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The necessity and objective of testing of pretensioned mechanical anchors

The necessity and objective of testing of pretensioned mechanical anchors

The necessity and objective of testing of pretensioned mechanical anchors

(OP)
Manufacturers, like Hilti, require applying certain torque to post-installed concrete anchors during installation. For example, a 20 mm Dia HSL carbon steel anchors needs to be twisted to 400 Nm (300 ft-lb) with a torque wrench. Although not torque controlled, a 20 mm Dia HDA carbon steel anchor is expected to experience a 300 Nm (221 ft-lb) torque. The corresponding clamp load (pretension) should be in the order of 100 kN (20 kip +). Hilti representative told me that the pretension load after applying installation torque is usually about 150% of the allowable load. But since he doesn't have answers to my following questions, I am here to look for help.

My questions are:
1. Since those mechanical anchors have been pretensioned by application of torque, is it reasonable not to ask for additional testing afterwards, if Contractor can prove that the initial torque is applied to each anchor?
2. If Engineer has concern that the installer did not apply the manufacturer specified initial torque, he can ask for verification testing afterwards. If additional testing is necessary, can he simply specify the manufacturer required initial torque as a testing criterion, instead of asking for tension proof loading?
3. If I have only 16 anchors in one job, is it reasonable to overdesign the anchors to avoid load test, considering each anchor test probably will cost more than $100.

Thanks,

RE: The necessity and objective of testing of pretensioned mechanical anchors

Hi George

Not sure how you would do additional tests on anchors already installed, what additional tests are you thinking of?

I'm surprised that the Hilti Rep said that after applying the installation torque the load is 150% of allowable load how does that work?

Torque applied to the anchor is only accurate to about +/-25%, all you could reasonably do is set the torque wrench to the installation figure and go round each anchor, if they are under the installation torque you will know straight away however if the torque applied was initially higher than the said figure it won't tell you a thing.

RE: The necessity and objective of testing of pretensioned mechanical anchors


Note carefully what the Hilti rep said: "the pretension load after applying installation torque is usually about 150% of the allowable load". Considering that the allowable load stated in Hilt's manuals is 25% of the ultimate load. the safety factor of 4:1 is not impacted.

Ralph
Structures Consulting
Northeast USA

RE: The necessity and objective of testing of pretensioned mechanical anchors

Use HSL-3-B anchors with the indicator caps that twist/shear off when the correct preload is applied. Link

RE: The necessity and objective of testing of pretensioned mechanical anchors

(OP)
Thank you all for replies. However, it seems only desertfox inferentially addressed one of my questions by staing " ... set the torque wrench to the installation figure and go round each anchor ...". However, I don't know if this is the valid approach used in job site.

For Ingenuity, you seem saying "Yes" to my Question 1. But please clarify:
Do you mean that since contractor can demonstrate the correct torque has been applies, the owner should be content with contractor's work? That is, verification test is not necessary?
Also, could please you weigh in on other two questions?

For desertfox,
"Not sure how you would do additional tests on anchors already installed, what additional tests are you thinking of?".
After contractor installs the anchor, the owner's representative usually wants to engage a third party to verify the contractor's work quality by ordering some verification tests. Coupling nut and bolt extension can be use to facilitate pull test if needed.
"Look at this link the HSL carbon steel anchor has an installation torque 200Nm so I'm not where your 400Nm figure comes from."
I think you are looking at HSL-3, which has the mechanism to limit the max torque, not HSL.

RE: The necessity and objective of testing of pretensioned mechanical anchors

Hi George

Page 3 of this link might help just above clause 3.1

http://www.fixingscfa.co.uk/media/58293/gn_site_te...

Yes I was looking at HSL-3 but the only HSL I can find is stainless steel but your post definitely says carbon steel, perhaps you can provide a link to the data sheet your looking at.

RE: The necessity and objective of testing of pretensioned mechanical anchors

(OP)
Check this link for HSL carbon steel, Section 3.3.3: http://submittals.us.hilti.com/PTGVol2CA/.
The paper you sent suggests anchors with European Technical Approvals don't need proof load test. I am in Canada. So I guess we can waive the proof load test requirement for anchors with certain jurisdiction approval, or thoroughly verified by a reputable test agent, such as ICC-ES.

RE: The necessity and objective of testing of pretensioned mechanical anchors

Hi George
Yes that's what I was thinking about waiving the tests and I guessed there would be something similar if you weren't in Europe.
Thanks for the link.

RE: The necessity and objective of testing of pretensioned mechanical anchors

GeorgeYin:
I think that the fastener (A.B.) manufacturer basically wants to proof test the fastener holding mechanism, be it mechanical, wedging of some sort or epoxy. They want to be sure that that is up to snuff, with some factor of safety. Are they holding, well beyond their working load, 150% +/-? This, because there is so much potential for poor installation practice, which you see fairly regularly. People are just not following the installation instructions, there is a reason for the instructions. I don’t see a reason for retesting, if there isn’t some intervening reason that the fastener may have been mistreated or loosened. The EOR might want a second test if he is insisting on some specific torque or bolt tension in the final application. Although, a meaningful and reliable relationship between torque and bolt tension, or clamping force, are notoriously unreliable.

RE: The necessity and objective of testing of pretensioned mechanical anchors

Is it not possible to just go out ant you test them to the owners delight? buy or rent a calibrated torque wrench, set it to 95% of the specified torque and get your workout in! or follow the contractor observing him/her checking each one.

It think that is a reasonable request if they are critical or if the owner requests or if the EOR requests it

RE: The necessity and objective of testing of pretensioned mechanical anchors

Hi George

This is an ETA

https://www.hilti.co.uk/medias/sys_master/document...


and from my previous link:-

Site tests are not required for objective b) above in cases where an anchor with an ETA is used and it is installed, as
required in the ETA, by a competent person working under supervision.

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