US Customary vs. metric units 3

[HgTX]

What's the common wisdom for dealing with a project designed in the US, using feet, inches, and fractions of inches, but to be fabricated elsewhere in the rest of the world where a decimal system is used? What is likely to lead to the least error--translating all the shop drawings into metric ones, or trying to teach the workers how to deal with the arcane US system? (Let's skip the first obvious solution, teaching the wisdom of the SI to the arcane US designers.)

Hg

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[PMR06]

Easy there HgTX... Here's the first obvious solution from an arcane US designer. If you know or suspect your jobs will be sent international, have your arcane drafter create a custom dimension style that labels in both english and SI units.

 

[youngstructural]

Hello Hg;

I'll first put my answer in context: I am a Canadian, born, raised and trained. We are forced by the wisdom of our neighbours to be fluent in the Imperial system (we never adopted the US version), and by Canadian law to be fluent in the Metric system. We're probably the only country where assignments in virtually every course through an engineering degree are 50% metric, 50% imperial. I'm not kidding...

When I design something, my frist decision is Metric or Imperial, and then everything follows. Occationally there is cause to change the units on a set of drawings, and I WILL NOT do this by hand. I finalize the drawings and specifications in CAD and then have the computer convert. I only do a check by hand. I DO NOT round or "clean up" the resulting units. 1/2" becomes 12.7mm and stays that way. It's a pain for the shops and on site, but in Canada they are fairly used to it. They complain, but get the job done correctly.

Going overseas is a unique problem for the US system. I live and work in Christchurch, New Zealand now, and the young engineers I work alongside do not know anything about the system. They have an understanding of feet and inches (a great deal of the world still uses them for height), but NONE for forces, loads or stresses of any kind. Slugs are bugs here, nothing more.

If you want my advice, bearing in mind the above, you should find out if the destination country has a recent imperial tradition (30year or less) or made the change to Metric early. If they made the transition early, you will HAVE to change. There just won't be any capacity for working with your plans and specs in that country. Otherwise, leave them and deal with issues slowly and carefully, one by one.

Either way, you must CLEARLY label your drawings as imperial. The rest of the world doesn't even bother marking a unit on drawings... It's all automatically mm, unless otherwise indicated. So otherwise indicate. Everywhere.

One more thing: Talk to a construction lawyer in the destination country. You may not actually have a choice: It may well be illegal (for good reason!) to issue drawings in any system other than Metric. That's actually fairly common around the world...

Good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[HgTX]

I myself am from the U.S. but spent a formative part of my childhood in The Rest Of The World and had to learn the US system at a later age than most. I hated it then and I hate it now. 'Course, I've also lost most of my ability to think in metric units, so all I have left is self-loathing...

U.S. engineering classes also give the homework problems about 50/50. At least they did a few years ago. On the other hand, the metrication movement for civil engineering projects seems to have died a thorough death; a number of standards that had briefly been issued in metric versions have now reverted.

I like the idea of having the CAD program do the conversion at the end after everything is done. That sounds like the safest way to go.

Hg

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[PEinc]

Solving the "problem" isn't as simple as just converting from Imperial to Metric. You need to consider hard vs. soft conversion and you need to consider the available manufactured products. For example, if someone converts a 2"x6" wood wall stud to 50.8mm x 154.2mm, the available wood product in another country may not be exactly the same size. After all, a 2"x6" isn't even 2"x6". Soon the structure will not be able to be built to the originally designed size. There will be many problems of proper fit. You need to design for the products and structural members that are available for the project.

 

[Beggar]

I've done some machine and fixture design to be built in Europe and Asia. In both cases, I found it safest to do the design using metric dimensions from the outset. I obtained a list of preferred dimensions for structural steel, machine tools, etc from contacts in the host country.

