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

 

[TehMightyEngineer]

I'd disagree, unless you're going to spend a lot more time working in metric your ability to look at calculations and quickly find errors, or you ability to apply rules of thumb, will be difficult in metric.

I'd simply do everything in imperial units and convert to metric as appropriate for your output. Using mathcad or other software that can switch units seamlessly will likely be highly valuable.

Whatever you do, don't mix and match systems (NASA...). Stick to one system and be consistent in when you switch over units in the design.

Of course, if you're collaborating or otherwise need to have your calculations reviewed by people experienced in science units, then I'd do as you say and start out entirely in metric.

In the end, it's simply a unit conversion and nothing should really change other than your personal efficiency.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[JoelTXCive]

I have done a couple retaining wall projects like this.

We had existing Excel spreadsheets that were all set up in Imperial units so they could not be converted easily.

Our solution was that we designed the entire project in imperial units, and then converted to SI at the end. We then had to painstakingly check the plans and adjust for rounding and conversion issues. Since these were large retaining walls, it did not matter too much if the footings or wall height was a centimeter too tall or short.

If you can easily convert to SI units on the front end then I would recommend going that way. If not, then I think you have to go the route we did. It was a little painful, but we got through it.

Good Luck

 

[rb1957]

if you're not using a metric design code, then you'll be in for a bunch of back and forth; no?

if you are, then does it matter ? dimensions, loads, allowables all metric (or imperial) ... the problem with existing s/sheets is most likely conversion factors (inches to feet, "g"). The biggest problem is having a sense of what these funny units mean (ie you know what imperial sizes mean).

another day in paradise, or is paradise one day closer ?

 

[TehMightyEngineer]

Imperial is so easy.... it's all fractions

(I kid, but doing things like carpentry you can see how imperial units were meant for people to do math in their head and metric was meant for science. How many 3/16" boards do we need to get 3/4"? 16/4 = 4, we need 4 plys. Metric = 0.750 / 0.1875 = ???)

Edit: I made an unfair comparison; metric would be something like 20mm / 5mm = 4 plys, but hopefully the point got across that decimal units are sometimes not as easy as fractions for doing math in your head.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[robyengIT]

I use metric units but when I had to do in imperial units I did as JoelTX said. I do not convert the formulas (too many conversion factors, easy to make mistakes) : I convert the results (also the intermediate ones) once I am sure that the results in metric are OK

PS : what I really do not understand are the tolerances. sizes in inch and ft and tolerance in decimal

 

[IRstuff]

I would recommend Smath Studio or Mathcad to do your calculations, which are pretty much the only two programs around that understand units and converts them on the fly. Additionally, you can setup empirical formulas with known units into forms like

which would allow you to use any compatible unit and the program will normalize to the correct units for the empirical formula.

btw, Smath Studio is completely free.

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[Ron247]

For the project in general, does the dimensions of the finished product have to be somewhat "even" numbers. For example, kitchen cabinets in imperial are 2' wide but metric cabinets are 600 mm. 2' is not equal to 600 mm. Do you want a floor level 10' above the ground or 3,000 mm? If you do need a 600 mm wide (not 609.6 mm) then I suggest determining the design dimensions based on mm, convert that to feet/inches and then use imperial for your calculations. I agree heavily with some previous posts, we do not identify metric numbers in our heads very easy. When you make a math mistake, it may appear correct.

Designs can start of with nice easy to remember numbers, but that is gone with the first few calcs. 25'x25' building becomes 23.2' center to center of column pretty quick. So the odd numbers are not that big of a deal

I can easily convert parts per million from imperial to metric and vice versus. You multiply imperial ppm x 1 and raise it the 1st power, then add 0. Other than that, I am lost.

 

[jayrod12]

I'm a bit backwards. I much prefer to run loads in imperial and even doing the analysis in imperial. I've memorized all the important conversions and convert the final analysis numbers to metric to do the design.

 

[Celt83]

I'd probably do something similar as jayrod12 run all my force analysis in the system I am most familiar with (Imperial for me) since 1 kip-ft vs 1000 kip-ft means a whole lot more to me from a gut feel reaction on the analysis result than 0.00135 Kn*m vs 1.35 Kn*m

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https://github.com/buddyd16/Structural-Engineering

 

[JStructsteel]

On a side note, we had a DOT require metric for all drawings, etc. Then they wanted a imperial set so they could check it easy enough and also something for the contractor to build from. We endeded up doing dual units on the drawings. Funny huh?

 

[TLHS]

As a person who consistently works in both systems, I wouldn't document in the one you're not comfortable with unless all those documents really need to be in metric or you're going to be doing this a huge amount in the future. It would take a lot to get me to document my math in imperial, even though I use it every day and I can convert most major engineering units in my head.

Do your math in the units you're comfortable with. Convert the input and output, as necessary. Then do the drawings in metric. You're not going to do a good job seeing mistakes in intermediate steps in a unit system you're not used to working with.

It's easy enough to pick imperial forces and dimensions close enough to round metric numbers that design is still fine.

 

[MGZmechanical]

I am in the case of robyengIT and I agree with him. Solve everything in your units system and convert just results.

 

[IRstuff]

If you're not in the habit of converting units, I would suggest at least using SMath to do the conversions and/or checking conversions. I've seen examples of unit conversions embedded in Excel worksheets and they can be a nightmare to trace through and verify.

In SMath, it's pretty simple; type in your quantity, with the original unit, and type an "="
Then select the little black placeholder and put in the desired unit

TTFN (ta ta for now)
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[TLHS]

For basic engineering, it's also completely reasonable to use the limited version of mathcad. You can't do the more advanced styles of calculation and solving, but for basic math and unit conversion, Mathcad express can get the job done.

 

[BridgeSmith]

Excel has a "Convert" function which is fairly simple.

 

[canwesteng]

I do tons of switching back and forth between metric and imperial (the joys of working in Canada), and can't recommend doing all your calcs in Smath or Mathcad enough. You're probably getting your inputs in metric, which leaves you to convert into a messy number at the start of the calc and convert back to a messy number at the end of the calc to get back into metric. Mathcad/Smath will you let you see intermediate results in imperial so you get feel for the answer, but you can dump all the final results out in metric at the end.

 

[hokie66]

Dave,
If you are only going to do one job in SI units, it is probably not worth your effort to learn to think SI. It took me 6 months or a year. But it is much simpler once you get used to it. Just one piece of advice...with length measurements, only use metres and millimetres. Discard the centimetre. That is for dressmakers.

 

[271828]

I'll second the ones who are saying to do your calculations in USC units and then convert to SI, unless you're going to work on lots more SI jobs.

A lot of our judgment comes from having an innate -- effortless -- sense about whether a number is reasonable or not.

 

[IRstuff]

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.

TTFN (ta ta for now)
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