Hilti ASD Factor of Safety: 4 vs 5 2

[David Deck Guy]

This isn't for any pressing issue, just something I'm looking to have in my back pocket if needed.

Hilti uses a factor of safety of 5. The hilti catalog says that a global safety of 4 to 8 has been industry standard for nearly three decades. Is there any scenario where you guys would use a factor of safety of 4 and let the DCR run up to 1.25, such as if its a one off non critical component for safety and you really trust your contractor? How would you justify this to a peer reviewer?

 

[skeletron]

Yikes buddy! You're basically asking for a firm definition of "engineering judgement" and, surprisingly, this isn't a consistent term in the Engineer's glossary. It would be a challenging conversation to have between the designer (who gets to make the assumptions) and the peer reviewer (who needs to determine the appropriateness of the design assumptions).

But as an eng-tips thought exercise I'll give some comments:
1. FS=5 is usually associated with plastic or ultimate failure (e.g. in lifting components), so that makes me think that I would consider how your Hilti is failing
2. 25% overstress may be accomodated by short-term loading, so that makes me think that I would consider the actual live load scenario vs. the design live load scenario and whether the effect of transient peaks in the loading could occur
3. Most Hilti products are sensitive to installation so as to require a high factor of safety, so that makes me think that I would consider being on-site during installation or having a Hilti rep observe the installation or force the Contractor to provide certification

Honestly, if it's a case of 1 anchor @ 125% versus 2 anchors @ 63%, I would take 2 anchors any day of the week. Redundancy is king. But if we are talking 12 anchors @ 125% versus the impossibility of putting in 15+ anchors, I would probably focus on sharpening the pencil and re-estimating loads and/or considering a different anchor setup.

 

[phamENG]

No, I would not. For proprietary fasteners and connection components, I let the manufacturer or the third party testing report tell me what the appropriate FoS is. I may make it larger, but I will not make it smaller.

 

[David Deck Guy]

That's all I needed to hear. I've designed all of my Hilti anchors to their product specifications with factor of safety of 5, but I'm always looking to understand the products better and what options I have when I get into a pinch. When I have connections that are getting bulky and I have architects and contractors pushing on me, I'll make sure to continue to hold my ground. Thanks.

 

[WinelandV]

The best part about using Hilti (or Simpson, etc.) is that you don't need to stand your ground, you just point back to Hilti.

"No, you can't use epoxy anchors for the PEMB anchorage - Hilti doesn't allow for 3/4"Ø anchors at 3.5" spacing."

Please note that is a "v" (as in Violin) not a "y".

 

[David Deck Guy]

If only it were that easy. They always push!

 

[phamENG]

Some days, I get a sort of sick pleasure from telling contractors with unreasonable demands "no." Other days, it's just normal pleasure.

 

[David Deck Guy]

I recently picked up "Philosophies of Structural Safety and Reliability" by Vladimir Raizer and Isaac Elishakoff with the hopes there is something in this book I can use to get my points across. We'll see....

 

[jerseyshore]

I agree with everyone above, especially skeletron's statement about installation. A Hilti rep once told us that less than 10% of all anchors are installed correctly per their spec. That SF of 5 seems plenty fair when you consider the average contractor installing those anchors has no clue what they are doing.

 

[jjl317]

In my mind, depends on the substrate. In the old days (prior to the implementation of the ACI anchorage guidelines), I typically used 4:1 into concrete, and 5:1 or 6:1 in masonry.

With the new ACI approach, the safety factors aren't really relevant. In fact, I often have to spend time documenting / explaining on the given project specifications I am working with (that are based on the old 4:1 against ultimate into concrete), aren't relevant anymore, and that many/most of the Hilti expansion anchor products (i.e. Kwik-Bolt TZ2) don't have product data to be evaluated in this way. We do many federal and state government projects, and this often ends up in a big argument. In fact, I know will recommend Dewalt in some cases, because some of their products (i.e. Power-Stud+ SD4) have technical documentation that allows the analysis to be done both way (current ACI approach, and 4:1 versus ultimate). And my understanding is that anchorage into masonry is heading in the same direction.

IMHO, this is a perfect example of what is wrong with the process of engineering these days. The analysis that used to take half a page, now takes 6-7 pages (and more likely a proprietary software package that we can't possibly back check), and what public service has that accomplished? Has all of this additional analysis, and the associated increase in risk of human error, created some huge economic benefit? Plus, all of the additional work required of the manufacturers to try and meet the all-powerful ICC? The amount of extraneous work required anymore to get something constructed can be maddening.

 

[human909]

A Hilti rep once told us that less than 10% of all anchors are installed correctly per their spec. That SF of 5 seems plenty fair when you consider the average contractor installing those anchors has no clue what they are doing.

Here in Australia testing of installed anchors is becoming more and more common. And the results are nothing to get excited by. I've seen anchors pull out at around 20% of the ultimate load. A safety factor is (usually) not supposed to be substituting for poor workmanship.

With the new ACI approach, the safety factors aren't really relevant.

I'm from the "new days" of LRFD (LSD). And safety factors are gobblygook to me when it comes to engineering. Sure I as a layman understand them, but I don't have a clue what they mean in terms of engineering design. If I see a safety factor I normally ignore it and go hunting for the design capacity of the factored design capacity which is often 3-5x more that the value with a mysterious safety factor on it.

IMHO, this is a perfect example of what is wrong with the process of engineering these days. The analysis that used to take half a page, now takes 6-7 pages (and more likely a proprietary software package that we can't possibly back check), and what public service has that accomplished?

We practice in different jurisdictions so I doubt the same challenges that we are both seeing have much to do with changes regarding safety factors. There has been a trend, especially in the anglosphere, of more box checking in order to perceived legal hurdles for safety. You can see this in engineering or in OSH.

Bureaucrats and administrators have tried to make safety controllable by administration, and this more and more is including structural engineering.