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beams braced with a welded flat steel plate
i have a pair of parallel beams that i'd like to brace with a flat steel plate (of at least 3/16" thickness) spanning between them. this plate would be continuously welded to the top flanges of the beams, and also to any cross-beams that might exist. this plate would also have angles welded beneath it to stiffen it for carrying live psf gravity loading. (for the purposes of this analysis, i'd like to ignore the contribution to beam bracing offered by the cross-beams and the angle stiffeners since there may be a situation in the future where these don't exist.)
so, i would like the plate to act as a beam brace and also be able to directly carry transverse loads itself.
i have researched the topic of "diaphragm-braced beams" by ordering and reading several papers published in the late 1960s to mid 1970s in the asce journal of the structural division. the paper "columns and beams braced by diaphragms" by errera, pincus, and fisher gives formulas for calculating the critical lateral-torsional buckling moment "mcr" when diaphragm bracing is present. however, their equations are dependent on a quantity called "q", the shear rigidity of the diaphragm, which is the product of diaphragm cross-sectional area "ad" and effective shear modulus "geff". the authors state that "shear rigidity ["q"] of diaphragms was determined experimentally because no general theory of diaphragm behavior was available. such theory is necessary for the general application of the results of the investigation to practical design situations".
none of the other papers i reviewed presented a general theory of diaphragm behavior applicable to my welded flat steel plate.
have any theories or data been published since then that i could use for my situation? the most helpful information would be values of "q" to use in the equations presented by errera, pincus, and fisher in "columns and beams braced by diaphragms". also, is there any information on how the bracing capability of the diaphragm is diminished when it is loaded transversely?
check out our whitepaper library.
aisc has information on bracing of beams against ltb that is much more up-to-date and relevent than papers from the 60's and 70's. have you read through the aisc steel specification?
jae,
thanks for responding. i've read a lot of different posts, and i always find the quality of your responses quite high. i鈥檓 glad that someone of your caliber has responded to my post. (i hope this doesn鈥檛 scare off other responders鈥擨 want to hear all thoughts on this topic.)
we design using the aisc steel construction manual. i use allowable stress design specifically. it's not the general bracing of beams that i'm concerned about. i have a pretty good understanding of how to design beam bracing, thanks to dr. joseph yura's paper "fundamentals of beam bracing". what i'm concerned with here is the specific situation of bracing beams with a welded flat steel plate. the paper that i referred to above would be extremely helpful if only i knew how to evaluate the shear rigidity of the diaphragm "q". in this paper, there exists an equation for calculating the lateral-torsional buckling moment of a diaphragm-braced beam. if i knew 鈥淨鈥? i could plug it into that equation, solve for 鈥淢cr鈥? divide by 1.67 if it鈥檚 in the elastic range, and that gives me my allowable moment. as i鈥檓 sure you know, the aisc asd manual makes no mention of diaphragm-braced beams, and i presume the new 13th edition black manual makes no mention of beam bracing with a welded flat steel plate. that's why i took off on my own and tried to find published papers on the topic of diaphragm-braced beams.
i know it would be easier to just use a bracing system that's easier to evaluate, but let鈥檚 pretend i鈥檓 constrained into bracing my beams with a welded flat steel plate and can鈥檛 use any other bracing configuration.
bwally - actually the aisc 13th edition does have information that would prove useful to you. you can download the specification for free at
very nice post there, willisv. bwally - does that answer your questions?
jae and willisv,
according to the research i've done, a flat plate diaphragm can brace a beam in 2 ways: one is with its weak-axis bending stiffness as a continuous torsional brace; the other is with its "in-plane shear rigidity". the later 3 of the 4 papers that i read neglect the weak-axis bending stiffness of the diaphragm because the authors claim that its contribution to bracing is much less important than the contribution from in-plane shear rigidity.
so i could use the equations from appendix 6, but i'd be greatly underestimating the bracing capability of the plate by not counting its in-plane shear rigidity.
i must be missing something. i was under the impression that you were welding a flat plate to the top flanges and the plate was spanning the beams so it could carry transverse loads. diaphragms are attached to webs and don't carry gravity loads. could you describe your geometry once again? thanks.
dinosaur,
you are correct in your re-statement of the geometry. the plate will carry transverse loads and diaphragm-brace the pair of beams.
diaphragms are not necessarily attached to webs. in the paper "columns and beams braced by diaphragms" by errera, pincus, and fisher published in asce journal of the structural division in 1967, their derivations are based on diaphragms attached to one or both flanges of the columns or beams they are bracing. the tests they performed were with corrugated diaphragms attached by fasteners to both flanges of a pair of columns and to the top compression flange of a pair of beams. they do not attach the diaphragms to the webs.
bwally,
thanks for the clarification. these diaphragms described in your paper, are they attached to both flanges? i think a plate attached to the top flanges only would provide a modest amount of support but does not qualify as a diaphragm. likewise, a flat steel plate of 3/16 inch thickness will not carry much load as compared to highway loadings i am familiar. good luck.
dinosaur,
the authors' formulas address diaphragms attached to one or both flanges. their beam tests were performed with the diaphragm attached to the top compression flange only.
the authors state the following at the end of their paper: "present theory and test results demonstrate conclusively that shear-resistant diaphragms, properly attached, can be highly effective as lateral bracing for slender columns and beams." and: "diaphragm bracing is also effective in supporting slender beams against lateral buckling. the yield moment of beams appears to be readily attainable using shear-rigid diaphragm bracing."
based on everything i've read so far on this topic, i think a plate attached to only the top (compression) flanges could possibly provide a great deal of bracing strength and stiffness. could you elaborate a little as to why you don't think it would?
i agree that the plate i end up using will not carry much transverse load unstiffened. it will have to be stiffened with angles welded beneath it to carry the psf loading.
thanks for your input.
come on, fellow engineers. somebody out there has to have dealt with this situation before.

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