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x-bracing design considerations 1

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nuche1973

nuche1973

Structural
Apr 29, 2008
300
In designing x-braces, using angles with a bolted connection in the center, how does one determine the unbraced length? I want to use both members and not ignore the compression member. Does this move my Lu to the midpoint? I've read the answers posted in the RISA forum (unbraced length of X-brace design & V-brace segmenting).
 
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In one direction, the double angle is braced at midspan by the other double angle. But in the other direction, it is unbraced for its full length.

DaveAtkins
 
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If you bolt at the center crossing point, the brace diagonal in tension has an unbraced, tension length equal to the full diagonal length.

The brace diagonal in compression has an unbraced length equal to half the total diagonal length.
 
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Dave and JAE:
Correct me if I am wrong, but I believe I read somewhere regarding the bracing of the compression diagonal by the diagonal in tension and it was suggested to use a unbraced length of 0.65 L. I believe it was based on research done at the University of Michigan.
 
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No wonder Michigan buckles under pressure every time! [smile]

Mike McCann
MMC Engineering
 
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I've seen references that recommend 0.75 L for the buckling length (L being the total length, not half the length).
 
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When they are connected in the center, the in-plane length is halved but the out-of-plane direction is still the full length (two drunks holding each other from buckling).

I don't understand reducing L, it should be the actual unbraced length under consideration. If you want to play around, do it with the "k" value, but be careful the codes often specify "k" by way of charts and nomographs.

Michael.
Timing has a lot to do with the outcome of a rain dance.
 
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JAE,

I don't see how the tension diagonal can brace the compression diagonal against out of plane buckling.

DaveAtkins
 
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Picard and Beaulieu in AISC Engineering Journal, 3rd Quarter 1987, pp. 122-126.

Picard and Beaulieu in AISC Engineering Journal, 4th Quarter 1988, pp. 156-160.

Stoman, S. H., "Effective Length Spectra for Cross Bracings." ASCE Journal of Structural Engineering. December, 1989.

DeWolf, J. T., and Pelliccione, J. F., "Cross-Bracing Design." ASCE Journal of the Structural Division. July, 1979.

El-Tayem, A. A., and Goel, S. C., "Effective Length Factor for the Design of X-Bracing Systems." AISC Engineering Journal. First Quarter, 1986.
 
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Looking at the K values, for a pin-pin the K is 1.0. For a pin-fix (as in the brace length being from the pin to the brace) the k is 2.0 or more. This assumes the brace is free to move out of plane laterally. Placing a restrant on the movement, as connecting it to a tension member, would reduce the movement some. So you may get a K of 1.5 (plus or minus) for the pin to brace length which would give you a K = .75 for the overall length. I hope the articals mentioned by slickdeals will shed more light on this.

Garth Dreger PE
AZ Phoenix area
 
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CCB1,

The tension diagonal definately does help the compression member due to the bowstring effect. It does not act as a full strength brace though due to the deflection. I would suggest you have a good read of the papers above.

Keep in mind that the restraint is only on one side of the member and therefore does not prevent twisting.
 
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From ASCE 10-97 'Design of Latticed Steel Transmission Structures';

"Tension/Compression system with members connected at the crossover point. If the member in tension
has a force of not less than 20% of the force in the compression member, the crossover point
provides support resisting out-of-plane buckling."
 
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Slickdeals, what are the highlights of the articles you posted?
 
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