Friday, February 15, 2013

Design of Foundations at Pile Head.

A general description of ground beams and pile caps is discussed in previously and restraints and cap/beam details are briefly mentioned.

In addition to providing restraint, the ground beam is also used to transfer loads from the superstructure to the pile and can be used with or without pile caps. For example, two alternative layouts are shown in Fig. 14.22 indicating a wide ground beam solution and a narrow beam using pile caps.

Alternative beam/cap layouts.
Fig. 14.22 Alternative beam/cap layouts.

Where the increased width of the beam needed to accommodate the pile diameter, plus the total of all necessary  tolerance, is only slight and where a reduction in beam depth helps to compensate for the additional concrete, a wider beam omitting the pile caps can be more economic.

Often the ground beam can be designed compositely with the walls above and by using composite beams a standard nominal size ground beam, dictated mainly by the practicalities for construction, can be used. This has the advantage of standardizing shuttering, reinforcement and excavation, making site construction simple, economic and quicker than the traditional solution. Many different beams designed ignoring the benefit of the contribution from the structure above can severely complicate the foundations (see Fig. 14.23).

 Composite action versus normal design.
Fig. 14.23 Composite action versus normal design.

When considering the use of composite action, consideration must be given to services which may pass through below ground level in these zones. It is often the case that in adopting composite beams the resulting shallow beams can be more easily made to pass over the services. The use of composite action should however be used with caution if there is a requirement to maintain flexibility of future layout. Any modifications involving the introduction of major openings in the walls would invalidate the design assumption that the wall and foundation act together.

A further help in standardizing a smaller and more economic section is that composite action often makes it
possible to precast the beams alongside the excavation and roll them into position, speeding up construction.

For building structures the basic alternative foundations for support on piles generally adopted consists of one or a combination of the following:

Type 1 Concrete ground beams with or without caps sup- porting the main superstructure load but with a floating ground floor slab between the main wall (see Fig. 14.24).
Type 2 Concrete ground beams and suspended in situ or precast concrete floor slabs (see Fig. 14.25).
Type 3 Flat slab construction (see Fig. 14.26).
Type 4 Suspended slab and beam foundations with voids or void formers (see Fig. 14.27).

The economic viability of the pile solutions for the above foundations will differ depending on many variables but, by applying the following basic principles, realistic cost comparisons can be made and piling options exploited:

(1) Minimizing pile numbers relative to pile length/cost and beam length/cost ratio.
(2) Maintaining axial loads on piles and ground beams wherever practical.
(3) Providing pile restraints from other necessary structures wherever practical.
(4) Standardizing on the minimum beam size which can accommodate pile driving tolerances, restraint stresses and pile eccentricity while exploiting any possible composite action.
(5) Minimizing the depth of excavations.
(6) Minimizing the required bending of reinforcement.
(7) Minimizing the shuttering costs by simple standard beam profiles.
(8) Use of simply supported design and simple beam cages wherever possible unless some small cantilever action can greatly reduce the number of piles per unit.
(9) Minimizing the need for pile caps wherever practical by the use of slightly wider beams.

Piles and floating ground slab.
Fig. 14.24 Piles and floating ground slab.

Piles and suspended ground slab.
Fig. 14.25 Piles and suspended ground slab.
Piles and flat slab construction.
Fig. 14.26 Piles and flat slab construction.
Piled suspended slab and beam construction.
Fig. 14.27 Piled suspended slab and beam

1 comment:

  1. I need a ground slab at the minimum depth possible on piles. What method would be best and how shallow could it be? I cannot disturb the ground either as it is in an RPA.