Page 55 - Curriculum Visions Dynamic Book

(Below) Massive modern construction with metal trusses. The Sydney Harbour Bridge, Australia.
Metal makes strong frames
Many designs need to be strong and rigid. One way to do this, and to keep materials to the minimum, was invented about 2500 b.c., during the Bronze Age. They made frames using straight pieces of material shaped into triangles. Each piece used to make the triangle is called
a truss.
The first trusses were made of
wood and used to hold up roofs. But trusses can be made thinner and even stronger with metal. The Romans were the first to make metal trusses. They used bronze because at that time they could not work with iron and steel.
The main advances in metal trusses came in the late 18th and 19th century, when bridges were made first of cast and wrought iron and then later of steel. Cranes and many pieces of machinery have trusses as a way of combining strength and lightness.
Metal was preferred to wood in building construction for fire safety reasons as well as for strength. Fire was a constant hazard in factories because they were lighted with candles and later with gas. But above all, the use of metal frames allowed buildings to get taller. Textile mills could not have grown to six or seven stories in height without metal supports. In this case hollow columns supported the floors and were joined by bars of cast iron. Once the frame was built, the outer brick walls could easily be constructed.
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Tension Compression