Structures fall into two categories ~ they can be natural or man-made. A tree is an example of a natural structure as is your own skeleton. It is likely that you are reading this sitting on a man-made structure, a chair or a table. Structures can be further divided into either frame or shell structures. A frame structure relies upon several parts to keep it held together whereas a shell structure consists of a continuous surface to provide the strength. An egg is a good example therefore of a natural shell structure and a crane is a good example of a man-made frame structure.
The forces which act upon a structure can be constant or static, or they can move around and vary in which case they are dynamic. A tree (natural, frame structure) on a dead calm day experiences a static force. A ship (man-made, shell structure) in a stormy sea experiences a dynamic force.
There are many different forces that can act upon a structure and you need to be able to recognise them and understand how they differ.
The force that you experience most of the time is compression, a squashing force. The legs on a chair experience a compression force, a submarine experiences a compression force because of the weight of water pushing down on it.
The opposite of this force is a pulling or tension force. Swing fro a rope and the rope will exper ience a tension force as will your arm. Inflate a balloon and there will be a tension force within the skin of the balloon making it stretch.
A twisting force is called a torsion force. A screw requires a torsion force in order to tighten it. An electric motor produces a torsion force capable of making a gear or a wheel rotate.
When two forces slide against each other it is called a shear force. The blades of a pair of scissors produce a shear force and a hole punch works by shearing away a little circle of paper or card.
The final force to consider is a bending force. This is a force which causes a structure to flex or sway. A bending force, however, is not a force in it’s own right but a combination of forces.
So what is a bending force then? Take a piece of cake, a chocolate sponge cake with chocolate icing, and bend it upwards and you will notice that the bottom of the cake splits apart whilst the icing on the top wrinkles up.  What is happening is that the bottom is being stretched apart (or experiencing a tension force), whilst the top is being squished together (experiencing a compression force). The centre of the cake is not particularly harmed by any of this because it is experiencing neither a tension nor a compression force. This is important because this line in the middle is called the neutral axis. Because the line doesn’t have to cope with any force, the material along the neutral axis contributes nothing at all to the strength of an object. Remember the experiment with a ruler? Try to bend it when it’s flat and nothing could be easier. Turn it on it’s edge and it is a different story.
To make a beam as stiff as possible it is necessary to have as much material away from the neutral axis as possible. How can this be done? By making it hollow. Tubes are so successful because there is nothing at all along the neutral axis. All the material is at the very edge where the forces are at their greatest. The larger the diameter, the stiffer the tube. Girders are another good example of concentrating most of the material as far away from the neutral axis as possible.
You can see the same effect of moving more of the material away from the neutral axis by simply creasing a piece of paper. All of sudden, something that was too flimsy to support it’s own weight becomes more rigid. You haven’t added any material merely altered whereabouts the material is. This feature is commonly seen on all manner of flat surfaces which need to be more rigid. The panels of a car are made from steel sheet less than 1mm thick but because they are folded and creased into complex shapes, they become much more rigid. An egg is a flat surface that is infinitely curved and creased which is why it is so incredibly strong (when you consider how thin the shell is).
Another way of stiffening sheet material is by adding ridges of material or ribs. Take a look at the underside of the maroon trays in any of the technology rooms and you can see this clearly. Forming material into structures using these techniques means that you can cut down on the amount of the material used which in turn saves weight, lowers the cost and reduces the environmental impact.
We have mentioned sheet steel as one material that can be formed into shell structures but others include polypropylene (used for the Millenium Dome) and more commonly concrete. Concrete is an excellent material in compression but lousy when under tension so, to make it suitable for bridges (which experience a bending force, a combination of compression and tension) it needs to be reinforced by a material which is excellent in tension. So, when pouring the wet concrete into the huge moulds to make a section of roadway, engineers will bury within the concrete a mesh of steel rods which will withstand the tension.
Many structures rely on frames for their strength and we are now going to look at the way these are designed. Most structures tend to be rectangular shapes; bridges, cranes, buildings etc. If we were to arrange a framework in this shape without any thought, problems could occur. The illustration below shows what happens to such a structure when it comes up against a shear force (such as a gust of wind) ~ it collapses easily.
The strongest arrangement of the pieces of the framework, or members, is in a triangle. It is very desirable to have this arrangement (triangulation) within frame structures and you can see it present in everyday structures (the passenger footbridge and the platform roof at Tonbridge station for example).
Within a framework structure it is possible to identify types of force present within the members. On the illustration below I have shown a section of a framework bridge and indicated the downward force caused by a lorry on the bridge. The arrows show the direction of the force in each of the members, a pulling or tension force in the diagonal members which causes a pushing or compression force on the horizontal member. Remember that if the framework is made from steel it is better for most of the members to be in tension rather than compression.
| 
