No Axial Load in Bottom Space Truss Members
No Axial Load in Bottom Space Truss Members
(OP)
I'm drawing a simple space truss (three almost equilateral triangle sides and a bottom side). I have the connections all pinned at the three bottom reactions. When I run it, my truss members on the bottom have zero axial force which I understand since my three bottom joints are pinned in all three directions. When I try to set the 3 X translations to free, I get an error to check the boundary conditions (when I set only 2 free, it is fine and some axial loads go to those three bottom members). I'm not sure how to set it up so that I can get my X and Z translation to be all free since the structure will simply by sitting on the ground.





RE: No Axial Load in Bottom Space Truss Members
Seems to me that the bottom members should have axial forces unless you have totally restrained the supports from movement. I do not think you have modeled this correctly if I understand the problem.
Set one support ficed in the X direction, and one other leg fixed i8n the Y direction. Set Z fixed for all supports. You should get forces in the bottom members with this as the supports will be able to move.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: No Axial Load in Bottom Space Truss Members
I would tend to restrain one of the joints in all three directions. The other joints I would restrain vertically and transverse to the main line of thrust at that joint. That may require an "inclined" support which is discussed in the Modeling Tips section of the RISA help file.
RE: No Axial Load in Bottom Space Truss Members
If you do that at only one joint, the figure may want to rotate about the Z axis. That's why I recommended two different supports to give a moment couple to try to limit that scenario.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: No Axial Load in Bottom Space Truss Members
RE: No Axial Load in Bottom Space Truss Members
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: No Axial Load in Bottom Space Truss Members
RE: No Axial Load in Bottom Space Truss Members
I actually could not make any of the supports fully pinned. Instead, one has only only has X and Y restraint (but is free to translate in the Z diretion (the direction of thrust). The other two have an "inclined support" which I created with rigid links oriented so that the zShear release on the link releases force in the thrust direction of the truss.... leaving the truss free to translate in that direction. When I do this, the only joint reactions that develop for a symmetrical loading are the vertical reactions.
So, we have support in two directions at all three joints.... It's the ORIENTATION of these supports that allows us to get the truss behavior that we're looking for.
RE: No Axial Load in Bottom Space Truss Members