O Level Revision : Integrated Science - Science in Structures and Mechanical Systems - Beams Structures
Human life is enhanced by structures. Every structure has a design that makes it best suited for its purpose and the material chosen to make the structure must have suitable properties. Beams are used to carry loads. A truss is made up of several beams joined together. Different building materials have their advantages and disadvantages. There are a variety of methods of joining materials.
- A beam is a supported bar which bears a load. A beam may also be defined as a bar of solid material which is subjected to forces along any points on its length.
- Beams can be made of materials such as
- A beam is supported at some points along its length so that it can perform its function.
- Structures e.g. buildings, bridges and dams are made up of beams.
- Systems making any such structure should be strong, light and cheap. Beams are put together to make the structure.
(i) Simply supported beam
(ii) Cantilever beam – supported at one end.
- When beams are loaded, they develop internal forces that are referred to as stresses.
- There are three types of these stresses namely compression, tension and shear.
- Compression is pushing stress, represented by
- Compressional forces cause buckling or shrinking.
- Tension is pulling stress
- Tensional force causes tearing, breaking or cracking.
- Shear = twisting stress, represented by Beam
- Shear forces causes twisting, deformation or change of shape.
- A beam can also be subjected to forces which tend to twist the beam.
- In a loaded beam stress is distributed depending on how the beam is supported.
- The stresses are inside the beam but near the edges.
- The central part is a neutral axis, there is no stress.
- Existence of neutral axis is the reason why most beams are hollow. Removal of central material makes the beams lighter (hence increase in strength – mass ratio).
Types of beams
- Hollow box beam
- Hollow cylindrical beam
- Solid beam
- L-beam f) T - beam
The strength of beams
- The strength of a hollow and solid beam can be compared by loading the beams each at a time thus determine the magnitude of the load which causes failure on the beam.
- For accurate results the following must be ensured when carrying out the experiment:
- beams must be of the same material.
- beams must be of the same length.
- Loading must be in the same manner (hanging the masses or placing them on the beam).
- Results will show that the beams will support the same load therefore the middle layer in a beam is redundant i.e. neutral thus can be removed with no effect on the strength of the beam.
- Single beams are characterized by high concentration of stress. This compromises their life.
- Several beams are jointed into frameworks called trusses.