The Effects of Shapes

Under a heavy load, a square distorts easily. It ends up looking like a parallelogram. If you put a brace diagonally across the middle of the square, you create two triangles and a much stronger shape. In fact, the triangle is the only shape that cannot be deformed without changing the length of one of its sides. Because it's not easily deformed, the triangle is an extremely popular building shape. Look for triangles in: bridges, railway trestles, hydro towers, geodesic domes, and skyscrapers.

What pops into your mind when you think of Romans? Noses, great pasta, and temples to the goddess Athena? The Romans were also known for arches. They used arches when building amphitheaters and aqueducts. And since the Romans built 80,000 kilometres of roads, they were bound to need a few bridges. When it came to crossing inconveniently placed rivers, arch bridges were their structure of choice.

To make an arch bridge, the Romans built a framework shaped like a half circle, on top of which they wedged stones (called "voussoirs"). They placed the stones starting at both ends. The last stone (the "keystone"), located at the top of the arch, held the others in place. Romans usually built the roadway on top of a group of wide arches. Citizens could cross the river without getting so much as a wet foot!

The strength of an arch comes from its shape. When a load is placed on top of an arch, its force spreads out and down the sides of the arch. At the ground, strong supports ("abutments") keep the arch from moving outward.

A dome can be thought of as a group of arches set around a central point--or as a 3D arch. Like arches, domes distribute loads down through curving sides to the foundations.

Besides being strong, domes are great for covering large areas without using walls or columns. That's why you often see domes in churches, astronomical observatories, and sports stadiums. Toronto's SkyDome was the first dome built with a completely retractable roof. The 98-metre high steel dome roof can be completely opened in about 20 minutes. Like conventional domes, SkyDome is a compressive structure.

But not all domes are compressive. B.C. Place Stadium, home of the B.C. Lions, is a 60-metre high fabric and cable dome. It's called a pneumatic dome--air pressure generated by 16 electric fans supports the roof. The double-layered roof is made of Teflon-coated, fibreglass-woven fabric. Even though each of the layers is only 0.85 millimetres thick, the roof material is stronger than steel! Twenty two steel cables give the dome its rigidity. This is definitely a tension structure (especially if the Lions are losing).

Return to Structures