Alright, today I'm going to show you how to determine whether each beam or member that makes up a truss like this is under tension or compression. You see, whenever a truss member is under tension, that means the beam or member is pulling on the joints at either end of the beam. And when it's under compression, that means the beam is pushing on the joints at either end of the beam. Now, to determine whether a truss member is under tension or compression, What I want to do is look at what would happen to the entire truss if we were to remove that truss member. So let's say we took out or removed this member right here that runs from joint C to D.
If we were to pull that out of there, the load on this truss would pull the entire left half of this truss downward, rotating around that pin right there. And as a result, this beam right here, BC, would get drug off to the left. And you can see as this entire assembly rotates downward or is pulled downward by the load, the distance from D to C shrinks.
Really what that means is in order to hold this entire truss up this member C-D actually has to be pushing these two joints apart. And remember if a truss member is pushing that means it's under compression. Now conversely if we look at this member right here CB If we were to remove this beam right here the left half of this truss is going to rotate downward because the loads pulling downward on it and This joint C is going to get pushed outward here by this other member And what that means is the distance between joint B and joint C is going to grow.
Ultimately what that means is that in order to hold the truss up, this beam BC has to be pulling these two joints together. meaning the beam or member is under tension. Now this method can be applied to more complicated trusses.
Looking at this seven beam truss we have, I see a lot of people get confused on whether or not this top member right here is under tension or compression. Well, if we were to remove that member and have our load pull downward here, you'll see the left half of this truss just rotates around our pin, and the right half gets drugged downward by the load and kind of slides over on our roller here. And if you look at the distance between D and E, after this whole truss has collapsed, you'll see D and E are much closer together than they started. Really what that means is that in order to hold this truss up, this member that runs from D, E had to be pushing these two joints apart from one another. Again, a beam that's pushing on both ends is under compression.
Now the last one, and this is the one that I think is most complicated for a lot of people, is this member right here that runs from D. B to E. You see if we were to remove this beam right here, this left side of the truss is going to collapse. And it can be kind of hard to imagine exactly what's going to happen as this whole thing collapses. I mean yeah it's animated here, but think about what's going on.
D is going to move to the right over here as B moves down and to the left. Well as D moves to the right, E is going to get pushed up this way. And as B moves downwards and C gets drug inward, the distance between B moving down and E moving up and to the right is gonna grow.
Meaning that in order to keep this truss from collapsing, beam BE has to be pulling these two joints together. Meaning this member right here was under tension. So there it is how to determine tension versus compression within a truss without using any math. So I hope you found this useful and on that note that's all for now.