Metric Units

[wannabeSE]

I decided to make a spreadsheet to convert units that are commonly used in structural engineering and I have a couple questions:

Force: Are kilogram-force measurements ever used? I have seen a Basis of Design by Americans that used kg/m² for live loads. Is this incorrect or do some areas use this in lieu of kN/m²? Same question for kg/m line loads.

Density/Unit Weight: Is density in kg/m³ or specific weight in kN/m³ typical used. Or are both used depending on location?

Moment of Inertia: What units are typically used, m[sup]4[/sup], mm[sup]4[/sup]?

Gravity: If kg is used for loading, is it safe to use g=9.80665 m/s? Or do some locations areas adjust for latitude (I typically use 3 significant digits, but for this spreadsheet, I'd like to use 6)

I have uploaded the unfinished spreadsheet that has not been back-checked. I'd like to know if I am missing any commonly used units. Let me know if there are any glaring omissions or errors. I am trying to make it easy to modify. Additional units can be added to or deleted from the drop-down lists by inserting or deleting lines from the tables on the second sheet. The conversions mostly rely on Excel's CONVERT function.

 

[Ingenuity]

Gravity: If kg is used for loading, is it safe to use g=9.80665 m/s? Or do some locations areas adjust for latitude (I typically use 3 significant digits, but for this spreadsheet, I'd like to use 6)

...good lord, I use 9.8 m/s2 for 'g', mabye 9.81 if using a computer/spreadsheet!

 

[hokie66]

kN should be used for force, kPa and kN/m for loading. MPa for material stress, except soil pressure is normally kPa. Density is usually expressed in kg/m^3 or tonnes/m^3. Property constants like I, Z, A, J in the appropriate multiple of mm.

Agree with Ingenuity. 9.8 is good enough, I use 10. After all, this is structural engineering, not watchmaking.

 

[IDS]

wannabe - You might find my spreadsheet link below useful. It's open source, so feel free to use anything you find useful.

http://newtonexcelbach.wordpress.com/2012/10/11/daily-download-24-units-for-excel/

Kilogram-force units were common in mainland Europe, but the new Eurocodes use proper SI units, so I don't know how much they are still used. The UK and Commonwealth countries have always used N for force since they went metric.

For area and moments of area both mm and m are commonly used. For programs that require consistent units it's common to work in MPa for stress with either m and MN or mm and N.

For g you should use 9.80665 m/s2, which is the official standard value. The actual acceleration varies of course, but so does everything else (density of materials, actual dimensions etc). For a computer conversion program you should use the maximum precision available.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

With the tables in the spreadsheet, you are converting from the units in the left column to the units at the top of the table, so for instance in the length table, 1 cm is 10 mm, but you show cm.

If you want to convert from the units at the top to the units on the left you need to adjust the formula. Also the base SI length unit is m, not mm.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IRstuff]

Or, you could try

http://www.katmarsoftware.com/uconeer.htm

which was developed by ET member katmar.

Not free, but very comprehensive, and probably worth it if you don't own a copy of Mathcad. There used to be a free version, back a LONG time ago, and it was pretty complete, even back then

TTFN
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[drawoh]

wannabeSE,

I create an extra sheet on my spreadsheet for unit values...

[pre]
in =.0254 m
ft =12*in m
lb =4.45 N
g =9.81 m/s^2
[/pre]

I name the cells as per the text on the left hand side. I can now use these to multiply units when I enter them in cells elsewhere on the spreadsheet...

=16.5*in
=42000*lb/in^2

...I get the proper SI units.

Here is a cute thread I started in Pat's pub... thread1088-343440

The photo has gone away. It was of a bridge with a sign limiting the capacity to 5914kg.



--
JHG

 

[jeanb88]

Al units with kg (kg/m³, kg/m² and kg/cm²) are still in use in Europe by the older generation engineers.

 

[bridgebuster]

Back when DOT's in the US were using Metric, we were using kN for force but estimating rebar by kG; never understood why.

 

[hokie66]

Because rebar is sold by mass, which is measured in kg or tonnes.

 

[bridgebuster]

but in the US it was a measure of weight

 

[hokie66]

I confess to using mass and weight interchangeably (for structural engineering purposes on Earth only).

 

[tempeng]

You'll also find in the UK that kPa for soil/bearing pressure usually gets converted to kN/m^2. It drives the purists mad. I haven't come across this, as a common practise, in main land Europe though.

 

[Archie264]

>>>I confess to using mass and weight interchangeably (for structural engineering purposes on Earth only).<<<

You're willing to forgo all that lucrative space colony work?

 

[bridgebuster]

hokie66,

If I weigh myself on your bathroom scale would it read 86kG or 845 N?

 

[jayrod12]

It would probably read 189.5 lbs. HAHA

 

[drawoh]

You'll also find in the UK that kPa for soil/bearing pressure usually gets converted to kN/m^2. It drives the purists mad. I haven't come across this, as a common practise, in main land Europe though.

A kPa is a kN/m[sup]2[/sup]! This makes it way easier to do unit balances in my calculations.

--
JHG

 

[IRstuff]

"A kPa is a kN/m2! This makes it way easier to do unit balances in my calculations."

Meh, Mathcad doesn't care, so I don't care. As long as the units are correctly defined in Mathcad, it can convert anything and everything until the metric cows come home, including furlongs per fortnight ;-)

TTFN
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[hokie66]

bridgebuster,
Don't you have a bathroom scale that measures in Newtons? And you call yourself a bridge engineer!

 

[bridgebuster]

I don't have a good relationship with the bathroom scale. It never gives me the answer I want.