AS 3600 Concrete Bearing Under Steel Base Plates

Every steel column eventually hands its load to concrete. The steel side of that handover is well covered — AS 4100 governs the base plate itself, the yield line that sets its thickness, and the welds back to the column. What gets far less attention on the drawing board is the other half: the concrete underneath, which is an AS 3600 problem. Bearing is where the two standards meet, and in our experience it is where base plate details most often come unstuck.

This is a detailer’s guide to what the concrete has to do under a base plate, and what the drawing has to show so that it can.

What the concrete under a base plate actually has to do

The base plate spreads a concentrated column load over an area the concrete can tolerate. Steel will happily carry stresses an order of magnitude beyond what concrete will, so the plate exists to convert a small, very high stress into a larger, much lower one. The concrete’s job is to accept that spread load without crushing locally or splitting the pedestal apart.

Two failure modes matter. The first is local crushing directly beneath the plate. The second — less obvious and more dangerous — is splitting: the bearing load pushes outwards inside the concrete, and if there is nothing to restrain that lateral bursting, the pedestal can split before the surface ever crushes. That second mode is why reinforcement detailing under a base plate is not optional dressing.

Why bearing capacity depends on what surrounds the plate

The single most misunderstood point is that concrete bearing strength is not a fixed material property you can look up once and reuse. It depends on confinement — how much concrete surrounds the loaded area.

A plate bearing on a large concrete raft is confined on every side: the surrounding concrete restrains the bursting forces and the permissible bearing stress rises accordingly. The same plate sitting on a pedestal barely larger than itself has almost no confinement, and its permissible bearing stress drops towards the plain compressive case. The governing relationship is between the loaded area and the larger supporting area geometrically similar to it — which is a drawing question as much as a calculation one.

The practical consequence for detailers: a pedestal that gets trimmed on site, or shown undersized on a drawing, can quietly remove the confinement the design assumed. Nothing about the plate changes, and nothing looks wrong, but the capacity the engineer relied on is no longer there.

Edge distance, pedestal size and the detail that gets value-engineered

Because confinement comes from surrounding concrete, edge distance is structural, not cosmetic. A column placed near the face of a blade wall or at the corner of a pedestal has confinement on fewer sides than one placed centrally, and the bearing check has to reflect that reality rather than the idealised case.

This is also the detail most likely to be eroded late in a project. Pedestals get narrowed to clear a service run, a column shifts to suit a setout, or a footing is reduced to save concrete. Each of those is a bearing change, and each of them should go back to the engineer rather than being absorbed silently into the shop drawings.

Grout, levelling and the load path you cannot see

Between plate and concrete sits the grout, and it carries the entire load. The assumption behind every bearing calculation is full, continuous contact. Anything that breaks that contact — voids, incomplete packing, a grout that shrinks away from the plate — concentrates load onto whatever is still touching.

Levelling method matters here. Where the plate is set on levelling nuts or shims, the column’s weight and early construction loads can sit on a handful of small contact points until the grout is placed and cured. That temporary condition is a genuine bearing case on a very small area, and it is one the drawing should acknowledge rather than leave to chance.

Non-shrink grout, a grout hole where the plate is large enough to trap air, and a specified bedding thickness are the three things that most reliably convert the designer’s assumption into site reality.

What the drawing has to carry

A base plate detail that respects the concrete side will show:

• Pedestal or footing plan dimensions — not just the plate, so the supporting area is unambiguous
• Edge distances from the plate to each concrete face
• Bedding or grout thickness, with the grout type specified as non-shrink
• The levelling method, and a grout hole where plate size warrants it
• Anchor embedment and the reinforcement that restrains bursting, including ties beneath the plate
• Concrete strength the bearing check assumed, called up on the detail rather than left to the general notes

That last one is worth insisting on. Concrete strength frequently varies between elements on the same project, and a base plate detail that does not state the strength it relies on is an invitation to pour the wrong mix into the one element where it matters most.

Where these details go wrong most often

Four patterns account for most of what we pick up in drawing review:

• The pedestal is not on the steel drawing at all. The plate is detailed perfectly and the concrete it bears on appears only in the concrete package, so nobody checks the two against each other.
• A standard detail is reused across different conditions. The same base plate detail appears at a central column and at a slab edge, where confinement is completely different.
• Bursting reinforcement is omitted or shown generically. Ties under the plate get drawn as typical when the splitting case is what actually governs.
• Grout is left as a note rather than a dimension. “Grout to suit” is not a bedding thickness, and it is not a load path.

None of these are calculation errors. They are drawing errors — which means they are the detailer’s to prevent, and they are cheap to prevent at the right moment.

Reading the two standards together

The cleanest way to approach a base plate is to treat it as two checks that have to agree. The steel side sets plate thickness, weld sizes and anchor design. The concrete side sets what the pedestal has to be, how it is reinforced, and what bearing stress is permissible given the confinement actually available. When the two are developed by different people at different times — which is normal — the drawing is the only place they meet.

Current editions of both standards are published by Standards Australia, and detailing guidance for the steelwork side is available through the Australian Steel Institute. If you are detailing the plate itself, our companion guide walks through the AS 4100 limit state procedure step by step.

If you would like a second set of eyes over base plate and pedestal details before they go for fabrication, talk to our drafting team about how we handle structural steel detailing review.