A fix for aluminum overheads: using innovative fiber-reinforced polymer repairs to correct distress in welded joints on aluminum sign structure trusses

Public Roads, Nov-Dec, 2003 by Paul Mooney

In the early 1960s, State departments of transportation (DOTs) began using aluminum trichord overhead structures to support signs along our Nation's highways. The advantages of using aluminum were obvious. It is lightweight, costs less than steel, and is inherently resistant to rusting. In 1994, however, the American Association of State Highway and Transportation Officials (AASHTO) started to include fatigue as a design factor for overhead signs.

"This development, combined with observations of cracking and occasional structural failures in aluminum signs," says Chris Pantelides, professor of civil engineering at the University of Utah, "alerted engineers to a potential life-threatening problem that may exist in States throughout the country."

Over time, wind forces create stresses on an aluminum structure, eventually causing cracks to appear in the welded joints of the truss diagonals, lf these cracks are not discovered and repaired, a welded joint eventually could fail and cause an aluminum diagonal to fall onto the roadway. "These problems are surfacing in large numbers now that many of these aluminum sign structures have been in service for 30 years or more," says Pantelides.

The consequences of a failed welded joint could be tragic, especially on high volume roads where traffic moves continuously and at high speeds beneath hundreds of aluminum overhead sign structures.

Fortunately, New York State DOT (NYSDOT) engineers, working with private industry and a research team from the University of Utah, found an inexpensive way to repair problematic structures to increase their safe and useful lives. Using a fiber-reinforced polymer (FRP) composite to wrap cracked joints, workers can restore the structural integrity of a cracked joint to virtually the same strength as the original aluminum weld. The cost of the material is minimal, and the repair can be conducted in the field. In sum, this solution can be applied with minimum difficulty.

A Sizable Problem

NYSDOT engineers first became aware of a problem when maintenance crews reported some minor failures of truss diagonals. Those small failures alerted the engineers to a potentially larger problem.

"It was decided to inspect all of the aluminum overhead signs on Long Island to determine the extent of the problem," says NYSDOT Senior Civil Engineer Harry White. "Initial inspections in one area of the State revealed that approximately 10 percent of all the overhead sign structures had some form of structural damage. We then decided to inventory and inspect every overhead sign structure in the State. If we hadn't inspected, the risk would still have been unknown. There are more than 2,000 of these types of structures in service in New York. Depending on the size and location, replacing an overhead sign structure can cost as little as $50,000 or up to $300,000 for a large structure carrying variable message signs."

With little warning and no time to prepare for sticker shock, NYSDOT suddenly was facing a potential $25 million problem.

Finding a Low-Cost Answer

Facing the tremendous cost and logistics of replacing hundreds of overhead sign structures spanning some of the busiest roads in the State, the agency needed to find a better solution. At the time, John Neidhart was a senior civil engineer for NYSDOT and head of the overhead sign structure unit. Neidhart recalls, "We were thinking of all kinds of crazy repair ideas, and someone directed us to an FRP [fiber-reinforced polymer] contractor." At the time, these composites already were being used to retrofit substructures in New York.

Fiber-reinforced polymers have been used successfully in many transportation applications, but this was the first time they were considered as a wrap for welded aluminum joints on overhead signs. NYSDOT and the Utah DOT organized a pooled-fund study to determine whether fiber-reinforced polymers could be used to repair cracked aluminum joints. Larry Cercone, a consultant with Air Logistics Corporation, contacted Chris Pantelides to conduct the study.

Testing a Practical Solution

The research was designed to test cracked aluminum weld connections that were either unwrapped or wrapped with fiber-reinforced polymers. Researchers wanted to see whether the cracked aluminum welds could be replaced by this composite material. The research also tested aluminum chords that had not been previously welded but were reinforced with fiber-reinforced polymers. For these specimens, a tack weld (for alignment) was applied and then wrapped with the composite material.

Testing involved placing repetitive forces simulating wind on the test specimens. A triangular load frame was built to enable the diagonal of the specimen to be positioned vertically, directly under an actuator (a device that converts hydraulic energy into mechanical energy). The actuator applied loads on the joints of specimens at the same angles that natural forces act on the structure in the field.

The results were promising. Cracked samples repaired with fiber-reinforced polymers achieved strength in excess of 115 percent of the original welded connection. In addition, samples fitted only with a tack weld and fiber-reinforced polymers reached capacities virtually equivalent to a newly welded aluminum connection.