Define "Nominal"...Am I right or wrong???

[65BCUDA]

I am wondering if I'm being too **** with a contractor.
My position is a Resident Engineer overseeing FAA projects.
Project: .5 mile pier into a river

Issue: Shop drawings that were submitted for the 1 mile of handrail note a 0.25 nominall exspansion joint between sections of railing. Accomplished with a slip fit joint. Two months ago when the handrail install began I brought to the PM's attention the fact that the joints were not consistant from the top rail to the bottom rail. Some were 0.25 at top, .50 -.75 at bottom or vise versa. This was approximatly 3 sections into the install. I was pretty much told to kick rocks and go pound sand.

The fix would have been to test fit sections and remove with a port-a-band the offending material. Instead he let his crews "stack" the problem. Now 1,800 feet into the problem he wants to argue the definition of "nominal". I am allowing him up to .375 which constitutes a 50% "nominal" margin. The fix now is to remove 1.5" or better of material and weld in a correct sized piece to better close the gaps = time=$$$

If he would have addressed the issue 2 months ago I would have been more apt to forgive further into the project.

Pride in workmanship is a dead art.

So am I right.....or just a pr#ck??? Be honest I can take it.

 

[38Chevy454]

How long of length is the slip fit section? That may determine whether the extra gap is really a problem or just cosmetic. I do agree that your tolerance of 50% of the nominal is pretty fair. The real issue is that the allowable gap is not specified correctly in the contract or bid package, you and the contractor have different definitions.

The slip fit has to handle some load amount. If the current overlap is sufficient, it is only a cosmetic appearance problem. If the overlap is not enough and the pieces could separate in extreme loading, that is a safety issue.

 

[65BCUDA]

38chevy.....I do agree as to it being cosmetic, at the same time there are spec's to the cosmetic's of the product......btw ...4" overlap in the joint.

Side note : Been a problematic contractor from the start....has not met any critical milestone dates.

 

[MattT]

Shop drawings that were submitted for the 1 mile of handrail note a 0.25 nominall exspansion joint between sections of railing.

Do the drawings have a general tolerance on them?

I'd say "nominal" for construction should probably look about 1/4". Not obviously more or less to an untrained eye which I guess would be +/- 1/16". A lot of us in the mechanical trades could eyeball it way closer than that but I don't think it would be fair to apply that standard to construction.

Not sure whether the gaps have grown enough to cause a problem. If not you're maybe being a bit ****. That said the contractor is an idiot for not spending a few minutes with a portaband to shut you up. The fix is so quick and easy it isn't worth upsetting an engineer over.

 

[Stephenw]

I work for the FAA.

In my experience, there is a nominal value. That is the ideal or preferred value.

A tolerance is usually specified, i.e. nominal value ± some number.

Both you and the contractor should have been provided a set of FAA standards relevant to the project.

 

[65BCUDA]

Stephen......FAA standards have been provided....the railing was originaly designed as a continuosly welded item as to provide a continuos grounding guard (as per 19e) for the ALSF-2 lighting system mounted on the pier. The design change came about to address exspansion and contraction issues due to weather changes.This was evidently over looked, not clarified, by the design engineer when the submittal was approved and is in the RFI proccess as of today.
As you well know cosmetic issues are a concern with the client as well as structural integrity.
I have an FAA RE to answer to as well.

As stated before I my concerns were ignored on what would have been as easy fix if attended to early on.
I have always approached these projects with a "help me help you " philosophy.

 

[aleccolin]

In my experience, there is a nominal value. That is the ideal or preferred value. A tolerance is usually specified, i.e. nominal value ± some number.

This is correct. As a contractor I deal with this sort of thing all the time, though typically it's not the engineer being told to pound sand. If the joints exceed the defined tolerance the deficiency should be documented in a non-compliance report; payment can then be held against the non-compliance until resolved by the contractor.

If the tolerance is not defined, then it's more of a fight, but a good contractor will do their due diligence to get answers to issues like this before the work is done. Banking on the issue being forgotten or forgiven is a reckless strategy.

