JULY 2016 Beam Support Reactions
There have been a number of Factsheets recently that have dealt with the technical aspects of timber member design such as load widths and load areas. The load widths and areas can be entered into software which will work out the member size as well as what the support reactions are. But what if you want to know how much of a load is put onto each support of a beam without relying on software?
Below are a number of common single span scenarios which give you formulae for the proportion of load transferred to each support. Note that load transfer to supports is cumulative. This means you can break down a complex beam into individual loads, work out the proportion to the required support for each individual load and then add these together for the total load on the support. The other important fact is that the sum of the forces must be in equilibrium. The combination of the forces acting on the beam and all the support reactions must be zero. So for a beam with two supports once you have worked out the proportion of load onto one support you know that the rest of load must carried by the second support.
NOTE: In diagrams above P is a load (kN) and w is a load per unit length (kN/m)
The formulae above can be used for many other applications apart from working out support reactions. They can be looked at as load distribution formulae with the concentrated loads (kN) replaced by areas (m2) and the distributed loads (kN/m) replaced by load widths (m). As an example you may have a bearer supporting another bearer which is not centrally located.
If you know the load area from the first bearer you can use the point load formula above to work out how much of that area is transferred to each post and therefore quickly check the design of these posts. As with all technical calculations, if you are unsure then it is best to ask a qualified professional. If doing work for the first time it is also wise to get someone else to check your working.