Homestead Bridge
Dateline: 10/27/97Contents
Los Alamos, New Mexico
Funding for Bridges
Materials Selection
Design
Installation
Grand Opening
Further Information
Photo Gallery
Trail
Map
Los Alamos, New Mexico
The county of Los Alamos is located on the Pajarito Plateau of Northern New Mexico, about a 40 minute drive northwest of Santa Fe. Home to the Los Alamos National Laboratory, it is probably best known for its role in the development and maintenance of nuclear weapons.
Often overlooked, however, is its unique location. Isolated from other towns and surrounded on three sides by the Santa Fe National Forest and Bandelier National Monument (the San Ildefonso Pueblo forms the other border), it is an outdoor-lovers paradise.
For hikers, bikers, or equestrians, a network of mixed-use trails are easily accessible from anywhere within the county. Over 40 miles of trails criss-cross the county; many forest trailheads originate in town; and hundreds of miles of trails are only a short drive away.
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Despite this proliferation of trails, getting around town is still not always easy. The town of Los Alamos sits on three mesas, each separated by steep canyons approaching 200 feet or more in depth. Getting from one mesa to another often requires following a circuitous route to a low point in the canyon, or a tough climb down and then up again (made more difficult by the 7500 foot-plus altitude). (Click on any of the small pictures to see a larger image; click on the "Back" button of your browser to return to the article.)
Funding for Bridges
County trails are maintained by the Los Alamos Pathways Association, or LAPA. LAPA volunteers meet every Saturday during the summer to repair existing trails or establish new trails.
LAPA and the County realized that many of the trails were used for commuting, to school or to work, and not just for recreational hikes. Use of the trails would increase, and traffic jams would possibly decrease, if easier routes between the mesas could be established.
About three or four years ago, LAPA and the County government began seeking funds to build pedestrian bridges across the canyons. The effort was led by Janie O'Rourke, LAPA chairman, and John Bradley of the County Traffic Department.
The County applied for and received federal funds to build the bridges. Because the money came from the federal government, the bridges had to comply with the ADA and be handicap accessible if such accomodations could be made with a reasonable effort.
The bridge was wide enough to accomodate wheelchairs, and ramps or slopes could easily be installed at either end (this would also help the bicyclists). But was this considered accessible?
The definition of reasonable was left up to the state highway department. They further required that the trails leading to the bridges had to be handicap accessible. One of the trails followed an old paved road and thus was easily accessible.
The state's requirement, however, was more strict. They wanted all trails leading to the bridges to be handicap accessible. LAPA and the County didn't consider this very reasonable. Some of the trails descended or ascended steep canyon walls, and widening trails in other areas would have required cutting down many trees.
Rather than modifying the trails (and causing much destruction to the surrounding forests), or fighting the definition of reasonable, LAPA and the County gave up on using the federal funds. At that point, LAPA was also ready to give up on the bridges.
Fortunately, Dave Riker of the County Public Works Department had other plans. He wanted to spend the money on sidewalk and bicycle lane improvements. This freed up County money, and the bridge project was back on track.
Materials Selection
About the time the bridge funding was finalized, I moved to Los Alamos and began helping LAPA with trail maintenance. One day, instead of doing trail work, we hiked to the new bridge locations.
Plans called for two bridges to cross Pueblo Canyon (the canyon that separates the main town area from one of the large residential areas). The main bridge would cross near the old sewage treatment plant where the canyon was over 100 feet wide and close to 200 feet deep. A paved road led almost to the canyon edge, so bringing in heavy equipment would be no problem.
The other bridge was to be much smaller. It crossed the canyon further up, where it was only 50 feet wide and 10 or 20 feet deep. This crossing was at the location of an old bridge built by the original Homesteaders in the early 1900s. Although the crossing was much easier, the nearest road was over 500 yards away.
Because of its remoteness, the County thought its only options were to bring in a steel bridge by helicopter or build a wooden bridge on site. When O'Rourke mentioned these problems, I told her about a composite bridge in the Haleakala National Forest I had recently read about. I gave her a copy of the article which she then passed on to the County.
Betsy Lucido, an Assistant Engineer in the County Public Works Department, was responsible for obtaining bids. Before sending out the Request for Quote (RFQ), she examined steel, wood, and composite designs. Steel was out for the obvious reasons. Wood was a candidate, but past experience had shown wood bridges do not hold up well in the Los Alamos environment.
For help in assessing composites, she contacted E.T. Techtonics, the designer of the Daniel Boone bridge. Based on those conversations, she determined that a composite bridge could easily be built in that location. Further conversations with personnel at Bandelier National Monument indicated that a wood bridge would most likely be more expensive than composite. Thus, the RFQ went out specifying a composite bridge.
E.T. Techtonics was the only bidder for the Homestead Bridge. They won the contract with an $18,500 design (not including installation). They also bid on the longer Pueblo Canyon Bridge, but came in slightly higher than the steel design. Installation would have been more difficult (the steel bridge was lifted into place using a crane), so composites were rejected for the large bridge.
Design
E.T. Techtonics has designed over 75 composite pedestrian bridges to date, about half of which are in parks and forests. Most of these bridges have spans of 20'--40', with several up to 80'. Eric Johansen, president of ETT, considered the Homestead bridge at 54' to be of "intermediate" span.
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The Homestead Bridge didn't present any significant challenges, but it was just long enough to complicate the design slightly. The longest continuous segment that can be shipped is 48'--this restriction forced ETT to put a splice joint in the center of the bridge. Also, above 50' or so, frequency begins to drive the bridge design, forcing the use of larger cross sections.
