At a glace, how can you tell if the force within a diagonal member of a truss is in compression or tension?

Question

I have read that there is a way to visualize a panel cut by a vertical plane or something like that.

Answer 1

I'd help you but i have no idea what you just said :)

Answer 2

Hopefully it's a simple truss you are contemplating and your looking for a quick glance, not a full blown analysis.

Visualize your system as a trapazoid. When you apply a force, does the diagnol member try to get longer or shorter? If longer it's tension, otherwise it's compression.

Answer 3

look at where the load centre of gravity is, and look at where the support is given to the truss. The lower span will be in tension usually, and the upper in compression. The struts between therefore will pass the strain towrds the compression member from the tension member.

Also we tend to triangulate members with the apex at a sheer point so they push into each other

Answer 4

Let's try for something a little bit more simple. Is the truss being pushed on by a weight, or is it being pulled on by a weight. You do need to be able to determine what situation the truss is in. Such as a column holding up anything. If it is anchored at one end, by, let's say the building foundation, and the other end is supporting a roof, that beam is in compression because it is being squeezed together. The same would hold true for a truss. Is it being squeezed, or pulled on,. compression, versus tension.

Answer 5

I'd like to add and amend what "fil Yerboots" said.

Diagonal members are pretty easy.  If the diagonal member points toward the load, and the truss is loaded from below, the member is in tension.  If the diagonal member points toward the load and is loaded from the top, the member is in compression.

For horizontal and vertical members, you have to conceptualize how a load will pull on the truss.  Usually, if the truss is loaded from below, and there are diagonal members, the lower horizontal members are in compression, while the upper horizontal members are in tension.  Reverse this condition of the truss is loaded from above.  Vertical members are usually in compression in concert with diagonal members.  Alone they are usually in tension.