unbraced length question
unbraced length question
(OP)
i have a steel platform with grating with no horizontal brace. see attached for partial plan.
unbraced length of this i set as total L for new structure or i add horizontal brace.
but this is existing and i dont want to retrofit this if not necessary. any chance this can work out as L/8. i've read about 2% flange force and also grating might help.
looking for some clarification.
my biggest worry is original engineer (this is a very old structure) might have considered L/8 as unbraced length for the beam which will cause them to fail in my new analysis if i consider total L.
thanks,
unbraced length of this i set as total L for new structure or i add horizontal brace.
but this is existing and i dont want to retrofit this if not necessary. any chance this can work out as L/8. i've read about 2% flange force and also grating might help.
looking for some clarification.
my biggest worry is original engineer (this is a very old structure) might have considered L/8 as unbraced length for the beam which will cause them to fail in my new analysis if i consider total L.
thanks,






RE: unbraced length question
In doubt, model the thing with initial imperfections in say RISA 3D and P-Delta activated, with the sounder representation of the joint constraints. Apply your load-case and see the stability of the thing.
RE: unbraced length question
I had a similar situation. See attached thread thread507-277139: Bar Grating as a Diaphragm
RE: unbraced length question
If they are WF, you could look at them to make sure they can support the bracing forces in weak-axis bending (provided the columns, in turn, form part of the LFRS.)
Look at AISC Appendix 6 for requirements.
You mentioned this is a very old structure.
Make sure you use the proper Yield Strength in LTB calculations; this can make a difference.
RE: unbraced length question
I'll check appendix 6. If i get confused or will take too much time, i'll just use total length and retrofit the girder. LOL
Dont like the idea of welding the grating. I have no experience doing that.
Thanks,
RE: unbraced length question
Grating is probably not much help since it has low strength and stiffness in the horizontal direction. We did a detailed analysis of grating to see if it helped as bracing on a steel stringer bridge and found it wasn't much help. Better than nothing but not close to providing fully effective bracing. Also, it had to be welded to the top of the stringers. If the grating is just resting on top as independant sections it really can't do much to prevent lateral movement of the beams.
RE: unbraced length question
I read about the global lateral buckling happens only if all girders are loaded but if not, some of the girders may help as brace to the loaded girder.
I will not be able justify this though as we design for WORST CONDITION so all girders have to be assumed loaded.
RE: unbraced length question
The W12x19 may also help as a torsional brace depending on the relative stiffness and conection details with respect to the main beams. I've analysed this problem using handouts and notes from a class on structural stability taught by J.A. Yura. Yura published an article on beam bracing that covered the basics but I haven't bothered to look for it since I already had the info. The basic theory should also be in more recent textbooks. The main parameters are diaphram length, diaphragm moment of inertia, beam length, beam moment of inertia, and distance between brace points. I can scan Yura's handout for you if you're unable to find a published source.
Connection details are important. Ideally you'll have a stiffener at your brace points (full depth or nearly full depth) to control cross section distortion at the connection. Stiffeners would be common on older structures. You're probably looking at an adequate torsional brace if the W12x19 diaphragm depth is at least half the beam depth. That was the old rule of thumb guideline for diaphragms in steel bridges.
RE: unbraced length question
BA
RE: unbraced length question
However, that picture of yours seems to show some sort of joist or purlin spaced at L/8 that is much more likely to be provide lateral and twist restraint to your girder. So, L/8 might make sense for the girders..... just not for the joists / purlins.
RE: unbraced length question
thanks,
RE: unbraced length question
Available by download for $10 at the AISC website.
I've attached stability class notes on bracing. Your problem is covered in Example 7 and the "Tension Flange Bracing" notes on page 5.
RE: unbraced length question
RE: unbraced length question
If that doesn't prove sufficiency of the bracing you could try more sophisticated computer modeling to get a direct solution of the full system. Some of the current software can model a 3-D system including effects of beam bracing and buckling.
RE: unbraced length question
RE: unbraced length question
Some of these parameters are listed in section property tables in the AISC Manual Beam Section for 36 and 50 ksi steel. For example if you have a W21 x 50 beam with a yield of 36 ksi, you can look up an Lr value of 16.2 ft which means you can develop first yield on the beam if it's braced at no more than 16.2 ft. Lp for this beam is 5.4 ft which means you need braces at 5.4 ft to develop full plastic bending capacity in the beam. For 50 ksi the numbers for this beam are 12.5 ft and 4.6 ft.
RE: unbraced length question
what are you talking about? i know how to put unt, unl, ly, lz, etc...
you were the one who said something like i will quote you "Some of the current software can model a 3-D system including effects of beam bracing and buckling."
i thought you were talking about automatic unt, unl, etc.. from computer similar to direct analysis method where you dont put K values anymore
RE: unbraced length question
The reason I suggested you may need more study is that the question of what unbraced length is required to carry a given load on a given beam is not a design program problem. You could use a design program to give you the answer by trail and error where you do repeated runs with different unbraced lengths until you converge on the required capacity. But I believe if you were more familiar with the theory of lateral torsional buckling and were familiar with the design equations in the AISC Steel Manual you could easily find the appropriate equation and solve it for unbraced length and by that method determine the unbraced required unbraced length for your beam.
If on the other hand you are only plugging values into design programs the design equations might be meaningless to you and in that case you are seriously in danger of falling victum to the dreaded "Black Box" problem. I've investigated two failures where the design engineer used a program without understanding its capabilities and limitation. The output said everything was fine but the bridge failed because the program didn't handle out-of-plane buckling and bracing issues. In one case the designer was very inexperienced and lacked sufficient knowledge of the issues. In the other the designer was quite experienced but didn't recognize a global buckling issue of girder pairs and unfortunately his program and the way he modeled the girder didn't handle the issue either.
RE: unbraced length question
if this were a new structure i would have no problem as i could easily add horizontal brace.
anyway, thanks!
RE: unbraced length question
RE: unbraced length question
yes, point taken. i'm probably on the "weak" side technically compared to other posters here. i need more reading and practice more hand calculations. maybe because i've been spoiled with good analysis and connection softwares eversince. or maybe because i'm naturally not gifted technically. haha.
anyway, all i was asking was can i use the grating or 2% of flange force to assume unbraced length as L/8.
i would normally use full L here if it's a new structure since it doesnt have horizontal brace.
p.s. as far i am concerned this thread is close and my question has been answered.
thanks,
although i dont agree i dont need staad for this structure. i just posted "partial" plan. i actually have the whole equipment struture to design with different levels.
RE: unbraced length question
Also ask yourself on any project- can I design this by hand, but software will make it a lot easier to reach a final solution? Always make sure you understand all the boxes you are checking and unchecking, and all of the variables and other inputs come from you, and that all of your actual design assumptions are reflected in the analysis. Its when you become too removed from all of these processes that software can become dangerous.
But to conclude on your problem I think unless the grates are welded or otherwise attached in a manner where they act like a diaphragm, they will not properly transfer the axial buckling force into the side beams (assuming they are properly braced)...
You can always add lateral bracing, but often its much cheaper in the long run to avoid bracing and the labor involved and just upsize the beams...