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[HgTX]

For after-the-fact conversion, youngstructural's advice for soft conversion seems the only way to go. If designing using metric to begin with were an option, I wouldn't be here asking the question. Obviously conversion after the fact is not an optimal way to go. It's not an unreasonable assumption that the project was to have been fabricated domestically. In my industry, the design is completed long before the fabrication contract is awarded so there's no designing with fabrication country in mind; the only thing we know is where the structure will be erected, which is in the U.s.

I'm thinking about steel projects primarily. Steel can be cut to any length. Plate thicknesses might be touchier, but I believe non-U.S. steel products are already used for domestically fabricated projects with U.S. customary units as it is; the U.S. market is big enough for the mills to bother with our silly units.

Hg

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[Rentapen]

Rentapen (mechanical)
Rentapen (product designer)
Rentapen (floor sweeper)
Until all the machine shops and factories in the USA start using new machine tools with metric readouts etc. the USA will use inch. I can not design a special machine or product that will be made in the USA with pre 70's equipment in metric. Now if the job will be made in Japan or Germany or in a shop with all new CNC centers then yes I'll do it in metric, it isn't hard, inch metric soft-inch hard-inch whatever. Its the guys who's finally using it. I am not going to go metric and sent it to another 50 year old using a 60's knee-port to make my detail if he has to figure out dim's.
chow or lunch or whatever;-)
just wandering
rentapen

 

[DaveVikingPE]

Soft metric is just about the only way to go in the US. For sheet piling, most of what I've designed used the Arbed (now "Arcelor") sections. Arcelor is a company from Luxumbourg and that's where they do their fabrication. Their sheet piling manual has metric and Imperial dimensions and 100% of the time I simply go with Imperial. However, the designations, for example an AZ18, are keyed to metric; the "18", corresponds to an eleastic section modulus of 1800 cm^3/m - though I'd use 33.5 in^3/ft...

The real pain in the a.. regarding metric comes when the contractor's shop drawings are submitted. That slick, metric design you did, especially those striking, crisp plans, including the notes with concrete strength in MPa, and the steel beams with designations like "W840 X 527," not to mention having all your measurements in mm... is rendered null and void because the contractor converted everything to Imperial/US units and thumbs his nose at your metric system... And the Project Manager won't side with you on getting the contractor to submit in metric because it would take too much time, despite what the contract might say.

My advice: create two sets of plans, one in metric and one in Imperial/US. This isn't hard to do at all: set up two dimension levels in the CAD files, one for metric, one for US. If you send out the metric set and get back a bunch of shop drawings in US, you'll be able to rapidly check the shops AND you'll have time to argue with the PM about dining the contractor for not following the contract.

 

[HgTX]

Two dimension levels? Sounds even better that just converting. Thanks!

Hg


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[TpaRAF]

The dimensional issues have been discussed throughly; what about the stresses and strengths? I'm not so sure about the dual-plans approach. I would stick to dual units on one plan for measures and capacities.

If you're the engineer, shop drawings have to be prepared to your satisfaction. It might not go quickly, but it's not going anywhere without your approval.

My own experience has been like a pendulum. For about 8 years, Florida adopted the metric system for FDOT work. I even worked on a few, very s-l-o-w projects that started in imperial units, were converted to metric and then had to be converted BACK to imperial units.

Personally, I found the metric system more rational to use. Either is fine; it's the conversions that cause problems. (Just ask JPL re: Martian orbiters). Using software like MathCAD might help you double-check your conversions.

Good luck with your project! RAF

 

[HgTX]

No stresses noted on the shop drawings; it's really only going to come down to dimensions. They should probably put dual designations for the material grades; that would avoid any confusion, not that I anticipate any on that score (it's not like someone can accidentally order 345 ksi material instead of 50 ksi or vice versa).

Hg

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[BigH]

HgTX - actually many ASTM and AASHTO specifications do have the dual designations (say M31M31M for steel rebar in AASHTO). Then, all you need to do is follow the one in use. I've had a lot of problems before when the specifications call for ASTM strengths and the steel is all coming from Thailand. Their SD40 is slightly under the 40ksi of ASTM. Most SD40 passes but the odd lot will be under (less than 1%).