Moment base plate and anchor rod design

[bnickeson]

I'm designing a base plate for a wide flange moment frame column. I have the plate design done without any issues, but my question revolves around the anchor rod detailing. I've typically seen that it's good practice to pre-tension the anchor rods, and I've also seen the detail used where the anchor rod is de-bonded from the footing concrete using a sleeve or something similar. However, how do you pre-tension the anchor rods if you typically use leveling nuts? Is it wrong to use leveling nuts in this case and, if so, what do you typically call out as a substitute? Shims?

Thanks.

 

[SandwichEngine]

If you're using typical anchor rods (F1554 Gr 36 or 55) you should never pretension the anchor rods because they'll snap off. They aren't high strength like an A325 or A490.

 

[HTURKAK]

- What is the reason for pretensionning ? In general preloading of anchor rods is an option if subject to dynamic loading and vibration ..

- The use of steel leveling nuts under the base plate is not a good idea even for anchors without preloading.

I will suggest you to look the following thread;

thread507-473362

thread507-474830

 

[bnickeson]

Sandwich, there's nothing wrong with pretensioning Gr 36 or 55 rods. They obviously don't get pretensioned to the same load as A325 bolts. Each size and grade of anchor rod has its own pretension load...I believe some are even given in Design Guide 1.

HTURKAK, it's my understanding that some pretension in a fixed-base moment connection is good practice to remove any initial slop that might cause excess deflection in the frame. I haven't done this before because I typically like to encase the bottom of a fixed-base moment connection in concrete which makes it quite rigid. I can't do that in this case, however.

The use of leveling nuts under a base plate is an extremely common method of erection in small to medium-sized projects nationwide. There's certainly nothing wrong with it.

 

[ProgrammingPE]

Pretensioning of anchor rods is typically only required if you have cyclical loads from some sort of equipment in order to improve the fatigue life. You can only pretension them against steel since the concrete will creep and remove the pretension so you would need sleeves to do this.

You can level the base plate using leveling nuts, setting plates, shim stacks, or leveling screws. Take a look at Section 2.9 and Appendix A of Design Guide 1 for more info.

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

http://www.youtube.com/channel/UCOj2mXXf3ZbZtgxTc6BtkGg

 

[bnickeson]

OK, thanks. That certainly makes the detail easier if there's no point in pretensioning the anchor rods.