4.5 Raft, pile, pier and beam foundations
|4.5 - Appendix 4.5A|
|Guidance for the design of semi-raft foundations on made ground|
Guidance for the design of semi-raft foundations on made ground
The following notes are to be used as a guide for Engineers designing raft foundations, but are by no means exhaustive. Special consideration will be required for certain sites.
- Raft foundations are to be designed by a Chartered Civil or Structural Engineer taking account of ground conditions and the results of the site appraisal and ground assessment.
- Sufficient internal beams are to be provided to adequately stiffen the slab.
- The area between downstand beams should not be greater than 35m2.
- The ratio of adjacent sides on plan should not exceed 2 : 1.
- The minimum depth of perimeter and party wall beams is to be 450mm. On larger dwellings some internal beams should be of the same depth as the perimeter beams.
- Perimeter and internal beams should be sufficiently wide at their base to carry their total loading at the allowable bearing pressure for the site.
- Beams are to be designed to span 3m simply supported and cantilever 1.5m.
- Beams are to use properly formed reinforcement in accordance with BS EN 1992-1-1
- Where mesh is used in beams, it should be delivered to the site pre-bent.
- All beams should be cast on a minimum of 50mm concrete blinding.
- Minimum cover to reinforcement should be 40mm.
- Floor slabs should be a minimum 150mm thick, with nominal top face reinforcement as a minimum and anti-crack reinforcement in the bottom face, if appropriate.
- Stools or similar should be used to support floor slab mesh during casting.
- Corners and junctions to beams should be adequately tied using similar reinforcement to the beams.
- A minimum cavity drain of 225mm below dpc is to be maintained.