huangyhg

reinforcing steel braced frame diagonal
i am working on a high rise addition, where the two upper floors of the existing high rise have braced frames. the diagonal members in these braced frames are overstressed when the addition loads are added.
the diagonal is a w18x143. i find that addding a c10x30 to each flange will increase ry sufficiently.
my question: do the channels need to run the full length of the w18? i am only trying to increase ry, not add area to the section.
daveatkins
hi dave! if the channels are not needed to carry stress they can be stop just short of the connections. are the connections adequate?
whew! that is a good question!
are you modeling this brace in? if so, is it inactive under vertical loading? if it's not, it will resist the compression of the columns due to the upper floor loading. i don't let the braces take vertical forces due to the gravity loads when i model something. might make me old timey, but i want to make sure my column can take the vertical load as is intended, and the braces take the lateral load as they are intended.
i know aisc published some papers in their engineering journals. one is titled "allowable axial stresses in segmented columns" and was published first quarter, 1979.
i have it in pdf format. if you need it, let me know and we'll find a way to get it to you.
dave, i would agree with jike. if you are just trying to increase ry and eliminate buckling concerns the added shape can stop just short of the connection.
if you run the channel even shorter, then the traditional buckling equations aren't valid as they are based on mono- properties along the length. this then would create a similar problem to that of step columns in buildings where there might be a crane beam at mid-height or something.
dave,
i would stop them 6" to 1' from each end.
then it is no different to the situation with gusset plates each end.
dave, just a thought. if you increase ry, does rx become the controlling r for your situation?
rgerk,
excellent question! i did not think of that, and i will check. but the existing braces are w18x143, so i don't think it will be an issue.
daveatkins

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