HILTI HIT HY-150 & CREEP 4

[GSigma]

After the I-90 tunnel in Boston and the NTSB report I am a little concerned using adhesive anchors when subjected to direct tension. Any tips or concerns will be appreciated.

Thank you

 

[MiketheEngineer]

I believe those were not made by Hilti - but if you have questions - call Hilti!!

 

[hokie66]

I don't use adhesive anchors in direct tension. Whether the specific adhesive is subject to creep or not, workmanship in installation is my main issue.

Back to your question about the Big Dig tunnel, Sika made the adhesive, Powers Fasteners packaged it, and the contractor used the wrong type.

 

[MiketheEngineer]

I think Hokie is right - I just don't like using names unless I know for sure!!

 

[ARKeng]

I believe all the manufacturers now have to meet certain creep test criteria under their testing for ICC approval. Make sure you read the code report and that it addresses this and call the manufacturer.

I believe the last time I asked saw HILTI or one of the other manufacturers, they showed me a graph that showed that the allowable creep is like 1/10th what it used to be and this particular manufacturer was bragging that theirs was like 1/10th of the allowable maximum. So there are epoxies out there that are way better than used to be available, but I believe most do require reduction in allowable loads or something to that effect as well.

 

[Teguci]

The other problem with overhead epoxy applications is the field installation. Photos from the failed connections showed drill dust encasing the epoxy when it slipped out (ie the epoxy bond was to dust and not to the concrete soffit). Overhead installation requires monitoring of the volume of epoxy being deposited (to avoid voids). Creep of the epoxy was a known and tested issue at the time of the big dig (there are creep tests on epoxy that are 27 years old), and, despite the NTSB's initial recommendation, should not invalidate the use of adhesive anchors under sustained loading.

The industry response has been as follows:
- ACI 318-11(not yet adopted by most jurisdictions) now requires certification of the installers for horizontal to overhead installations
- ACI 318 now has a methodology for calculating the failure mechanisms for bonded anchors
- Powers has become very sensitive to this issue
- Additional testing and approvals have been performed on adhesive anchors

 

[ToadJones]

I had a question about Hilti the other day and was chastised here for not calling Hilti directly.
When I did call, the guy was very helpful and very knowledgeable.

 

[dik]

Shortly after the Big Dig problem, Hilti reps made a 'tour' of the end users to separate themselves from the problem. The report stipulated it was the fast setting epoxies that are prone to creep. Hilti clearly noted that their products were not susceptible to this...

There are/were a few threads on Eng-tips... do a search for big dig...

Dik

 

[TXStructural]

Creep is not a primary problem where the correct adhesive is used, even in constant tension applications (i.e., suspended items.) In the Big Dig, as noted above, the issue was installation practices, principally hole cleaning and adhesive installation. Most of the ICC-ES reports I have examined address creep.

CRSI and ACI have developed an installer certification program at the request of NTSB/FHWA, and it is now being rolled out ahead of ACI 318-11 adoption in IBC. Certification is required for all adhesive anchors installed between horizontal and vertical overhead. In preliminary testing of experienced installers, we found that very few (nearly none) could install the adhesive adequately for long term performance. The testing procedure involves a 75 question written test (computer testing now in development) and a two-part practical exam involving downhand drill and install in real concrete, and overhead simulated adhesive installation which is then dissected to determine proper hole filling with adhesive.

 

[dik]

TXS...
I'll have to dig out my copy of the final report, but, if memory serves, it identified the problem as creep caused by a fast setting epoxy adhesive... not the quality of the hole... and noting that the wrong material was ordered/supplied...

Dik

 

[hokie66]

If it is all that difficult to do (and it is), why bother? Use a more reliable type of fastener, like the Hilti undercut mechanical anchors which were used to retrofit the hangers.

 

[SAIL3]

agree with TX & hokie...creep is not the main issue...quality of workmanship and installation is what leaves a great deal of doubt in my mind....I am not able with a sufficient degree of certainty to ensure the integrity of the final instalation..so I don't do it for constant tension situations and besides there are other alternatives

 

[dik]

From the NTSB report:

The National Transportation Safety Board determines that the probable cause of the July 10, 2006, ceiling collapse in the D Street portal of the Interstate 90 connector tunnel in Boston, Massachusetts, was the use of an epoxy anchor adhesive with poor
creep resistance, that is, an epoxy formulation that was not capable of sustaining long-term loads. Over time, the epoxy deformed and fractured until several ceiling support anchors pulled free and allowed a portion of the ceiling to collapse. Use of
an inappropriate epoxy formulation resulted from the failure of Gannett Fleming, Inc., and Bechtel/Parsons Brinckerhoff to identify potential creep in the anchor adhesive as a critical long-term failure mode and to account for possible anchor creep in the design, specifications, and approval process for the epoxy anchors used in the tunnel. The use of an inappropriate epoxy formulation also resulted from a general lack of understanding and knowledge in the construction community about creep in adhesive anchoring systems. In addition, Powers Fasteners, Inc., failed to provide the Central Artery/Tunnel project with sufficiently complete, accurate, and detailed information about the suitability of the company’s Fast Set epoxy for sustaining long-term tensile loads. Contributing to the accident was the failure of Powers Fasteners, Inc., to determine that the anchor displacement that was found in the high‑occupancy vehicle tunnel in 1999 was a result of anchor
creep due to the use of the company’s Power‑Fast Fast Set epoxy, which was known by the company to have poor long-term load characteristics. Also contributing to the accident was the failure of Modern Continental Construction Company, Inc., and Bechtel/Parsons Brinckerhoff, subsequent to the 1999 anchor displacement, to continue to monitor anchor performance in light of the uncertainty as to the cause of the failures. The Massachusetts Turnpike Authority also contributed to the accident by failing to implement a timely tunnel inspection program that would likely have revealed the ongoing anchor creep in time to correct the deficiencies before an accident occurred.

Dik

 

[hokie66]

What has always intrigued me about anchoring with epoxy is this: why do engineers so readily accept that development length for adhesive anchors can be much less than for cast in anchors?

 

[ToadJones]

hokie-
Agree 100% with your last comment.
I have been forced recently to use some epoxy anchors (owners request) but thankfully they are not in direct tension.

I will not use any post installed anchor if I can avoid it.

 

[TXStructural]

Yes, I am fully aware of the various reports issued following the Big Dig collapse. Examination of anchors revealed that the adhesive on a significant number of anchors withdrew from the holes intact but covered in dust. Anchors which were similarly loaded (even those holding the same attachment) did not act similarly under load in many cases. Some sections which remained in place and holding load following the collapse were held by a single embedded anchor after the other in the pair withdrew. These findings indicate that creep due to material choice is not the exclusive cause of failure of adhesive anchors in sustained tension.

Installation is not all that difficult once you are trained and practiced, and use a system designed for overhead installation. But you cannot expect the least expensive adhesive, installed improperly, to perform properly.

 

[asixth]

I had a hilti rep come into the office the other day and said that hilti epoxy doesn't creep. My preference is hilti.