huangyhg

diagonal bracing methods
i have seen round bar, cable, angles and tube sections all used as diagonal braces in building frames. is there any reason that flat bars could not be used?
in remodeling an existing single story, 30' eave height building we plan to add x-bracing in some of the bays. since the use of any "three dimensinal" brace installed on the column centerlines will require cutting or slotting the wall girts to install the braces, the question has come up regarding flat bar. the erector's thought would be to install the flat bar across the "face" of the girts and weld them to the inside face of the column flanges. now, this would introduce some twisting into the columns since the connection would not be on the column centerline. the load would be (column depth/2)*(tension force). one solution would be to put the flat bar on both the inside and outside of the column.
i think this would be similar to the flat stock straps used to brace in light gauge stud walls, except that the loads will be higher. generally, the flat stock is welded to each stud, would welding the flat bar to each wall girt be a benifit or a problem?
any thoughts on this?
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i have seen flat steel straps used successfully as x-bracing in some pre-engineered metal buildings. this strapping was somewhat flexible, like the metal bands that are used to bind bulky items to wooden shipping pallets. your idea of flat bars should be fine. having the bracing on the column flanges should be ok too. this location is no different than the way many wooden structures are braced. of course be sure to limit the extent of welding on the column flanges.
it is probably best to keep both of the x-braces in the same plane if possible.
welding to the wall girts should not be necessary and could cause unexpected problems.
just a thought... no problem with the flat straps in principal but you could pierce the neutral axis of the column put the flat bar right thro and fillet weld both sides of the web. i have done a similar thing with round tube before. clearly the tube has much better weld area so the welds are less critical. as you point out introduction of additional torque to a lateral torsional buckling problem is undesirable. because the straps will spend 50% of their time in tension or compression, in compression they are inneffective and will buckle laterally by a large ammount, restraining this effect with additional structure would be ill-advised.

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