Steel bridge or concrete crossing

Start with what the two things actually are

“Steel or concrete” sounds like a choice of material for one structure. On a private crossing it usually is not. It is a choice between two different structures that solve the crossing in different ways.

A prefabricated steel bridge spans the gap. It bears on an abutment at each end and the channel underneath it is left alone.

A concrete crossing, on most rural and residential sites, means one of three things:

• A pipe or box culvert with fill and a driving surface over it. The structure sits in the channel and the water goes through it.
• A precast concrete slab or plank span, which does span the gap, and which arrives on a truck the way a steel bridge does.
• A cast-in-place structure, poured and cured in position.

Those have very little in common with each other. Comparing “steel” against “concrete” as if concrete were one option is where most of the confusion on this question comes from.

The question underneath the question is what the water does

This is the part that decides it more often than anything else, and it is worth working out before comparing anything else.

A culvert puts structure into the channel. However it is sized, it narrows the opening the water had. That is fine on a drainage swale that runs a few days a year. On a creek that comes up fast, a restricted opening is the thing that catches debris, and a culvert that plugs sends water over the top of the crossing and around the ends of it.

A span leaves the channel open. The bridge is above the water and the banks carry it. A crossing that has washed out before, or one where the neighbours describe it moving in a storm, is usually asking for a span rather than a bigger pipe.

We are not the right people to size a culvert for you, and this guide is not going to pretend otherwise. What is worth knowing is that the hydraulic question comes first and the material question comes second.

Access decides more than people expect

A prefabricated steel span arrives built. It is set on prepared abutments and the set is often completed in about a day once the site is ready. What it needs is a route in for a truck, and room for a crane to reach.

Cast-in-place concrete needs the opposite kind of access. It needs ready-mix trucks able to reach the crossing, on a schedule, and it needs the weather to cooperate while it cures. On a site at the end of a soft two-track, that is frequently the constraint that settles the whole decision.

Precast concrete sits in between: it arrives built, like a steel span, and it is heavier for the same clear span, which changes what has to reach the site to place it.

When a crane genuinely cannot reach, a bolt-together steel kit is the format that exists for that case. Delivered assembled, or as a kit is the comparison for that decision.

Span length narrows the field on its own

Culverts are a short-crossing answer. As the opening gets wider, the structure needed to carry fill over it grows quickly, and at some width a span is simply the sensible structure.

Roadrunner’s steel spans run from 16 ft on the Trail family to 72 ft on the Utility and Pedestrian families, and the longer Utility spans are built from standard modular sections bolted at a plate. If your crossing is 40 ft bank to bank, the comparison is not really steel against concrete. It is a span against a much larger civil project.

Both of them need something to bear on

This is the most common misunderstanding about prefabricated bridges and it is worth stating plainly: choosing steel does not remove the foundation question.

Concrete foundations are recommended for many applications and may not always be required. What your crossing needs depends on soil, bank geometry, and the load crossing it, and it gets settled during the site evaluation rather than assumed from a table. The full version is in Can I install a bridge without concrete?

The difference is how much concrete, and when. A steel span typically needs a bearing surface at each end. A cast-in-place structure is concrete for its whole length, placed in position.

Maintenance is a real difference and it runs both ways

A powder-coated steel bridge is maintained by looking after its coating. Roadrunner publishes a 65 year design life, which is a design basis rather than a warranty, and it assumes the coating is intact. Where a coating is damaged it is repairable in place.

A raw steel finish behaves differently on purpose, and Raw steel or powder coat covers what that means before anyone chooses it.

Concrete does not need a coating and has its own maintenance profile, including what happens where reinforcement is exposed to water and salt over time. Both of them want looking at occasionally, and a comparison that tells you one of them never will is worth reading twice.

What we would actually tell you on the phone

Roadrunner builds prefabricated steel bridges. That is worth saying out loud in a guide like this, because it is the reason to check the reasoning above rather than the conclusion.

The honest summary:

• If the crossing carries real water, or has washed out before, look at a span first.
• If it is a shallow drainage ditch that runs occasionally and the span is short, a culvert may be the proportionate answer, and it is not one we sell.
• If access is difficult, whether a structure arrives built or is placed wet is likely to decide it before anything else does.
• If the span is long, the comparison stops being a comparison.

Nothing above sizes your crossing. Span, load, and what the banks will carry are settled during the site evaluation, when someone walks the location with you before anything is fabricated. Start a configuration if you want to work through the size and format first, or talk to engineering if the site is the part you are unsure about.