Tuesday, January 8, 2013

Structural Design of Foundation Members.

This section covers the design of the foundation elements in terms of the structural resistance to the applied forces, but does not cover durability factors which are catered for by reference to BS 8110(1)or BS 5950.(2)

The vast majority of foundations are constructed from concrete, either plain or reinforced, precast or in situ, though a few foundations utilize masonry or steel grillage systems.

Each of these materials are currently designed using limit-state design methods familiar to most practising engineers.

The simplest to design are the mass concrete or plain masonry foundations which rely on natural load spread through the foundation to enable the point or line loads at the top of the foundation to be distributed out to the full area of the base of the foundation. The load spread is usually taken to occur along a 45° line such that the thickness  at the base of the foundation should be no less than the maximum outstand between the edge of the column or  wall applying the load to the foundation and the edge of  the foundation (see Fig. 10.19). No other structural design  is required for such foundations providing they are not required to span over soft spots. It should be remembered, as with any structural element, that the worst case loading condition needs to be determined and the loading case which produces the highest column axial load may not be the one which creates the worst bearing pressure or elemental stresses. This is particularly so when considering foundations which are required to resist column base moments and/or wind loads (it is frequently the case that the size of a base on a bracing line is determined by the minimal dead load and maximum wind uplift) or when designing balanced bases.

 Fig. 10.19 Load spread in mass concrete foundation.

In the normal case the total unfactored column/wall load from the superstructure will be of the form

and the factored load from the superstructure will be

Table 10.4 Typical load cases for ultimate limit-state design of structural foundation members

The unfactored (characteristic) foundation load has previously been expressed (see Fig. 10.16) as

For ultimate limit-state calculations it should be rewritten as

and the factored load from the foundation will be

and the total factored load at the underside of the  foundation is

In simple cases where wind loads are not critical the calculations can be made simpler by using an overall combined partial load factor γP for the superstructure load such that

Frequently γP is taken conservatively as 1.5 (being half-way between γG = 1.4 and γQ = 1.6 for the dead + imposed case)  on the basis that very few building structures support  a total imposed load greater than the total dead load.

Alternatively a closer assessment can be made on the ratio between dead and imposed loads and the value of  γP obtained from Fig. 10.20.

Similarly combined partial safety factors γF and γT can be used for the foundation and total loads where

Again these may be obtained from Fig. 10.20. The use of these combined factors is illustrated in the design examples later.

Having calculated the factored loads it is then necessary to establish the factored foundation pressures, and to determine the resulting moments and shears in the foundation elements, which should be designed in accordance with the appropriate British Standard.
While the loads already utilized to establish that the allowable bearing pressure is not exceeded are unfactored service loads, the factored loads are required for the design of  the members. Some discipline is therefore required when designing the superstructure to keep the dead, imposed and wind loads separate so that they can be easily extracted.
This can be achieved either by recording the working load reactions separately so that the loads can be used directly in the determination of bearing pressure and factored up for the design of the elements, or by recording the factored reactions separately so that the loads can be used directly for the design of the elements and factored down for the determination of bearing pressures. While there is no  particular advantage in which way it is undertaken it is  recommended that a consistent approach is adopted for each project to avoid errors.

Fig. 10.20 Combined partial safety factors for dead + imposed loads.

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