TOPIC: STRUCTURES AND FORCES
In this topic, learners are required to be able to:
- define structures
- differentiate between natural and artificial structures
- give examples of structural materials (cement materials, steels & wires, timber, etc)
- give elaborate examples of (1) natural structures and (2) man-made structures
- describe some techniques for strengthening structures (principles of structural design)
- make illustration of structural members under load of the following forces: tension, compression, bending, torsion, shearing, distortion.
1.0 Definition of Structure
A structure is a group of elements somehow united to support a load with stability. Everything that can be seen or touched has a structure. For example: humans have skeleton, bicycles have a frame, eggs have shells, houses have columns, beams and other structural members. In principle, all such members are designed to maintain a state of equilibrium; resisting internal/external loads to avoid collapse.
1.1 Definition of Force
A force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its shape or form. A force has both magnitude and direction, making it a vector quantity.
1.2 Differentiate between natural and artificial structures
To differentiate between natural and artificial structures and give elaborate examples, see slides named, Introduction to Natural and Artificial Structures.
Examples of materials for making man-made structures
Candidates must be able to give these examples and describe their practical uses.
- Cement materials (bricks, blocks, reinforced concrete, etc)
- Steel materials (Structural beams, wires, etc)
- Wood (beams, planks, etc)
- Other materials (plastics, glasses, etc)
1.3 Techniques for strengthening structures
By form: Some shapes and forms are stronger and resist loads better than others. For example, with the same material and the same mass, an I-beam support lateral loads better than rectangular beams. Some forms are stronger than others depending on the direction of the force we need to resist. Consider the use of domes, honeycomb and monolithic structures.
By material: Some materials are stiffer than others. Choosing a stiffer material over a less stiffer one can help resist loads. Also, materials have different types of strengths. The type of resistance needed can determine the type of material needed. We know that when compressive strength is needed, materials that are of compressive capacity are used. In the same vein, when tensile strength is needed, materials that have tensile strength should be used.
By arrangement of weight: The weight of different materials arranged in a certain manner can make a structure stable. For example, in structural principles, stability can be achieved by bringing heavier weights down and lighter weight up. We know that the location of the centre of gravity of objects has a significant effect on the stability of the object/structure.
By triangulation: Triangulation involves the use of triangular shapes to give stability to structures. It relates particularly to pinned or hinged structures. Usually these types of structures offer no resistance to bending moments when a force is applied. Some examples of triangulated structures are strussed bridges and roofs.
1.4 Behaviour of Structural Members under Loads/Forces
When members of a structure come under load, they behave in certain ways. The following are some examples.
Under compression load/force, pressure is applied to the member to flatten or squashed it. Check the example below.
Fig 1: Material under Tensile load
Under Tensile Load
Tensile force is the stretching forces acting on the material and has two components namely, tensile stress and tensile strain. This means that the material experiencing the load is under tension and the forces are trying to stretch it. The cross-section of the material tend to become slimmer if it is ductile.
Fig 2: Material under Tensile Load
Under Torsion Load
A torsion force is a load that is applied to a material through torque. The torque that is applied creates a shear stress. If a torsion force is large enough, it can cause a material to undergo a twisting motion during elastic and plastic deformation.
1.4 Under Bending Force
Bending force is the amount of energy which tends to bow an item from its natural shape or condition. A bending force is a combination of tension and compression.
Fig.4: Material under bending load
1.5 Under Shearing Forces
Shearing forces are unaligned forces pushing one part of a body in one specific direction, and another part of the body in the opposite direction. When the forces are aligned into each other, they become two compression forces.
Fig. 5: Material under Shearing Load
Behavior of a Beam under Sagging
Students are to analyse Fig. 6 to understand the behaviour of a beam under load in the middle of the beam.