Nut Supported vs Grout Supported Columns 1

[jheidt2543]

This question came up recently and I would appreciate some input from the "Cyber Group".

I've seen many light poles, sign columns and steel towers that have their base plate set on leveling nuts then bolted down without any grout under the plate. I haven't seen this detail in buildings.

1. Why no grout under many outdoor column installations?

2. If no grout is used to help level the plate and distribute the load, should we be sizing the bolts and nuts differently than a grouted condition?

3. Where in the AISC manual do I find the capacity for nuts on threaded studs?

 

[rmw]

Having seen those for some years now, my assumption was (speaking as a ME with minimal sturctural background,) that this was so that the pole could shear off in the case of an impact by a vehicle, minimizing damage to the vehicle, and injury to the passengers.

However, I am sure the experts will tell both of us what the deal is.

rmw

 

[mrengineer]

See thread "Grouting column base plates"

 

[jheidt2543]

mrengineer:

Thanks for the reference to the thread, I missed that one on my search and it has some good points. However, there really isn't a definitive answer there either.

Yes, I agree that corrosion prevention and perhaps freeze/thaw concerns may be a reason to omit the grouting of outdoor base plates. The break away arguement is a little HARDER <g> to agree with. If the bolts can support the structure and the moment induced bending, then a vehical gets really dented hitting it and I'm not sure that it breaks away. Most break away connections I've seen are light plate splices of the column or holes drilled in wooden posts.

My basic question is where do we find the code requirements for the allowable nut/thread bearing for a conection like this?

 

[JAE]

I always thought that they'd just left out the grout and got away with it.

 

[Taro]

I don't think it's necessarily a code requirement. Often, the owner (State Department of Transportation, etc.) has typical specifications or details that require the type of base plate support based on their past experience with maintenance or serviceability issues.

 

[structuresguy]

Well, here are some other thoughts on the topic.

1. The loads from a light pole or power line pole are much lower than in a reasonable building. I have personally designed steel columns which fluctuate from 500 kip of gravity load (DL + LL) to -150 kip of uplift (.6DL + WIND). I doubt you see many light poles with these kinds of forces. Well, maybe here in Florida....

2. You ask about the strength of the nut/washer. Well, it exceeds the tensile strength of the rod/bolt material by specification, or you would not be allowed by code to use it. AISC is quite specific about which grade of nut/washer to use with structural bolts. For example, I can use a nut and washer on a grade A325 bolt, and yield the bolt before rupturing the nut every time. (I have personally seen this in lab testing.) So as long as the nut/washer grade matches the rod material, the nut/washer will not be the weak link.

3. In the case of anchor rod supported structures, you have to design the rods for buckling under the compression load over the unbraced length of the rod. Also, the rods have to be designed to take all the shear, and the bending moment due to the eccentric shear loading. So you might have a load case where you have very high axial compression, shear and bending simultaneously. But if you design for it, you can make it work obviously. Also, you may want to check the concrete bearing stress due to rod shear at the rod entry into the foundation as it will resist all the shear forces.

 

[DWPENGR]

I agree with structuresguy. I work of an electric utility and have designed many steel pole type structures supported by the anchor bolts without grout. Most of our structures are hot dip galvanized members with drain holes located in the bottom of the base plates. We omit grout under the base plates so water (due to condensation) inside the poles can drain out and not rust the inside of the poles. The base plate must be appropriately designed for axial and bending. For the anchor bolts, we follow the guidelines in the ASCE Manual # 72, “Design of Steel Transmission Structures”. It states that if the distance between the bottom of the base plate and the top of the concrete does not exceed two times the bolt diameter, a bending stress analysis of the anchor bolt is not required. As an example, say the distance between the bottom of the base plate and the top of the concrete is 3”. Then the anchor bolts must be checked for axial stress (due to axial & overturning moment) and bending (due to shear & torsion x 3”). Development length of the anchor bolts is very critical. Not unusual for us to use # 18J rebar, A615-GR 75 ksi, threaded at the top to accept A563 2.25" nut.

 

[DWPENGR]

Correction - the inflection point of the anchor bolt is half the clear distance. Therefore, the anchor bolt must be checked for axial stress (due to axial & overturning moment) and bending (due to shear & torsion x 1.5”).