5.3 Steel foundations
The use of a welded steel support structure for the turbine-generator has several advantages:
- A steel support structure is lighter than one of reinforced concrete, reducing the static load on the sub-foundation.
- Steel is a uniform material with accurately known and dependable properties, giving greater accuracy in analysis.
- Offsite fabrication is possible, independent of the weather, and site erection time is reduced. A site erection programme time of three months would be typical for this type of support structure.
- Plant erection can be done immediately the support structure is complete, as it is not necessary to wait for curing or shrinkage. This has obvious advantages for the construction programme.
- A steel support structure gives improved access for pipework and generator connections.
Steel support structures are fabricated as a 'table top' of cellular construction supported from the sub-foundation on box construction columns. The 'table top' is designed to provide support to the bearing pedestals, turbine cylinders, generator and exciter. Examples of steel support structures are shown in Figs 2.50 and 2.51.
A steel support structure is more flexible than the equivalent reinforced concrete structure and is always low-tuned. The flexibility of the support columns isolates the dynamic forces produced by the rotating shafts from the sub-foundation to some degree and may allow a reduction in sub-foundation mass. This isolation also helps the causes of unusual vibration behaviour of the plant to be determined as the effects are more localised than with stiffer support structures. Any attachments necessary on the support columns are strictly controlled to ensure that the structure characteristics and integrity are maintained.
With a steel foundation, the responsibility for designing and supplying the support structure usually rests with the turbine-generator contractor, rather than with the civil engineering contractor. This is generally regarded as an advantage, as the same contractor is responsible both for the plant which generates the dynamic forces and the structure which must respond to, and partially absorb them. Shrinkage is not a problem with a steel structure and working support structure temperatures are below those liable to give creep in the steel. The use of good-quality weldable steel is very important to accommodate the static and dynamic loads, and the steel must have good impact properties. Steel to BS4360 grade С is typical of the types used. Thorough non-destructive testing (NOT) is used on a random check basis for all welds, with particular emphasis on critical and site welds, to ensure freedom from defects.
The thermal expansion coefficient of steel is less than that of concrete and, together with the good thermal conductivity, reduces problems of differential thermal expansion upsetting shaft alignment in the vertical direction. In the axial and transverse directions, the thermal expansion characteristics of the support structure are almost the same as those of the static parts of the plant which it supports: this expansion is readily accommodated by flexing of the support columns. Attachment of bearing pedestals to the support structure is quite simple for there is no need to use grouted-in fixing plates, as on a concrete structure.
The design analysis includes for the effects of rotating out-of-balance loads and considers the fatigue conditions at welded joints to ensure the long term integrity of the structure.
The accuracy of analysis possible with a steel structure gives confidence in the performance of the structure in service and post-commissioning problems have only rarely been experienced. If problems do occur, the structure can be 'tuned' more easily than a reinforced concrete structure by three methods:
- Adding steel struts to the structure.
- Adding weight external to the columns.
- Adding weight inside the sections of a box construction column.
Differential settlement can also be corrected by jacking-up each of the affected columns and inserting packing between the column base and the sub-foundation.