Pretensioned Anchor Rods 1

[EBF]

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?

 

[flamby]

Although I am not clear about the details of the structure, the idea of using pretensioned anchor bolts does not appeal to me. Upon bending, some bolts will be subjected to increased tension over and above their pre-tension. This is not a good use of material. As far as 'no rotation' is concerned, a reasonable fixity is generally obtained even without using pre-tension.

Sorry EBF, I am not with you.

 

[Ingenuity]

flame,

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.

 

[flamby]

Ingenuity,

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.

 

[Taro]

Some amount of pretension is beneficial in stiffening the connection and reducing deflections. The pretension and applied tension will be additive and should be considered. See "Anchorage of Steel Building Components to Concrete" by Marsh and Burdette, AISC Engineering Journal, Q1 1985.

 

[EBF]

I was wondering if anyone had heard of a similar application and what a reasonable amount of pretensioning would be. We are using 1-1/4" A354BD bolts for our case. I designed the base plate and checked the anchor bolt capacity (per 1997 UBC Section 1923) for load combinations including the seismic overstrength factor (2.8 for moment frame). This is the level of pretension force that i was planning to specify (roughly 85k per bolt). I'm just looking to see if i am in the ballpark of what seems reasonable for a pretensioned anchor bolt, since i've never done this before.

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!

 

[flamby]

EBF,

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.

 

[dik]

Although not often, I've used foam pipe insulation as a bond breaker to develop a length of anchor that can be 'stretched' to provide a better guess at the tension... also used Dywidag threadbar with a proprietary 'nut'... worked well. I generally use threadbar where there are significant uplift on AB's...

Also used foam insul on PT cables to provide a little lateral gap for tensoning parallel to joints...

 

[g0203447]

For this certain case, I have just seen once when designer wanted to improve friction between base plate and concrete surface only. The post-tension load was 10% of the bolt strength.
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.

 

[Ingenuity]

I am not familar with Taro's reference ("Anchorage of Steel Building Components to Concrete" by Marsh and Burdette, AISC Engineering Journal, Q1 1985) but the DIRECT addition of the pretension force and the applied external tension would be incorrect IMO.

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.