Stabilizing Elevated Pole in High Winds 2

[psychedomination]

Hi there,

I have a project, where the client recently erected a lightning rod to his chimney and mentioned that even in low winds it was making a loud rattling noise. He wanted to check if it was structurally sound.

The rod is connected to the chimney (hollow CMU block construction), with two 3/8” wedge anchors embedded 1.5” into the wall. Assuming confast, which have a pull out capacity of 5.4kN ea.

https://at3w.com/FichasTecnicas/AT-057A_EN.pdf

(mast specifications - note the weight is incorrect on this I believe it is supposed to be 20070 g). I’m not sure if the 1 ½” diameter is inner or outer?

https://at3w.com/FichasTecnicas/AT-042B_EN.pdf

specification for anchorage to wall

I checked the wind load acting on the pole and in a hurricane it would have about 3.6 kPa, which results in a wind force of 0.6kN and a moment of 1.2kNm.

I did a check of the section properties using an online calculator and if I did this right seems that the mast tube has a :

Second moment of area value of 33523 mm4 and section modulus of 1640.46mm3.

Assuming the grade of steel is 275MPa, this would have a resisting moment of only 0.45kNm, which isn’t enough and would have a deflection at the tip of 583mm, which would be too large.

Are there any simple ways to stabilize something like this in high winds? Anchor points would most likely need to be in the chimney,

Any help would be appreciated.

I suspect that the rattling noise is probably just the copper line moving inside the pipe but best to address any structural issues if there are any.

The pole is 4m tall (from supports) with an apex of about 12m from the ground.

 

[skeletron]

Big long span with a small backspan...yep, it'll happen. Mast and connection seem light but I'm not going to bust out the calculator to check.

If it is a deflection via vibration issue, is increasing the mass between supports (so as not to cause huge moment increase on anchors) possible? Or can you add some sort of damping type system?

 

[psychedomination]

@Skeletron, thanks for your comments. Yes, the connection seems a bit light to me as well. I suspect that the connection may fail before the mast/pole reaches the full deflection. When you say increase the mass, do you mean to put it inside a thicker steel tube to stiffen it? I think that may possible, although I am unsure if that will impact the lightning rods functionality. I've done something similar for stiffening a much larger marine beacon tower in the past. I wasn't sure if there was something relatively cheaper/easier for something of this scale? I was considering the use of guidelines as well but the connections for this system are a point of concern for me, the CMU hollow block is prob 8" thick and with an embedment of only 1.5", the capacity and breakout is questionable.

 

[dik]

I generally don't like using 'wedge anchors' with masonry.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dauwerda]

The complaints and description sound like a vortex shedding issue under low to moderate winds causing excessive vibration. This happens when the excitation from the vortices is close in frequency to the natural frequency of the pole.
If this is the issue, you can either take measures to change the natural frequency of the pole (add mass, increase stiffness, add dampers, etc.); or you can add helical strakes (or something similar) to the outside of the pole to add turbulence to the airflow around the pole so that vortices won't form.

I would definitely revisit the connection to the chimney, that looks like it is just waiting to fail.

For what it is worth, the local utility here has historically used 1-1/2" and 2" diameter pipe that is 10-15 feet tall on top of some substation structures as lightning protection and have had many failures where the pipe itself breaks off just above the connection due to the fatigue caused by vortex shedding. Your dimensions seem to be pretty similar to this.

When the client hears the noise, they should go out and see (and video) if there is excessive movement to confirm if it is a vortex shedding issue.

 

[psychedomination]

@Dik I haven’t specified wedge anchors for anything I’ve done in the past, I normally specify chemical anchors. This lightning rod was erected before I was involved. However, this is hollow CMU, so a chemical anchor wouldn’t be very effective. What alternatives would you recommend to a wedge anchor going through a hollow CMU block? Through bolt to the other side? Or what sort of connection would you recommend?

@dauwerda thanks this is a very helpful comment. When you say to add mass, do you mean adding another thicker pipe around the existing and through bolting/welding? The helical stake sounds interesting but I can’t visualise this as I haven’t seen this done before. Would these be connected to the chimney as well? I also fully agree with the client taking a video.

I think a damper would be a good idea regardless, I’m searching for different companies now. Would you recommend a specific type for this application?

 

[dauwerda]

There are a few ways to add mass. The simplest would be to plug the bottom of the pipe and fill it with sand or grout. If sand is used you would probably want to use a screen at the bottom so that moisture could find its way out and seal the top to prevent moisture from getting in. You could also use a pipe with a thicker side wall X-Strong or XX-strong. A standard practice for large static masts in substations (think 50 to 100 feet tall) is to hang a large chain (1" diameter links) inside the mast that is 5-10% the weight of the mast (chain is surrounded in flexible hose to prevent clanging inside mast). This adds weight and acts as an internal damper. For something this small, I don't know what type of dampers would be available or practical.

Here is am image showing different attachments that have been used to prevent vortex shedding:

 

[1503-44]

Lightening rods don't need to have such a tall reach. Just higher than the next highest point is normally sufficient, given that they offer the path of least resistance to ground.

http://www.electrical-knowhow.com/2...or-the-Best-Positioning-of-Air-Terminals.html

 

[XR250]

I agree with dik, wedge anchors don't work in hollow CMU. Screened chemical anchors do. Thru bolts maybe - not really code anymore IIRC, but there are a few threads about it somewhere

 

[dik]

I'd use an epoxy with HDG F1554 55 or SS anchors, and I'd probably drop the bottom connection by a few feet to increase the back span.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Liam Nesson]

There are some cool windvortex shedding videos out there, i would second getting a video of the rattling from the client.

https://lytepoles.com/info/wind-induced-vibrations/

 

[3DDave]

Were I to desire a bracket with the least resistance to side-to-side and twisting modes of vibration I am unlikely to have done any better. Adding a second stand-off at each of the existing stand-off locations, but off at an angle - 45 degrees or so - will considerably stiffen the supports. Adding a spiral to promote uneven/irregular vortex shedding would be additional protection.