Working in Metric 8

[DaveAtkins]

For the first time in my career, I will be working on a project which requires the use of metric (hard metric at that!) units.

I am curious about the experience of those who normally work with imperial units like I do. My hunch is I should not try to convert units back and forth, but instead dive right in and do calculations and drawings in metric. Train myself to think in metric, so to speak.

DaveAtkins

 

[IDS]

The "Convert" function in Excel seems to have a seemingly arbitrary set of conversions. Certainly, furlongs/fortnite isn't available, but others, like cubic centimeters or even kilograms aren't available, while cubic picas and Admiralty knots are.

It seems that for each unit type Excel Convert offers just one base SI unit, so m3 (orm^3), or g, but I agree, for practical use it is pretty hopeless.

For those who want to do unit conversion in Excel, my EvalU spreadsheet will do unit aware calculations, with a wide range of units, and you can add more if you want to.

If working to a metric code I would recommend working in metric units, then review the final design in imperial.

Apart from anything else, there are many code specified values that have inconsistent units, such as tensile strength etc.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IRstuff]

minor nitnoid, the gram is not a base SI unit, it's the kilogram. The gram is a base unit of the CGS system.

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[IDS]

IRstuff - quite right. I knew there was something a bit strange about having g as the base unit.

DaveAtkins- What is "hard" metric by the way?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

Since this is the structural forum, 271828's caution about developing an innate sense as to whether a number is reasonable or not means consideration of the main units we use. In the US, you use lb/ft^2 for loading, while in the SI world, we use kPa. For stress, you use psi or ksi, we use MPa. For bending moment, you use kip-ft (or ft-kip), we use kNm. For force, you use kips, we use kN. It takes a while to develop the innate feel he is talking about.

 

[Lomarandil]

I recently moved to a metric country. SMath is my best friend -- I have it open constantly. Easy to do calculations in either units, both side-by-side for gut checks, or even mixed units when I'm in a rush and don't want to look up a constant that I know in the other units (eg unit weight of concrete).

Mathcad prime is fine also, but I got tired of them breaking compatibility with each release.

Talking about calibrating our mindsets -- it's still weird to me that a kN is much less than a kip... But a kNm and kip-ft aren't that different.

----
The name is a long story -- just call me Lo.

 

[DaveAtkins]

With soft metric, a 1/2" diameter bar is called a 13 mm bar, even though it is really 12.7 mm diameter.

With hard metric, a 13 mm bar would actually be 13 mm in diameter. So conversions to imperial would not be exact.

DaveAtkins

 

[IRstuff]

Another advantage of SMath and Mathcad is the ability to define convenience units, like microflicks, useful in hyperspectral work, and my favorites, furlong, or fortnight, although the more recent versions came with them already defined. I suppose that if you use a custom function, you can do likewise in Excel, but then you're talking VBA modules that you have to make portable.

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[drawoh]

IRstuff,

In Excel, you can set up a page of named cells, with unit conversions...

Now, you can type =23*ft+16.5*in, and convert the result to whatever units you want. The evil stuff got cut off. It is kgf and lbm. Evil.[ ]

--
JHG

 

[BridgeSmith]

drawoh, Excel also has the CONVERT function, which has many of the common conversion factors already programmed.

 

[ThomasH]

Hi

I have worked with metric units all my life and find the imperial units fairly confusing. But when I have done several projects in countries that require imperial units.

The solution is simple, give two results, one in each unit system. In MathCad it is simple. "By hand", not that difficult. That was an approach we used with a project in the US several years ago, suggested by a PE. And it worked and works just fine. It is often just a matter of talking to the "other guy" and to agree on a reasonable approach.

But I would not build a FEM-model based on imperial units, the software I use has no built-in units. That would not feel comfortable at all since I would loose many "sanity checks". What is a reasonable result?

Just a few thoughts.

Thomas

 

[Tmoose]

Based on where where your PE and SE are registered ( Illinois and Wisconsin ) I'm guessing the materials you will be using/specifying are Imperial/inch.

As others said, calling 3/4 inch plywood, 1/2-13 fasteners, or 14 g steel by their "hard metric" equivalents is just looking for trouble.

AutoCad, for one, supports automatic dual dimensioning making drawings, at least, fully functional.

The real metric folks don't do that when they use "our" stuff.

Very popular "G" BSPP threads

https://www.gewinde-normen.de/en/whitworth-pipe-thread.html

Wheel diameters. Inch, even when the bolt circle, etc and etc are real metric.

https://www.wheel-size.com/size/bmw/3-series/2014/

Of course some like Volvo, have "inch" bolt circles, and maybe even inch center hub pilot bores, but probably inheriting them from the early days.

 

[IDS]

I suppose that if you use a custom function, you can do likewise in Excel, but then you're talking VBA modules that you have to make portable.

Save it as an add-in.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[skeletron]

There's lots of good advice in this thread. Document in your familiar units or use a calculation program. And know how to communicate in the unit system so that other disciplines are on the same page.

I remember my first "imperial job" after graduating...frustrating, but not worthless as an experience. One of the big lessons I learned through construction/fabrication jobs is that so much of the downstream activities are inherently imperial that you need to have an awareness of the geometry more than anything. I don't believe "hard metric" is ever 100% applicable. Somebody, somewhere is purchasing/installing/thinking/substituting imperial...

 

[BUGGAR]

I'm in Excel right now trying to find the metric conversion for radians. I can convert to grads and then find the metric (or imperial?) equivalent? This will be for a project in a less than one "g" environment so I have to consider force "g" vs. force "f"?

 

[hokie66]

Angular measurement is different than linear measurement. There are essentially two systems, as described here:

http://wmueller.com/precalculus/families/angles.html

I don't think either system can be called "metric".

 

[BridgeSmith]

Buggar, radians measure a ratio; there are no units, metric or otherwise.

 

[atrizzy]

A point of disagreement to most that have posted here: as someone who's had to learn both systems, I'd suggest trying out the calculations in metric, then converting the important results to a 'gut feel' number in imperial that you can get the sense of quickly. If all seems right, you're probably set. If something seems off, keep digging.

In the end, it's just math. Keep your units straight and you'll be fine.

 

[IRstuff]

While angles are ratios, the accepted SI unit is radian

https://physics.nist.gov/cuu/Units/units.html

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[BridgeSmith]

Radians are also used with imperial calculations. Radians, degrees, pitch, slope, percent grade, etc. can all be used seamlessly in conjunction with either system, since they are unitless values.

 

[Agent666]

Definitely stick to just metric when documenting the drawings, its usual practice/good practice to coordinate/locate structure to the nearest 5mm to avoid odd measurements that imperial conversions can give you.

Make sure if converting either way that you make up the missing millimeters. Over a long distance across a whole building you might end up missing 10-15mm of length. Certainly seen it happen before on some jobs I did early in my career in American Samoa, we provided both imperial and the nearest/rounded metric measurement to the nearest mm on the drawings. If you added up all the mm dimensions they didn't quite add up to the imperial measurement due to rounding errors inherent in this way of doing things.

We designed it all in metric using UBC97 metric equations, but using equivalent imperial thicknesses and lengths converted (i.e. 12" wall was designed as 305mm thick). Then for the contractor basically had dual measurements in autocad as someone else noted. So as long as you use 300mm = 11.811023" no problems and stay away from thinking in 8th's/16th's of an inch no problems!