Consequences and Precautions

Consequences and Precautions

Bridges made of steel and concrete expand in hot weather. To allow room for expansion in a bridge an expanding joint must be made in the road surface. The concrete or steel span is usually mounted on rollers or rockers which allow it to move without straining the vertical support pillars.

Railway tracks have been bent and seriously damaged during a very hot day when the gap allowed for expansion was too small. To prevent damage and allow for temperature changes concrete sleepers are used which can withstand large forces caused by the temperature changes. At the ends of welded lengths of track a tapered joint is used so that some movement is possible in the track without the jarring caused to trains by gaps. Some lengths of track are heated when they are welded together which leaves the track in tension when it cools down. Then on a hot day when the track expands, all that happens is the reduction of the tension.

This technique is also used in construction of the stainless steel pipes which carry the cooling gases or liquids into the hot core of a nuclear reactor. When the reactor is being built, the pipes are welded together under tension, so that when the reactor begins to operate and gets hot in its core, the pipes will not expand and cause damage. The rise in temperature of the pipes only reduces the tension in them.

Pipelines in the chemical industry which carry liquids and gases over long distances must have flexible expansion joints built in them at regular intervals.

Telephone wires should be placed high so that when they expand and begin to sag they do not interfere with activities on the ground.

If installing telephone wires in the summer, there should be some sag so that in the winter the telephone lines do not snap.

For some types of glass, a thin glass will be better able to hold hot water than thick glass. Thin glass warms up more quickly because it is thin and not subject to differences in temperature within the glass itself that cause internal stress. With thick glass, the surface directly in contact with the water may be hot, but the surface furthest away from the water may still be cold. With the two surfaces at different temperatures, the hotter surface can expand more than the colder surface and cause the glass to crack.

Expansion and contraction is used in rivetting to get a tight joint. A hot (expanded) rivet is pushed through a hole in the two plates to be joined. Then the end of the hot rivet is hammered to form another head. As the rivet cools it contracts and pulls the two plates together more tightly.