II. 6 Reduced loadings in main beams and columns

similar data processing equipment. It is usual to add 1 kN/m2 to allow for partition loads. The imposed loads referred to are used in the design of floors and secondary members. For main beams and columns it is usual to allow a reduced loading as discussed in Section

11.6of.this chapter.

11.4Cranes

A variety of cranes is provided in power stations, and on conventional stations the largest are those in the turbine halls (see Chapter 2, Fig 2.37). -

The start point for determining the design loadings to be applied to a crane support structure is the maximum static wheel load. For conventional design on simple craneage, to allow for dynamic effects, the maximum static wheel loads are enhanced by 25% for design purposes. Horizontal loadings induced by operation of the crane are taken to be 10% of the maximum static wheel loads acting transverse to the long travel rail and 5% acting along the rail.

Loads for heavy or high speed cranes need to be given special consideration, as do the effects of more complex forms of crane gantry construction.

Elements of the crane support structure subject to significant fluctuations of stress are checked for fatigue endurance. The criteria for this is obtained from consideration of the crane’s duty and is determined from the number and magnitude of lifts expected over the design life. This may include periods of construc­ tion, operation and decommissioning.

Although single cranes are used to lift the heaviest loads, it is sometimes advantageous to use two smaller cranes individually for the smaller loads and linked together with a lifting beam on the two main hooks for the heavier loads. The latter arrangement gives a greater spread of loading on the crane gantry girders with potential reductions in the cost of foundations and structural frame. The use of two cranes for smaller loads improves availability during the erection programme.

Cranage and their support structures in nuclear stations require special consideration. Where failure could lead to a nuclear hazard the cranage and its support structure are designed and manufactured to high integrity standards. This entails the provision of a conservatively designed support structure to withstand the normal operating loads. In addition to these normal loads there is a need to consider the effects of accident conditions and extreme events. The loading applied from such hazards, earthquake, etc., are the subject of special evaluation. The design requirements for the structure under the action of these loads will vary with circumstances but will at least require that it does not fail or collapse and may require that it remains service­ able following the event.

Loadings

11.5 Wind and snow loads

The allowable pressure and suction on the vertical faces of walls and also sloping and horizontal surfaces of roofs due to wind are covered by the recommendations of building Code of Practice CP3: Chapter V: Part 2

[16]-

The dynamic pressure (in Newtons per square metre) of the wind is given by the expression q = kVs2 where k = 0.613 and Vs is the design wind speed in metres per second. The design wind speed is derived by multiply­ ing the basic wind speed V by three factors Sj, S2 and S3. The basic wind speed (i.e., the 3 second gust speed at 10 metres height estimated to be exceeded once in 50 years) for any chosen site in the UK is determined by consulting the isopleth map in the code. The factor Si is a topographic factor, the value of which is usually taken as 1.0. However, in certain very exposed hill slopes and in valleys shaped to produce wind funnelling Si is increased to 1.1. The advice of the meteorological office is sought if there is any doubt about a particular site.

Factor S? takes account of the combined effects of ground roughness, the variation of wind speed above ground and the size of building or component part under consideration. Most new power stations are likely to be in terrain category 1, i.e., in fetches of open level country with no shelter. Factor S3 is based on statistical concepts which take account of the degree of security required and the period of exposure of the structure in years. A building life of 50 years and ,a probability level of 0.636 is usually adopted and this gives a value for S3 of 1.0.

Having obtained the dynamic pressure q for a parti­ cular surface, the surface pressure is calculated by multiplying q by an external pressure coefficient Cpe, values of which are given in the code for buildings of various shape and proportion. The code also gives advice on determining the appropriate internal press­ ure coefficient Cpi. Thus the wind load F acting in a direction normal to the individual structural element or cladding unit is F = (Cpe — Cpi) qA, where A is the surface area of the structural element or cladding unit.

Typical wind loads on cladding and roofing on the main building of a modern power station boiler house are of the order of 2.5 kN/m2 in both pressure and suction. Boiler house roof corners and edges where wind eddies and vortices can occur are designed for suctions in the order of 4 kN/m2.

In the cases of important buildings, or components, of unusual shape such as domes, where there is little data in the code and where limited advice is available,

to determine the appropriate pressure coefficients directly. This is an expensive and time consuming procedure and the possibility of adopting a very conser­ vative code-based value will suffice in most cases.

Civil engineering and building works

Two types of construction can be used in the frames of the power station main buildings — the turbine hall and boiler house:

(a) Portal frame construction This consists of a system of heavy unbraced columns and beams which rely on the bending stiffness of these members and the rigidity of the connections to provide the required stability. The main advantage in this type of construction is that it provides large clear spans that simplify plant access and main­ tenance.

Chapter 3

(b) Braced frame construction This consists of a system of heavy columns and beams braced together in the traditional manner to provide the required stability. It is lighter in weight than portal frame construction and consequently cheaper.

A combination of the two types of construction is normally used in modern power stations, portal struc­ tures being limited to areas where the need for clear spans is considered essential.

A typical portal frame moment connection is shown in Fig 3.41. The typical box column shown here

BENT BARS WELDED