Looking into modeling up a curved steel stair stringer made out of flat plate (including welded steel plate treads). In doing so, I've got discreet members that run from the ground to the landing that I was hoping to model up as a plate section (say 3/4" x 10" for sake of discussion). I have no issue modeling this up or assigning section sizes - the issue I have is when looking at my design results. Although the system is behaving as I'd expect, it appears that the design results (are only taking into account the yielding mode from AISC in equation (F11-1). No matter how slender I make my plate element (1/2" x 10", 1/4" x 10", etc), I do not see any checks for the LTB of the bar section per equation (F11-2). Is there a known workaround to this in Elements?
Here is some steel design output from version 15.00.00.18 for AISC 360-10 LRFD design using a 12" tall, 1/4" thick plate with major-axis moment only.
Is this different from what you are getting? What version and design code are you using?
Thanks for the response! When I put in your 12" tall , 1/4" thick plate I do actually see some results. When I had run my previous section, I was only seeing the yield failure mode checked (no LTB). I agree that your member should be checked for LTB (so that checks out), but technically my section meets the criteria meets the criteria in F11.2.
In my example Lb = 12", d = 8", t= 1", E = 29000 ksi, Fy = 36 ksi. Checking (L_b*d)/(t^2) I get a ratio of 96. Comparing this to 0.08*E/F_y (value of 64), I see that 96 is greater than 64 and thus I need to check LTB per (F11-2). When I go into my results, though, I'm only seeing the yield checks.
I've currently got version 14 installed, so I'll look into see if upgrading to 15 changes anything here quick.
If you save your file (as a zip with custom data included) and send me that I can check it in v15 quickly.
Emailed it to you to check out - thanks for looking.
Try inverting the section such that height is 8", and width is 1". You can rotate the members 90 degrees to keep the same analytical model.
Answer Verified By: Nulukkizdin
Sorry - the version I sent had the wrong member orientation (was oriented such that 1" was the depth). The original orientation (8" as the depth) still produces the same issue. These were all based on a section property with a height of 1" and a widthof 8".
That said, changing the section property such that the height is 8" and the width is 1" produces the correct result no matter what the orientation is in the model. It appears that for the slenderness checks it will use t = width and d = height no matter what the orientation of loading, correct? Either way, I think that does the trick - thanks!
Yes, the expectation is that height is the larger dimension. I now remember this came up once before.