Expansion Anchors vs Mechanical 1

[Bubik]

Hi everyone,

In the base of a column of a lift shaft there is a requirement to install M20 8.8 Hilti chemical anchor, however the contractor for unknown reasons installed expansion M20 anchors of similar properties. The base of a column is in the proximity of a lift pit ( the closest bolt is about 500mm to the edge) and also it needs to be mentioned that high vibration occurs as the building is a Cement Mill.

I know that chemical anchors are preferable to install close to edges (and probably in high vibration environment as I heard)
What my other arguments would be to the contractor, for example pullout strength of concrete around chemical anchor is better that in a case of expansion anchor??

 

[SlideRuleEra]

...there is a requirement to install M20 8.8 Hilti chemical anchor...
...contractor for unknown reasons installed expansion M20 anchors of similar properties.

Have the Contractor explain to you why expansion anchors are acceptable.
Don't play the game trying to "convince" the Contractor they are wrong... you will loose.

With that said, if the Contractor can come up with a good reason, accept it.

http://www.SlideRuleEra.net

 

[JGard1985]

Check your Hilti tables but I think you're fine (M20 anchors don't have a derate until 300mm near edge).

I'd leave it alone and consider contractors point of view. In my mind:

Chemical anchors require very clean holes for adhesive to bond with adjoining concrete (Google boston big dig ceiling failure). In my mind a cement mill (very, very fine dust) is a terrible environment to try and keep holes clean during epoxy anchor installation. If you have the necessary capacity (confirm by embedment) I'd go with the mechanical anchors and leave it alone


Jeff
Pipe Stress Analysis
Finite Element Analysis

http://www.xceed-eng.com

 

[TEDstruc]

If you have tensile loads or even moderate shear loads on the anchors, you should probably verify that the expansion anchors are adequate. In my experience, the same size expansion anchor will have less tensile capacity than a properly installed epoxy anchor, but it may depend on embedment and spacing of anchors. It is easy enough to check, especially using Hilti's free Profis Anchor Software. You might also want to read through the ESR reports for the expansion anchors and epoxy anchors and see if it says anything about vibration. The anchor design software and ESR reports can be downloaded from Hilti's website.

 

[Strong]

As a vibration control person, I never recommend expansion anchors. Expansion anchors tend to loosen from vibrations whether inline or in shear. As others have stated properly installed epoxy anchors are the best choice.

Walt

 

[BridgeSmith]

We never spec epoxy anchors for tension applications. That said, I'd be leery of wedge anchors subjected to vibration, as well, unless the anchor manufacturer is willing to give approval for such use.

You could possibly investigate a hybrid system of some sort, by injecting a low-viscosity epoxy or resin around the fully installed mechanical anchor, you could prevent movement of the anchor from the vibration.

 

[TLHS]

If you had a reason to specify the epoxy anchor in the first place, then that's the reason you need it, and that's what you tell them. Then it's on them to convince you otherwise. That's the perfect world, although I totally understand that it's sometimes better to write a couple of paragraphs of engineering opinion to justify a hard 'no' answer, rather than spending weeks on a back and forth.

However, if you're not sure you had a solid reason for specifically calling out epoxy anchors, then I'd suggest that you try to evaluate the situation as a whole, rather than approaching the problem to prove that you're right. Depending on your install situation, you're going to have some people who would never use epoxy anchors, for valid reasons, and other people who would never use mechanical anchors, for valid reasons as well. There's definitely an solid grey area of engineering judgement between the use cases for both technologies. If strength works out for both, then you have to evaluate the whole situation and decide if the quality control issues that can occur with epoxy anchors justify the increased strength and serviceability. Since they're already installed, then in this case I guess it's really more a question of whether the increased strength and serviceability of an epoxy anchor is required to meet your project goals.

As an aside, given that your contractor can't even buy and install the right types of anchors, I would suggest that you want to have full time inspection during at least the first few epoxy anchor installs to make sure they prep the holes right if you decide to go that route.

