Methods to laterally brace a cantilevered steel pipe column? 1

[structure567]

I have an 11' existing cantilevered pole that is embedded onto the ground. I would like to brace the column somehow and was wondering if there are any ways to laterally brace a pipe column at the mid height. Will it be possible to install a seismic clamp and attach a high strength rod? The pole does have a 5'x5' sign that is attached at the top and is located at an open field where wind force will govern.

 

[oldestguy]

A photo would help a lot.

 

[r13]

And reasons for bracing - foundation problem, or else.

 

[structure567]

I have attached a basic model of what I wish to accomplish. The existing design does not meet load requirements. I want to install a brace in order to decrease the reactions on the existing foundation.

 

[r13]

If this pole footing is defective in resisting horizontal load, then the retrofit should have at least 3 ties. What is the problem does not satisfy the code?

 

[STrctPono]

Of course it is possible. The most common and safe thing to do would be to mount the guying device up at the point of your applied lateral load. If not possible, then you can surely account for the bending of the pole and the kick load at the base.

Personally, I would try and weld a shear connector with a pad eye to the pipe column. Attach a turnbuckle at the top and connect a guy wire, loop it over and secure it with clamps. Attach said guy wire to your deadman.

 

[structure567]

If this pole footing is defective in resisting horizontal load, then the retrofit should have at least 3 ties. What is the problem does not satisfy the code?

The existing foundation does not meet the required depth in order to resist the horizontal load. Please ignore the sketch of the new foundation. I have drafted this model just to show the conditions visually.

 

[STrctPono]

If you are needing this to take compression as well then you will need to go to some sort of pipe brace. Look at the pipe braces made by EFCO. Take some ideas from that. Check buckling.

 

[oldestguy]

Your diagram of the "fix" appears very weak compared to the ability of the foundation to hold things in place. Put some numbers to your loads, etc, including the sign, Where is that photo that will tell the story? I take it wind from other directions no problem?

 

[structure567]

If you are needing this to take compression as well then you will need to go to some sort of pipe brace. Look at the pipe braces made by EFCO. Take some ideas from that. Check buckling.

Thank you for your reply. My initial idea was to just install the anchor rods as the brace for the 4" pipe. The horizontal load at the top is less than 2 kips.

 

[structure567]

Your diagram of the "fix" appears very weak compared to the ability of the foundation to hold things in place. Put some numbers to your loads, etc, including the sign, Where is that photo that will tell the story? I take it wind from other directions no problem?

I do not have any pictures right now but there is a 5'x5' sign on top of the pole 11' from the ground level. I do wish to weld another steel column as a brace at the side of the column but I can not find any information on welding for a pipe column...

 

[STrctPono]

My initial idea was to just install the anchor rods as the brace for the 4" pipe. The horizontal load at the top is less than 2 kips.

Threaded rods are too slender to resist any real type of compression load. Even with 2 kips and assuming a 45 degree bracing angle, you are looking at about 2.8 kips of load in that threaded rod. Best to go with an angle or a pipe brace if you are wanting to minimize your braces and need them to take compression. What is the source of your load? Wind? Even with a tension/compression element, you are still going to need a minimum of 2 braces. I guess one brace could work out if wind is perpendicular to sign face. If wind can occur on the side of the sign face or wind load on post in orthogonal direction is a problem, then you would need a 2nd brace.

 

[structure567]

Threaded rods are too slender to resist any real type of compression load. Even with 2 kips and assuming a 45 degree bracing angle, you are looking at about 2.8 kips of load in that threaded rod. Best to go with an angle or a pipe brace if you are wanting to minimize your braces and need them to take compression.

I shall install 2 rods on both sides of the sign. The existing column is strong enough to handle the wind load coming to the side of the sign face. Do you still recommend going with a pipe brace? It seems as though those products are used for shoring and will be a temporary solution. I also dont think there is a pipe brace compatible with a pipe column.

 

[STrctPono]

Ok, so you are going with 2 tie down points on opposite sides. I would still use guy wire. It's just a standard of practice thing. Not to say that threaded rod won't work but most exterior cantilever poles are supported using galvanized guy strand. Follow this link for strand and hardware that you would need.... Link

I've used threaded rod before but typically for temporary exterior applications or permanent interior building bracing. Either way you choose, as the Engineer, you will need to design the connection to the pipe column and how it is going to anchor to your concrete deadman. I would be thinking about corrosion protection at the connection point to the pipe column.

You are correct, pipe braces are typically for shoring but if you were trying to use only one anchor point then either that or an angle would have been one of your options. No matter what you would have had to design a connection plate to make the transition from the brace to the post.

 

[BridgeSmith]

The hardware for the retrofit would likely to be more expensive than just replacing the pole with one that is strong enough, with an adequate foundation, to act as a cantilever.

That would most likely be easier and quicker to design, also. The interaction between the bending resistance of the pole and the resistance of the brace foundation is complicated, involving the strain compatibility of 2 foundation elements both embedded in soil, which has a non-linear elasto-plastic stress-strain curve. That's a fancy way of saying unless you design the brace for the full loading on the pole, you're in for a major headache trying to figure out how much load is carried by each component. Unless you really like that sort of thing, then go for it.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Just make sure the deflection of the sign support column will not introduce excessive bending demand in the pile below, which was the root problem.

 

[azcats]

If you've got the space around the pipe to add guys and foundations, can you excavate earth next to the foundation and add more concrete to increase bearing area of the soil? If the foundation is way short, you can add a spread foundation around the pole and use that to resist the overturning.

Not sure the scale or how much add'l strength you need...

For a 4" pipe and a small sign, it's not going to take much reinforcement just simply replace the pole & footing.

 

[structure567]

If you've got the space around the pipe to add guys and foundations, can you excavate earth next to the foundation and add more concrete to increase bearing area of the soil? If the foundation is way short, you can add a spread foundation around the pole and use that to resist the overturning.

I was actually thinking about this route. I was planning on adding additional concrete at the base of the foundation to create a fixed connection. The existing foundation would be enough to support the sign with a fixed condition.

 

[BridgeSmith]

...create a fixed connection. The existing foundation would be enough to support the sign with a fixed condition.

I understood the pole was embedded in (cast into) the foundation. Is that not the case?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

The spread footing idea was the thought I refrained from suggesting. It can work, if designed correctly to counter the tendency of lifting (slab lose touch with ground), and properly interpret the state of soil stresses under the slab, and around the pile. But worth give it a try.