4. Loads

The gravitational force on a structure can be divided into dead loads and live loads. Dead loads can be calculated accurately because they rarely change with time and are usually fixed in one place. Live loads are always variable and movable, so no exact figures can be calculated for these forces. Structures must also resist other types of forces, such as wind or earthquakes, which are extremely variable. It is impossible to pre­dict accurately the magnitude of all the forces that act on a structure during its life; we can only predict from past experience the prob­able magnitude and frequency of the loads.

Engineers never design a structure so that the applied loads exact­ly equal the strength of the structure. This condition is too danger­ous because we can never know the exact value of either the applied loads or the strength of the structure. Therefore, a number called a 'factor of safety' is used. The safety factor is defined as the ratio of the probable strength of the structure and the probable loads on the structure. This factor may range from 1-1 (where there is little un­certainty) to perhaps 5 or 10 (where there is great uncertainty).

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5. Walls

The external walls are made up of brick cladding, wall planks, win­dows, doors, heads and sills, stanchion casings and inner lining panels. While the steel frame is being erected, the wall planks and floor units are fixed. At the same time, the stanchions are enclosed in casings which serve the function of resisting fire. The precast concrete floor units are capable of carrying a load of up to 5 kN/ sq m. The wall planks are designed to be weatherproof and to sup­port the outer cladding. The aluminium heads, sills and windows are then fixed from inside the building. After this, the 900 mm and 1 800 mm wide external doors are installed. These doors are either aluminium framed and pre-glazed or hardwood framed and glazing is done on site. Finally, the internal sills and lining panels are installed. These form a cavity for the heating and electrical ser­vices. A grill underneath the sill, together with an air intake at skirting level, enables air to circulate up past the finned heating element. The lining panels are capable of being removed to give access to the services.

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6. Heating

One method of heating a building is to circulate hot water through radiators which are located in each room. The water is heated in a boiler by a burner and is kept at a constant temperature by a thermostat device called an aquastat. The aquastat is located on the outlet pipe from the boiler. The pipe runs in a continuous loop from the boiler to the radiators and back to the boiler. The function of a radiator is to transfer the heat from the hot water to the air in the room. Therefore they are made of a material which has the property

of high thermal conductivity. The shape of the radiator is also important because the greater the surface area the more rapidly it gives off heat. One type of radiator, called a fin tube, consists of a number of thin fins shaped like a circle which are welded to a pipe passing through their centres.

A thermostat in the room can be set to the required temperature. When the air temperature in the room decreases, the thermostat switches on the pump which is located on the return line from the radiators. When the room reaches the set temperature, the thermo­stat switches off the pump.

An expansion tank is provided to allow for expansion of the water as it heats. A safety valve, located on the pipe leading to this tank, serves to relieve the pressure in the boiler if it is too high.

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