Using a Previous Design 2

[SKIAK]

A project I am working on is going to replace a fence. The fence is being replaced because, basically, it’s old, weathered, and falling apart. The fence is in a very remote location and it is very expensive to get materials and people out to the location, so the previous engineer used a unique type of anchoring for the fence; it was easy and cheap to transport and construct. I would like to use a similar (almost identical) design for the new fence, but I can’t actually calculate or verify that the existing anchor system works, but it seems to have worked on the previous design for at least 20 years. Nobody else that I have talked to knows how to calculate capacity for these anchors either (other structurals and geotechnicals).

But my question isn’t technical; I have talked with other engineers in the office and I have talked to geotechnical engineers that are involved in the project. I’m a relatively new engineer, just got my PE a year ago, and would probably be stamping this drawing. Different anchorage systems for the fence that have been suggested by other engineers and the geotechnical engineers would be much more expensive (on the order of 10x more from my cost estimates) for the sole reason that we can’t calculate or actually verify that the previous design “works.”

My immediate thought is that it seems to have worked for the past 20 years, and conceptually I can see how it works, I just can’t calculate it. It’s not a life safety issue, if the anchorage failed the fence would just fall over (I suppose there is a chance somebody could be walking beside it, but that is very unlikely) which of course is undesirable but I also think is unlikely.

Would it be unethical, or bad engineering practice, to use this previous design since I can’t actually verify it for myself?

 

[MintJulep]

I'm a bit confused by your concern about frost jacking in permafrost.

Frost jacking requires freeze-thaw cycles.

Permafrost never thaws.

 

[SKIAK]

The top layer of soil in permafrost is called the active layer, it will freeze in the winter and thaw in the summer.

 

[SKIAK]

So I guess that attachment didn't post, I'll try again.

 

[SNORGY]

That picture is worth a thousand words.

You might want to just do the math around what is drawn, because *that* is way different than what I had pictured, and now that I see it, I can see *that* working rather well.

 

[ukengineer58]

Seems from sketch in order for anchors to pull out the opposite side would need to fail in bearing or the whole system overturn. Would simply adding weight work?

 

[SKIAK]

Yes, adding weight would work. The problem with adding weight is the shipping cost. The materials are getting barged out to the job site and the shipping cost is based on weight and volume. The shipping cost estimate for just the fence materials is already hovering around $100,000 which makes that option very unattractive. It would be less weight to use new anchors.

 

[GregLocock]

Well, that is a very sensible design, even if you can't think of a way of analysing it. The simple case where some depth of the ground suddenly increases in height, lifting the base of the fence, is easy enough, if you know the properties of the underlying permafrost layer. The tricky bit is finding out what would happen if the melting layer is melting at different rates, lifting the base of the fence unevely, so tending to pull one tent peg out at a time.

Given that the army and the oil industry have a fair amount of experience building stuff in permafrost I'm guessing the basic design parameters for permafrost are understood. So doesn't it boil down to examining (a) what is the likely distribution of uplift forces on the base of the fence? (b) waht force is required to pull one of the pegs out at what angle (c) how good is a given length of permafrost at resisting the forces in (b).

I'd expect some sort of statistical distribution as an answer, and you make the pegs longer to shift the pass/fail rate.

Cheers

Greg Locock


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[SKIAK]

Yup Greg, that's pretty much it. The uplift at the base of the fence from the wind is easy to calculate. There is also uplift that is created from the active layer freezing (the frost jacking forces). The basic force balance is uplift from the wind plus uplift from the frost jacking has to be less than the allowable anchorage force of the rebar in the frozen soil.

The frost jacking and skin friction forces have been fairly well studied for typical posts and piles and these are easy anchors to design given the soils information, which we will have. We don't have adequate information about how rebar performs in these situations to be able to determine the answers to your questions (b) and (c). The difference is in the surface of the material you are embedding into the ground, a rougher surface will develop more force but not necessairly a proportional amount comparing the anchorage force and the frost jacking force, because they use different mechanisms to develop the force. The problem with making the pegs longer is they are more likely to buckle as they are hammered into the frozen ground since rebar is pretty flimsy compared to, say, 4" diameter std pipe. The geotechnical engineers involved in the project are very familiar with permafrost conditions, and we have some information about rebar available, but not enough to come up with anything consistient.

 

[KENAT]

Can't you just 'repair' the old fence even if that means replacing every component in it?

