Pretensioned Anchor Rods
Pretensioned Anchor Rods
(OP)
We are using fixed base columns to limit the drift of a small ordinary moment frame building with high floor to floor heights. The building is in a high seismic zone. We are planning to pretension the anchor bolts to ensure a fixed base condition with no rotation at the base. Is this
reasonable use of pretensioned anchor rods? If so, are there any guidelines for determining the adequate pretension force to be used?
reasonable use of pretensioned anchor rods? If so, are there any guidelines for determining the adequate pretension force to be used?






RE: Pretensioned Anchor Rods
Sorry EBF, I am not with you.
RE: Pretensioned Anchor Rods
If I understand EBF's pre-tension application correctly then I disagree with your statement that "...some bolts will be subjected to increased tension over and above their pre-tension". If the bolts are pre-tensioned, the act of pre-tensioning will impart compression into the base. Upon application of an external load this precompression must be first overcome before any additional tension is applied to any individual anchor rods.
Very similar to a prestressed concrete tension member - the initial P/A will helps with deformation control under external tension forces. The additional stress in any line of anchor rods needs to be analysed by considering the increment in strains, remembering to account for the initial pre-compression that will be imparted into the concrete at time of pre-tensioning.
EBF - I am not familar with directly similar applications to what you discuss, but pre-tensioned tensioned anchors rods (aka prestressed ground anchors) have been done before. Example - 4 x vertical rock anchors fixing a concrete foundation base (maybe a crane base), where the rock anchors are pre-stressed and locked off prior to application to any external loads. Depending on the pre-tension levels you will probably require 150 ksi (1000 MPa) threaded high-tensile bar.
RE: Pretensioned Anchor Rods
Your reasoning is sound and uncontestable for pre-tensioned members as far as the anchors do not come out of concrete and do not end up as bolted connections with the base plate. And this is what I assumed the case is. I assumed that the pre-tensioned anchors come out and end up in the base plate by nuts or whatever. If this is the case, consider the small part that ends up in the bolts. This portion will not be able to share the load with concrete and will be subjected to loads over and above the pre-tension. Correct me if am wrong.
Regards.
RE: Pretensioned Anchor Rods
RE: Pretensioned Anchor Rods
flame,
I'm not sure I understand what you mean about the portion "...that ends up in the bolts". Are you refering to the part of the bolt above the base plate and nut?
Thanks!
RE: Pretensioned Anchor Rods
I mean the portion of the bolt between concrete surface of the foundation and the nut. Actually I wrote 'bolt' where I should have written 'nut'. Sorry for the confusion created there but prabably my boss was passing by my table when I wrote that
I think you will be benefited by referece cited by Taro. I am not in a favour of pre-tension as it will mean bigger requirement of better steel (high tensile). However tightening of bolts to a particular force (strictly speaking - post tensioning) can be done.
RE: Pretensioned Anchor Rods
Also used foam insul on PT cables to provide a little lateral gap for tensoning parallel to joints...
RE: Pretensioned Anchor Rods
I think this application is to some extent similar to the use of high strength bolts when we use them to resist shear force by friction and protect water attack to the connection.
In my opinion, steel in tension have low quality of fatigue so it's not recomended in the case we use to resist inverse load directly but in the case making use of friction, when stress in steel bolt do not change too much during the impact of inverse load.
RE: Pretensioned Anchor Rods
The increase in force above the pretension level depends on the magnitude of the external force and the bolt-to-bearing plate areas - ie the pretension P/A. Whilst this is additive, it is not simply Tf = Tp + Te
where:
Tf = Final resulting bolt force.
Tp = Pretension bolt force.
Te = Applied external tension force
There is no significant increase (there is some but it is relatively small) in bolt tension from the pretension load level, until the external load equals or exceeds the pretension force.
The MINIMUM bolt tension as per LRFD or ASD to AISC for fully-tightened bolts is actually 70% of the min. tensile strength of the bolts. Which is coincidentally about equal to what most POST-tensioned high-tensile bars get stressed to in concrete structures.
To be meaningful in a connection, an applied pretension of 10% of UTS is too small in my opinion. SNUG tightening will probably give you more than 10% - but difficult to practically measure.