Inquiry about the theory of Tension-Only Bracing
Inquiry about the theory of Tension-Only Bracing
(OP)
I've got a question about the theory of bracing for steel structures, primarily vertical bracing but it applies to horizontal bracing also. Bracing primarily provides a load path for the lateral loads to be transferred to the foundation by being able to take tension and compression, right?
It seems that it is popular to design the bracing as tension-only bracing because the member sizes will apparently be smaller and lighter, than if it were to be designed as taking both tension and compression. Is this fundamentally true?
The compression has to go somewhere, though, doesn't it? And if not to the bracing - when the bracing are designed as tension-only bracing - then to the main framing members, such as columns or beams. Is this a case of "robbing Peter to pay Paul"? That is, my bracing sizes may be smaller because they can take only tension, but my other framing members might have to be larger.
Also, are there cases where it is more economical to design bracing as taking both tension and compression?
Hope someone experienced in the design of bracing members (bracing engineers, anyone?) provide some feedback. All are welcome to answer, though. Thanks a bunch.
It seems that it is popular to design the bracing as tension-only bracing because the member sizes will apparently be smaller and lighter, than if it were to be designed as taking both tension and compression. Is this fundamentally true?
The compression has to go somewhere, though, doesn't it? And if not to the bracing - when the bracing are designed as tension-only bracing - then to the main framing members, such as columns or beams. Is this a case of "robbing Peter to pay Paul"? That is, my bracing sizes may be smaller because they can take only tension, but my other framing members might have to be larger.
Also, are there cases where it is more economical to design bracing as taking both tension and compression?
Hope someone experienced in the design of bracing members (bracing engineers, anyone?) provide some feedback. All are welcome to answer, though. Thanks a bunch.






RE: Inquiry about the theory of Tension-Only Bracing
Yes, the tension only brace does introduce extra force responses into the column/beam system around the brace.
In low seismic areas, tension only bracing is common. In areas where design and detailing must follow the AISC Seismic criteria, tension only bracing is prohibited.
RE: Inquiry about the theory of Tension-Only Bracing
But if your concern is to use tension only bracings in normal cased (low seismic zones) then i don;t see any problem, except the fact that they tend to exert more compression on beams and columns at the junction. But in this case try to use more slender members so that during compression they can easily buckle , otherwise in case of stocky member they may tend to resist the compression and in case of high compression permanent yeilding may occur which eventually lead to the failure of these members and subsequently leaving these members unsuitable to take any further tension.
RE: Inquiry about the theory of Tension-Only Bracing
RE: Inquiry about the theory of Tension-Only Bracing
A = T/(Allowable Stress * 0.85)
What is the 0.85 factor in the denominator of the above equation?
Another question is that since my bracing members are all tension-only members, is it correct to say that the slenderness ratio (KL/r for tension members = 300 max) is not to be considered in tension-only bracing. Thanks.
RE: Inquiry about the theory of Tension-Only Bracing
RE: Inquiry about the theory of Tension-Only Bracing
Generally, compression braces or struts can take tension as well.
RE: Inquiry about the theory of Tension-Only Bracing
Where did your 0.85 come from? For ASD, the required area would be T/(0.6Fy). There is no need for an extra 0.85 reduction.
RE: Inquiry about the theory of Tension-Only Bracing
If so, then the 0.85 factor may be to allow for attachment to one leg only and the uneven stress distribution resulting from that.
The Australian codee uses this factor, I cant recall seeing it in US codes though.
csd
RE: Inquiry about the theory of Tension-Only Bracing