Kind of like the janitor getting ready to retire who boasts about having had the same broom for 50 odd years, "and in all that time it's only had 5 new handles and 7 new heads".

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[KENAT]

Oh, and in the process of dismantling the old fence any chance you can 'test' some components of it?

Back in aero defense our stress man would occasionally clear things by analogy, of course he wasn't a PE though.

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[SKIAK]

I considered the option of detailing repairs for all the individual types of failures, I even wrote up a narrative explaining all of the details; I decided against that for a few reasons. The shipping costs go up even more if you consider shipping out materials in several smaller packages. The cost of labor is a similar story since there is nobody on site who could do the repairs. You could of course ship all the materials out at once, but then why wouldn't you just do all the repairs at once when the laborers are out there? Also, the existing rebar, although almost all of it is still in place and appears to be working, is pretty heavily corroded and likely won't last a lot longer so then we're back to the same question: do we use the old design or do we come up with something new.

It would certainly be an interesting research project for some of the permafrost geotechites around here, but probably not enough time to figure out something useful for this project.

 

[zelgar]

Here's a pdf about installing piles in permafrost:

http://www.google.com/url?sa=t&rct=...ign&source=web&cd=3&ved=0CDgQFjAC&url=http://

http://www.uaf.edu/ces/energy/housi...-oD4CA&usg=AFQjCNFxfdPLphaxvyGhjVN8Myg6abjTpQ

Also, here's a link to a Snow fence guide for Illinois DOT website:

http://www.dot.state.il.us/blr/l002.pdf

 

[SKIAK]

I have seen several of the manuals produced by the Permafrost Technology Foundation and they are usually very good. I have quite a bit of experience designing in permafrost conditions. To get your PE in Alaska you have to pass a class in Arctic Engineering which coveres all these topics. (that wasn't meant to sound meant to sound elitest, just that I have a pretty good understanding of permafrost).

It's interesting to me that the link from the Illinois DOT suggests a similar type of anchorage for the fence. I didn't see where it shows any design data to determine the depth of embedment of the rebar which further suggests to me that this more of a "tried and true" method.

Thanks for the links!

 

[ukengineer58]

my last comment. i assume your being employed to provide a proffesional design service. if you cannot verify it i suggest you do somethinh you can. any fool can copy the old drawing

 

[SKIAK]

Is it foolish to copy a design that appears to have worked satisfactorily for 20 years? Or is it foolish to create a new design that costs roughly 10x more to “ensure” it works? (just playing devil’s advocate)

Again, this is a fence out in the middle of nowhere… literally. I wouldn’t consider doing anything like this if I felt in any way that it endangered anybody.

Either way, I appreciate the input.

 

[SNORGY]

SKIAK:
You're a class act, man. (gender neutral "man").

 

[truckandbus]

@Kenat - the 'repair' solution is the elegant solution to this problem.

@OP - no, you don't release designs that you don't understand the principles behind.

 

[SKIAK]

I considered the repair option but I get back to the same situation because the rebar would likely have to be “repaired” (replaced) since a lot of it is rusting out. Again I am in the situation where I am putting that detail on my drawings and signing off on them. Is that any different?

I guess I take exception with the comment that I "don't understand the principles behind" the anchor. How many engineers understand load transfer between the threads of a nut and bolt and can calculate that the threads are capable of transferring the required loads? I would say that we all (okay, most of us?) understand the principles behind that load transfer, but not many are capable of the actual computation. A lot of the designs we use rely on testing, how about any of those post installed concrete anchors? Who can calculate those pullout values? I think we still understand the principles behind them.

There are 246 existing snow fences, with (6) rebar anchors each that appear to have successfully withstood 20 freeze-thaw cycles, 246 x 6 x 20 = 29,520 test cycles of this design. That’s probably not really fair since none of you had that information, but that’s getting a little more specific than I intended. Or maybe it doesn’t change anybody's opinion anyways?

 

[ukengineer58]

Suppose it comes down to how confident you are the worst case loading required has been applied together with the weakest support conditions

 

[SKIAK]

Yeah, that's the one thing I can't be sure of. I haven't yet looked into finding information about the wind speeds experienced over the last 20 years compared to design speeds. It could certainly be possible to experience a higher wind speed in the next 20 years and exceed the loading previously experienced… but I suppose if that happened I could just blame it on climate change!