I am being asked to design a custom truss with a completely unbraced bottom flange. The dead load on the truss is so light that there is a net uplift with wind load. This puts the bottom chord in compression. I don't like this condition, but I would like to have more than "I don't like it" for a answer, and if there is a safe way to design for this condition I would like to know how. I checked the bottom chord as a unbraced column the length of the joist and it worked fine. Also, KL/r would be less than 200 if K is 1.0. Is there some reason that K would be more than 1.0? Is there some reason other than structural stability to brace the bottom chord? The truss is basically a bar joist made out of tube steel. I have two competing ideas in my mind. The one is bar joists where any joist with uplift always has uplift bridging. The other is a crane beam where the compression flange has no bracing. Is there a way to calculate a stiffness that would make compression chord bracing unnecessary?
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Unbraced compression truss chord 2
- Thread starter WCW
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kslee,
If the bottom chord is totally unbraced, the value of k cannot be determined. It could be argued that it is infinite.
Probably not, but I can't think of a better reason.
The K value for web members may be taken as 1.0. The K value for the bottom chord may be conservatively taken as 1.0 when L is the length between brace points. When considering uplift, the axial load in the bottom chord should be the maximum axial load in any panel within the length L unless a more detailed analysis is performed.
Such an analysis may be found in "Theory of Elastic Stability" by Timoshenko and Gere. Practicing engineers usually do not go to such lengths, however.
Best regards,
BA
If the bottom chord is totally unbraced, the value of k cannot be determined. It could be argued that it is infinite.
Probably not, but I can't think of a better reason.
The K value for web members may be taken as 1.0. The K value for the bottom chord may be conservatively taken as 1.0 when L is the length between brace points. When considering uplift, the axial load in the bottom chord should be the maximum axial load in any panel within the length L unless a more detailed analysis is performed.
Such an analysis may be found in "Theory of Elastic Stability" by Timoshenko and Gere. Practicing engineers usually do not go to such lengths, however.
Best regards,
BA
BA:
Please read my posts/responses again, I was disputing the notion that K shall be taken greater than 1.0 for truss design. You have confirmed my point, for all practical reasons, K = 1. Some call it conservative, some not, but it is the magic number to use, which usually leads to satisfactory designs.
"K" is a 2D geometry factor of defelected column. A stright wire subjects compression (by finger tip), it deflects into sine curve that conform to column with ends pinned, this observation leads to the believe, K is somewhere close to 1, if not exact. I agree, for unequal end forces the K value can't be determined by simple observation, but it is one special case beyond "oridinary".
Structural instability is dependent of member stiffness, to derive which requires reasonable assumptions to begin with. I firmly believe K = 1.0 fits the task (truss design). While I have no question about the need of stability investigation for unbraced compression chord, I do have reservation on tension chord requirement (except for construction purpose), especially for the case with free end pannel joints. I wouldn't offer any thought before I read/understand all justifications.
Your ending sentence cleared a lot of air. AGREED.
Please read my posts/responses again, I was disputing the notion that K shall be taken greater than 1.0 for truss design. You have confirmed my point, for all practical reasons, K = 1. Some call it conservative, some not, but it is the magic number to use, which usually leads to satisfactory designs.
"K" is a 2D geometry factor of defelected column. A stright wire subjects compression (by finger tip), it deflects into sine curve that conform to column with ends pinned, this observation leads to the believe, K is somewhere close to 1, if not exact. I agree, for unequal end forces the K value can't be determined by simple observation, but it is one special case beyond "oridinary".
Structural instability is dependent of member stiffness, to derive which requires reasonable assumptions to begin with. I firmly believe K = 1.0 fits the task (truss design). While I have no question about the need of stability investigation for unbraced compression chord, I do have reservation on tension chord requirement (except for construction purpose), especially for the case with free end pannel joints. I wouldn't offer any thought before I read/understand all justifications.
Your ending sentence cleared a lot of air. AGREED.
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