Plain cruciform (not flanged cruciform) member design using AISC 360
Plain cruciform (not flanged cruciform) member design using AISC 360
(OP)
We have a situation where we are using a plain cruciform shaped beam in a simply supported situation. The cruciform is symmetric about the vertical axis but not about a horizontal axis (see attached image).
The beam cross section is nearly a inverted "T" shape, however, the bottom of the flange of the beam is not flush with the bottom of the web of the beam. My understanding is that this is called a "plain cruciform". I apologize if the terminology I am using is incorrect. I borrowed the terminology from:
http://www.newsteelconstruction.com/wp/wp-content/...
What I am trying to determine is if I can use the provisions in AISC 360 F9 for this situation. Table F1.1 shows a cross section of F9 that is not exactly like my situation. That section would cover yielding, LTB, and FLB.
Any recommendations or guidance would be appreciated. Thanks very much.
The beam cross section is nearly a inverted "T" shape, however, the bottom of the flange of the beam is not flush with the bottom of the web of the beam. My understanding is that this is called a "plain cruciform". I apologize if the terminology I am using is incorrect. I borrowed the terminology from:
http://www.newsteelconstruction.com/wp/wp-content/...
What I am trying to determine is if I can use the provisions in AISC 360 F9 for this situation. Table F1.1 shows a cross section of F9 that is not exactly like my situation. That section would cover yielding, LTB, and FLB.
Any recommendations or guidance would be appreciated. Thanks very much.






RE: Plain cruciform (not flanged cruciform) member design using AISC 360
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
stemsflanges are in tension during bending). This has a significant effect on the calcs since in the as-shown position the stems do not brace the compression part of the beam.www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
SlideRuleEra - Did you mean to say that the flanges (not stems) do not brace the compression part of the beam?
All - I need the additional stiffness provided to me by the flanges at the bottom, so I'm inclined to treat it as a t-shape. As mentioned, the flanges are too far from the compression stresses in the top of the stem to be of any real help, but that brings me back to my problem on which I should have mentioned in my first post.
Section F9.1 has criteria for (a) stems in tension and (b) stems in compression:
For tension .........Mp = FyZx <= 1.6 * My
For compression...Mp = FyZx <= 1.0 * My
Section F11.1 has criteria:
.................Mp = FyZ <= 1.6 * My
Am I missing something? If I meet the Lb*d/t^2 criteria in F11 and can use the equation above, my rectangular bar will have a much higher capacity than if I treat it like a tee-beam with the small flanges attached near the bottom (at least, if the stem is in compression)?
Thanks again.
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
--> Lb*d/(t^2) <= 0.08E/Fy for LTB to not control/occur.
Unless you're dealing with an incredibly short span (or have the beam braced at very short intervals) you're going to be looking at Equation (F11-3).
--> Mn = Fcr*Sx
where Fcr = (1.9*E*Cb)/(Lb*d/(t^2))
Also, don't forget the LTB check for the T-shape either. There's no quick "if this ratio is less than this ratio" check to see if LTB controls for the T shape, so you get to check for LTB regardless of your braced length.
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
My point was, for the situation where yielding would control (either a rectangular beam or tee beam), the capacities appear to be quite different. If the beam was proportioned such that LTB, FLB of tees, and local buckling of tee stems did not occur, I would have a limit on the nominal moment of My for a tee (stem in compression) and 1.6*My for a rectangular beam.
Is this odd to anyone? Am I missing something obvious?
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
It seems odd to me. What do you mean be 'nominal moment' and why would the limit be 1.6*My for a rectangular beam?
BA
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
The other interesting thing here is the commentary explicitly states to use Cb=1 for WT with stems in compression, citing testing that shows premature failure, but allow for the Cb factor to rectangular plates.
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
Hi BAretired - Rectangular beams meeting the geometric criteria in AISC 360 Chapter F11.1 would use the nominal strength criteria shown in equation F11-1. I'll attach it for reference in case you don't have AISC 360 handy. The upper limit happens to be 1.6*My, but for a rectangular shape I'd imagine that Fy*Z would govern, where Z would just be 1.5 times the elastic section modulus for a rectangle.
Nominal is just the term they use for the strength before it's knocked down by the resistance factor (phi) or the safety factor (omega). I'm using AISC and their terminology.
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
I don't know that this is the answer but it may be related.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Plain cruciform (not flanged cruciform) member design using AISC 360
BA