Bolt Shear Problem? 9

[swearingen]

A previous post (thread507-165823 - shear transfer at base plates) reminded me of something I've always wondered about.

Have you seen base plates, normally under signs and electricity/light poles, where the anchor rods protrude from the concrete and the base plate is held on the rods above and below with nuts? I've seen this detail on everything from the order menu at McDonald's, to 150' tall, 20 bolt, galvanized transmission line poles, to the large light poles in mall parking lots, all with gaping space under the base plates and between the anchor rods.

When I first noticed these, I dismissed it as a contractor neglecting to put the grout in. However, since they seem so prevalent, that theory is in doubt.

Does anyone know why this is done? Are those anchor rods really designed to take that axial load and shear over that unsupported length?

 

[feadude]

Nuts and washers under the base plate could be for leveling purposes, ie keeping the pole on a vertical axis.

 

[rb1957]

i agree withyou, i've looked nerviously at those base plates on highway lights, mounted on top of the new concrete barriers. i haven't looked too closly (being more preoccupied with avoiding the other idiots on the road rubbering necking about) but i don't think there's a shear spigot in the middle of the joint. so i think you are left with a dozen bolts holding onto the 100' light post "in quite desperation".

 

[civilperson]

This non grouted base is standard in the the electric transmission industry and in electrical sub-station for support of bus, components and switches. The anchor rods are designed to take the combined shear and tension from the enormous moments generated by wind and seismic. These are vertical beams and not axially loaded columns as in buildings.

 

[mrMikee]

swearingen,

The following link is to a design maunual for monopoles which discusses anchor bolt practices related to transission towers:

http://www.risatech.com/

http://www3/c-conceptsinc/files/Technical_Manual_1.pdf

This is part of the RISA site now which took over ERITowers software.

There's some interesting info and is a good page to browse.

http://www.risatech.com/risatower.asp

Regards,
-Mike

 

[IceNine]

Star for you Mike.

Great Link!

 

[swearingen]

I guess that answers the question - yes they are designed, and the reasons for this design appear to be internal corrosion and the ability to adjust plumbness.

Thanks Mike!

 

[JStephen]

If you have a heavy object with moment, it makes sense to support the object on the foundation, with bolts to hold it down. But where the weight of the object itself is minimal, then the uplift on one side of the bolt pattern is pretty much the same as the compression on the other side. So long as you don't get local bending of the bolt, it should work fine.

I have seen this used on transmission towers 5-6' in diameter, with 2.5" or larger bolts.