What causes rust staining on bridge girders and soffits
Rust staining on a girder web or soffit comes from one of a handful of sources, and the source matters more than the stain itself. A streak can mean nothing more than runoff drying on bare steel overnight, or it can mean rebar is actively corroding behind a crack you haven't spotted from the ground yet.
Where the staining actually comes from
Most rust staining on steel girders traces back to water finding a path it shouldn't. Deck joints that have lost their seal let runoff drip straight onto the top flange below. Once that water sits on bare steel, or on steel where the paint system has chalked and thinned, it oxidizes the surface and the rust washes down with the next rain. You'll see this most often at joint lines, bearing seats, and anywhere a drain or scupper has clogged and started overflowing onto the structure instead of carrying water past it.
On concrete soffits, the mechanism is usually corroding rebar. Chloride from deicing salt or marine spray gets into the concrete, reaches the reinforcing steel, and starts the steel expanding as it rusts. That expansion cracks and eventually spalls the cover concrete, and the rust bleeds out through the crack as a reddish-brown stain, often spreading wider than the actual spall. A stain with no visible crack or delamination nearby usually points to surface contamination sitting on the concrete face. A stain with a hairline crack feeding it, or a hollow sound when you tap near it, points to corrosion already underway.
Girder webs near bearings tell their own story. Bearings that have failed or frozen up concentrate load and moisture in one spot, and the staining there often runs alongside section loss on the web or flange, not just on the paint film. If you're seeing stains cluster at bearing lines across several spans, that's usually a drainage or bearing maintenance issue across the whole structure, not a one-off.
Efflorescence vs rust staining
The two get mixed up constantly because both show up as streaks on concrete, and both come from water moving through the structure. Efflorescence is the white, chalky, powdery deposit left when water carries dissolved salts out of the concrete and the salts crystallize on the surface as the water evaporates. It signals water movement through the concrete, which matters for durability, but it isn't corrosion. Rust staining is reddish-brown to orange, often darker and wetter-looking at the source, and it means iron oxide is leaching out. Somewhere upstream of that stain, steel is corroding, whether that's rebar, a tie wire, or an embedded steel component.
The two frequently show up on the same soffit, sometimes even in the same streak, because the water causing one is often the water causing the other. A soffit with white bloom and no red in it is a joint or drainage problem worth noting. The same soffit with rust mixed into the streak means you've likely got active reinforcing corrosion under that spot, and it belongs on the priority list for a closer look.
Why this matters before you send a crew
Staining by itself, viewed from the ground or from a photo, isn't a measurement. You can't read section loss off a stain. What staining does well is flag where to look. An inventory with forty structures and a few hundred bearing lines doesn't need a snooper truck or rope crew parked under every stain. It needs a first pass that tells you which elements actually show spalling, staining, and the visual signs of section loss, so the access equipment gets scheduled against a short list instead of the whole book.
That's the gap Structure Screening is built to close with an annual drone pass over the structure and a flagged element list you can hand to the crew before you rig anything.
If staining has been piling up on your inspection notes without a plan behind it, that's worth a conversation before next season's access equipment gets booked.