Tuesday, January 8, 2013

Select Foundation: Questioning the information and proposals.

Having gathered the information together for the design, the engineer’s first considerations should take account of the following:

(1) Is the investigation sufficient to design a safe and economic foundation?
For example, a ground investigation that was undertaken which collected samples and arranged testing  in the light of background information and which predicted a piled solution may have lacked detailed investigation of the upper strata – this would be necessary in order to consider a raft as an alternative (see Fig. 10.2).

Fig. 10.2 Borehole log.

An alternative example would be a ground investigation based upon boreholes and sample collection/
testing at shallow depth envisaging surface spread foundations – this would not provide the information
necessary to consider a piled solution.

If a piled foundation solution is subsequently found to be necessary, important sub-soil information would not be known.

In some cases the engineer is only called in after the initial site investigation has been completed and the
alternative foundation solution is only appreciated at that stage, making further sub-soil investigation neces- sary in order to design the most economic foundation.

(2) Is the initial proposed scheme appropriate for the ground conditions identified?
Could the proposals be modified without detriment to the  successful functioning of the building and give significant savings on foundations or significant reductions in predicted differential settlement?

For example, where piling is necessary for a singlestorey building the economic span for ground beams,
etc., often produces loads in the piles which do not fully exploit their load-carrying capacity. In such situations consideration should be given to changing the building form from single-storey to multi-storey (see Fig. 10.3).

Fig. 10.3 Typical situation for low-rise construction.

(3) What blend of superstructure and foundation should be employed?
For example, in active mining areas, the combination of superstructure and foundation can be very critical in
accommodating movements and the whole structure must be carefully considered when taking account of subsidence (see Fig. 10.4).

Fig. 10.4 Effect of subsidence on pinned/fixed frame.

(4) Is the arrangement of the superstructure supports very critical to the foundation economy?
For example, the design of the superstructure should not be made completely independently of the foundation economy. In the same way the foundation economy should not be considered independently of the superstructure. A typical example of this kind of problem is the use of fixed feet on portal frames which often create greater additional costs on the foundation than they do savings on the superstructure (see Fig. 10.5).

Fig. 10.5 Arrangement and effect of superstructure support on foundation.

(5) Is the proposed layout and jointing of the foundation exploiting engineering knowledge to provide the most economic foundation?
For example, the choice of the lengths and jointing of continuous ground beams can have extreme effects on
the foundation moments and forces and hence on the costs, as is shown by the following example.

Consider a series of six columns at 10 m centres, the four outer columns having a load of 500 kN and the two inner columns having a load of 250 kN (see Fig. 10.6).

Fig. 10.6 Continuous foundation beam 1.

Assume that a ground beam is positioned under these columns in one continuous length of 50 m. The total load on the beam is 2500 kN and it is symmetrical.

Assuming a stiff beam, a uniform distributed pressure below the beam of 50 kN per metre run would result.
Referring to the diagrams shown in Fig. 10.6, it can be seen that the point of zero shear occurs at the mid-
length of the beam and that the maximum resulting shear force is equal to 500 kN. Since the maximum bending moment is equal to the area of the shear force diagram to one side of the point, the maximum bending moment is as follows:

It also follows that the bending moment diagram would be approximately that shown in Fig. 10.7.

If the resulting bending moments are considered, it can be seen that for its full length the beam is hogging
and the resulting deflected shape would be of convex outline (see Fig. 10.8).

It is also apparent that much smaller bending moments and shear forces would result if a deflected
shape similar to a normal continuous beam supporting a uniform load could be achieved (see Fig. 10.9).

If, therefore, we set out to achieve this and keep in mind that the fixed forces in this case are the column loads, then we must aim for a continuous uniformly loaded beam with reactions equal to the column loads.

If we now refer back to the original loads and consider them as reactions, we can then place upon them
a beam uniformly loaded with similar reactions (see Fig. 10.10). The beams in Fig. 10.10 have been chosen by ending the beam near the smaller loads and cantilevering out over the heavier loads.

Fig. 10.7 Bending moment diagram beam 1.
Fig. 10.8 Deflected shape beam 1.
Fig. 10.9 Desired deflected shape beam 1.
Fig. 10.10 Jointed foundation beam.
If these beams are now adopted, assuming that ground conditions and site boundaries will allow this, the revised bending moments and shear forces can be assessed. The total load on each beam is now 1250 kN and the length of each beam is 25 m. Let the resultant load act at a  distance x from the 250 kN load (see Fig. 10.11). Taking moments about this column’s position:

Fig. 10.11 Jointed foundation beam – resultant load location.

Since a 0.5 m cantilever has been given to this end of the beam the resultant load acts at 12.5 m from each end and hence is symmetrical.

The resulting pressure, again assuming a stiff beam, is 50 kN per metre run, as for the previous beam.
Referring to the shear force diagram for this beam (see Fig. 10.12), it can be seen that a number of zero shear points occur. Consider point A and again using the  area of the shear force diagrams to obtain the bending moments:

Fig. 10.12 Jointed foundation beam – shear force diagram.

Bending moments can similarly be obtained for each position on the beam, but by inspection the maximum value will be 506.25 kNm at A.

If the result on this beam is compared with that of  the previous beam it will be found that the maximum bending moment is less than one-tenth of that of the earlier solution and the shear force has been reduced to approximately one-half, both emphasizing the importance of the  selection of the foundation beam arrangement to be used.

The previous example illustrates the need for the engineer to use a basic knowledge of structures to exploit the conditions. It can be seen that it is not economic to have a continuous beam foundation which bends in either a hogging or dishing form under a number of loads unless site restrictions prevent alternative solutions.

The aim should therefore be to achieve bending more in the form of a normal continuous beam being bent in alternate bays in each direction. To achieve this aim it is necessary to inspect the loads and to relate these to continuous members which would have similar reactions (see Fig. 10.13).

Fig. 10.13 Beam loads.

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