consistent units
consistent units
(OP)
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
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,
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RE: consistent units
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RE: consistent units
Learn something new all the time...
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RE: consistent units
David
RE: consistent units
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
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RE: consistent units
TTFN
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RE: consistent units
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.
RE: consistent units
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.)
RE: consistent units
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.
RE: consistent units
TTFN
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RE: consistent units
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.
RE: consistent units
RE: consistent units
I'm not sure this would fix my problem. If Mathcad divided 'b' by ft, it may save the result as dimensionless feet.
RE: consistent units
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
RE: consistent units
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
RE: consistent units
thanks,
Q
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
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RE: consistent units