plate/member interaction

[RPSCHMITZ]

I've modeled plates supported by members a number of times but just recently noticed an odd effect in the members being support. It might just be the scale of the model, but anyway I'm just wondering if anyone has seen this as well. The problem:

I'm modeling an old barn with a wood floor. There are three bays, about 11', 13' an 10' with 2x10 floor joists at 12" o.c.
To model the deck I'm using a 1' sq plate with a plywood material similar to the "How to Model a Semi-Rigid Diaphragm" on the RISA website.
The plates are fixed and I'm using the "plane stress" option. I'm using them to take a surface load (could use line loads on joists as well I know).
I'm also using rigid links to offset the joist passing over the supporting beams. Here's what I've observed after solving the model for just gravity loads.
I've noticed that some members, those in the middle bay, have a spike in shear, Vz, and there shouldn't be one there. Also, the column reactions are skewed or shifted
from those observed running this model using RisaFloor. Now if I release the in-plane force Fx then the member Vz force is zero and the column reactions
are what I would expect.

I should also note I'm using a couple of membrane diaphragms to get wind loads but they aren't at the level of the plywood deck.

Anyone come across this effect before? If so, is there a fix for it or modeling method? Thanks.

 

[JoshPlumSE]

Applying a plate surface load transverse to a "plane stress" plate strikes me as being inconsistent. If the plates are plane stress then you are only concerned with diaphragm type behavior. Therefore, you shouldn't be loading the plates with vertical gravity loads.

Regarding the spike in Vz shear (weak axis shear in the joists), it is possible that the mesh is too coarse to capture the diaphragm behavior in some regions of the deck. Try sub-meshing 2x2 to see if those spike go away or dramatically improve.

 

[RPSCHMITZ]

Hello Josh,

Yes, that's true. One shouldn't be using a density of 31 lbs/ft^3 for the plywood material either. Anyway, removing the plane stress option and refining the mesh didn't make any difference. The center bay still has members that "lock up" Vz forces. The mesh is about 6"x6". Attache is a photo and a pdf of the detail results of a center bay member.

 

[RPSCHMITZ]

Josh,

Attached is the floor photo. Guess you can't upload more than one file with a post.

 

[RPSCHMITZ]

I think I found the problem. It appears that rigid links used to pass the floor joists over the supporting beams was the problem. One end was pinned the other free. When I pinned both ends the shear force Vz in the joists disappeared. The floor is tied into the walls/arches so stability wasn't a problem. Attached is a screen shot of a section thru the floor with some comments.