RISA3D help

[JStructsteel]

Having an issue with RISA3D and Pdelta. My model is attached.

No outrageous loads, just 10psf DL and 20psf LL. Moment frames 4 directions.

It gives me alot of locked nodes for My.

If I do Pdelta analysis, wont even run.

Anyone good with Risa3d and see anything out of the norm?

 

[JStructsteel]

So by locking to 2D, I can get it to run with Pdelta. I then add an area load for a RTU, and pdelta blows up again. Not sure why its so sensative with pdelta.

EDIT - Just saw my model is not attached. Here is a link End Frame.r3d

 

[JAE]

Sometimes this happens when your model has too many member end releases and is unstable as a result.

Other times your frame is too flexible and the PDelta doesn't converge.

Either of those two reasons - but pay close attention to the locked degrees of freedom to see if it's more likely the first option.

 

[JStructsteel]

Thanks. Right now I am only chekcing gravity loads, no lateral load included. The loads are small.

I will try fixing the moment frame bases and see if that stiffens up the model, I can check them seperate.

 

[Aesur]

A few thoughts on this:
1. Lock the Y rotation as a reaction on each pinned base, you will then see the reaction is 0, sometimes this is just an annoyance that comes up in RISA 3D.
2. Column M7 is set to pin at the top, remove that as the beams framing in are set to pins this is why N8 wants to be locked for rotation, again a RISA annoyance where you can't have two beams pinned and then release the column as well.
3. Not sure if this is required, but it's a preference of mine to split the members at supports where you have CFS members spanning 2 or 3 bays as a single member. Probably not a big deal, but as long as the members aren't changed to pinned ends should still act as a single member if you are going for multi span members.
4. You are missing all unbraced lengths and k factors for all members, maybe you haven't gotten that far yet, but this is the first thing I always check as it's missed 90% of the time in models I see.
5. May not be a big deal, but I have often found using categories for load cases and load combinations are a bad idea as sometimes they work and other times they don't. My preference and requirement at my company is to put BLC 1, BLC 2 etc in load combinations and not use "category" ie DL, LL, etc.. this has caused many mistakes when it comes to lateral loading typically.
6. For DL Gravity, just a tip, you can put -1 in the load case for dead load and it will add in self weight so you don't need 2 DL categories.
7. Verify that you intended for areas loads to be 2 way distribution and not one way, it appears you have joists, so I would think you intended 1 way, this may have minimal impact, just a suggestion.
8. Run a model merge.
9. The sum of Z direction reactions not being equal to the reactions is most likely due to moment frames deflections. There are a few moment frames in the model. I'm not sure I would worry to much about this as the load is negligible.
10. There are a few CFS members that the design is not running due to connector spacing, see code check, CFS tab. I didn't look into this any further.
11. There are various members, I believe edge of units like M74 there are fixed at ends and the CF joists double also supporting members appear fixed at ends, this is probably causing some weird stuff to go on in the backgrounds, I believe these should be pin-pin.
12. There are various nodes where you don't have anything bracing the top, ie see N28. If you put a x reaction global fixity here, it solves the P-delta issue when you change all the CFS members support the equipment to pin-pin. The reaction comes back as 0, so I believe it's looking for a diaphragm or some sort of brace at that location.

As a whole the model is extremely stiff from what I see, there are very little movements.