Iron expands or contracts about 1/150000 of its length for every degree on the Fahrenheit scale.
It is therefore important to make provision in all large roofs for the expansion and contraction caused by changes of temperature.
This is generally done by leaving one end of the truss free to move horizontally. In roofs of great span the chair or saddle at the free end of the truss is supported on oiled rollers, so that it can move outwards and inwards with ease under changes of dimension caused by the effects of temperature.
In smaller roofs the same object may be attained by supporting the chair on a sheet of lead, and making the holes for the bolts which secure it, slots of an oblong form, so that the chair can move slightly backward and forward on the lead.
It is better to fix the end from which the heaviest gales are most likely to blow.
" In countries liable to hurricanes extra precautions must be taken, and not only should the roof be strongly braced together by wind ties, but the entire structure should be well anchored to the ground. The latter precaution is especially necessary in buildings open at the sides or ends, and liable therefore to severe wind pressure below the roof." 1
Fig. 301 shows the arrangement adopted to allow for expansion and contraction in the roof over the former Lime Street Station, Liverpool, the details of which will be found in Dempsey's Railways, or in his work on Iron Roofs.
1 Works in Iron, by Ewing Matheson.
The objection to this arrangement is that when the principal rafter is depressed under the force of the wind, an uneven stress comes upon the joint, the roller on the inner side is more compressed than the others and is liable to be crushed. To prevent this the joint may be formed as in Fig. 302, which shows the expansion arrangement adopted for the roof of the Cannon Street Railway Station.1 The junction of the roof with the upper part of the casting which rests upon the rollers is hinged with a circular joint, so that, whatever the inclination of the principal rafter may be, the stress passes through the centre of the group of rollers and is distributed evenly over them.