Set-Downs And Penetrations In New Concrete Slabs

Every set-down and every penetration in a new concrete slab changes how that slab carries load, so both belong on the structural drawings before the pour. A step in the top surface for a balcony or a bathroom, or a hole for a pipe or a duct, is easy to allow for in design and expensive to fix once the concrete has gone off.

The difficulty is that these items come from other consultants, and they tend to arrive late. The hydraulic engineer, the mechanical engineer, the fire engineer and the architect all have holes and steps to ask for, and the slab has to answer every one of them at once.

Why set-downs exist

Wet areas and balconies need room above the structural slab for falls to a drain, a waterproofing membrane and the tiles. On a flat slab all of that sits on top, and the finished floor ends up higher than the room beside it.

Architects usually want a level step from the living room to the balcony, or from the hallway into the bathroom, so the slab drops in those areas instead. External membranes fall under AS 4654.2 and internal wet areas under AS 3740, and the depth of the set-down comes from what those systems need, plus the tiles and their bedding.

Depth and deflection

A set-down makes the slab thinner, or makes it step, and both matter. A thinner section carries less and deflects more, so the reinforcement gets checked for the reduced depth.

Balconies are the sharpest case. A cantilevered balcony works hardest along the line where it leaves the building, and that is exactly where its set-down starts. Keeping enough depth there can take a thicker slab, a downturn beam or a step in the soffit, and the cantilever side is covered in [cantilevered awnings and balconies](/blog/cantilevered-awnings-and-balconies).

Penetrations near columns

A hole in a flat slab close to a column cuts into the zone that resists [punching shear](/blog/punching-shear-in-flat-slabs). AS 3600 reduces the effective shear perimeter around a column when openings sit close to it, so a riser beside a column can change the slab thickness, the column size or the shear reinforcement.

Move the same riser a little further away and the problem often disappears. That conversation belongs with the services engineers while the risers are still lines on a sketch.

Post-tensioned slabs

Tendons in a [post-tensioned slab](/blog/post-tensioned-concrete-when-and-why) can be swept around a penetration, provided the designer knows it is there. Cored later, the same hole can cut a stressed tendon, which turns a drilling job into a structural repair.

So PT drawings show the penetrations and the tendons together, and large openings get their own trimmer bars and tendon layout. Anything that turns up after the pour goes through the checks in [core drilling and penetrations in existing slabs](/blog/core-drilling-and-penetrations-in-existing-slabs).

Sleeves and cast-in items

Small penetrations are formed with sleeves fixed to the formwork before the pour. Cast-in ferrules, anchor channels and plates for balustrades, facade brackets and services hangers go in at the same time.

Each one needs a position, a size and clearance from the reinforcement. A cluster of sleeves squeezed between two bars leaves too little concrete around them, and the steel fixer still has to get a hand in to tie the bars, which makes it a buildability question as much as a design one.

Coordination before the drawings issue

The reliable way to handle all this is a single coordinated set, where every penetration and set-down appears on the structural plans and has been checked against the slab design. The architectural plans, the services layouts and the structural model get overlaid, and anything that doesn't line up is resolved before the slab drawings are issued for construction.

Late requests still happen. A plumber asking for a new hole just before the pour is part of normal site life, and it gets a quick check rather than a guess.

On site before the pour

The pre-pour inspection checks the set-downs and sleeves against the drawings as well as the reinforcement. A set-down formed in the wrong place, or a sleeve sitting on a bar, is a quick fix with the formwork open and a far bigger job once the concrete is in.

How we approach it

We ask for the services layouts early and mark the risers, set-downs and large openings on our slab plans, then run them through the slab model before the design is fixed. Where a penetration lands in a bad spot, we would rather ask the services engineer to move it than thicken the slab across a whole floor.

The best penetration is one the slab was designed around.

ACSES provides structural engineering, including slab design and documentation, across Sydney. Talk to us about a project.

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