Forces in structures
In order for a structure to perform as it was designed to, it must be able to withstand all the forces that it is subjected to when loaded. In order to understand how a structure will behave when loaded it is important to investigate the forces acting on the external surface of the structure, and those acting internally. This section will deal with the forces found in structures and the effect they have on the structure.
Before you can understand how a structure will behave when loaded you must first have a knowledge of the different types of forces that may be present in a structure.
There are five main types of force, these are :
Tension:a force which causes an object to stretch when it is applied is called a
Tension force.
The member above is subject to a tension force when it is pulled as shown.
Compression: a force which causes an object to be squashed or to buckle when it is applied is called a
Compression force.
The member opposite is subject
to a compression force when it
is pushed together as shown.
Bending: a force which causes an object to bend when it is applied at an angle to that object is called a
Bending force.
The member opposite is subjected to a bending force when the load is
applied as shown.
Shear: a force which acts across a object in a way that causes one part of the structure to slide over an other when it is applied is called a
Shear force.
A teapot maybe subjected to a shear force
between the handle and the body when the teapot is full of water. The reaction on the handle is upwards, while the load is downwards, this will cause a shear force between the body and the handle.
Torsion: a turning force which causes an object to twist when it is applied is called a
torsional force.
The member opposite is subjected to a torsional force when it is twisted as shown.
All structures will have some force or a combination of forces acting on them when they are loaded. If the resultant forces cause the structure to act as it was designed to do, then the structure is suitable for its purpose. However, when excessive loads are applied to a structure it may fail. The failure could result in the structure cracking, or may cause it to deform excessively. A much more potentially dangerous failure could result in the total collapse of the structure.
You may initially find it difficult to determine the type of forces acting on a structure.
It may help you to remember the simple rule shown below.
A structure will change its shape when a force is applied to it.
As an example of this consider what happens to a plastic ruler when a force is applied at the centre. The deflection of the ruler will clearly be seen, however with engineering structures the deflection is not always so obvious.
When a load is applied, and the member deflects, then it may be stretched, compressed or affected by other types of external force. As a result of this, internal forces are set up which push back against the external forces produced by the load.
When assessing members for shear, torsion and bending, it is usually quite easy to
visually determine what type of force is acting. The table below summarises these types of force.
For members subjected to a tensional force, wires, cables and flat sections are generally used. However such sections are poor in compression. In many structures thin members or cables are used to resist tension, an example of this are the ropes or chains used on a children's swing in the park.
The ropes or chains used on a swing in the park are subjected to tensional forces when children are swinging on them.
The rope or chains of the swing are clearly
good in tension as indicated opposite.
Thicker members could be used to support the
load of the girl, however using such members
would not be an efficient use of materials since
thin members or chains are more than
satisfactory for the purpose.
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