Tension-only Bracing and AISC 341 1

[amendale]

Hi,

I know AISC 341 defines width-to-thickness limits for compression members, but I was wondering if these apply to tension-only braces? Although you design these members as tension-only, compression and buckling will occur in these members upon load reversal, and it might be a good idea to avoid local buckling and plastic deformation of the section to maintain tensile capacity. So I was wondering if the width-to-thickness limitations should still apply to tension-only bracing members?

 

[IsaacStructural]

Is this an SCBF? AISC 341-05 Section 13.2d Width-Thickness Limitations states:

"Column and brace members shall meet the requirements of Section 8.2b."

It makes not distinction between compression and tension braces, so I think the answer to your question is that the braces are required to meet these requirements.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[SteelPE]

IsaacStructural

This is in regards to a OCBF.

Requiring a minimum width thickness ratio for a tension member makes no sense to me. The system is designed to act in tension. When the the members inside of the system receive compression the will probably be slender (KL/r) an buckle globally allowing the opposite tension brace to pick up the load.

 

[IsaacStructural]

Actually looking at OCBF section 14.2 says

"Bracing members shall meet the requirements of Section 8.2b"

in the grey commentary box immediately below it says:

[highlight #888A85]User note: Bracing members that are designed as tension only (that is, neglecting their strength in compression) are not appropriate for K, V and inverted-V configurations. Such braces may be used in other configuration and are not required to satisfy this provision. Such members may include slender angles, plate or cable bracing, which are not excluded by section 6.1.[/highlight]



M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[JoshPlumSE]

Use of tension only braces in any configuration is not permitted for SCBF's (per commentary and user note for section 13.1 of ASCE 341-05) Also, they will not be permitted in K, V, Chevron OCBF frames because of maximum slenderness restrictions per 14.2.

Other than that, I would think it would be permitted to use tension only braces. And, I wouldn't think the element slenderness restrictions would apply because they are not designed to take compression.

Another way to look at is do these braces experience damage when subject to compression? And, does that damage them badly enough that their tension capacity could be compromised when load reversal occurs? If the answer is no, then you can use them. If you think the answer is yes, then you probably shouldn't use them.

 

[Gumpmaster]

While the tension only braces aren't designed to take compression, they will still buckle as you stated. The compactness requirements are to provide repetative ductility during successive low cycle fatigue cycles. For instance, in a non-compact HSS member, after a couple of cycles, you get tearing at the locally buckled section and not only loose compressive capacity, but also tension capacity. I would imagine the same thing happens with a tension only member.

Look at what type of members you're using (are they HSS or pipes?) and look at the testing and see how many cycles you get out of non-compact members. While the code doesn't require it, you may want more cycles out of your system than the testing would indicate for non-compact elements.

I'm only familiar with the testing for HSS and pipes, but I personally wouldn't want to use slender members just because you may get less than 10 cycles with them. Wide flange sections and angles likely have better performance due to no cold rolling.

Something to think about.

 

[amendale]

Gumpmaster, I agree with you completely. If plastic deformation occurs (local buckling) then the tensile capacity might be reduced. However I'm not certain about the code requirements for this case.

 

[SteelPE]

Would the member in tension, in a frame that has tension only members, limit the drift of the system and therefore reduce the probability of the member in compression from buckling locally?

If you are that concerned about it then just make the members in tension comply with the b/t ratio's of table D1.1.