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Foundation beam continuity!

AS2870-2011 the australian code that governs the design of residential slabs and footings specifies the minimum foundation and slab sizes as well as design rules. However, those sizes provided in chapter 3 of the code are subjected to a very important assumption, which is beam continuity. From experience, if a residential foundation slab fails it most likely has failed due to incorrect beam layouts and therefore not have all beams continuing.

Unfortunately, most engineers we worked with assume that beams need to go under load bearing walls! which is completely against the mission of AS2870. The designers need to realise that the main force residential slabs are designed for is soil shrinkage and swelling not bearing pressure!

The figure above is a direct snipet from AS2870. It is clearly seen that load bearing walls might not rest directly on foundation beams. However, the walls need to be at a maximum of 1m on either side of the beam, otherwise slab thickenings can be used to support the wall.

Furthermore, AS2870 states and I quote "Internal and external edge beams shall form an integral structural grid in accordance with Clauses 5.3.8 and 5.3.9."

Clauses 5.3.8:

"5.3.8 Beam continuity in rafts

Where the raft design includes internal beams, the structural continuity of internal and

external beams in stiffened rafts, including waffle rafts, shall be maintained in accordance

with the following criteria.

Internal beams shall be continuous from edge to edge of the slab. Where beams are at

different levels, as may occur in two-pour systems, special detailing is required to provide

continuity. The requirements apply to stiffened rafts, including waffle rafts. Internal beams

shall be located to provide continuity with the edge beams at re-entrant corners."

Therefore, according to the code which is the only source of reference in Australia to the design of residential slabs on expensive soils, the beams need to be continuous from edge to edge.

However, the code does acknowledge that in some cases this might not be possible due to the shape of the building and specifies the following detailing rules to mitigate it.

This is a very large topic and the reason behind these rules will be the topic of future blogs. I hope this can assist engineers and builders to better understand the requirements of AS2870.


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