Structures Question Frames

hudsonriver

Preety basic question here but cant find anything on it and wont be in the library till tommorow

In lectures we have only done frames with members that are perpindicular and all forces perpindicular.
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Now I am trying a frame thats members are sloped, I cant seem to figure out how to get/draw the shear and benging moment diagrams.

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Sorry about the awful diagrams,(had to turn the blank spaces to x's cause boards deleted them)
say a vertical force at Y

Do I resolve all forces so that they are either perpendicular or parallel to the member?

Thanks for any insight

bnt

Well, as I understand it, to analyse any given member, you need to resolve the forces on it in to axial (in line with the member) or perpendicular to it (shear forces). This goes for self-weight too. The axial forces are compression/tension, and the perpendicular (shear) forces generate a bending moment in the member.

Possibly oversimplified example: if you have a beam of length L planted in the ground at 30° to the vertical, with only its self-weight w (no applied forces), you treat it as a Cantilever (which is what it is, just not horizontal) when it comes to calculating its shear force and bending moments.

• You'd have axial forces of w.L.cos(30°), and max shear forces at the base of w.L.sin(30°).
• The bending moment would also be calculated in the usual way for a Cantilever, using the reduced shear force: the maximum would be at the base, ½ w.L².sin(30°)

When calculating whether a particular member is up to the job, you have to make allowances for axial forces and bending moments. This is all covered under Limit State Design in the Eurocodes - I don't know whether you've looked at those yet. (If not, well, you will!)