Where steel meets concrete

Three tasks in one connection

1. Load transfer

The force passing from the steel member into the concrete: compression, tension, shear. The solution may be a base plate, a cast-in anchor, a post-installed anchor or a combination.

The capacity on the concrete side has to be checked separately: edge distance, concrete grade, reinforcement, cone breakout. That is the step most often omitted.

2. Tolerances

The execution tolerance of concrete is an order of magnitude larger than that of a manufactured steel member. So the connection has to include a means of levelling:

• shim plates,
• grouting or a mortar bed,
• slotted holes for adjusting the position,
• adjustable components where warranted.

Without them erection happens by force, and the force loads the connections. Setting out and foundation tolerances.

3. Corrosion and chemistry

Fresh concrete is alkaline, which attacks the zinc coating. So:

• a galvanised member has to be separated from wet concrete,
• ground moisture must not reach the steel directly,
• moisture must not be trapped in the connection.

Protection against ground moisture · Material pairing and corrosion.

The typical connections

• Column to foundation: base plate, holding-down bolts, grouting.
• Beam to wall: bearing, ring beam or cast-in component.
• Thin-walled frame to slab: fixing of the bottom track, damp-proofing, levelling. Fixing the bottom track.
• Steel beam to concrete floor: composite action or separation.

The question of sequence

Fabrication of the steelwork and the concreting proceed on separate programmes. The connection has to handle that:

• with cast-in anchors, the setting out is decided before the pour,
• with post-installed fixings it is more flexible, but the capacity on the concrete side has to be rechecked.

The point

The meeting of accurate steel and inaccurate concrete always calls for levelling. If it is not in the drawings, it will be improvised on site — worse.