Grating Bar Allowable Bending Stress

[ibrov]

Grating bars (2" high, 1/4" thick and 36" length) are
spaced 2-3/8" on center. On top of bars there is 14GA
perforated diffuser plate welded to bars (no crossbars).

How to calculate allowable bending stress and
unbraced length for these bars. What is required
size and spacing of welds that connect the perforated
plate to the grating.

 

[whyun]

Allowable bending stress can be obtained from chapter F in the AISC ASD Steel Manual, ninth edition. Fy of the bar material shall be obtained from the grating supplier. Unbraced length shall be distance between the members that provide lateral support at the compression fiber, in this case the top of the bar. in the case of perforated diffuser plate welded to the bars, perhaps the weld spacing can be used as being unbraced length.

Usually grating manufacturer has load tables for various spans. Typically, engineer need not calculate these by hand. For a given loading and the rated maximum span, just be sure your span of 36" less than the allowable span.

 

[ibrov]

Whyun,
Thank you very much for your input.

This is a "custom" grating - owner does not want to use prefabricated one. Chapter F in the AISC ASD Steel Manual deals with I shaped members not with rectangular bars. Can I use Equation (F1-2) for calculating Lc using thickness of bar as bf, e.g. Lc=76*bf/sqrt(Fy)=76*(1/4)/sqrt(36)=3.2 in?
If welds are every 12" (Lb=12">Lc) then Fb=6.1ksi (F1-7)
Is this proper approach? Looks very conservative!

Regards,

Ibro V.

 

[whyun]

hmm. it is not rectangular plate acting in weak axis thus can't use Fb = 0.75Fy. using thickness of plate in place of bf sounds too conservative.

other's can comment on this but my gut feeling tells me Fb = 0.6Fy sounds reasonable for this case. When i have my books available, and find anything useful, I will make a posting then...

 

[austim]

ibrov,

It is ages since I worked to ASD, and I have never worked to US standards, but this may still help.

Your assumption that the effective length of your bars would be the distance between welds could be conservative in itself. If the welds are of any significant length, then they will provide relatively more bending restraint to your 1/4 inch thick bar than you would get from a typical lateral brace to an I section beam. That is, looking at each clear span of bar in plan, there is a tendency for it to deflect more like a fixed ended strut than a pinned strut. Perhaps you could take Lb about 0.7 * 12 ??

But if you don't like the thought of using Lb < 12 inches, then just ask yourself what compressive stress would you be allowed in a 1/4 inch thick pin-ended strut 12 inches long. (That is, assume that the top fibres of the bar will behave much the same whether they are being compressed as part of beam or as part of a strut).

You have l/r about 166, for which the old Australian ASD code gave an allowable stress of 33 MPa ( = 4.8 ksi) in a pin ended strut. Just maybe that 6.1 ksi is not all that conservative.

 

[austim]

P.S. I am not suggesting that 4.8 ksi is the right stress for you to use. My comparison with a simple strut was purely to find a reasonable lower bound.

The critical buckling stress in your load bars in pure bending would be somewhat higher than in pure compression, which makes your 6.1 ksi look pretty reasonable to me.