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What is a structure?|
What are the types of structures?
What is structural analysis?
Structural analysis is the determination of the effects of loads on physical structures and their components.
What structures are subject to structural analysis?
Structures that must withstand loads.
|Aircraft: Is it a stucture, machine, or both?|
How is the conclusion of structural analysis reached?|
How do you do structural analysis?
What is the intended use of the structure?
Is the structure fit for intended use?
Here are further guidelines.
What is a structure? |
A structure is something that will support an object or a load.
A structure must be strong enough to support its own weight and whatever load is put on it!
What are the types of structures?
There are three types of structure :
Mass Structures are solid structures which rely on their own weight to resist loads. A single brick is a mass structure but so is a large dam.
Frame structures are made from many small parts (called members), joined together. Bridges, cranes and parts of this oil rig are some examples.
Shell structures are made or assembled to make one piece. Tin cans, bottles and other food containers are examples of shell structures. Larger things such as car and aeroplane bodies are examples of more complicated shell structures. Most shell structures are made from thin sheet material (which makes them light) and most have ridges or curves moulded into them (to make them strong).
Structures are not new, nature produced the first structures long before humans were able to. A leaf is a natural structure. Its veins provide support and carry nutrients. A tree has to carry the weight of its own branches as well as resisting strong winds.
A manufactured structure is quite simply a structure built by human beings.
Many of Nature's structures have been copied by humans. The shell of a snail and the body of a modern car are both shell structures designed to protect their occupants.
TYPES OF FORCES
Forces acting on Structures
Forces can be either static (stationary) or dynamic (moving).
Static forces are usually forces caused by the weight of the structure and anything which is permanently attached to it.
Dynamic forces are caused by things such as wind, waves, people, and vehicles. Dynamic forces are usually much greater than static forces and are very difficult to predict. These are the most common reason for structural failures.
Forces can be internal or external
An external force is a force placed on the structure from outside, by the wind perhaps or perhaps by someone sitting or standing on it.
Internal forces are the forces which the structure must provide within itself to resist the external forces placed upon it. If the external forces are greater than the internal forces, a structure will collapse.
Forces acting on and within Structures
External forces or loads cause internal stresses to be set up in a structure. Not all forces or loads act in the same way. Forces can bend, pull, press, or twist. Each of these types of force are given special names.
Tension : Is the name given to a force which tries to pull something apart. A structural member in tension is called a tie. A Tie resists tensile stress.
Compression : Is the name given to a force which tries to squash something together. A structural member in compression is called a strut.
A strut resists compressive stress.
Torsion : Is the name given to a turning or a twisting force.
Shear : A shear force is created where two opposite forces try to cut tear or rip something in two.
Bending : Bending is a word you will have met before. A structure which is subjected to bending is being stretched and squashed at the same time.
THE EFFECT OF FORCES ON MATERIALS
When a material is bent, stretched or compressed but returns to its original size when the load is removed, we say it behaves in an elastic way. Elastic bands are good examples.
When a material is bent, stretched or compressed but does not return to its original size when the load is removed we say it behaves in a plastic way.
Moist clay is a good example of a 'plastic' material.
A simple experiment with a paper clip will show the difference between elastic and plastic behaviour. Up to a certain point, a paper clip will spring back into shape when you bend the end outwards and let go.
If you bend it too far, it springs back slightly but stays permanently bent. When this happens, it has been bent beyond its elastic Iimit.
MEMBERS IN A STRUCTURE
The different parts of a frame structure are called members. Each type of member has a different job to do in supporting the structure.
A Beam is a piece of material supported at either end.
When a beam is loaded the top is compressed and the bottom is in tension.
They are usually supported by two or more Columns. Ideally beams should be able to span a wide gap and support a load without deflecting.
Beams used in larger structures take many different forms, some are simply solid, some are hollow, and others have special cross-sections to provide strength and rigidity.
A cantilever is a beam which is supported at one end only. Cantilevers are used where it is not possible to have a support at both ends (a diving board for instance).
When a cantilever is loaded, the top surface is in tension and the bottom is in compression.
Frame structures achieve most of their strength and rigidity from the way they are assembled.
Most frameworks are built using a combination of struts and ties to make triangles. Triangles make very strong and rigid structures. Using triangles in this way is called Triangulation.
Most shell structures achieve their strength and rigidity from the way they are shaped. Shell structures very rarely have large flat surfaces they tend to be designed and made with ribs to act as stiffeners.
Egg and light bulbs containers are good examples. Both eggs and light bulbs can withstand considerable static forces if they are applied carefully.
What they are not good at is resisting dynamic forces.
This is why their containers are designed to absorb impact.