Any structure must be designed to resist loads of various types. A building must withstand wind, snow and floor loads as well as its own weight. A bridge must resist the weight of traffic. An offshore oil rig must resist the force applied by the sea.
All structures must be anchored in solid material (usually ground). This anchor point is called the foundation of the structure. The purpose of a structure is to transfer loads from the places where they are applied through to the foundation. The designer must ensure that all the elements of the structure are strong enough to fulfil that role.
Most structures are fairly complex and are made up of a number of individual members (beams, columns etc.). Each member is in fact a mini structure in its own right.
The majority of structures resist loading in one of two ways, either by bending of the members or by axial load (compression and tension). Consider the two simple structures below.
The diagram above shows a simple beam fixed in a wall with a weight hung from its end. The beam will take up a shape similar to the one shown and the movement at the point of load will be relatively large.
Now consider if a second member is added which supports the first member from below.
In this structure when the load tries to move downwards it will be stopped from doing so by the presence of the second member. Because of the angle of the second member, the load cannot move vertically downwards without causing an axial compression in the second member. Similarly, if the load tries to move outwards and downwards (as it would do in the absence of the first member) it cannot do so without causing an axial tension in the first member. Any movement of the load therefore causes axial force in the two members. A structure that resists loads in this way is called a truss. Because the members are much stiffer when they resist loads by compression and tension than by bending, the overall movement of the load is much smaller.
However complicated the structure is, it basically resists load in the same way. Each loading is restrained from moving by the members that directly support it and those members are, in turn, restrained from moving by members adjacent to them. This situation continues until eventually solid foundation is reached. The purpose of the structure is to make a path for the loads to be transferred though to the foundation. Members should positioned to allow this to occur in the most efficient way possible.
To illustrate this further consider the structure below. (The numbers refer to the joints of the structure)
The load is directly resisted by member [1-2] at end 1. Member [1-2] is restrained from moving vertically by members [2-3] at end 2. Members [2-3] push joints 3 downwards and sideways and they are restrained, in turn, by members [3-4] from moving downwards and by members [1-3] and [3-5] from moving sideways. This behaviour continues until the foundations (joints 5) are eventually reached.
This explains why this type of structure is made up of a series of triangles. Loads must be moved from the point of their application across (or down) to the foundation. The loads are transferred from one member to the next at the joints. Since the loading in each member is carried in a direction along its axis, there must be a significant angle (usually 30-60 degrees) between the adjacent members for the load to be transferred efficiently. All joints are usually fully triangulated. If any joints are not, they will often form a weak point and the structure will not be able to carry any load.