STRUCTURAL FORCES

1) Structures a) What do structures do? There are different kinds of structures. A few examples could be a tower crane, an electricity pylon (tower), the body shell of a car, a building, etc. All structures must: -

Protect the various parts of an object without deforming.

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Support the load without collapsing

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Span a gap.

Protect the contents

Support something

Span a gap

b) Types of structures There are two main types of structures: -

Frame structures, made of bars joined together to form a “framework”. Examples: could be the electricity pylon and the crane. It is an economical way of structure as it needs less material

Electricity tower

Tower crane

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Shell structures, which are made of panels. It is a quite different structure. Examples: the car body shell and the appliances casings (photo cameras, etc.)

Car body shell

Tin of food

- Structure of a building

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2) Types of forces All structures have loads applied to them. To compensate against these external loads, structures react with internal forces. There are different types of forces:

Tension: Tension is the force that tends to lengthen an object. It is a pulling or stretching force.

Compression:. Compression is the force that tends to shorten an object. Compression is a pushing or compressing force.

Torsion: Torsion is a turning or twisting force.

Bending force: So that a bending force tends to bend.

Shear Force: Shear force tends to cut an object. A shear force is a sliding force, pulling in opposite directions.

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3) Design of frame structures Structures have been designed to be rigid and stable.

a) Making structures rigid When a structure cannot be deformed (only by breaking the structure) it is said to be rigid. If you want to make a structures rigid you have to add an extra bar between corners A and B. This new bar has formed triangles, and this effect is called triangulation. Most of the frameworks are triangulated to become rigid.

Exercise: In the following picture you will find some non-rigid structures. Add whatever bars you needed to make them rigid:

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b) Making structures stable A structure which will not topple easily when acted by a force is said to be stable. In the drawing the girl is applying a force to her model, and when the force is removed, the structure will fall back to its original position. This is a stable structure.

In the following picture when the boy pulls the thread the structure collapses. This is an unstable structure.

Centre of gravity For stability of structures, it is necessary to understand what the centre of gravity is.

The point in which the pull of gravity (or weight) acting around your finger is equal is called the centre of gravity. The ruler is then balanced. Finding the centre of gravity Using a cardboard: 1. cut the shape you want to find the centre of gravity 2. hang the shape and a plumb line from and nail or a large pin 3. draw the line directly underneath the plum line 4. now, hang the shape from a different point, and draw again the plumb line 5. where the two drawn plumb lines cross, that is the centre of gravity.

Exercise: Where do you think the centres of gravity of these shapes are? Try to guess where the centre of gravity will be for the given shapes.

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Why is the centre of gravity so important for stability?

If the centre of gravity moves outside the base area, gravity will make the structure topple over. Some general rules to make structures more stable -

The lower the centre of gravity is, the more stable the structure will be.

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The wider the base of the structure is, the more stable is will be.

This tractor will NOT topple over since the vertical from the CG moves inside the base

This tractor WILL topple over since the vertical from the CG moves outside the base

So that, the tractor on the left has more stability as its CG is lower than the one on the right

In the given position the structures WILL topple over since the CG moves outside the base. You could make these structures more stable by lowering the CG or making the base wider

This structure is really stable since the CG is very low and the base is rounded and moves, so that it will be always underneath the CG. It will be very difficult to make if fall

Exercise: Label the following structures with an S for stable, and with UN for un stable

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Structures