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]

If you can understand conceptually how it works then you can quantify how it works and therefore you can analyse it.

 

[MiketheEngineer]

It's your seal and it's your call. If you can somehow verify it will work....

 

[bootlegend]

Maybe a sketch would help?

Then you may get some comments that help erase the mystery.

 

[SKIAK]

I understand the question seems vague but I purposefully avoided getting into any specifics about the design just because I feel that I’ve exhausted all options of coming up with a way to calculate the anchor capacity. The situation seems fairly unique and I didn’t want to provide details that would make the project obvious to anybody familiar with the specific technical issues (or maybe I’m just paranoid?). The anchor design isn’t revolutionary in that it uses soil to resist uplift forces, but the way that the anchor engages the soil isn’t very well known since this object is rarely, if ever, used for this purpose. It was more of a “this is readily available, light and compact, and cheap to throw on a barge.” I suspect little calculation was done with this anchor and was perhaps something that this engineer had previous experience with but I have not been able to contact the previous engineer. I’ve looked in all kinds of books, Google searches, literature searches, emailed everybody I know who might know something about this and have come up nothing.

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.

 

[bootlegend]

You've probably considered all of these, but here are some thoughts.

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?

 

[dik]

A sketch may help... we may have encountered one of these 'critters' before...

Dik

 

[ukengineer58]

I person sly would not if I could not verify. Recently in UK a wall collapsed and killed a three year as she passed as ground was being back filled behind it. Unlikely yes. Put that on the gravestone. Would you want it on your concience

 

[GregLocock]

I must confess the OP's attitude strikes me as so cagey I wasn't inclined to help, but for goodness sake, get one of these magic sekrit things, install it in the ground, and pull it over with a tractor. repeat with a normal system. You have now validated the fence post. Total cost, installation of two fence posts and hire of a tractor and load cell for one day. Sheesh.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[SKIAK]

Sorry for appearing cagey, there were two reasons I didn’t post the specifics of the situation: I didn’t want to incriminate anybody and I didn’t want to discuss the technical aspects of the situation here. Maybe it would be hard to incriminate anybody from this site, but just wasn’t sure and didn’t think it was really necessary.

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.

 

[MintJulep]

Google "anchors for permafrost"

About 1,990,000 results (0.39 seconds)

 

[SKIAK]

I feel like I haven't done a good job of communicating my question...

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.

 

[TStaples]

If you can clearly use the existing anchors installation with 20 years of field service as the test data that qualifies the design, and 20 years of service meets the design criteria for the new installation, then document it that way, stamp the drawings and move along. I see no ethical issue with that. Quite frankly, I prefer calculations + testing + field data. However, field data is always trump as it literally is as close as you can get to real world testing!

-Tony Staples

http://www.tscombustion.com

 

[dik]

What is the consequence of the fence failing? Can you design it for 5 or 10 psf or whatever, depending on the fence type? and the spacing? You can calculate the moment resistance of the rebar using the plastic section, and can ignore deflection?, seems to me like you can put some real numbers to it and show that it works or doesn't work.

Dik

 

[TenPenny]

I must admit, I'm intrigued by the idea of several engineers designing a snow fence that seems to be built from an 8ft rebar driven into the ground...sounds rather complex.

 

[MintJulep]

OK, to answer the question.

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?

 

[SNORGY]

Given your situation, without knowing anything else, I would be reluctant to provide professional endorsement just yet.

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.

 

[SNORGY]

Actually, upon reflection...

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.

 

[SKIAK]

Okay, well, maybe I should have just done this sooner but here is a quick sketch of the fence. The fence supports are made up of 2x6's built into a triangle shape. I have data to obtain wind speeds and wind pressures. I can do the wood design. With statics it is very easy to resolve the forces and obtain an uplift reaction. The fence is attached to the rebar just a few inches above the ground surface, so the bending of the rebar ends up to be more-or-less negligible. The problem that I have is I am unable to determine the uplift capacity of the rebar. There are some things that complicate the calculation, like trying to determine a frost jacking force when the active layer freezes, that we just don't have good information on.

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.

 

[JAE]

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?

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.