 

[MD11]

now it all makes sense....

 

[Torque1st]

It sounds like the contractor is also using the 'nominal' gap to cover slope changes in the installation. I agree with Stephenw's definitions. It also sounds like bonding between the sections would be more of a concern. I think the contractor is right and nobody will notice except you.

 

[Charles (in GA)]

I work for the FAA.

In my experience, there is a nominal value. That is the ideal or preferred value.

A tolerance is usually specified, i.e. nominal value ± some number.

Both you and the contractor should have been provided a set of FAA standards relevant to the project.

I agree (dang, I never thought I'd agree with the FAA on anything ) To have a nominal number, there needs to be a minimum and a maximum, the nominal being the desired number. Sometimes the nominal is dead in the middle of the tolerances, sometimes it is toward one end or the other of the specs.

.250" +/- .125" (ie, 1/8" to 3/8" with the middle being preferred)

.250" +.125/-0.0 (ie. 1/4" to 3/8" with 1/4" being preferred)

Pretty tight tolerances for a railing on a pier with slip couplings over the joints, if you ask me.

Charles

 

[TommyK]

Allow me to play the scumbag contractor for a moment:

Your the same guy who would be complaining in August when the rail binds up and deforms because the "nominal" clearance wasn't adequate even though the shop drawing was approved, right?

There are compliance issues which are critical to achieving design/engineering intent that must be enforced and then there is what we call ball busting.

 

[nehog]

Definition: Existing in name only.

So when they say .25 inch nominal gap, then that is the gap they'd like, but it could be almost anything (which is what you are getting!)

I am allowing him up to .375 which constitutes a 50% "nominal" margin.

IMHO, there is no such thing as 50% of 'nominal'. It does sound like the sections were poorly made, or are somehow otherwise flawed (bent?) but I don't (can't) visualize exactly the shape of the sections (maybe a picture would help?)

 

[demographic]

Anywhere I have worked with tolerances has had an ideal then a plus or minus percentage or a maximum and minimum gap.

Otherwise how the hell do you work it out? Right pain in the neck.

 

[Falcon67]

Offer to pay the contractor with a similarly precise "nominal amount", reflective of their precision in construction.

 

[steven083008]

Offer to pay the contractor with a similarly precise "nominal amount", reflective of their precision in construction.

I wish that is how things worked! I'd love to see people actually held accountable for the work they turn out.

As for the railings... it should be really simple... do the railings match the tolerances on the drawings? If they do, then the contractor is okay, and if they don't, then he needs to fix it. If there aren't tolerances on the drawings, then he is completely off the hook.
I've never seen an entire drawing package that didn't have tolerances however. If it isn't on the actually drawing for the railings, it should be in there somewhere.

 

[Torque1st]

I've never seen an entire drawing package that didn't have tolerances however. If it isn't on the actually drawing for the railings, it should be in there somewhere.

Usually the general job tolerances are superseded when called out on a drawing. In this case I would read that 'nominal gap' to supersede the general tolerance. I would also read it as a cumulative gap. If one section was only 1/8" then a 3/8" gap nearby would make up for it. There was no specification for maximum gap or cosmetic issues. It basically adds up to 'design intent' which in this case means adequate gap to prevent buckling due to thermal expansion.

 

[steven083008]

Unless "nominal gap" is defined somewhere, it can't supersede a stated job tolerance. Putting the word "nominal" after something is only viable when referring to something like u-bolts or pipes where the actual size is different than the commonly referred to size (i.e. 1" pipe being 1.315".)

If it is called out as "1/4" nominal gap" and the job tolerances are +/-1/4" then the gap can be 0"- 1/2".
Even though nominal is used in general conversation to mean "roughly", this isn't implied by putting it on drawings.

 

[NitroPress]

Unless "nominal gap" is defined somewhere, it can't supersede a stated job tolerance. Putting the word "nominal" after something is only viable when referring to something like u-bolts or pipes where the actual size is different than the commonly referred to size (i.e. 1" pipe being 1.315".)

