Issues Modelling Braced Frame in SAP2000

[terrance1]

Hi everyone, Im a student intern trying to model a one-story steel warehouse in sap2000. Im new to using the software so Im having some issues with the model. Primarily Im having two main issues, and Im assuming the same issue in the model is causing both. The first issue is that the joists in my model do not carry the same load. Some carry a lot of load and thus have large bending moments, and others have very little load and moments. The second issue is that under the auto-generated wind loading (NBCC 2020) it generates cases wind-2 to wind-12, and for these cases none of the members or braces show any axial forces. But it also generates cases for wind-32, wind-42, wind-52 and so on to wind-122. For these it does display axial forces in the members and braces. Below im going to list some information about the model, as well as some screenshots of the model.

• the beams, joists, and braces have moment 2-2 and moment 3-3 released at both ends, as well as torque released at one end. the columns are left as continuous with no releases.
• all the supports are pin supports
• the roof deck is a shell-section thin with material property G40-350W(plate(t>65mm))
• a semi-rigid diaphragm has been assigned to the joints at the roof level
• uniform dead and snow loads have been applied to the area sections

Moment 3-3 diagram under dead and snow load.

axial force diagram under wind-2.

deflected shape under dead load





Thank you for any help that can be provided.

 

[BulbTheBuilder]

The first issue is that the joists in my model do not carry the same load

In CSI product, one-way loads are distributed in the local red axis. Review how your panel was modelled and in which direction the load is being applied.

The second issue is that under the auto-generated wind loading (NBCC 2020) it generates cases wind-2 to wind-12, and for these cases none of the members or braces show any axial forces. But it also generates cases for wind-32, wind-42, wind-52 and so on to wind-122. For these it does display axial forces in the members and braces.

I don't know which directions are for the load cases but all I can say check if you are using rigid or flexible diaphragm.

the beams, joists, and braces have moment 2-2 and moment 3-3 released at both ends, as well as torque released at one end. the columns are left as continuous with no releases

I would leave torsion restraint and release only major and minor axis.

For semi rigid diaphragm, release shear in the minor axis so that beams don't take lateral forces in weak axis. You can define the plate to be a membrane to take in-plane stress only.

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All that being said, the first step would be fixing the issue model to work correctly under gravity. Create a simple bay with joist and see how the panel directions and moment releases affect your analysis. Afterwards, return to fix the issues you have.

For the lateral review, I won't recommend auto load generation. Apply arbitrary load, say 5kips to each frame in both direction so you can manually track the loads and verify your stresses.