5.2 Concrete foundations
The traditional foundation used in the CEGB for turbine-generator units up to 500 MW was reinforced concrete. For small units, the support structure was a rigid monolithic block on a substantial sub-foundation, with openings to accommodate plant and pipework. This type of foundation was invariably high-tuned and movement at the bearings was restricted at the expense of high bearing forces. The dynamic energy associated with machine vibration was transmitted through the support structure and sub-foundation and absorbed by these structures and the subsoil.
As unit size increased, the problems of designing a high-tuned foundation with adequate access for pipes, connections and ancillary plant became more severe, and the larger units now in service with concrete support structures generally use a low-tuned design. Examples of low-tuned reinforced concrete support structures are shown in Figs 2.48 and 2.49. The structure is generally in the form of a series of portal frames, linked with horizontal beams at the top to form the platform on which the turbine and generator are mounted. The structure is designed to limit the differential settlement of bearing supports to about 0.5 mm, although for bearings close together differential movement of half this value is more appropriate.
When designing a concrete foundation, margins on calculated natural frequency have to be allowed because of the wide range of values of Young's modulus for concrete which can be met in practice. Concrete also shrinks during curing and this can continue for a long time after construction. Most of the shrinkage occurs during the first six months, so turbine-generator erection on the support structure is normally programmed to occur nine months after construction. Further shrinkage continues to a lesser extent for some time and minor changes to machine alignment may be necessary for up to four years. Concrete also creeps under stress and this continues throughout the life of the support structure. In fact creep is one of the mechanisms by which the stresses resulting from shrinkage are relieved. Special concrete mixes have been developed for minimum shrinkage to alleviate this problem but these are sometimes difficult to pour and compact.
Thermal expansion of concrete is significant and, together with its poor thermal conductivity, can give rise to alignment changes between hot and cold conditions due to differential expansion. Thermal shielding of parts of the concrete support structure is therefore employed to reduce these effects and one UK power station has been fitted with cooling pipes embedded in the concrete to control temperatures directly.
As concrete is a site-prepared material and its structural properties are likely to be variable (within limits) between sites, the risk of resonance or excessive vibration might be expected to be greater than with other types of support structure. This has not been the case in practice, and post-construction tuning of reinforced concrete support structures is rarely necessary. If tuning is necessary, it is much easier if specific provision is made for this in the original design.
Methods of achieving this, reported from outside the UK include:
- Incorporation of redundant members in the support structure which can be removed later if it is found necessary to reduce the stiffness of the structure.
- Incorporation of attachment points for the later addition of structural members to increase stiffness.
- Constructing the supporting columns in a pocket in the sub-foundation. The free column length can then be later reduced by filling the gap between column and sub-foundation with additional concrete.
As unit size increases, the mass of the support structure necessary to provide a sufficiently rigid foundation for the plant and to limit bearing deflections increases. Access for pipes and ancillary plant is also restricted so, despite the economic advantages of concrete in many situations, some 500 MW and all larger CEGB turbine-generators are mounted on steel support structures.