SHEETING RAIL

CLEATED TO COLUMN

Fig. 3.49 Lightweight wall cladding

advantages over traditional materials such as brick­ work, stonework or-similar materials are:

® The lightness of this form of cladding reduces considerably the dead load on the structure.

• Speed of erection enables plant installation to proceed earlier in the overall programme under sheltered conditions.

16.4 Durability

Some forms of cladding, especially the plastic-faced metal sheets, are comparatively new and the develop­ ment of these materials has been very rapid. Acceler­ ated weathering tests, as developed for paint testing, have been carried out using closed carbon-arc lamps to simulate sunlight. Also exposure to ultraviolet rays to determine colour fading, and washing with artificial solutions of seawater and water containing dissolved sulphur dioxide can determine resistance to corrosion.

Roofing

However, the final proof of a material’s durability is found only after use in a particular location.

Care must be taken when using different metals in close proximity such as sheets and fixings. If they are sufficiently different in the electrochemical series, cor­ rosion cells will be formed unless adequate insulation and sealing of fixings and joints is used. The use of PVC tape and plastic washers together with layers of insu­ lating board between purlins and sheets helps reduce the risk of corrosion through electrolytic action.

17Roofing

17.1Structural elements

These usually consist of profiled sheets of aluminium or steel protected in a similar manner to wall cladding. They are laid across steel or concrete beams to form the roof covering. The sheets are fixed together and to the roof beams by self-tapping screws, cartridge fixings or approved fixing clips.

Metal decks are designed in the same way as any other structural unit, the panels being designed as wide beams spanning between purlins. Panels with different elastic structural properties can be obtained, and it is only after considering the spans of purlins and decking that the most economic arrangement of roofing can be designed. Excessive deflection may occur due to load­ ing which in itself will not overstress the metal deck­ ing. This distinction between elastic modulus and the material’s permissible stresses in the design of roof sheeting systems may be of crucial importance. It is usual to limit deflection to one two hundred and fortieth of the span.

17.2 Insulation and weatherproofing layers

An insulating layer of Perlite (or PU) above 40 mm thick is bedded in bitumen on top of the vapour barrier and metal decking. On this is laid a weatherproofing layer consisting of two intermediate layers and a cap sheet of high performance polyester-based bitumastic felt. The joints and individual felt layers are stuck together using hot bitumen and a final surface of small white mineral chippings set in bitumen is provided to reflect heat and increase the resistance to the spread of flames in case of fire. Insulation board is kept clear of steam pipes passing through the boiler house roof, where steel plates with collars and flashings are used to exclude the weather. Glass reinforced concrete tiles are used to form the principal walkways across the roof.

A vapour barrier consisting of a sheet of hessianreinforced aluminium foil cored felt is essential between the metal decking and the insulation board to prevent moisture being absorbed into the insulation board. Such moisture intrusion leads to both structural deterioration and reduction in insulation performance.

Purlin levels arc arranged to give a minimum gra­ dient ol 40 nun in 3 tn, although much highci slopes arc strongly advised.

17.3 Application to power stations

Provided'the felt is laid properly on this type of roof no trouble should be encountered for many years. These roofs, however, are subject to mechanical damage and care must be taken to restrict traffic to the areas which are specifically designed for it.

The advantages applicable to wall cladding regarding lightness and speed of erection apply equally to this form of roof construction. Figure 3.50 shows a typical form of roof construction. There are limitations in its use, however, for example it should not be used within 2 m of boiler escape pipes. One necessary precaution to be undertaken by the structural engineer is to provide, with one of the high steel columns, a 150 mm bore mild steel dry vertical fire main with standard fire brigade fittings at the ground floor and at turbine hall and boiler house roof levels. There should be one main to each unit. The same main can be used for boiler and tank fillings.

17.4 Durability

Roofs of this type, if constructed properly, require little maintenance when only subjected to light traffic con­ ditions. The insulation layer, however, deteriorates

rapidly if the waterproofing layer is damaged. Experi­ ence from both within and outside the power industry lias shown this form of consli uction to be iclali\ely very fragile and serious examples of major deteriora­ tion or lailuic arc not uncommon.

17.5 Rainwater disposal

In view of the large areas of roofs to be drained and the consequent large amount of water to be dealt with, roof drainage and rainwater pipes require careful considera­ tion at the design stage. If rainwater pipes are spaced at large intervals water can be running quite deep in the vicinity of the rainwater outlet during heavy storms, unless large box gutters are formed to take rainwater discharged by cross-falls and the gutters are sloped to carry the water comparatively large distances to rain­ water pipes. On tall power station buildings large quantities of rainwater are precipitated directly onto the face of the building.

On some stations, water from the roof is allowed to cascade down the glazing forming the elevations of the main buildings and is collected in large gutters at operating floor level. This has the effect of washing the glazing and reduces the number of rainwater pipes.

Rainwater pipes are located preferably on the out­ side face of the building. Ducts are provided in welfare, offices and similar buildings to conceal internal pipes and the large pipes used tor turbine and boiler houses are often fixed in the box columns of the structure. I his