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shapes which are used in structures are very important. This
means both the shape which is formed by the pieces when they
are put together and the shape of the individual pieces. If
you look at structures, you will see certain shapes over and
over again |
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A
triangle is a very strong shape. If you hold up a strip of wood
or card you will be able to move the ends from side to side.
Tape two more pieces together to form a triangle. Try to move
the pieces at each of the corners |
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will not move easily because each piece stops the others moving
in all directions. Depending on which way you push or pull,
some parts are in tension or stretching
and some are in compression (squashing).
Can you say which is which? |
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Make
a square with strips joined at the corners. The square is free
to move from side to side. |
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put in a diagonal to form two triangles. This makes the square
rigid. A triangle resists forces on all corners equally. |
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An
arch supports loads and is very strong because it is compression.
A straight member bends because it is in tension.
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two pieces of wood or pencils together to form a straight sided
arch. Push down while holding them together, you can feel them
pressing together into a strong shape. Two straight sided timbers
will resist loads imposed from above. The load is shared by
each side and the ground pushes back with equal force. |
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A
curved arch distributes loads more evenly than the straight
sided arch. The sides are in compression and push back into
the load. If you curve a strip of card and hold the ends on
the table, it will take the weight of a small object. If you
try the same with the card flat or too shallow a curve, it will
bend downwards and collapse. |
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is because the load
on top of the arch is pushing equally down each side of the
arch, spreading or distributing
it to each side and down into the table. |
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Arches
are used in bridges and buildings to support loads. Look over
the top of many doorways and windows, particularly in old buildings
and you will see an arch supporting the brickwork above the
window. |
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beam is a straight member
which supports a load along its side. For instance, if you place
a ruler across a gap and put a load on it, the ruler is a beam.
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If
you try this you will see that the ruler easily bends. This
is because it is a thin flat strip. If they are to support loads,
beams have to be either thick enough or they have to have a
special shape. |
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you put a cardboard box over the gap, the box stays quite stiff
and will support a load. A box uses more material, however,
and can twist out of shape. |
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Another
way of making a stiff beam is to make it so that it is
the shape of an I when you look at the end. An I beam is stiff
and resists bending. The shape of a beam when you look
at its end is called its section. |
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Look around for beams such as those which support a flat roof
in the school hall or gap over a modern window. There are also
many simple bridges which are made as beams |
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Beam railway bridge. |
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I-beam girders at East India Dock development. |
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Overhead
gantry for railway power lines. |
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