ETT used a truss design for this bridge. All components except the decking were made from Extren Series 500 fiberglass pultrusions, manufactured by Strongwell. The Series 500 pultrusions have a natural olive-green color which blends in nicely the surrounding landscape.
Strongwell also uses a UV inhibitor in all of its Extren pultrusions. ETT has conducted tests on how UV affects their bridges, but it is still not clear what long-term effects it has. ETT has been monitoring its bridges for eight years, and the weathering is highly dependent on geographic location.
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Extren pultrusions are available in a wide variety of profiles: angles, channels, I-beams, round, square, and rectangular tubes, square bars, rods, plates, and special sections (corner posts, hat sections, kick plates, etc.). These profiles, and their properties, are detailed in the Extren Design Manual.
ETT's design for the Homestead Bridge uses a combination of channels and square tubes. The truss chords consist of two channels placed back-to-back. Square tubing is sandwiched between the truss chords to form the vertical posts and diagonal members. The horizontal rails are formed by a square tube covered by a small channel. A flat plate is used to splice the truss members at the center of the bridge.
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The lateral cross beams under the bridge are of the same back-to-back design as the truss chords. Six outriggers on each side provide lateral stability for the upper truss chords. The outriggers are bolted to the lateral support beams like the vertical posts, and connected to the upper end of the vertical rails.
All fasteners on the bridge are double-dipped galvanized. ETT sometimes specifies stainless steel, but these add significantly to the cost.
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The decking is simply pressure treated lumber bolted to the inside channels of the main beams. Composite decking was considered but rejected as too expensive. ETT has sometimes used TREX (a wood-reinforced plastic) but tends to avoid it because it has no structural properties.
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Eric Johansen would have liked to have won the contract for the larger Pueblo Canyon Bridge. Not only would it have been an interesting project, but with a single span of 100', it would have also been the longest span composite bridge in the US. (The total span of the Pueblo Canyon Bridge is 140', with a concrete support 40' from one end.)
Installation
After ETT designed the bridge and ordered the materials, Structural Fiberglass of Bedford, PA cut and drilled the parts to size. All of the bridge components, along with general installation instructions and detailed drawings, were shipped to Los Alamos for installation.
Sam Gardner, of SG Western Construction, had won the contract to install both the Pueblo Canyon Bridge and the Homestead Bridge. Gardner had never worked with fiberglass before, and he didn't really know what to expect.
"It was great," he said, "easier to work with than steel. Just about anyone could assemble it." In this case, it was probably also easier than wood. All of the pieces had to be carried 500 yards over a narrow trail, and fiberglass was simply the lightest material.
Although the material was very easy to work with, the project was more difficult than he originally thought it would be. In large part, this was because of the central splice. To assemble the bridge, he had to first construct a temporary support bridge.
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To make the temporary bridge, Gardner built two 2x12 beams across the canyon, placed scaffold planks across the beams, and braced the entire thing to the bottom of the canyon. He then assembled the composite bridge on top of the temporary bridge, pumped in concrete to form the footings, and removed the supports after the concrete had cured.
Total construction time was five days. It took a three person crew three days to assemble the temporary bridge, and a four person crew two days to assemble the composite bridge--and the last two days were very long days.
The bridge went up with very little difficulty. Gardner had to drill a few extra holes and cut one or two pieces to length. The holes were drilled with a battery-powered drill and high speed bits; the cuts were made with a demolition saw and ordinary blade. The materials cut easily, and he had no problems with overheating or itchy skin.
The most difficult time came towards the end of the construction, when the bridge pieces didn't fit together as easily as they had earlier in the project. Gardner originally attributed this to thermal expansion, but it was most likely reduced flexibility as the bridge neared completion.
Grand Opening
LAPA and the County held a grand opening celebration at the Pueblo Canyon Bridge on October 11. Turnout was quite good--approximately 100 people showed up despite the unseasonably cold weather. Everyone I talked to was pleased with the bridges.
The only complaint I have heard is that the bridge seems too wide for the area it is in. The outriggers do add significantly to the width, but ETT required them for lateral stability, especially under snow loads. Other designs were possible, but would have increased the cost of the bridge.
The Public Works and Traffic Departments have a few more pedestrian bridges planned around the county, and they are definitely considering composite designs. The prices are competitive with other materials, and damage from installation is minimal.
Further Information
For more information about pultrusion, see my earlier feature on the Pultrusion Process.
Related Mining Company sites include:
If you're ever in (or near) Los Alamos and you want to see the Pueblo Canyon and Homestead Bridges, here's a trail map. If you need better instructions (or want a guide), feel free to e-mail me at composite.guide@about.com. The trails are passable almost all winter, and the bridges make a nice side trip if you're in Los Alamos to ski.
For more information about Los Alamos in general, check out the following Web sites:
Outside Magazine ran a story about Los Alamos, called "Bomb City, USA," in April 1996. For a magazine dedicated to the outdoors, they did a good job of ignoring the wide range of activities available in Los Alamos. In one of their few references to outdoor activities, they actually found something sinister in the fact that Los Alamosans like to excercise and spend time outdoors.
E.T. Techtonics doesn't have a Web site, but you can reach them at 800-854-0957, or e-mail Eric Johansen at etteric@aol.com. You can read about some other bridge projects on the Applications: Infrastructure page.
Strongwell does have a Web site, and it includes some basic information about Extren pultrusions and their other products (the design manual will cost you $35). The Owens Corning Web site has an article about a tram station that has a roof built with Extren pultrusions.
Sam Gardner at SG Western Construction can be reached at 505-662-3852.
Finally, I have collected all of the photos from this article in a Homestead Bridge photo gallery. This gallery includes a few pictures not used in the main article, and I've added some more descriptive text.