Section 3 deemed to comply
Site classifications
Soil type and reactivity is classified by soil testers in accordance with the provisions of Section 2 of AS2870.
The core reactivity classes are S, M, H1, H2, E and P.
Nowadays, to reduce their liability, soil testers are classifying most sites Class P – Problem Site. They still specify a reactivity of the site but the overarching responsibility for good design then rests with the structural engineer specifying the footings.
Reading a soil test
Allowable bearing capacity
Site maintenance provisions
Understanding Class P sites
Continuity requirements
Spacing requirements
Cover, Durability and concrete grade
Slab reinforcement
Waffle slabs
Raft slabs
Shear flow reinforcement
Tie-down forces
There is nothing in AS2870 that lets a designer not design a footing and slab system for roof uplift/tie-down forces from wind loads.
In a modern trussed roof, the use of girder trusses (trusses that support other trusses), concentrates uplift forces into 2 to 5 really highly loaded locations. The footing and slab system must be designed to resist these forces.
Slab thickenings
We use slab thickenings where concentrated loads would sit on unthickened slab portions and the slab and foundation material do not have sufficient bearing capacity to resists these loads.
Start with the bearing load and bearing area on the slab. Distribute the load through the slab with a load spread of 45 degrees if no other load spread angle is known. At the bottom of the slab the load area is therefore increased by the load distribution through the slab. Compare bearing force with known allowable bearing capacity.
Don’t forget, when we’re designing with design loads we’re using ultimate loads. When designing for bearing capacity we use allowable loads. These are different load/resistance systems.
We provide slab thickenings under all bracing walls that are not already on a strip footing or edge beam. This is because of the large reactions at both ends of a bracing wall. In uplift, these forces are transferred into the slab system with a tie-down bolt and the bolt has to be embedded at least 100mm. Without a slab thickening, this bolt will punch right through the slab.
Re-entrant corners
We provide re-entrant corner reinforcement at internal re-entrant corners to comply with Clause 5.4.7 of AS2870. We prefer to use the same trench mesh across the corners as are being used on the site in the footings. That reduces the chance that different thicknesses of reinforcement will be mixed up on site.
The code says to use 3 bars of trench mesg x 2m minimum.
Make sure we show the entrant corner bars on the slab plan. Make sure label one of them.
Polished concrete
Polished concrete comes with its own peculiar stresses – the stress of a homeowner requiring a near-perfect, unblemished concrete surface that is hard-wearing, durable with even stone exposure throughout. Concrete shrinks and when it is restrained it cracks. So the requirement for a unblemished conrete slab results in the need to change the specification of the portions of concrete that will eventually be polished.
- Use a smaller bar spacing of slab mesh. This increases the distribution fo the reinforcement through the slab and keeps any cracks that do form narrower. Use either SL81 or SL91 (really heavy and not nice to use on site) or use two layers of slab reinforcement with the top layer offset from the bottom layer.
- Specify a slightly thicker slab. Grinding the slab surface reduces the slab thickness marginally. Maintain cover to reinforcement by making sure there is plenty of concrete thickness to work with.
- Specify Special Class concrete grade. S32, for example, ensures the concrete supplier knows that this is a special slab.
Strip footing design
Bored pier design
Bored piers and site reactivity