Reinforcement, construction and main reasons to use raft foundation

The main reasons why the designer choose matt/raft foundation instead of single footings: 

1- Very weak bearing capacity of soil which leads to high subsidence in the soil.
2- When high loads are applied (such like multi story buildings) with poor soil bearing as well.
3- In the case of private structures such as (towers, chimneys, and water tanks).

Raft foundation reinforcement:

Raft foundation reinforced with two lower and upper steel mesh. The main steel reinforcement is in the long direction, because the moments are greater in the long direction.

Additional reinforcement:

* Additional reinforcement is in placed at the maximum negative moments.
* Bottom mesh of steel reinforcement is placed at the columns bottom.
* Additional steel reinforcement is placed at midspan.

Permissible places to overlap reinforcing steel

* Bottom steel connections and overlaps are permissible in the middle of the span between columns.
* The upper steel connections are permissible at the columns.
* The length of development length is according to the code 60D, where D is the diameter of the reinforcement used.

Raft foundation steps of construction:

  1. Excavation works at the required levels.
  2. Pouring the normal concrete under the raft foundation.
  3. Formwork of the raft foundation sides.
  4. A cover is placed in concrete spacers with a thickness of 7.5 cm in the form of strings every meter.
  5. Reinforcement works according to the plans, which forms two steel meshes (a lower steel mesh and upper steel mesh) with steel chairs to carry the upper mesh with a diameter not less than the iron used in the two meshes.
  6. Columns and walls are made with the upper mesh, and the length of the overlap is not less than 60D or 1 meter, whichever is greater.
  7. It's recommended to put steel marks of the thickness of the raft foundation required for concrete pouring.
  8. Pour concrete with the required thickness.
  9. Concrete must hydrated by with water for a period of 7 days, which starts 24 hours from the concrete pouring.

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