HSS tube column (LED sign) with a 3" diameter hole for electrical - torsional issues

[TNich]

I am designing an LED sign on HSS tube columns (similar to what you see in front of schools). ASCE 7 says you need an offset for wind forces equal to .2 times the sign width and the extra torsion created gives me problems because the column needs a 3" diameter hole for electrical purposes. I have quite a few books that show me ways of calculating the torsional stress for an open section member and it's not looking good stress wise for this size column that I want to use... But this is also assuming the entire member is open sectioned and not just at the one location. Do I have any method to calculate the benefit of the rest of the member not being open? I.e. stresses will go around the hole (preferably a reference that I can look into), similar to allowable hole sizes in LVLs and the like. Any other ideas to look into? The reason I am searching so much is because other designs I have seen are VERY similar and nothing special was done around the openings, so I felt I missed something but cannot find anything regarding holes in steel members.

 

[SAIL3]

you did not mention the size of the HSS member....openings in steel stacks have been addressed with the hole size limited to 2/3 of dia but also with reinforment..I would personally reinforce the hole and move on....

 

[TNich]

Doh. I definitely thought I did. Thanks for pointing that out. Columns are HSS 6x6x1/4. And the reinforcing is what I'm probably going to be doing, but my curiosity gets the better of me sometimes.

 

[KootK]

I've done a few utility poles with this issue. Hated every minute of it for just this reason. Is there a document that provides guidance on how the reinforcement should be done SAIL? All I could find was "replace the lost Z/I/A" recommendations. Pretty crude. Certainly not appropriate for torsion.

There's no need to get into open section torsion here TNich. The intact section either side of the hole will restrain warping at the hole. Saint Venant torsion will do so long as nothing buckles or fractures due to stress raisers.

Some relevant information in Tubular Steel Structures by Troitsky.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[SAIL3]

There may be, but not aware of one.....just follow a sound engineering approach....say the flat is about 5" and less 3" would leave a 1" flat each side of hole...not usually enough IMO. So I would use a one-piece pl that would cover or extend over the radius of the member on both sides of the hole. This would mean the reinf pl would have to be bent to a radius at both ends. Use a hole dia of 3.5" to allow a fillet weld around hole. If the total horiz shear is well below the allowable, then I might consider not extending the reif pl all the way around the radius...it's an engineering judgement call...