Weathering Steel Bolts and Fasteners: Getting Connections Right

Most weathering steel problems blamed on the plate actually start at the connection. Bolts, faying surfaces and joints are where water sits, where dissimilar metals meet, and where the protective patina never gets a chance to stabilize.

Why an ordinary bolt is the wrong bolt.

Put a bare or zinc-plated carbon steel bolt into a weathering steel assembly and you create a galvanic couple. The bolt is small, the plate is large, so the bolt becomes the anode and corrodes far faster than it would on its own while the plate around it stays almost untouched. The result is predictable: rusty bolt heads on sound steel, and brown streaks running down the face of the structure within a few years.

The four realistic options.

Slip-critical joints need their own decision.

In a slip-critical connection the faying surfaces, not the bolts, control the slip factor. Uncoated weathering steel can only reach Class A or Class B when both surfaces are blast cleaned to the specified profile before assembly, and any rust formed in storage has to come off first. Many specifications therefore paint or blast clean the faying surfaces and protect them until erection. Decide this at the drawing stage; it cannot be fixed in the field.

Detailing that keeps connections dry.

Water is the deciding factor. A connection that drains and dries behaves; one that ponds fails, whatever bolt you choose.

Details should shed water instead of trapping it. Avoid upward-facing crevices and back-to-back angles that hold a water film. Seal or back-seal gaps. Keep bolt groups out of ponding zones and below the drip line of joints. Use washers to separate dissimilar metals, and never mix galvanized and bare steel in the same wet detail.

On coastal sites, or anywhere de-icing salt is used, assume the connection needs protection even when the plate does not.

Inspection after erection.

Verify bolt markings against the fastener certificates. Confirm the tensioning method, whether part-turn, torque or direct tension indicators, matches the specification and that records exist. Walk the structure after the first wet season and look for rust streaks that start at bolts: they point to the connection rather than the plate, and they are much cheaper to correct early.