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A bridge must be able to carry the forces imposed on it, high in the air, safely down to the ground.
The forces are many and various and difficult to predict - bridge builders call these external forces.

Some are natural like the wind, snow, earthquakes, flooding and the bridges own weight (bridge builders refer to this as 'dead' weight).
Some are man-made coming from the weight and impact of trucks, lorries, trains and pedestrians.

As these external forces are carried safely to the ground a flow of force through the bridge is created.
Bridge builders call these internal forces.

The smoother and easier the flow the better the technical quality of the bridge.

One way to think about how the internal forces flow at any level is to think about what is happening to any particular 'piece' of the bridge.

This piece can be at any level from letters (say a small cube of material such as concrete) through sentences (say a sizeable piece of steel plate) to chapters (the whole deck of a bridge).

Imagine this piece (whatever size it is) behaving just like you would as you try to move purposefully through a crowd of people as in the picture below where the red matchstick man is trying to push his way past other people.
You know that if you can move freely then you feel no pressure - there are no forces on you - no jostling, no pushing or shoving.

However as soon as you bump into someone then you feel a force and you cannot move easily in the direction from which that force is coming.
You may be able to move a little but you cannot move freely and you feel you are being pushed and shoved.

If the crowd is so dense that you just can't move at all then you are being constrained from moving in every direction - you are stuck!
No matter where you try to move you are stopped forcefully.

If there are one or two directions in which you can move then you will take them as you try to get to your destination. But you are having to do this under duress - you move but not freely.

In a bridge every piece of bridge at all levels is trying to move.

The destination of each piece is decided by the external forces on it. One of those forces is always downwards - its weight due to the earth's gravitational attraction. Often there are other forces in other directions too. Forces like wind pressure or the shaking from an earthquake.
However the piece of bridge cannot move much in response to these forces because of all the surrounding pieces of the bridges that stop it or constrain its movement - these constraining forces are internal forces.

In a bridge we identify the various directions of constrained movement and call them 'degrees of freedom'.

Bridge builders capture them all by translating them into just three directions - up/down, side to side and forward/backward.

This is because these three directions are independent i.e. you can move in one direction without moving in any of the others if you so choose. As a consequence using mathematics you can convert any movement in any direction as having components which are one or more of these basic three.

If you are familiar with mathematical co-ordinate geometry then the three orthogonal directions are usually designated x, y, z as shown in the diagram.
But there's slightly more to it than that. A piece of structure might also want to rotate or twist and be constrained from doing so freely.
Again it's possible to convert rotations and twists into one or more of three rotations about each of the directions up, down and sideways.
So all in all there are six possible degrees of freedom for all the pieces of the bridge at any level.

Read more about the three ways in which materials are strong in these degrees of freedom…………

Using computers bridge engineers can calculate the forces generated when any piece of bridge at any level of definition is stopped from doing what it is impelled to do by the natural and man made forces on it.

We can see this through the ever present impelling force on the surface of the earth - which is weight - bridge engineers refer to dead weight.
Each piece of bridge will fall to the ground under its own weight unless it is stopped from doing so.
The only things stopping it are its neighbours - i.e. the other pieces of the bridge at the same level of definition to which it connected.
It is like a team of gymnasts standing in a human pyramid.
The person at the top stands on the shoulders of the persons underneath.
In turn that person's weight is transferred down too.
Eventually their weight is transferred to the persons at the bottom and they transfer the total weight to the ground.
The ground must be strong enough to resist the weight of all the gymnasts or else the pyramid would topple.

So every piece bridge at every level has internal forces generated and imposed on it by its neighbours.
Just as if someone pushes you in a crowd then you have to push back if you are to stand your ground and try to move in the direction you want.
However to do that you may have to rely on pushing against someone else behind you.
If there is a solid wall nearby you might push against that.
But you must be strong enough to do this or else you will be crushed by the crowd.

Likewise a piece of bridge must be able to resist the internal forces imposed on it or else it will be crushed or damaged in some way.
In other words the strength of the piece must be greater than the internal force flowing through it.

You can see more detail of how this works out for different basic structural forms by reading more about
B for beams……
A for arches……
T for trusses……
S for suspension or hanging structures……

Every bridge sits on foundations - read more.....

Now you can try looking at more examples by clicking here or by clicking on the header - Notable Bridges - at the top of the screen.
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