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ecosta1-Visitor
1-Visitor
November 16, 2015
Solved

Unit combination

  • November 16, 2015
  • 2 replies
  • 5411 views

Good morning,

I have a quick question regarding units. I am new using mathcad. This might be a silly questions but i am trying to input the units at the end of a calculation such as wind pressure in psf, but when i do it changes automatically to Pa^1*psf. that's not what i want. how can i change this?

I am attaching the file. first page, last equation. It doesnt have any units at this moments, but as soon as i double click on the unit section and enter psf of lb/ft^2 is also adds Pa^-1.

Thanks.

Ernesto Costa

EN Engineering.

Best answer by MJG

Fred,

I see that you assumed the 0.00256 factor to be an approximate value for the density of air in slug/ft^3 (perhaps we could call it "heavy air" - maybe to account for air pollution ).  However, looking online,  I found this example (see the reference to ASCE 7-10 Equation 27.3-1 near the bottom of page C-10, and continuing with numerical examples on page C-11).  This appears to confirm my initial thought that the equation is set up expecting certain units (mph) and resulting in certain units (psf).

Convinced, however, that you were correct in assuming the density of air should be used, I noted the following:

Therefore, I assume a more "pure" form of the equation would be:

I doubt Ernesto will find this equation very useful, however, since anyone reviewing the calculations will expect the equation to be in the form presented by ASCE.

2 replies

R
RichardJ19-Tanzanite
19-Tanzanite
November 16, 2015

You can't assign arbitrary units to a result. Allowing this would defeat the purpose of Mathcad's unit handling, which is to give you the correct units for the result given the units of the defined variables. You defined your variables with no units, so the result has no units. If you then type psf into the units placeholder Mathcad puts in the default SI unit that is the inverse of that so the the result remains unitless. You need to add units to the definitions of Kz, Kzt, Kd and V. qz will then be displayed in the default SI units of Pa. If you type psf in the units placeholder the numeric value of the result will change.

    M
    MJG1-Visitor
    1-Visitor
    November 16, 2015

    Due to the format of the equation, I assume the K values are unitless factors, V has a unit and the value 0.00256 is the unit conversion.  Unfortunately, this is often how engineering equations are presented.  In this case, I would recommend you multiply V by its unit (you show mhp in the adjacent text box - I assume you meant mph?) and multiplying 0.00256 by the expected unit over the V unit squared.  See below & attached.

    M
    MJG1-Visitor
    1-Visitor
    November 16, 2015

    Alternatively, you could divide each variable by the expected input unit and multiply the entire equation by the expected return unit:

    F
    Fred_Kohlhepp23-Emerald I
    23-Emerald I
    November 16, 2015

    Richard beat me to it, but since I deal with dynamic pressure I added density units to the second expression to get pressure.

      M
      MJG1-VisitorAnswer
      1-Visitor
      November 16, 2015

      Fred,

      I see that you assumed the 0.00256 factor to be an approximate value for the density of air in slug/ft^3 (perhaps we could call it "heavy air" - maybe to account for air pollution ).  However, looking online,  I found this example (see the reference to ASCE 7-10 Equation 27.3-1 near the bottom of page C-10, and continuing with numerical examples on page C-11).  This appears to confirm my initial thought that the equation is set up expecting certain units (mph) and resulting in certain units (psf).

      Convinced, however, that you were correct in assuming the density of air should be used, I noted the following:

      Therefore, I assume a more "pure" form of the equation would be:

      I doubt Ernesto will find this equation very useful, however, since anyone reviewing the calculations will expect the equation to be in the form presented by ASCE.

        F
        Fred_Kohlhepp23-Emerald I
        23-Emerald I
        November 16, 2015

        Mark;

        All of what you said is correct.  Without the full ASCE documents and the time to read thru them I can't tell, and the factor of 1/2 is certainly in the definition of dynamic pressure.  Ernesto, you, and I are all caught in the snare of n equation presented without adequate definition.

        I like the fact that Mathcad understands and converts units.  But it's difficult for someone not used to that fact to discard the terms in the old engineering books that are simply unit conversion terms that don't need to be in the Mathcad version of the equation.

        The debate over whether to use units or not has endured for as long as I've followed Mathcad.  It's still alive and raging.

        As for me, I'll use them; and I'll try to understand equations like Ernesto's that confound that use.

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