"Stiffeners" at torsional connection

[a7x1984]

Have a torsional connection at the top of a concrete footing (supporting a relatively light item with a lot of surface area). Therefore, wind is my evil loading...See attached sketch.

Torsional moment is 9.5 k-ft - not large, but trying to get an estimated weld length has been difficult. The only way around it, that I could see, was to flare "stiffeners" from the HSS section to get the required weld length.

Basically (3) things I am unsure of:
1. Now that I have introduced these elements, is it even worth checking their capacities? If yes, is there a rational procedure floating around? My intuition says this small loading can be handled by inspection with not much additional analysis.
2. I am concerned that as the HSS begins to torque there will be an uneven distribution of stress in the welds, maybe causing the 'front-loaded' welds to crack. Any ideas in configuring this connection in a different manner?
3. I can't find a prequalified weld between the HSS and the base plate. The closest I could find was a flared weld, but it didn't show the HSS bearing on the plate to be welded to.



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[wannabeSE]

Sorry, I am a slow typer, I see you have it figured out

 

[BAretired]

BAretired,
With strength design in the US, a 70 ksi fillet 70ksi weld is good for 1.392 kips per 1/16". I am guessing a7x mixed up 1/2" in the denominator with 8 (for 8/16=1/2").
For a 1/16" fillet weld Φ0.6 FexxAw = 0.75(0.6)(70)(0.0625)/sqrt(2) = 1.392 kips.
If the applicable code allows, a flare bevel groove weld (with reinforcing fillet if need) may work.

I agree that a direct weld is a possibility, but my preference would be to use angles each side, bolted to the foundation.

BA

 

[a7x1984]

Without the prequalified weld, I would rather go with BA's suggestion. Thanks, guys. At least we know my previously designed fillet welds aren't going anywhere.

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[dhengr]

a7:
Nonprofit structures act essentially the same way as full paying clients structures act, and they require the same full attention to detail and design and cost. You mean the 5x5 tube is being twisted about its own axis, with a torque T = 9.5 ft.kips? Put a 1/2 or 5/8" end plate on the tube, 6" high and 6" wide at the top, and 14 or 15" wide a its base, at the base plate. Center four A.B’s. on either side of this end plate, two on each side of the HSS. If you wish put another plate 5" wide x 12" long under the tube and atop the main base plate with a couple A.B’s. in it to guard against the tube acting as a 4' long lever arm against lateral loads.

 

[a7x1984]

dhengr: My point is that, as a service to the community, I am willing to spend more of my own personal time to shave off every dollar I can (i.e. explore various options and configurations). There isn't always a lot of additional time spelled out in the proposals for narrowing down to the dollar. When a community organization has a budget hinging on a few dollars, they need some extra help. That is what I see as the difference for me - nothing about sacrificing a sound design.

I am having a hard time visualizing your idea at this time in the night. I will try again in the a.m. =)

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[swearingen]

I'm having trouble seeing this as being much of a problem. Because the plate is bolted, the end of the weld (nearest the extension of the HSS off of the pedestal) is may NOT be where the greatest weld stress is. It could be close to the bolts depending on plate thickness.

Also, do you truly have a force couple creating this moment, or is there an uplift component as well? In other words, is this a 1k load, 9.5ft away pushing up or something similar? This matters, because not only do you have to account for the moment, but the uplift as well.


-5^2 = -25 ;-)

http://www.eng-tips.com/supportus.cfm

 

[a7x1984]

swearingen: It is a true force couple - no uplift. I agree with you on the anchor bolt comment; makes sense.

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[swearingen]

If that's the case, then if the internal moment arm is about 9" (center of bolts closest to free end to end of HSS), then the tension on the bolts will be about 12.7k, or 6.4k per side. This is ignoring the other bolts, which do help. With this force, a small J-groove weld would easily hold it - no stiffeners and their requisite cutting, fitting and welding needed. You need to remember to design the plate thickness/bolts for prying action.

Forgive me for bugging you, but I am VERY curious as to how this thing is loaded in pure moment. Can you expound on that? I'm having trouble seeing how a setup like this can be loaded in such a way...


-5^2 = -25 ;-)

http://www.eng-tips.com/supportus.cfm

 

[a7x1984]

No prob: The HSS is about 6 ft long. There are two vertical HSS (about 6 ft tall) centered on the HSS beam about 2 ft apart. This framing supports large, plate-like permanent artwork (elephants in this case) that sandwich the vertical HSS's. The panels of the artwork are 16 sq.ft in surface area and made up marine-grade plywood. Not much dead load, but a lot of wind load on the panels.



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[dhengr]

a7:
Then that 5x5 tube has a small vertical cantilever load on it, mostly just DL; it has a lateral canti. load on it, (wind load)(face area on 1.5 to 2 face areas); plus the torsional load you have been talking about. All a vibrating and potential fatigue mess back at the conc. pier. And, if it is someplace where a 300# drunk can try to chin himself, don’t forget that. Quite a different problem than what you have been talking about.

 

[a7x1984]

dh:
Fatigue from wind on a structure like this? That seems like an unreasonable amount of analysis. The curb surrounding the displays is not mountable, and I think the lateral component is negligible in this case: I can't see 500 lbs of lateral load doing much to affect any interaction equations.

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