1.  Forces and structures

1.1 Forces

A force is anything that can deform a body (static effect) or change its state of movement or rest (dynamic effect).

There are many different forces in nature: gravity, wind, mechanical forces, pressures of gases, the force created by our own muscles, heat,… etc.

1.2 Structures

A structure is the set of elements in a body that are intended to resist the effects of the forces that act on it. A structure prevents a body from breaking or becoming too deformed.

1.3 Load

The forces that act on a structure are called loads.

There two main types of loads:

  1. Permanent load: they don’t vary over time.
  2. Variable loads: they are occasional and changeable.

2.  Stresses

Load act on bodies, causing internal tensions, which vary depending on how they’re applied, and the specific shape of the objects. These tensions are called stresses.

Main types of stresses

  1. Traction. Forces try to stretch the object they are acting upon:
  2. Bending. Forces try to bend a body.
  3. Compression. Forces try to flatten or compress a body.
  4. Torsion. Forces try to twist a body.
  5. Shear or cutting. the forces act like two scissor blades: they are very close together, one moving upwards and the other downwards, as they try to cut an object.

3.  Structural conditions

Stability: the capacity of a structure to remain upright and not tip over. Its centre of gravity must be centred over its base. If the centre of gravity is centred in the object and close to the ground, the structure will be more stable. We can give a structure stability by:

– Adding mass to its base.

– Bracing it.

– Burying its botton section underground.


Resistance: the capacity of a structure to bear tensions without breaking. Resistance depends on:

– The material use to build the structure.

– The quantity of material used.

– The shape of the structure.


Rigidity: all objects become slightly deformed when a force is applied to them, but the deformation of an object must not prevent the object from fulfilling its funtion. Rigidity comes from:

– Welding joints.

– Making sure the structure has the appropriate shape.

– Triangulation.


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