Double-nut moment connection of Cantilever over-head sign structure

[damorim]

Hello Eng-tips community,

I'm in Manitoba, Canada and I am designing several large overhead sign structures (12 sided galvanized steel structures) utilizing double-nut moment connections at the base. I've been using the 2015 AASHTO code for Highway sign structures and luminaires. Some background, in Manitoba we typically dry-pack grout underneath the base plate... this grout is not structural. In my experience, I've discovered that the Contractor rarely gets the grout the whole way under the base plate and around the anchor bolts anyways... its usually a poor job that leads to trapped moisture beneath the base plate and rapid corrosion of our bases.

The City is now wanting to implement rodent screens in lieu of grout pads... My question: What are the things I must consider before simply swapping out the grout for the rodent screens??

I've limited the exposed anchor bolt length to under 1 bar diameter. Additionally, I've included the additional bending stresses on the bolt due to shear at the base plate in my calculations (which AASHTO lets you ignore for exposed lengths less than 1 bar diameter). I did this because there has been research stating that this is not conservative, and AASHTO's beam method should be applied for ANY amount of exposed length. OK, check! I've specified the torque for the top nut (1/3 turn of nut for my 1.5" anchor rod).

Anything else that is required?? The reason I am asking is because I noticed that the Florida DOT does not allow substitution of the grout pad with a rodent screen for cantilever or high torsion carrying structures... I'm curious why this is the case if the connection is properly designed?? Keep in mind, Florida does not dry-pack the grout... they use a hi-flow grout that would provide a much better quality pad which can even be relied on structurally perhaps. But still, why can't it be used?

Should I be concerned about my bottom levelling nut loosening if there is no grout pad?? Should I include a jam nut on top??

Looking for some advice from individuals experienced in the design of these cantilever overhead sign structures specifically.

Much appreciated!

 

[BUGGAR]

Those monopole utility towers in SoCal have bases as you describe - no grout, nuts above and below base plate, no jamb nuts, limited anchor (bolt) cantilever, but no rodent screens. These are common here. ASCE has substation design guidelines that will give consensus on this.

 

[damorim]

Thanks Buggar. Do you think there is any concern with the vibrations loosening the nuts? Sometimes I see a jam nut and sometimes I don't, and it plainly seems to be a matter of preference...