For a small project with a few anchors I'm personally on the 'call-out epoxy anchors for almost everything' bandwagon at this point, with the exception of some tension cases or small nominal loads. The cost delta on a small project isn't significant, and there are definitely some better products now that help alleviate the quality control issues. If a contractor calls up and wants to substitute mechanical anchors in a situation where it doesn't really matter, though, I'll happily let them.

 

[MIStructE_IRE]

I wouldn’t use expansion anchors in vibration.

Have you got the Hilti Profis software? As far as I know you can download for free.

 

[BridgeSmith]

Would roughening the sides of the holes for an epoxy anchor improve the performance in tension?

 

[MIStructE_IRE]

I’m not sure how roughening the sides of the hole would help since you’re designing on the basis of a cone failure, not just a loss of friction between resin and original concrete. That said, it would be an interesting test!

 

[OldBldgGuy]

More than once I've seen lovely pieces of threaded rod with a nice epoxy coating, perfectly smooth and dusted white, hanging from the bottom of baseplates. I've never seen mechanical anchors in the same situation. If the anchors have sufficient capacity by their rating, mechanical anchors are far more likely to be installed correctly than adhesive anchors. That isn't to say that either one is inherently better than the other except in particular circumstances, but installation is of prime importance and adhesive anchors require much more attention to install correctly. It's not rocket science, but the installers aren't rocket scientists, either.

 

[BridgeSmith]

MIStructE_IRE, assuming careful cleaning and proper preparation of the holes, so that the epoxy fully bonds to the concrete, there would be no advantage to roughening the sides of the hole. However, given the apparent rarity of that happening in the field, poor preparation could be mitigated by having the epoxy interlocked with an uneven surface. Of course, if the two parts of the epoxy aren't properly and thoroughly mixed (as we've also observed often), even roughening the holes won't help.

"If the anchors have sufficient capacity by their rating, mechanical anchors are far more likely to be installed correctly than adhesive anchors."

I think we're all in agreement on that. Under static loading conditions, mechanical anchors would be the way to go if they provide the capacity (their capacity is generally slightly less than an epoxy anchor of the same size and embedment). However, there was mention of vibratory loads that would possibly being applied to the anchors, and there appears to be alot of uncertainty about how a mechanical anchor would react when subjected to vibration.

 

[Bubik]

Guys, Thanks so much for your replies..wonderful amount of help here.

 

[TheDW]

It's always interesting seeing the differences of opinion here. Just last week our office had to fix an issue where mechanical anchors were installed very poorly...in one location, they installed it too close to the edge and the expansion spalled off half the CMU face, exposing most of the anchor.

In another location, they kept tightening the nut and pulling the anchor up, not realizing that the expansion mechanism wasn't 'biting' the material at all, just sliding up through it. They pulled an 8" anchor so that there was almost 5" of threads above the concrete...and nowhere near the specified 6" of embedment. We wouldn't have known about it all but for noticing it in a picture highlighting something else.

Of course I know people have their horror stories about epoxy anchors as well, but I tend to feel more comfortable specifying them anyways.

 

[TehMightyEngineer]

I've seen people screw up both types as well (I installed the wedge anchors on my parents garage when we built it and actually had the wedge not bite just like yours Daywalker). I vary my install methods based on risk/redundancy and expected installer skill. If they're installing a baseplate with 4 anchors; wedge. If they're installing a single dowel heavily loaded in our shop; epoxy. Plus often I just need the strength or edge distance minimums that epoxy gives.

Above all, if they're installing overhead or anything with permanent tension; wedge or other mechanical only unless I can personally observe the install for epoxy.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[OldBldgGuy]

I think Daywalker illustrates exactly what I was getting at, & when I said I haven't seen mechanical anchors hanging from a baseplate I didn't mean it doesn't happen, but I did mean that you'd know it will happen before it does. If you blow out the edge, it happens immediately, you were too close, and you weren't meeting the specs. If you torque the mechanical anchor and it keeps pulling out, you know it when you're doing it and you know it doesn't meet capacity. Epoxy anchors can't be torqued until they've cured, and they can hold long enough then for an inexperienced installer to think they're okay. It still boils down to proper installation for whatever type of anchor you use.