Summary

Ohio DOT pulled aluminum overhead sign trusses after cracks started at stainless bolt holes. Lab tests confirmed galvanic corrosion between stainless fasteners and aluminum flanges. Match the fastener to the metal and the weather, not just the load.

Full Article

Routine sign work in Ohio Department of Transportation District 12 turned into a teardown. Crews found cracks in the flange connectors of aluminum overhead sign trusses. Every cracked joint used a six-bolt hexagonal flange instead of the circular eight-bolt flange on the standard drawings. The worst trusses came down immediately and sat beside the roadway until researchers could map the damage.

A joint team from Ohio University, the University of Cincinnati, the University of Toledo, and E.L. Robinson documented the joints in October 2015, then again in March 2018. The trusses had been in service since the late 1970s. Cracks started at the bolt holes and ran outward. Seven roughly 12-foot sections went to the lab. The official record is FHWA/OH-2019-21, Aluminum Truss Overhead Sign Support Flange Damage Assessment.

What failed at the bolt hole

This was not a missing fastener and not a torque miss. The bolts were still in the holes. Energy-dispersive X-ray spectroscopy showed the bolts and washers were stainless steel, typical 18% chromium and 8% nickel. The flange was a cast aluminum alloy. Visual inspection found the failure surfaces and the spaces around each bolt packed with a dense corrosion product.

The University of Toledo confirmed the mechanism. Galvanic corrosion was running between the stainless fasteners and the aluminum flange. Aluminum sat on the anodic side of the couple, so it dissolved first. Fourier-transform infrared spectroscopy found no organic residue in the corrosion product. EDS showed oxygen moving into the aluminum while metal section disappeared and oxide took its place. X-ray diffraction identified aluminum oxides as the dominant product. Scanning electron microscopy showed crack patterns consistent with stress corrosion cracking, heavier attack next to the bolt, and ductile fracture only at the outer surface.

The sequence is straightforward. The stainless bolt and washer stayed noble. The aluminum lost density and strength. Stress corrosion cracking started on the inner surface of the bolt hole. Wind load and truck-induced gusts then drove those cracks from the hole to the outer face of the flange.

Why “stainless” was the wrong call here

Stainless looks like the conservative choice on a wet interstate. It resists rust. It photographs well on a bill of materials. In an aluminum structure it can become the cathode of a battery.

A galvanic cell needs three things: two dissimilar metals, electrical contact, and an electrolyte. Rain, road spray, and winter salt give you the electrolyte for free. Thread the stainless shank through an aluminum hole, seat a stainless washer, and you have contact. The fastener does not have to rust for the joint to fail. The parent metal does the rusting.

The report’s own potential fixes start with that physics: remove one factor in the couple. Coat the joint so water cannot complete the circuit. Shorten inspection if cracking is already moderate. Jacket a badly cracked flange and stop the corrosion instead of hoping the remaining ligaments hold.

Load testing made the same point. A section from I-480 eastbound at mile marker 17.6 reached the 10,000-pound design-equivalent load without joint failure. A more damaged section from I-90 westbound at mile marker 173.2 failed near 8,000 pounds when a crack at a bottom bolt ran around the weld that tied the flange to the chord. The worst flanges were already past the point where “it has not fallen yet” is a specification.

The fastener that belongs in that hole

Match the fastener to the metals it touches and to the weather that will sit on the joint for forty winters.

• If the structure is aluminum, do not treat bare stainless as a default. Isolate the couple with a dielectric washer, sleeve, or coating system that stays intact under clamp load, or specify a fastener alloy and finish the owner has qualified for aluminum in chloride service.
• If you keep stainless for strength or appearance, break the electrical path. A noble bolt in wet contact with aluminum is a planned anode.
• Grade and coating still matter, but they answer different questions. Grade answers load. Coating and alloy answer environment. This Ohio fleet needed both answers on the same hardware list.
• Geometry is not a substitute for metallurgy. Researchers asked why eight-bolt circular flanges did not show the same dramatic cracking. More bolts can share load. They do not stop galvanic attack if the same metals sit in the same salt.

The report also notes a detailing mismatch: cracked trusses used six-bolt hexagonal flanges, not the eight-bolt circular flanges on the standard drawings. Selection includes the pattern on the drawing, not only the alloy in the bin.

Right fastener for the job

A highway sign truss lives in wind, grit, and chloride. The fastener is part of that environment the moment it seats. Ohio’s labs did not find a mysterious new alloy failure. They found a couple every maintenance shop already knows how to avoid: stainless hardware, aluminum parent metal, and water.

Specify the joint as a system. Confirm the bolt, washer, and flange can share load without turning one member into a sacrificial anode. Then inspect for the first hairline at the hole, because that is where this story started.

The right fastener for an aluminum overhead sign is the one that holds the chord and leaves the flange intact after decades of spray. Stainless that eats the hole is the wrong fastener, no matter how clean the head still looks.