Curved Canopy - Beam Capacities 2

[EngineerRam]

Hi,

I am currently modeling a curved canopy. The RISA model is attached. The member named "Curved Beam" is giving me high D/C ratios. at the tail end of the canopy (0.81 for envelope). The suggested design members seem unrealistically large. On the detail report I am seeing that equation H1-1b is controlling for the code check. However, when I do this code check with my enveloped Mz, My and A, I get that it should actually be closer to 0.17. Can someone let me know if there is anything wrong which I am doing in the model? Also, of note, I set my unbraced lengths to 0 at the top and bottom of the beams.

Thanks

 

[jayrod12]

your model isn't attached,

however did you also set your unbraced length for axial to zero (or near zero) if you're trusting both your top and bottom flanges to be fully laterally supported.

 

[EngineerRam]

Sorry about that. Here is the file link

http://files.engineering.com/getfil...7328-49d8-8c51-a59dcb123d3f&file=Eng_Tips.r3d

I have both axial and flexural set to near zero.

 

[jayrod12]

I don't have an answer on the code check anomaly, perhaps JoshPlum will come in and comment on that regard.

As for the overall analysis, were you intending on doubling up your wind loads? LC 5 and 6 both account for Wind A and B. That means on the back half of your canopy you're pushing 160 PSF downward wind force at service levels. Seems extreme. Due to this, you're getting deflections under factored loading in excess of 5" (@ N342 and N348) which could be some of the cause of the code check issues.

Just my 2 cents.

 

[JoshPlumSE]

It's the torsional warping stresses that are causing those higher code checks. Take a look at the Member Torsion results spreadsheet for member M392. You will see warping stresses on the order of 44 ksi. These are analogous to weak axis bending stresses. Therefore, RISA includes them in the weak axis portion of the code check.

Now, if you do not believe these warping stresses are real (or want to account for it in a different way) you can turn off this effect by going to the solution tab of the (Global) model settings and unchecking the checkbox that says Torsional Warping. This affects the stiffness of the model as well as the stress calculations.

Warping is a tough concept and RISA accounts for it in an imperfect way. Better than most structural programs, but certainly not perfect. So, if torsion because an important part of your project, it is always a good idea to take a closer look at those forces / stresses and make sure you agree with them. In your case, I wonder if your connections are really going to be detailed out in a way which will impart torsion into those beams.

 

[jayrod12]

I think that confirms my thoughts about the 5" of deflection at the outer corners.

 

[EngineerRam]

Thanks for the help JoshPlum and jayrod12!