Cover Story / Autumn

When a Bridge Gusset Lies to You.

Three field samples, two hardness tests, and a microscope. What a cracked K-brace in a secondary truss can teach anyone still pulling a stinger for a living.

By the Apex bench · 14 min read

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01We test. We photograph. We write it up plain.

02No marketing department. The welders write the columns.

03Print run of 900. Mailed to the shops that asked.

The Lying Gusset

A county inspector pulled a cracked plate off a pedestrian overpass in late May and shipped it to us in a five-gallon bucket. The paperwork said likely fatigue. The plate said something else.

Everything about the sample looked textbook at first. A quarter-inch gusset, A572-50, fillet welds on three sides, roughly nine years in service. The crack ran clean along the toe of the weld, bright on the fresh face, scaled brown on the rest. You'd call it fatigue from across a parking lot.

We didn't. Here's why.

What the hardness map showed

Rockwell traverse across the HAZ came back ugly. We expected a smooth climb from base metal into weld and back down. What we got was a spike — 32 HRC in a narrow band about 1.5mm off the fusion line — that had no business being there on a mild structural steel. Somebody had rewelded this gusset in the field, hot, and walked away without preheat. The martensite that formed in that band had been cracking quietly for nine years before the load finally found it.

Fatigue was the symptom. The cause was a repair nobody logged.

What to do about it on Monday

If you're the one running the gun on field repairs: preheat the plate. If you're the one signing off: ask for the repair log and don't accept "it's fine" from a foreman who wasn't there. If you're the one writing the spec: require post-weld hardness on any field repair over 3/8".

We'll run the full micrograph set on the back cover. Photos are worth more than our opinions.

"Fatigue was the symptom. The cause was a repair nobody logged."

The samples

Three gussets total. One from the failed bent, two from adjacent members pulled for comparison. The adjacent pieces were clean — normal HAZ profile, normal grain. Only the failed plate carried the hardness anomaly. That localization is what convinced us this wasn't a bad heat of steel. It was a bad day on a ladder.

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Back Page

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The Quarterly is mailed, not emailed.

900 copies, four times a year, to the shops and inspectors who asked to be on the list. The list is currently closed. When a spot opens we hear about it through the trade.

Lab Work

New testing engagements considered through existing relationships. Referrals from prior clients, inspectors, and shops we've worked with are how we stay busy.

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