S14 Design and analyse basic structures

Posted: October 10, 2010 in Systems and Control

Initial knowledge – 0

Current working knowledge – 2

Although structures are evident throughout my life I have never really taken the time to analyse them.  When I think of structures the first thing that comes to mind are buildings or bridges,  large, seemingly solid structures.  Using these as examples it indicates that the purpose and function of a structure is to primarily support itself, but to also support the load that it has been designed to take.

So what are the different classifications of structures?

There are mass structures, these types of structures rely on their own weight to resist loads.  So in theory a single brick could be a mass structure.  As could a Dam made of numerous bricks as it too is a solid mass structures.  I resisted the opportunity to use a house as an example as nowadays these can often use frame structures as well.

the pyramids are a mass structure

Framed structures are supported by a skeleton that can be made of materials such as metal, wood and re-enforced concrete.  The rigid frames have fixed joints that enable the frames to resist lateral forces.  Some other frames require diagonal bracing such as the Eiffel Tower or shear walls and diaphragms for lateral stability.

framed structure

There are also shell structures.  If I think of one of the largest recent shell structures the Millennium Dome it has a large shell that has been bent to give it stability.  A more common example would be a car.

the dome, a shell structure

Of course all structures are put under a certain amount of strain or force, and this too can come in different forms.  Compression would be squashing something, whereas a tension force would be stretching.  Torsion is a twisting force and a shear force would be a fracturing.  Finally there is bending which is pretty self-explanatory!

The amount of force is measured in Newtons (N) with 1N being the amount of force to hold up a weight of 100g

These forces can all be applied in different ways too.  If I was to stand still and hold some weights without moving, the force that my body would be put under is called a static force.  If I was standing still and the wind was blowing against me this moving or live force is known as a dynamic force.

A concentrated force is applied at a single point on a beam or structure.   Beams are commonly used for structural support in homes, commercial buildings, and bridges  and so the beam must be designed to withstand forces and stress, while minimizing weight, space requirements, and material cost. Incorrectly designed beams can fail prematurely and this would obviously have catastrophic effects.  The two most important characteristics of a concentrated load in the designing of a beam are the magnitude of the force and the location where it is applied.  How a beam or structure is supported plays an important role in its ability to support this type of load.   A concentrated load applied at the center of a long beam, which is supported at both ends, will behave very differently than the same load applied to the end of a cantilevered beam.

A concentrated load can cause a beam to deflect, or bend, when the force is applied so the design and construction of a beam will influence its ability to resist bending when exposed to a heavy weight. The deflection of a beam is a function of its cross-section, how it is supported, the material it is made of, and where the forces are applied.

Steel beams are most commonly used in commercial buildings due to their strength and resistance to bending, but beams are also manufactured using other materials, such as wood and aluminium.

If the weight is spread evenly across a beam this is described as UDL or uniformly distributed load and so each unit of length has the same amount of load as the others.

Of course a lot of structures must be reinforced for additional strength.  A rectangle is not a particularly strong shape and would easily turn into a rhombus with applied pressure .  It is often common for triangular trussing to be used within a square or rectangular shape to add additional stability.  When a force is applied to a triangular frame, two of its members stretch the third one, making it tense. This in turn will pull the other two members towards it, making the structure rigid and spreading the force between all three members of the triangle.  Triangulation is used in most construction and in the building of bridges and other large structures.

steel structure with triangulation

Forces acting on the outside of a structure such as gravity pulling it down are known as external forces, these in turn cause internal forces or stresses  to the materials that the structure is made of.  These stresses can cause movement and a change in shape or the size of a structure.  This is known as deformation and can lead to repair or the permanent damage of a structure.

In a structure, the various parts can either be in compression (being squashed) or in tension, ( being stretched).  A member in compression is called a strut and a  member in tension is called a tie.  A simple way to look at them would be a triangular house roof:

On a more complex level if I look at a bridge I can see a whole range of struts ties and the triangulation that offers it stability.


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