Steel Columns and Beams Encased in Concrete

Steel columns and beams are encased in concrete for three reasons: fire resistance, corrosion protection, and extra strength when the steel and concrete are designed to work together as a composite member. In Ontario, the minimum concrete cover for a fire-resistance rating comes from MMAH Supplementary Standard SB-2, and the encasement has to be reinforced with light wire or mesh so it stays in place in a fire. Encasement is heavy and labour-intensive, so in new buildings it's used where its other benefits are wanted, not just for fire protection.

This post explains when encasement makes sense, what the code tables cover, how the steel is prepared, and what to watch for in existing buildings.

Why encase steel in concrete

Steel loses strength as it heats up. In buildings where the code requires a fire-resistance rating for the structure, the steel has to be protected. Concrete encasement was the traditional method; today, spray-applied fireproofing, intumescent coatings and gypsum board are more common because they're lighter. Our guide to structural steel fireproofing in Ontario compares the methods.

What the Ontario code tables cover

The 2024 Ontario Building Code relies on MMAH Supplementary Standard SB-2, "Fire Performance Ratings", for assigning fire-resistance ratings to many common assemblies. For concrete encasement, SB-2 includes:

• Table 2.6.1.A.: minimum thickness of concrete or masonry protection to steel columns, for ratings from 30 minutes to 4 hours. It covers monolithic concrete (with the spaces between flanges filled), concrete masonry units, brick and hollow clay tile.
• Table 2.7.1.: minimum thickness of cover to individually protected steel beams exposed to fire on three sides, including concrete with the beam spaces filled solid.
• Article 2.6.6.: ratings for hollow steel columns filled with concrete.

A few points from those tables matter in practice:

• Concrete type matters. SB-2 classes concrete as Type S, N or L based on its aggregate, and the required thickness differs between them, particularly at the higher ratings.
• Cover increases with rating. For both columns and beams, the minimum cast-in-place concrete cover listed for ratings up to 1.5 hours is 25 mm, and it climbs for 2, 3 and 4 hour ratings.
• The cover must be held on. The notes require cast-in-place concrete cover to be reinforced with light wire wrapped around the member or wire mesh. Without it, concrete can spall away from the steel in a fire, exposing it.
• Masonry on beams isn't rated in the table. SB-2 notes that the fire resistance of protected beams depends on how the protection is held in place, so it doesn't assign a rating to masonry units on beams in that table.

Which rating applies to a given column or beam depends on the building's classification under Part 3 or Part 9. In most single-family houses no rating is needed for basement beams and posts; see do steel beams in houses need fireproofing.

Composite columns and beams

When steel and concrete are designed to work together, the member is called composite. CSA S16, the Canadian steel design standard, includes design rules for composite columns and beams:

• Concrete-filled HSS columns. The tube acts as formwork and confines the concrete; the concrete adds capacity and fire resistance. SB-2 gives a method for rating them as well.
• Partially encased composite columns. Typically a welded H-section with concrete cast between the flanges, held by transverse links welded between the flange tips. The flanges stay exposed.
• Fully encased sections. A steel section completely surrounded by reinforced concrete, with longitudinal bars and ties.
• Composite beams. A steel beam connected to the concrete slab above with shear connectors (usually welded studs), so the slab acts as part of the beam's top flange.

Composite design is engineering work: the designer sets the concrete strength, the reinforcement, the shear connectors and the construction sequence, including how the steel carries wet concrete before it hardens. For a wider comparison, see steel column vs concrete column.

Corrosion: protected inside, vulnerable at the edge

Concrete protects embedded steel through its alkalinity, which is why rebar lasts for decades in sound concrete. Encased structural steel benefits the same way, with some caveats:

• Cracks let water in. Shrinkage cracks, impact damage or poor compaction can expose the steel.
• Carbonation and chlorides. Over time, carbon dioxide lowers the alkalinity of the concrete near the surface, and de-icing salts bring in chlorides. Either one can start corrosion on steel near the surface.
• The interface. Where a column leaves a slab or footing, water and oxygen collect at the joint. This is where steel posts set in concrete most often rust. Our post on embedding steel posts in concrete vs using base plates covers that detail.

Rust takes up more space than the steel it came from. In old buildings with steel frames encased in concrete or masonry, corroding steel can crack and push off the encasement or cladding. If you see rust staining, cracks following the line of a beam or column, or concrete that sounds hollow, have an engineer look before patching over it.

Preparing steel for encasement

Steel that will be encased is usually handled differently from exposed steel:

• Leave encased surfaces bare. Unless the specification says otherwise, surfaces to be encased are left unpainted. Paint isn't needed for protection inside the concrete, and bare steel bonds better. See does structural steel need paint or galvanizing.
• Clean surfaces. Loose rust, oil, grease and dirt should be removed so the concrete bonds to the steel. The specification sets the surface preparation.
• Attachments for reinforcement. Composite members may need studs, holes for bars to pass through, or welded links. These are shown on the drawings and made in the shop.
• Coordination with rebar. Encasement reinforcement, ties and any longitudinal bars must fit around the steel section and its connections. Clash points are usually at beam connections and base plates.
• Formwork and loading. Wet concrete is heavy. The steel and its connections must be able to carry the encasement and formwork during construction.

Encasement in houses and small buildings

In houses, true concrete encasement of beams is uncommon. More often you'll see:

• Posts with their bases embedded in the basement slab, which partly encases the bottom of the post. The OBC requires exterior steel columns susceptible to corrosion to have at least one coat of rust-inhibitive paint on the outside surface (Article 9.17.3.3.), and good detailing at the slab line helps interior posts too.
• Beam ends set into concrete or masonry pockets, where the beam bears on the foundation wall. These need adequate bearing and a gap or seal so moisture doesn't sit against the steel. Our guide to steel beam bearing length on concrete and masonry covers pocket details.
• Steel boxed in drywall, which is the usual way to finish and, where required, fire protect a basement beam.

If you're thinking about encasing an existing basement beam or post in concrete for appearance, talk to your designer first: the added weight goes onto the beam and footings, and covering the steel makes future inspection impossible.

Getting steel ready for encasement

We fabricate beams, columns and posts with studs, holes, links, base plates and connection plates to your shop drawings in our CWB certified shop, and we can leave encased surfaces bare while priming or galvanizing exposed ones to suit the job. As a rebar supplier, we also detail, cut and bend the reinforcing steel for the concrete around them, and deliver both across the GTA and Ontario.

Send your drawings for a quote, or see the steel products we fabricate.