Precisely. As said above, "nominal" means "in name" - so a spec of "1/4 inch nominal gap" is semantically null and invalid as an engineering spec. The only part that means anything is "1/4 inch"; if you wanted exactly that gap, it should have said so; if there's an acceptable installation range, it should have been specified as a tolerance.

Expecting a contractor to build the railing with gaps that were 1/4 inch plus or minus some tiny but arbitrary amount is faulty logic on your part.

 

[65BCUDA]

Thanks for the input guys,

I agree the design engineer (employed by the client) definitely dropped the ball when the shops were submitted and they approved them with out a set +/- tolerance.

Still awaiting the RFI response to see how the dust settles.

I guess 1/4" nominal could mean any thing from **** > **** ( No exspansion capability) to 1" ( which looks like ***). The biggest problem is the contractor is not consistant with the joints, some are 1/4",1/2', 3/4" and not all the same top rail to lower rail at the same location. Again poor craftsmanship. A simple shim / gauge during install would have made a world of difference.Their railing supplier has done them no great deed.

From now on when I see nominal on a print a I'll read it as "get clarifacation"

As a side note : I came up out of the field, been on the other side of the print table, always went the extra mile to be able to walk away slow without having to look over my shoulder.

 

[1948]

im confused as to where this gap is? you have some sort of joint made out of pipe that these other pipes slide into? the pipe doesnt come out past said joint that its inside? but you know its short on the inside? im lost..... are these welded? screwed in? please post a picture of what youre talking about if you got one.

 

[65BCUDA]

Slip joint, interior pipe welded into one section, slip fit into the next to allow for expansion/contraction. These are at the same location, top rail, bottom rail.

The inconsistencies vary top to bottom > bottom to top, some joints are fine.
The prefabbed sections come in 12-14' sections.
How hard would it be to test fit, decide what needs to be removed, port-a-band off the offending material, quick shot of cold galv and install.

A consistant 1/2" would have been better than a hit and miss result.

 

[Torque1st]

I still read the 1/4" nominal as an average gap to prevent buckling due to thermal expansion.

Unfortunately grinding, cutting, or adjusting those gaps will ruin the nice powdercoat and galvanizing. If those protective coatings are disturbed rust will show up quickly. If the railing is shimmed upward to correct for a grade change then the uneven gaps may appear at another point.

Some gray paint may help the gap appearance.

 

[ducktapeguy]

In this case, I would say the contractor is right. Nominal is a vague term that can be interpreted differently. In some cases you can almost translate it to mean "roughly" or "approximately".

If the designer intended to have a 1/4" gap, then it should have been stated on the drawing with a toleranced dimension. Otherwise "nominal 1/4" can mean "whatever it ends up being as long as it works", which is probably how the contractor interpreted it. It may look like ****, but since it wasn't specified you really don't have a case.

I'm not sure what the design looks like, but this could have been avoided if the slip fit pipe was on the outside (like on top of one of the upright supports), so that any variation in the gap would be hidden

 

[65BCUDA]

Torque, I wish this was the only issue.
Unfortunately the contractor has dozens more cut, grind and welding areas to attend to due to the railing being manufactured with short and long flags that meet up to other sections at a 90* angle to adjust for the required 4" max between sections. Some areas are so badly bowed from welding draw up heat that they need to cut the welds to straighten the railing.
Any and all cutting , grinding activities require a 2 coat application of high zinc powder containing coating, followed by a compatible primer, then the reapplication of the high performance epoxy coating.

The previously mentioned cold galv application would be a temporary rust inhibitor to prevent flash rust until the proper coatings are applied.

The referenced shim was meant only as a spacer gauge to set the gap to.

Yes I will have to agree the contractor has been handed an out to practice poor craftsmanship.
We will see in the a.m. when the RFI response is obtained from the design engineer.

 

[Toomanytools?]

