Can Torque applied be converted reliably into Anchor lock off load?

[Useeeer]

Hello Everyone!

For an anchored sheet pile wall (say 9m height), prestresses strands with a preload of 15T were recommended to limit deflection. Bridge abutment behind the sheet pile wall.

However, during execution, contractor has replaced strands with micropiles (fy 500 to 600 MPa). We've been asked to carry out the validation. It was conveyed to them that if the anchors are not prestressed, deflection criteria will not be met.

We have been informed that they have applied some preload. Data supplied was Torque and pressure cell calibration certificate for hydraulic set up used for the head connection.

Question is whether the torque applied to tighten the bolt head be considered as pretension??

We've been asked to assume a pretension of around 40T (bar capacity is 85T). Calculation was not provided. When I checked the torque to axial tension relation (T=KDF/1000) applied for usual bolt tightening, the force is found to be of the order of 40T. However, is it even correct to consider the tightening torque as preload for ground anchors? Normal bolts will be of say 1-2m length, with clamping arrangements.

The overall length of these anchors is around 10m. Basically hollow bars filled (core filled with concrete) and grouted around

 

[MTSOE]

I personally haven't pre-tensioned an anchor using torque (I've only ever used a hydraulic jack), but the Williams Form website says the following about torque: "The second method of prestressing is to use a torque tension method. Unlike competing products, Williams full, concentric, rolled threads allow for torque tensioning when applicable. This is accomplished by simply turning the anchor nut against the anchor bearing plate with a torque wrench. By using a “torque tension relationship” provided by Williams, the installer can correlate the torque reading to a corresponding anchor tension load. Although not as accurate as direct tensioning, it is often used for fast, economical installations in areas where hydraulic jacks would be cumbersome or difficult to utilize. Torque tensioning is recommended to be done using a high-pressure lubricant under the hex nut to resist frictional resistance."

This is from just one bar supplier, but if they allow it, I'm inclined to think others would as well.

I'd keep pressing your contractor for more information about the specific equipment and methodology they used to tension these bars.

I hope this helps.

 

[brucesilver]

Hello Everyone!

For an anchored sheet pile wall (say 9m height), prestresses strands with a preload of 15T were recommended to limit deflection. Bridge abutment behind the sheet pile wall.

However, during execution, contractor has replaced strands with micropiles (fy 500 to 600 MPa). We've been asked to carry out the validation. It was conveyed to them that if the anchors are not prestressed, deflection criteria will not be met.

We have been informed that they have applied some preload. Data supplied was Torque and pressure cell calibration certificate for hydraulic set up used for the head connection.

Question is whether the torque applied to tighten the bolt head be considered as pretension??

We've been asked to assume a pretension of around 40T (bar capacity is 85T). Calculation was not provided. When I checked the torque to axial tension relation (T=KDF/1000) applied for usual bolt tightening, the force is found to be of the order of 40T. However, is it even correct to consider the tightening torque as preload for ground anchors? Normal bolts will be of say 1-2m length, with clamping arrangements.

The overall length of these anchors is around 10m. Basically hollow bars filled (core filled with concrete) and grouted around

Yeah, torque can give an estimate of preload, but for ground anchors that long, it’s not that simple. Bolt formulas don’t fully apply here—losses due to elongation, grout bond, etc., are different. Usually, proper prestress means using a jack with a load cell to confirm tension, not just torqueing the head. I’d push for actual tensioning data, not just torque.

 

[Useeeer]

I personally haven't pre-tensioned an anchor using torque (I've only ever used a hydraulic jack), but the Williams Form website says the following about torque: "The second method of prestressing is to use a torque tension method. Unlike competing products, Williams full, concentric, rolled threads allow for torque tensioning when applicable. This is accomplished by simply turning the anchor nut against the anchor bearing plate with a torque wrench. By using a “torque tension relationship” provided by Williams, the installer can correlate the torque reading to a corresponding anchor tension load. Although not as accurate as direct tensioning, it is often used for fast, economical installations in areas where hydraulic jacks would be cumbersome or difficult to utilize. Torque tensioning is recommended to be done using a high-pressure lubricant under the hex nut to resist frictional resistance."

