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wbell8-Gravel
8-Gravel
October 29, 2015
Question

Mixed units in matrices

I am trying to set up the equations for the solution of a matrix displacement problem in structural analysis, i.e., F=KU, where F = force matrix {4X1}, K=stiffness matrix {4X4}, and U=displacement matrix {4X1}.  However, Mathcad will not accept the definition of these matrices with mixed units.  Do I have to define these matrices as unitless and track the results by hand?

Thank you for your assistance.

9 replies

RichardJ19-Tanzanite
19-Tanzanite
October 29, 2015

Mathcad 15 does not permit mixed units in matrices. Mathcad Prime does (it's one of the few things where Prime is better).

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MikeArmstrong1-Visitor
1-Visitor
October 29, 2015

Richard Jackson wrote:

Mathcad Prime does (it's one of the few things where Prime is better).

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Werner_E
Werner_E25-Diamond I
25-Diamond I
October 29, 2015

Thats a drawback in older Mathcad versions and one of the very few benefits of Prime.

in Prime you can have different dimensions in a matrix, but in MC15 and below you unfortunately can't.

WE

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MikeArmstrong1-Visitor
1-Visitor
October 29, 2015

Am I right in thinking that an older version of Mathcad accepted mixed unit arrays?

Mike

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RichardJ19-Tanzanite
19-Tanzanite
October 29, 2015

No, it's not been possible in any version prior to Prime.

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MJG1-Visitor
1-Visitor
October 29, 2015

I agree with Richard & Werner.  However, it might still be possible to achieve your overall goal without switching to Prime or omitting units.  Would you be able to achieve what you want by simply splitting your calculations into two portions: one for load/displacement, and the other for moment/rotation?

Finally, I would encourage you to include your units in the definition of the variables rather than in the equations.  For example:

Use:

Y1 := 1*kip

F12 := Y1...

Instead of:

Y1 := 1

F12 := Y1*kip...

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ValeryOchkov24-Ruby IV
24-Ruby IV
October 29, 2015

But only in Mathcad PM (Prime Minister)

we can do so - change units of separate elements in outputted array:

v-units.png

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