Well right wrong or what that looks like $*%! 1/2" gap for a handrail. Good luck.

 

[onewaydave]

I have nothing to add to the question under discussion except, to the poser of a question, the details always matter more than to anyone else. I have been accused of pickin nits when I was absolutely right. It is almost inpossible to bring others to your table.

This is as honorable as one can get:

I came up out of the field, been on the other side of the print table, always went the extra mile to be able to walk away slow without having to look over my shoulder.

But I am sure enjoying this discussion.

Dave.

 

[65BCUDA]

Thanks Dave...............

Contractually I may be wrong....

But I know wrong when I see it.......

 

[Torque1st]

The referenced shim was meant only as a spacer gauge to set the gap to.

Yes I will have to agree the contractor has been handed an out to practice poor craftsmanship.

I am talking about a shim to raise or lower the railing to equalize the gaps.

And yes I know all about cold galvanizing compounds which are not nearly as effective as actual galvanizing. Anything that disturbs the manufactured coatings will be a maintenance headache for the life of the project.

Unfortunately the design engineer could have picked better ways of expressing his intentions. At this point to get an aesthetically pleasing gap appearance will be expensive and the contractor will probably have to be paid extra for a change order. The least expensive option may be the gray paint.

The welding problems are due to substandard welding or 'operator error' and are avoidable if proper welding practices are followed. The contractor should fire his 'welder' and get qualified help.

 

[Shadowdog500]

Sounds like you got a **** contractor that just wants to get done and get paid. I don't know if it is too late to have him stop work on the current project, but definitely scratch him off your list for the next project. Talk to your contact in the FAA, it is just a handrail for their techs to service the approach lighting. If it meets safety standards they may not care how the gaps look this late in the game, especially if it will cost extra time and money to correct. I'm sure the next crew already has the light install dates pencilled in.

I'm figuring a quick way to clean them up would be to temporarily put some sort of non marring pipe jaws on each end of a cheap HF chop saw, then stack two metal cutting blades with a spacer between the blades so that the outside edges are the distance you want. You will have to cut into the bed with one of the blades.

Clamp, chop, next.


Chris

 

[pop pop]

I'm retired now. Registered PE. I've done your job many times. I never had a contractor tell me to pound sand the second time. That's all I'll say. Do your job. Make friends after work.

 

[NitroPress]

Slip joint, interior pipe welded into one section, slip fit into the next to allow for expansion/contraction. These are at the same location, top rail, bottom rail.

IMHO, that's a terrible design. I don't think I've ever seen a handrail with expansion/flex joints over an interior collar, especially with black powdercoat over a bare interior where the difference would show so dramatically.

Getting a consistent gap over 12-14 foot sections, especially with the loose nature of the rail, would be difficult in the first place and even more difficult to maintain over time.

 

[steven083008]

I would love to see the drawing for the hand rails. Although a gap may not have been accurately called out, it would be one sloppy drawing if the lengths of the top and bottom pipes weren't called out and I would imagine the location of those pipes share the same dimension. If this is the case, then the fabricator is 100% wrong in his construction. You guys may not have individual fab drawings for each piece on the job, but if there is, I would be shocked if the blame for the varying gaps (between upper and lower on the same weldment) doesn't fall squarely on the fabricator.

Below is an example of a railing drawing we did recently for a job. This is only the second time we have done railings, so this drawing still isn't perfect, but it does prevent the ends from not lining up (even if there weren't piece connecting to the rails.) The '2X' and '4X' on the vertical and horizontal length show that not only do they have to be the same length, but they share the same locating dimensions. These could be off by the +/-1/8" tolerance that is shown in the title block, and that is it.

 

[Jazz]

It's a nominal dimension given as .25 and not as 1/4" I'm guessing it should be tighter than what you're saying. Often an engineering standard can be referenced or the drawing border can give a reference for nominal dimensional tolerances. Personally I'd say if you can see a difference without even measuring it, then it's probably NOT within the nominal tolerances. 1/2" is not nominally 1/4".