This is from just one bar supplier, but if they allow it, I'm inclined to think others would as well.

I'd keep pressing your contractor for more information about the specific equipment and methodology they used to tension these bars.

I hope this helps.

Thanks for sharing. I believe their statements are limited to certain conditions. Currently, we have asked the contractor to confirm the preload that's applicable for the ground anchor after accounting for all losses. I'm pretty much sure they are going to put it in bolt torque and clamping force relation and give it back again. Honestly, that is tricky and may be adequate. Still looking out for some solution/ tests at site which could ascertain the preload on the anchors

 

[Useeeer]

Yeah, torque can give an estimate of preload, but for ground anchors that long, it’s not that simple. Bolt formulas don’t fully apply here—losses due to elongation, grout bond, etc., are different. Usually, proper prestress means using a jack with a load cell to confirm tension, not just torqueing the head. I’d push for actual tensioning data, not just torque.

Exactly! I guess they didn't tension it.

From all the limited conversations I've been to they've tightened the bolt. That's it. They have the cell pressure from the system and are telling that the bolts were applied torque until the cell pressure has reached the maximum and started dropping down. We didn't even get any proper reading. Just snapshots showing cell pressure (how can we confirm if that's the maximum applied?)

Then they are comparing this max cell pressure (400 to 500 Bar) to the torque values (7000 to 30000 Nm depending on the anchor dia) as per the hydraulic system's calibration certificate.

Now though we asked to confirm the preload themselves, they may throw some values which may give good analysis results but may not be the reality.

 

[MTSOE]

Thanks for sharing. I believe their statements are limited to certain conditions. Currently, we have asked the contractor to confirm the preload that's applicable for the ground anchor after accounting for all losses. I'm pretty much sure they are going to put it in bolt torque and clamping force relation and give it back again. Honestly, that is tricky and may be adequate. Still looking out for some solution/ tests at site which could ascertain the preload on the anchors

I imagine your contractor would be reluctant to re-stress all the anchors using a hydraulic jack, but perhaps you could do a do a lift-off test of a single anchor. Basically, mount the jack on the anchor and extend the ram until the hydraulic pressure abruptly increases. This pressure can then be converted to the lock-in load. It's your decision whether you can accept this sample as being representative of the anchor population.

 

[MTSOE]

I imagine your contractor would be reluctant to re-stress all the anchors using a hydraulic jack, but perhaps you could do a do a lift-off test of a single anchor. Basically, mount the jack on the anchor and extend the ram until the hydraulic pressure abruptly increases. This pressure can then be converted to the lock-in load. It's your decision whether you can accept this sample as being representative of the anchor population.

Sorry. I should have said until the increase in hydraulic pressure abruptly slows.

 

[Useeeer]

Sorry. I should have said until the increase in hydraulic pressure abruptly slows.

Let me know if my understanding is correct. Assuming the anchor is prestressed by X by torque ( unknown here) and then by subjecting it to max pressures, we'll come to know net stress to reach yield strength fy. We know the actual yield strength of the rod. So fy-y should let us the prestress applied (x).

 

[MTSOE]

Let me know if my understanding is correct. Assuming the anchor is prestressed by X by torque ( unknown here) and then by subjecting it to max pressures, we'll come to know net stress to reach yield strength fy. We know the actual yield strength of the rod. So fy-y should let us the prestress applied (x).

I'm not suggesting that you yield any of the anchors. Basically, you apply a tensile load to the anchor with a hydraulic jack until the nut is just barely lifted off the bearing plate/pile/wall/whatever it's bearing against. This point can usually be surmised by an abrupt reduction in gain of hydraulic pressure on the jack gauge. The hydraulic pressure required to do this is then used to determine what load the anchor was holding. This is referred to as a lift-off test in PTI documents.