No Axial Load in Bottom Space Truss Members

[WTFD4V1D]

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.

 

[msquared48]

"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."

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

 

[JoshPlumSE]

I THINK I understand what you're talking about. Though I cannot be sure without first seeing your model.

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.

 

[msquared48]

Josh:

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

 

[WTFD4V1D]

So here is the file originally with the pinned connections with restraints in all three directions. So right now, all of the three joints have reactions for all three directions. I changed one joint's X translation to fixed and a different joint's Y translation to fixed and tried to run it. It tells me that the sum of Y forces does not equal to my load combination.

 

[msquared48]

The pyramid bottom leg tension forces are internal within the system. The foundation supports points will see primarily vertical forces, and some movement laterally in line with the diagonals at all three supports. X and Y forces should be minimal, if any.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[WTFD4V1D]

Yes, those are the results I get. The thing is that I'm drawing this as a building block for a structure that is basically comprised of many of these into a large pyramid. Basically there are going to be five supports on each of the three sides at the base. If I release the X and Z direction for like 9/12 supports (not the three main ones at the corner), axial loads will show up in most of the bottom members (only like one still says zero). I still can't release the X or Z in the three main corner ones. Thanks for bearing with me. I just started using the program yesterday so I'm trying to learn as much as I can right now.

 

[JoshPlumSE]

Good, that model is simple and is almost exactly what I imagined from your initial post. Take a look at the attached model. Solve it and take a look at the results. I think you will see that it behaves as you would expect.

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.

 

[WTFD4V1D]

Thanks Josh! This is what I was looking for. I'll try it out on my larger overall structure and see how it works. Thanks again.