Tension Only Bracing - Single Angles 1

[Everynameistaken]

Hi All,

I see there have been several threads on this already but do not seem to answer our specific question.

We have a low rise building < 15m using a system that has low ductility. As such we are permitted to use tension only bracing and no strict capacity design (CSA S16 conventional construction)

The most efficient is to use cross braces angle, we use two bolts at each end and connect in the middle with a single bolt.

I have usually considered the central connection point as the length when checking the KL/r limit for the tension brace.

Ie. 10m node to node, with a cross so the length is now halved so my kL = 1.0 x (0.5 x 10m). I know there is quite a bit of conflicting literature on this point, but since we are not relying on this brace point for a compression member I am on with this.

My question is more about which r should be used when checking the kl/r limit. For a system with very low ductility (R < 2.0) the limit for kL/r < 300.

Since the angles are held square at each and by the gusset and at the centre we have typically used the r about the geometric axis. These are the higher r values from the code. The principal axes has a very low r in one direction and could impact the angle size due only to the slenderness limit.

My understanding , for tension only systems with low ductility, this kl/r < 300 limit is more about sag and if the sections are held in place and connected together, the geometric axes seem to make sense.

Let me know your thoughts

 

[human909]

Sag and vibration are the only big concerns if you are on the slender side. Sag can be an issue for horizontal cross bracing but much less of an issue with vertical cross bracing. Vibration can be an issue if there is vibrating equipment or it is exposed to wind.

The code I use doesn't have slenderness limits. But I've been burnt by pushing slenderness limits in situations involving vortex shedding. The actual member was easily strong enough, but with laminar winds at the right speed there was severe resonance. (L/r in my case was around 200)

 

[dik]

Is 300 the limit, or is it just recommended? I seem to recall there is an exclusion for 300. Most PEMBs use rod with a KL/r greater than 300.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[XR250]

I think just about any rod bracing will exceed 300

 

[OldDawgNewTricks]

My comments are based on AISC, not CSA, so take them as you will.
1. 300 is a recommendation, not a hard limit. It is advisable to comply with the recommendation because experience has shown that it helps to avoid problems. But you may exceed it if you have fully considered the ramifications (sag, vibration, etc.).
2. The recommended limit of 300 is not applicable to rod bracing.
3. The limit also applies to the fabricated length, not just the unbraced length in the completed structure. Very slender members are also susceptible to damage during fabrication, transportation, and erection.
4. I would check the Z-Z axis, not just the X-X and Y-Y axes.

 

[KootK]

1) As others have noted, there are plenty of cases where I wouldn't sweat the 300 limit.

2) Were I to enforce the limit, I would use the geometric weak axis. Regardless of the rotational restraint at the ends, I feel the geometric axis will be the most flexible with respect to movement and, therefore the most critical.

3) The benefit of the rotational restraint points will be a function of how long the member is between those points and how torsionally stiff the cross section is. A very short piece probably would be forced into its geometric axis patter. A long piece with crap torsional stiffness (single angle) probably would not.

 

[m7rhman]

The effective length (k[sub]z[/sub]) is 0.85 times the half diagonal length considering the radius of gyration in the z-axis, r[sub]z[/sub] and (k[sub]x[/sub] = k[sub]y[/sub] = 1.0) times half length in r[sub]x[/sub] and r[sub]y[/sub] (see screenshot from El-Tayem and Goel). All the three limits (0.85 * (L/2) /r[sub]z[/sub]), (0.85 * (L/2) /r[sub]x[/sub]) and (0.85 * (L/2) /r[sub]y[/sub]) to be in range of 200 to 300 (recommendation as mentioned above).

The paper I mentioned is available free of charge in AISC:

https://www.aisc.org/Effective-Length-Factor-for-the-Design-of-X-bracing-Systems

 

[lexpatrie]

If nobody minds I'll slap that into an FAQ for the AISC forum.

[link

https://www.eng-tips.com/faqs.cfm?fid=2919

]1986 - Tension Only Bracing (effective Length Factor for the Design of X-bracing Systems)[/url]

If you're looking to cite the article, here's the official version of that, from AISC.org.

El-Tayem, Adel A.; Goel, Subhash C. (1986). "Effective Length Factor for the Design of X-bracing Systems," Engineering Journal, American Institute of Steel Construction, Vol. 23, pp. 41-45


If there's anything else or anybody knows those two articles I remember reading on the subject, if you'd like you can use the feedback feature on the FAQ.