All structures support loads. Your chair supports itself plus your weight. The floor supports you and the chair as well as itself.
In order to design a structure, ie the individual beams, columns, trusses etc., you must be able to quantify the loads acting on them. This unit helps you get started.
For calculation purposes, loads are treated as:
- Point loads (forces)
- Uniformly distributed loads (forces spread along a length or over an area), or
- Applied moments.
Point load (or concentrated load)
A point load is a load that can be considered to be acting at a single point, eg. the load of a person sitting on a beam. On a diagram of a structure we represent a point load by an arrow.
Uniformly Distributed Load (UDL)
A UDL is a load with a constant magnitude along a structural member, eg a shelf supporting a number of food cans (total weight = 1 kg) along a length of 500 mm.
Load = 1 kg/0.5m = 2 kg/m = 20 N/m (taking g = 10 m/s2).
Or a beam supporting a number of sitting people! The most common UDL is the self-weight of a structural member like a beam. This is usually expressed in units of kg/m (mass) or kN/m (weight).
Applied Moment
As well as transverse forces (point loads or distributed loads), beams can be subjected to applied moments. Consider two people pushing on the beam shown. There is no net force applied to the beam (the two horizontal forces cancel out), rather there is an applied moment trying to rotate the beam about the point of application. An applied moment can be represented by a curved arrow. In 3 dimensional analysis, applied moments are sometimes represented by a double headed arrow. The direction of the arrow gives the direction of the applied moment in accordance with the right hand screw rule.
An applied moment is a bending or a twisting action. It most commonly is caused by another element of the structure.
Consider a sign board under vertical load. The applied moment M on AB due to the weight of the sign, M = W x d. This moment acts about an axis at right angles to the page, as shown.
