Concrete Beam vs Steel Beam: How the Two Materials Differ

Is a concrete beam or a steel beam the better choice? That question has no general answer, because the two materials behave differently. The useful question is how they differ, and why a concrete beam almost always carries steel inside it.

What Makes a Concrete Beam and a Steel Beam Different

The core difference is tension. A steel beam resists tension and compression on its own, while a concrete beam leans on embedded steel to carry tension. From there the two materials split again on fire, corrosion and weight.

This article compares the materials, not a design. The beam for a real job is sized by a licensed engineer to the applicable code, usually ACI 318 for concrete and AISC 360 for steel. Those code texts were not read for this article, so nothing here is a code rule. To estimate the concrete for a planned concrete beam, you can use our concrete beam calculator.

The table below sums up what the sources read for this article actually say. Where they do not cover a topic, the cell says so rather than guessing.

Why a Concrete Beam Carries Steel Rebar

A concrete beam carries rebar because concrete cannot hold a tension force well. TeachEngineering, an engineering teaching curriculum, puts it plainly: concrete is “extremely strong in compression, it is extremely weak in tension.”

Steel behaves differently. The same source says steel is very strong in both tension and compression. So a concrete beam embeds steel bars where the beam would otherwise pull apart, and the pair works as reinforced concrete.

In a simple beam, the bottom face stretches under load, so that is the tension zone. The bars sit low in the section to take that pull. The numbering and sizing of those bars is a separate topic, covered in our guide to rebar size numbering.

How the Two Materials Behave in a Fire

Concrete and steel react to heat in opposite ways. Concrete is non-combustible and holds up well, while structural steel keeps its strength only up to moderate temperatures and then drops off. That difference drives how each is protected.

For steel, SteelConstruction.info reports that strength loss begins at about 300 C and increases rapidly after 400 C. By 550 C the most common hot rolled structural steels keep about 60 percent of their room-temperature yield strength. The page gives no single failure temperature, so there is no one critical number to quote.

Because of that, structural steel usually gets added fire protection, such as an intumescent coating, boards or concrete encasement. Concrete is different. ConcreteNetwork, an industry site, calls concrete non-combustible and fire-resistant but not fireproof, and offers an industry rule of thumb: an inch of cover over the rebar protects it for roughly one hour. Treat that as a rule of thumb, not a code value.

Does a Steel Beam Rust, and How Is It Protected

Bare structural steel can rust, but only where moisture and oxygen are both present. SteelConstruction.info describes corrosion as an electrochemical process that needs both at once, so a dry, heated interior carries a low risk.

Where the risk is real, steel is protected. The same source points to good surface preparation plus paint systems, or hot-dip galvanizing, or sprayed metal coatings in aggressive settings. The method is matched to the exposure.

A concrete beam handles this another way. The concrete cover around the rebar shields that steel, so the protection is built into the section. The detailed mechanism is outside the sources read here, so this stays a general point.

Weight and the Load on the Foundation

Weight is a clear split between the two frames. Designing Buildings, a construction knowledge wiki, reports that a steel frame can be up to about 60 percent lighter than a comparable reinforced concrete frame.

That weight matters below ground too. The same source notes that a lighter frame might allow a less expensive foundation system. This article does not price that out, and cost depends on many project factors.

One common claim, that steel spans farther with fewer columns, could not be checked against a source read for this article, so it is left out here. A deeper look at slabs and supported members sits in our guide to how thick a slab should be.

What the Concrete Beam Calculator Estimates

The calculator estimates concrete volume only. It does not size reinforcement, check capacity, or compare a steel section. It takes a concrete beam’s length, width and depth, adds a waste allowance, and returns cubic yards, bags and weight.

Here is the tool’s own worked example for a concrete beam, with the arithmetic written out.

1. Enter the size. A grade beam 20 ft long, 12 in wide and 18 in deep. In feet that is 20 x 1 x 1.5, which is 30 cubic feet.
2. Add waste. With a 10 percent allowance, 30 x 1.10 is 33 cubic feet. Divide by 27 to get 1.22 cubic yards.
3. Read the bags. At 0.60 cubic feet per 80 lb bag, 33 / 0.60 is 55 bags.

A steel beam has no concrete volume, so the calculator does not apply to one at all. For other concrete members, see our structural concrete calculators. These sources are engineering-information and trade pages, and the design codes ACI 318 and AISC 360 were not read for this article.

FAQs About Concrete and Steel Beams

Sources and Further Reading

This is a materials comparison from engineering-information and trade sources, not a design or selection guide, and the codes ACI 318 and AISC 360 were not read for it. Concrete and steel beams behave differently in tension, fire and corrosion, but the beam for a real job is sized by an engineer to the applicable code. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-21.