Monday, January 14, 2013

Design of Semi-Fexible Raft Layouts

Rafts form a dual  levelling-out function. Firstly they take concentrated loads (stress concentrations) from the superstructure and spread them more evenly onto the ground below; secondly they mitigate the effects of soft spots (local weaknesses) in the supporting ground through local spanning action.

In order for rafts to fulfil this role it is necessary to avoid areas of undue stress concentration and zones of weakness within the raft structure itself. The arrangement and  frequency of raft thickenings – and movement joints  where necessary – to achieve this involve as much art as science. Nevertheless, the following guidelines should be considered when creating a suitable raft layout.

(1) Continuity of thickenings
Thickenings should be continuous wherever possible, with no abrupt terminations or changes in direction. Although a thickening will often be located to coincide with the main load-bearing elements of the superstructure, this is generally not essential. The priority should always be to achieve a consistent and robust arrangement of thickenings. It is recommended that there be sufficient thickenings in both directions to limit the aspect ratio for slab bays (i.e. length : width) to a maximum of 2 : 1.

(2) Avoidance of areas of weakness
Areas of weakness generally occur at re-entrant corners (see Fig. 13.13) and where re-entrant corners occur on both sides, resulting in a significant reduction in width of the slab. Re-entrant corners should be dealt with by ensuring that both of the external thickenings meeting at the corner are continued past the corner as internal thickenings on the same lines. Such areas of weakness within the raft should  be suitably strengthened, with additional thickenings if  necessary; alternatively, the weakness can be ‘acknow-ledged’ by positioning a movement joint at this location (see below and Fig. 13.17).

Thickening layout for raft stiffness.
Fig. 13.13 Thickening layout for raft stiffness.

Crust raft design example – plan layout and loadings.
Fig. 13.17 Crust raft design example – plan layout and loadings.

The other main possible source of weakness in a slab is via poor detailing at thickening junctions and intersections.

Suitable tying reinforcement should be provided at all  corner, tee, and  cross junctions, to ensure thickenings can interact and share load with each other as necessary.

(3) Appropriate use of movement joints 
Movement joints are used to break up a large raft into  a number of smaller rafts. This may be done to reduce  bending moments and shrinkage stresses in a large raft/superstructure, or to avoid areas of stress concentration.

Stress concentrations can occur at local reductions in width within a slab, or at junctions of  limbs with the main mass  of the raft. It is  usually easier to deal with the strain energy associated  with these stress concentrations by introducing a movement joint and allowing movement to occur, rather than strengthening the area by introducing additional reinforcement and thickenings.

It is good practice to avoid excessively large or excessively elongated rafts, and thus a larger building may well need  to incorporate a number of movement joints. It is recommended that rafts be limited to an aspect ratio (length : width) of approximately 4 : 1, and in general a maximum length of 20 m.

It is important that, when a movement joint is introduced into a raft structure, thickenings occur on both sides of the movement joint, to ensure the two halves do both act as independent rafts. The movement joint should be carried up through the superstructure walls and suspended floors; however it is generally not necessary to continue it through a tiled or slated timber roof structure, unless large differential foundation settlements or longitudinal strains are expected.

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