Pylon Sign HSS Torsion Tube Connection

[disoswam]

I am looking for advice making connection to HSS square tube with torsion load, wall thickness of the HSS tube, and bracing or stiffening requirements.

My customer has an existing two column pylon sign and wants to use the existing foundation and 18" of existing 10"x6" HSS columns spaced 104"OC. The new sign has 8"x4" HSS columns spaced 87"OC, factory welded 16"x12" base plates, and 4 - 1.25"bolts spaced 13"OC and 9"OC. I plan to: weld end caps to a horizontal 114" long 10"x10" torsion tube; weld 16x12 match plates spaced 87"OC to top of torsion tube; weld 4 - 3"x3" gussets spaced 4-5"OC to bottom of match plate and sides of torsion tube. Each column has 1500 lb wind load from sign face at 15' above grade so 20 kip.ft moment at 18" above grade top of existing column.

 

[BAretired]

Are you planning to field weld the 10x10 torsion tube to the existing stubs? This could be problematic as it involves uphand welding and welding two HSS members of the same depth along the radius of the torsion tube.

BA

 

[SAIL3]

a sketch would help get a more informed solution....

 

[disoswam]

Yes, they would need to field weld since the existing columns are embedded in foundation. Is there a better way to attach than field weld? Remember these are sign installers so they shouldn't have a problem welding; just look at a any sign column.

I attached a sketch. Any comments would be appreciated.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1431525694/tips/150454_Sign_Engineering_-_PPE-21839_wy7jbi.pdf[/url]

 

[azcats]

I share BA's concern. That weld from the top of the cut stub to the radius of the torsion pipe is doing a lot of the work and will be tricky. If you're not able to change sizes, I'd consider a plate between the two members so you can use vertical fillet welds.

 

[disoswam]

What about a 12"x6" tube to eliminate radius weld issues? Should the torsion tube be lower height rectangle or square? 12x6. J=124in4, C=32in3 vs 10x10. J=220in4, C=44in3. Is there any bending disadvantage in 10" column not lining up with side wall of 12" torsion tube? Would gussets help? Or would it be better to do vertical welds on plate straps and a small continuous weld to seal it or just caulk to seal it? Should the the closed ends of the torsion tube line up with side wall of existing columns or would that require too much precision?

 

[SAIL3]

attached is a rough sketch showing a general approach...attempted to make the conn as rigid as possible....my gut feeling is that there may be a stability issue with the setup. The lateral wind load is resisted at first by the top col in bending, then the torsion tube in torsion and bending and finally the existing cols in bending. If you run the numbers on lateral stiffeness of each stage and find that the torsion tube and it's connections to the cols is weaker than the upper cols..what does this due to the stability?...on the otherhand, the vertical load is probably small and may not matter.....

 

[BAretired]

I don't think the client would appreciate a bunch of unsightly plates exposed to view. My preference would be to use an 8x8 HSS torsion tube 98" long fitting between the stubs (site check exact dimension) and weld all around to the inner wall of each stub. The stubs would be 18 + 8 = 26" high and could be strengthened by adding plate material inside the existing stubs before finally capping them off with a top plate. Similarly, the torsion tube should be strengthened using hidden plates welded inside the tube, enabling it to sustain the bending moment from the upper columns.

BA

 

[SAIL3]

piggy-backing on BA's concept and accepting the fact that an ugly design is probably not the optimal solution available, I attached an alternate concept....

 

[disoswam]

Thanks for all the input. It show there are more ways than one. I like SAIL's first sketch, 1. gussets from base plate all the way down each side of torsion tube seems to add stiffness where you need it (I normally do that but this time in TN 115 mph wind speed I skimped), 2. side plates to existing 101 column seem easier to install and get a good weld than uphand and tube radius welds.

My initial concern was similar to SAIL's stability concern. Since the new 8x4 is OK in bending and during Load Case 2 each column carries 70% of total sign wind load the existing 10x10 columns are much stronger and stiffer.

??? I'm not sure how to compare torsion tube strength and stiffness to upper column. ???

 

[SAIL3]

I would take the easy way out and model the setup on the computer and obtain the deflection at the top of the cols. Then I would take another case 2 where the model is comprised of one cantilevered col comprising of the existing stub col and the upper col. I would then compare the deflections @ the top of the cols. This would give me a handle on how soft the torsion bm setup is compared to the ideal case.
I have a few aversions in design and one of them is cantilevering a col off a bm. In spite of the fact that all the forces may be accounted for, I still would go to any length to avoid it. So I would prefer sketch 2 as the connections are more clean and direct and it also gives one the flexibilty of varying the length "L" to obtain forces that can more easily be dealt with.

 

[BAretired]

You need something to prevent the 10x10 tube from racking in two locations, one at the new columns, the other at the existing columns. The gussets shown on Sail3's first sketch will help and so will the cap plate at each end of the torsion tube, but the latter is not aligned with either wall of the column, making for an indirect shear transfer.

BA

 

[dhengr]

Disoswam:
Why not use a 12" o.d. pipe by 110" long (out to out of the lower columns for the torsion tube. Cut the top of the lower columns to fit the 12" pipe and apply an end pl. which laps the outside faces of the lower columns. Cut the base plates off the sign columns and cut their bottom ends to fit the 12" pipe. You will want to install inside stiff. pls. in the torsion tube, at the two webs of each of the sign cols. and at the inner webs of the lower cols. If you put one end of the 12" pipe up on a saw horse, and roll the pipe as you weld, these stiff. pl. welds are all down hand. Weld the end pls. on the torsion tube and weld it to the sign columns. Then the only field welding is around the lower columns to the torsion tube and end pl.