SAP2000 How to design shear walls using shells

[CorporalToe]

I am trying to model shear walls in my model. I am using shell elements, however when I add them in I am getting axial load in my beams, when my load case is only self weight. I do not know why this is happening. I tried putting diaphragm constraints in the Z-axis but this did not work. Also How am I suppose to handle the connection between the beams and the wall?​

 

[milkshakelake]

Doesn't it make sense that you'd have axial loads in your beams? Something has to transfer the loading to the shear walls, and you have not modeled whatever that would be.

If you want diaphragm to take all the lateral loads and dump them into the shear walls, you need to model floor diaphragms.

If you have modeled the diaphragm, you want to check for any meshing and errors. SAP2000 is quite picky about the tolerances in the software. If you're having issues, a good place to start is to cut up the diaphragm at each beam.

 

[CorporalToe]

I currently have no axial loading. It is only the self weight of the members, so why would the beams experience axial load?. When I did not have the shear walls only beams and columns, everything acted only vertically, ie no axial loads in beams.

 

[STpipe]

What is providing stability to your frame? Even under pure gravity load you need to take that into consideration.

Said another way, take a simple moment frame that's pinned at the base. Apply self-weight to that frame. What will be the results?

 

[milkshakelake]

I don't know what your background is, but SAP2000 is different than RAM Structural System. All members are for both gravity and lateral. Even self weight causes lateral loads. The numbers should be quite low.

You can check the live loading case, for example. All axial loads should be zero.

 

[CorporalToe]

Well my frame is pinned at all connections. And when I model this type of frame in a smaller scope, I get 0 axial force. The shear was should be fixed at the supports, I forgot to do that but that shouldn't make a difference.

 

[Celt83]

The central gable roof has diagonal bracing.

Under gravity load the self weight of the braces and beam over the braces will transfer to the framing below as horizontal and vertical loads. If all beam column joints are pins this horizontal load will traverse the framing in any means possible to reach the wall panels which are the only elements with lateral resistance.

 

[CorporalToe]

Okay I have a follow up question because when I scale up my 2D Simplified Model I get come small (not nearly as large as my main model) axis forces in the beams. Why does this occur?

 

[canwesteng]

If you are running geometric non linear analysis, the columns shrink under axial loads. The shear walls likely don't shrink as much, and so the beams attached to wall shift a bit and pull the columns in to the shear walls, so the columns have a lean and now the structure has axial loads in the beams

 

[Celt83]

not overly familiar with SAP:

Are the beam ends hinged at both the wall and columns?
Is the wall panel continuously supported or only supported at the nodes with the support shown graphically?
Is the column base pinned?

 

[Celt83]

When doing verifications with smaller simple models I find if you plot the deflected shape and set the scale factor fairly high you can usually spot where some phantom forces are being generated.

 

[CorporalToe]

Okay so using diaphragm constraints in the Z-Axis on the shear walls along with a Area edge constraint so the shear walls have 0 resistance to out of plane forces. I was able to remove most of the axial forces in the beams. Besides the front middle. I do not know why it is experiencing so much axial load. I think it is due to the slight angle of the front of the building. Although I do not know why they are so large.

 

[Celt83]

That bay is acting like a catenary cable between the two shear walls.

 

[Celt83]