consistent units

[Qshake]

All -

Am working in MathCAD 14 and have a problem with unit consistency.

I have defined kips per cubic foot as kcf and have multiplied that density by an area. I was expecting to have resulting units as kips per foot klf but instead have lb/s^2. What gives?

How do I get klf?

thanks

Regards,
Qshake

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[IRstuff]

Can you post an example. I'm not familiar with your units.

TTFN

FAQ731-376

 

[Qshake]

IRStuff - Thanks for the help, but it seems that once I type over the units of lb/s^2 with klf, which is recognized by the program as a typical unit, the answer automatically updates to the correct value for those units.

Learn something new all the time...



Regards,
Qshake

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[zdas04]

Whenever you type K:4*kcf and then type "K=" it gives you an answer, units, and a little square to the right that you can type the desired units into. It will then try to convert and any leftover units show up between the number and the desired units.

David

 

[Qshake]

Thanks, David, that's what I wound up learing...

Regards,
Qshake

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[IRstuff]

That feature has been a critical part, and one of the best parts, of Mathcad's utility.

TTFN

FAQ731-376

 

[jhardy1]

And don't forget that dimensionally:

kip / ft = F / L = (M * L / T^2) / L = M / T^2

lb / s^2 = M / T^2

That is, the two units are dimensionally equivalent.

Mathcad has no real way of knowing which choice of dimensionally equivalent units you want, until you tell it.

 

[jhardy1]

P.S.

And don't make the common mistake of thinking that "1 lb" (one pound mass) and "1 lbf" (one pound force) are interchangeable.

(This may be "teaching you to suck eggs", but it is one of the most common cases of "unit confusion" that I come across.)

 

[Dinosaur]

Another important point about uints I had to learn:

If you ask Mathcad to round something using "floor" or "ceil" or something, it will use the "base units" to do this rounding. I wanted to round an answer to the nearest inch. I would often get '0 in' for an answer. This is because G:=floor(b) would round say '4.25 inches' to 0 ft. Then when I ask Mathcad to convert the answer to inches, it just showed 0 in. To get Mathcad to round as I wanted I had to do this ...
G:=floor(12 x b) / 12
If you want sixteenths of an inch or something else, you have to change you pre/post multiplier to get the right thing you want.
Mathcad is awesome, but it can bite you once in a while.

 

[IRstuff]

A cleaner approach might be floor(b/ft)*ft

TTFN

FAQ731-376

 

[jhardy1]

Nice catch, Dinosaur! I can't say I had ever noticed the problem of Mathcad rounding down in its base units before - probably because I rarely want to round a dimensional calculation result down or up in my work, but I can imagine cases where you might need this.

You would use the "Floor" function of you really want a rounded down value for subsequent calculations. However, if you are just rounding up or down for display purposes, a safer approach would be to not use the "Floor" function at all, but just format the displayed result rather than manipulating the value itself.

 

[IFRs]

Back to the OP - just change the default units to SI and your answer will appear as you wish it to. In M14 you can also define your own unit system (I think)

 

[Dinosaur]

IRStuff,

I'm not sure this would fix my problem. If Mathcad divided 'b' by ft, it may save the result as dimensionless feet.

 

[philipoakley]

You can use (MC11 help)
Floor(z, y) Returns the greatest multiple of y ? z
Round(z, y) Rounds z to the closest multiple of y.
Trunc(z, y) Returns trunc(z/y)*y
For Floor, Ceil, Trunc, and Round, z and y must both be either dimensioned or dimensionless

i.e. the value y is the unit that you want in the Floor / Round / Trunc.
Philip

 

[Dinosaur]

Qshake,

Another tip: I have a default worksheet for imperial units and for metric units, which contain a number of special units defined globally at the top of the sheet (kips:=1000*lbf, klf:=kips/ft, pcf:lbf/ft^3, ...). I even have a number of basic functions for loads defined as well such as the reaction at the end of a simple span with an HS20 truck at position 'x'. I also define ORIGIN:=1 so there is no confusion in my matricies. Good Luck

 

[Qshake]

Dinorsaur that is good advise...

thanks,
Q

Regards,
Qshake

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[miecz]

I have sheets of standard units, customary and metric, ORIGIN=1, similar to dinosaur's. It also contains our company logo, rebar diameters and areas. I reference this file into all of my worksheets, as I believe this cuts down on file sizes.