Anchor rod pretension

[BAGW]

HI,

Is it required to pretension the anchor rods in high seismic category? I am not able to find any reference that calls out for it.

If PT is required, do we need to introduce a sleeve or can PT be achieved with just double nuts on the top of base plate?

 

[WinelandV]

Without checking, I believe that AISC Design Guide 1 has comment about needing to pretension anchor bolts if it's necessary for the performance of the structure (like, you don't want the movement associated with the initial loading of the rods in tension).

I believe there are also statements in there about the best way to do the pretensioning.

 

[BAGW]

AISC DG is not very clear on PT requirement for high seismic areas.

Doing some reading and seems like without a sleeve PT in anchor bolts cannot be maintained. Not really sure if that's accurate.

 

[WARose]

I'm not aware of any requirement for seismic considerations. (A lot of folks just do it for high cycle loads....like machine bases.)

If I am going to use pretension in a bolt, I normally use something like Rowan bolts (that do have a sleeve).

 

[dauwerda]

As far as I know, pre-tensioning of anchor rods is not required by typical codes and standards (IBC, AISC, etc.)
It is a common practice in the oil and gas industry when anchoring tall towers (such as flare stacks). Guidelines given by PIP are for towers more than 150ft tall, towers with height-to-width ratios of more than 10, and for dynamic machinery such as compressors.

For pre-tensioning to be effective, you do need enough stretch length, this can be achieve through the use of anchor sleeves (wilson sleeves), or even wrapping the anchor in tape. If the anchor is not de-bonded from the concrete by use of sleeves/tape and instead a stretch length is provide by using a base plate with chairs you will run into more pre-tension loss as initially the bond between the concrete and anchor will carry some of the load but over time this bond will relieve itself until the load is truly being carried by the anchor head.
Double nuts on top of a base plate won't do anything to help with pre-tension.

Keep in mind, high strength anchors need to be used for pre-tensioning to be effective (Grade 36 will relax over time), at a minimum this should be grade 55 anchors, ideally grade 105. Also anchors tend to lose their pretension over time due to concrete creep (typical losses are in the 20% to 60% range). According to ACI 355.1R, the reduction of the initial preload can be reduced by about 30% by retensioning (1 week after initial tensioning).

 

[AskTooMuch]

I've seen this done in the field for our compressor foundations. This require construction with experience doing this and not just any GC in my opinion.
So OP who is trying to figure out PT for the first time, and if the GC is not used to doing this also may come back to OP with questions he may not be experienced to answer.
How much pull down is acceptable for example?

 

[HTURKAK]

As far as i know, pretensioning is required if;

- fatigue is a concern ( incase of stress reversals ) wind , seismic ..Copy and paste from PIP STE05121
(...Pretensioning should be considered for vertical vessels that are more than 150 feet tall or for those with height-to-width ratios of more than 10..),

- for vibrating eq. foundation when recommended by the equipment manufacturer..

You need to introduce a sleeve so stretching of anchor bolt will be effective.