles that look the same along the roadway, but they might be 50 different lengths," Gunn said.
Repeatability. Many of the poles the company manufactures must be made to replace a damaged pole from a traffic impact. "So repeatability is important because they have to match," Gunn said. "And what we make this year has to match what we sold last year."
Paint and Paint Prep. The aluminum shaft must be rotary-sanded, chemically etched, or shotblasted to prepare it for the finish, depending on whether the pole is round, tapered, or fluted. "The main concern is getting the metal surface free of grease, oil, and oxidation so that the powder paint will adhere to it," said David Oakley, vice president of product management. "Obviously, you can't get a sander into the grooves of a fluted shaft, so we chemically etch or shotblast those types of surfaces."
The poles are thermoset [thermostatically powder-finished], rather than anodized. "You get a more uniform appearance with thermoset powder finish," Oakley said. "The anodizing doesn't do well with castings and doesn't do well with welds. It causes a very
dark, almost black, color when you anodize castings and welds, and you get much color variation in anodizing. The powder coat finishes are exceptional in terms of uniformity and durability, so we found it does real well in the field," he said.
"Also, in terms of the variation you get in colors and effects with powder coat—it's constantly improving, and there always are new textures coming out, such as antiquing, which we can do," Oakley said.
Hapco's customizing-centric philosophy counters Henry Ford's famous quote: "The customer can have any color he wants so long as it's black." The company offers more than 400 colors, and although most of its customers request black and bronze, they still have options. "We have about 25 different blacks, about 30 different bronzes," Oakley said. "Our poles are very much custom in every sense of the word."
"What's really unique about our company is our attitude and policies toward customer service," Oakley said. "Everybody says they put a high value on customer service, but we truly do. That really is the driving force behind Hapco—the relationships that we build based on the customer service that we provide. I really think that's how we've become a leader in the decorative pole industry."