LESSON 5: Method of Sections


In using the method of sections you should begin by analyzing the truss as a whole. After analyzing the truss as a single structure, make a cut down the truss and pick either the left or right side of the cut to analyze. If given a truss that looks like this:


The proper way to make a cut using the method of sections would look like this:



The cut can be made wherever is most convenient. If the problem insists on finding the values of a couple specific members of the truss, then make the cut through these specific members. You can pick either the left or right side of the cut to analyze. If you’re super cool maybe you’d like to analyze both sides… (note: it is possible to make a horizontal cut, in which case you’d pick either the top or bottom to analyze.)

When analyzing either side of the truss, treat the broken members as forces, just like you would as if they were never part of the truss at all. Just like in the method of joints, each member of the truss is assumed to be in tension, so if you get a negative value for one of the broken members, this means that the member is actually in compression. So, a positive value means the member is in tension and a negative value means it is in compression.
If you are asked to find the value of each member in a truss, it may be easier to use the method of joints, because with the method of sections you can only find the value of the members that are cut. You would more than likely have to use more than one cut to find the value for each member.

A link to an example problem is here: click to view problem. This is the same example problem as the example problem from the last section, except instead of being solved with the method of joints it is solved using the method of sections.