Using a Previous Design
Using a Previous Design
(OP)
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?
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?





RE: Using a Previous Design
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Then you may get some comments that help erase the mystery.
RE: Using a Previous Design
The question is just basically: would you, personally, sign off on something without being able to verify the adequacy if it had been in place and apparently working satisfactorily for 20 years?
Obviously there are holes and technicalities in the question, like seismic design and extreme design events like that related to structures that might not have happened in the 20 years, there are probably other long term concerns in other disciplines that I’m not familiar with, but you are of course welcome to include that in your opinion.
Considering the additional cost and low importance of this fence I’m inclined to say it’s okay, I have also mentioned this situation to the owner, but the only response I get from anybody is a shrug and “if you want to.” As a newer PE I was just curious if anybody else had done something similar, or has chosen not to do something similar. Sorry if that’s a long explanation.
RE: Using a Previous Design
Maybe the fence was designed for lower loads. Maybe reverse engineer and see what wind your fence is capable of carrying.
Similarly, maybe the original designer used a lower safety factor. I've seen it done for continuous systems like this where the overstressing of one anchor doesn't mean the whole fence will overturn.
Also, I've never used soil anchors, but I assume they depend highly on type of soil, compaction, etc. Sounds like you are removing the old anchors. Won't this disturb the soil and require compaction in a "remote" area?
RE: Using a Previous Design
Dik
RE: Using a Previous Design
RE: Using a Previous Design
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Using a Previous Design
The original design was a #6 x 8’-0” rebar hammered into the ground and attached to a snow fence with a “U” clip that is installed in western Alaska. The problem really comes in when considering that this is going into permafrost. A standard driven pile would likely have to be embedded around 20’-0” into the ground just to prevent frost jacking considering the depth of the active layer. I can conceptually understand how the rebar is able to resist uplift but I have no references to understand how this will behave in frozen grounds. To test this I would have to wait until next fall to install the rebar when the ground is thawed and then wait the entire winter to see if it jacks out of the ground, but considering that these have been already installed for 20 years and appear to be working I didn’t think that was necessary. My question arose because it seemed like people around me said it wasn’t good enough to stamp something unless you can calculate it, which I didn’t think was necessary since they seem to have been working but I couldn’t find anybody that agreed and was just looking for some more input.
RE: Using a Previous Design
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RE: Using a Previous Design
I am capable of designing several types of anchors in permafrost, I am not capable of determining whether or not the specific anchor used on the previous design is adequate. Regardless of my capabilities, I have several engineers involved on the project that are helping me to address that issue. We have several options for other types of anchors but they are much more expensive. I am okay with specifying a more expensive anchor but I would like to think there was a justification for that expense.
My question has to do with the ethical implications of signing off on a design you can't numerically validate. It was just meant in a general sense.
RE: Using a Previous Design
-Tony Staples
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RE: Using a Previous Design
Dik
RE: Using a Previous Design
RE: Using a Previous Design
It only matters if it fails and you are sued.
At that point :
I copied something that worked before, even though I didn't understand it.
I didn't anticipate the conditions that led to the failure.
I made an error in my calculation.
Are effectively your defense options. There's no real difference in outcome is there?
RE: Using a Previous Design
Who provided the professional endorsement of this design in the previous twenty years? Perhaps that (those) engineer (engineers) will be able to provide guidance with respect to the design parameters involved. Otherwise, I believe you would be in a position that, if professional endorsement of the design is required, some supporting calculations are probably also necessary to have on file.
How tall is the fence and what wind loads will be imposed? From that you can probably calculate a moment about the anchorage point; in combination with knowledge of the cable tension and the estimated force required to pull the rebar out of the ground, this might boil down to something that is adequately addressed by a calculation involving first or second year statics, and that might be good enough to put a stamp to. But I think at the very least, that level of analysis is warranted before stamping.
RE: Using a Previous Design
I don't know where I got the notion of a cable from...
If it is just a vertical pile with a fence clipped to the top, oscillating loads will eventually loosen the rebar. I wouldn't stamp this.
RE: Using a Previous Design
The area is almost completely free of people, the only likely outcome of a rebar anchor failure is a section of fence would tip over and the snow fence would not function as intended. Although a lawsuit would certainly be possible, I don't think that is a likely outcome. Of course if it did go to litigation I would be very susceptible and have little defense.
It surprised me that almost all of the engineers I have talked to around here (structural and geotechnical) seemed very obstinate about not using similar rebar anchors and seemed completely unwilling to consider that they have been (apparently) performing sucessfully for the past 20 years. I would not consider this if it was a life-safety issue, but I don't see it that way. I was just wondering if anybody had used a similar argument to back up any designs that they have stamped.
Anyways, most of the people I've talked to seem against it. I think I will concede and design new anchors at the higher cost. Once the estimates come in we will see if the client will want to revisit the previous anchors.
I appreciate the input.
RE: Using a Previous Design
The correct answer is NO.
As a licensed engineer you have an obligation to provide for the safety and welfare of the public.
Even if something worked for 20 years and yet you had no idea how it works, or you knew from calculations that it didn't provide adequate safety against failure, then you shouldn't sign off on a new design of that same assembly or structure.
I think your decision to design a new anchor system is exactly what a good engineer will do - think, and think some more, and come up with a solution that works and is safe.
RE: Using a Previous Design
Frost jacking requires freeze-thaw cycles.
Permafrost never thaws.
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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.
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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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RE: Using a Previous Design
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.
RE: Using a Previous Design
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".
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Using a Previous Design
Back in aero defense our stress man would occasionally clear things by analogy, of course he wasn't a PE though.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Using a Previous Design
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.
RE: Using a Previous Design
Also, here's a link to a Snow fence guide for Illinois DOT website: http://www.dot.state.il.us/blr/l002.pdf
RE: Using a Previous Design
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!
RE: Using a Previous Design
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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.
RE: Using a Previous Design
You're a class act, man. (gender neutral "man").
RE: Using a Previous Design
@OP - no, you don't release designs that you don't understand the principles behind.
RE: Using a Previous Design
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?
RE: Using a Previous Design
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We all use codes and standards and cookbooks to do our work, but between education and experience they need to be applied correctly
RE: Using a Previous Design
So, since this is cost driven, is it worth inspecting all or some of the units in the field and checking that they have all performed as expected?
Cheers
Greg Locock
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RE: Using a Previous Design
Once that was done, one could go on to quantify a theoretical maximum wind speed, a safety factor, etc. until one obtained an "acceptability envelope".
If the results were acceptable, then certainly that would be good enough to stamp, would it not?
In the absence of calculations or proof of design acceptability by others, I would view it as incumbent upon myself to apply my own analysis, as right or wrong as that analysis might be, before considering stamping it. The other way to look at it would be, "OK, so if I have a fence that conceptually looks like this, how would I design it and back it up with calculations before I would be prepared to call it my own?".
The active word there is "own", because once you stamp it, you own it.
RE: Using a Previous Design