Warehouse Floor Slabs And Racking Loads

Racking post loads and forklift wheels govern a warehouse floor slab, far more than the uniform load written on the brief. A slab that looks generous at a given number of kilopascals can still crack or punch under a single loaded post, so the design needs the racking layout and the forklift details before the thickness is set.

The floor is also one of the few structural elements in a warehouse that gets used hard every day. A tenant won't notice the roof. They notice the joint that rattles every forklift for the next twenty years.

Why a uniform load misses it

A brief often asks for a floor rated at a uniform load, and that number makes sense for pallets stacked on the floor. Racking changes the picture. A tall pallet rack gathers all of its weight into small steel base plates, which push down on the concrete at a handful of points.

Two back-to-back racks put their posts close together, so those loads overlap in the slab. The design checks both bending and punching at those points, because a post can fail the slab in either way.

What we ask the racking supplier

Racking is designed by the supplier under AS 4084, and a slab design needs their numbers. We ask for the maximum post load, base plate size and post spacing, including the spacing between back-to-back rows. We also ask for the rack height, because a tall rack is far less forgiving of a floor that moves.

If the racking isn't chosen yet, we take a design case from the tenant's likely use, and put it in writing on the drawings. Anyone changing the layout later can then check their new racks against the limit the slab was built for.

Forklifts and joints

Forklift wheels run the same paths thousands of times, and a solid tyre puts a high pressure on a very small area of concrete. That chews up an unprotected joint.

The usual reference for this design in Australia is the guide to industrial floors and pavements from Cement Concrete and Aggregates Australia, known as T48. It sets out how slab thickness, wheel loads and repetition work together, and the [loading docks and heavy vehicle slabs](/blog/loading-docks-and-heavy-vehicle-slabs) article covers the truck side of a similar problem.

The subgrade carries the floor

A slab on ground is only as good as what sits below it, because the concrete spreads the load and the soil carries it.

On a Western Sydney clay site, that means dealing with reactive soil and with moisture changes under the floor. Sub-base thickness, compaction and proof rolling all count, and the fill under the slab has to be tested as it goes in.

A soft patch under one rack line doesn't show up until the rack is loaded. Then it shows up as a crack the tenant will send you a photo of.

Joint layout and shrinkage

Every concrete floor shrinks as it dries. Joints give that shrinkage somewhere to go, so we set out the joint layout with the racking plan in hand, keeping posts clear of the joints wherever possible and letting the main forklift aisles cross as few as they can.

Dowels transfer the wheel load across a joint so that one panel doesn't step down relative to the next. Saw cuts go in at the right time, while the concrete is young enough to crack where we want it. The wider logic is in [expansion and control joints in concrete](/blog/expansion-and-control-joints-in-concrete).

Slabs also curl upward at their edges as the top dries faster than the bottom. A curled edge under forklift traffic is how a joint starts to break down, which is why mix design, curing and slab thickness all get decided together.

Flatness for tall racking

Tall racking and narrow aisle trucks need a flatter floor than a general warehouse. A mast at full height magnifies any tilt at the wheels, so a small dip in the floor becomes a real lean at the top pallet.

That requirement has to be written into the specification before the pour, along with how it will be measured. Grinding a finished floor to fix it is slow, dusty work in a building someone is waiting to move into.

Fixing into the slab

Rack base plates are bolted down, and those bolts sit close to the edge of joints and to each other. Post-installed anchors are designed to AS 5216, and they need enough edge distance and concrete depth to work, as described in [anchors into concrete and why they fail](/blog/anchors-into-concrete-and-why-they-fail).

Mezzanines bring the same issue with larger post loads.

When the layout changes

Tenants change. The next one brings different racks, and a rack that puts a post in a new spot, near a joint or at a higher load, is a new load case on an old slab.

So we note the design limits on the drawings and, where the building allows, keep the capacity for a reasonable change of layout. If you already own the building, a quick check against those limits costs very little compared to a cracked floor full of stock.

How we approach it

Our structural and civil engineers treat the floor, the [tilt-up panels](/blog/tilt-up-concrete-panels-for-warehouses) and the hardstand as one job, tied to one set of site levels and one earthworks plan. We ask about the forklifts and the racks in the first meeting, because that is what the slab is really being designed for.

A warehouse floor gets judged every day by the person driving across it.

ACSES provides structural engineering for industrial and warehouse buildings across Sydney. Talk to us about a project.

Building Relationships Beyond Structures