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Multiple Units - Calculation Error

Forum|Forum|3 years ago
December 20, 2022
3 replies
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Hello

There seems to be a calculation error in my fred (marked with red box).

When I do hand calculation / Excel-calculation the value is ca. 1.68.

It is correct that the units drop out of the function as I hand calculated.

Could someone figure out the problem?

*here are again the inputs

rho = 1.2 [kg/m3]

ze = 7.64 [m]

zg = 380 [m]

alpha.r = 0.19 [-]

qp.0 = 0.9 [kN/m2] or ca. 91.74 [kgm/s2/m2]

f0 = 1.64 [Hz] or 1.64 [1/s]

Best regards,

Samuli Turunen

1_Hz.pdf

Best answer by ST_10533845

I've spotted the error. The Mathcad-sheet works correctly.

The normative needs units N/m2 without any conversion to arrive to the correct answer when doing the

hand calculation with calculator. The excel also needed the correct unit conversions and therefore the numbers

to arrive to the correct answer. 1.68 is incorrect when considering the gravitational acceleration and converting the units

into 91.74 kg/m2

( F = m x a = kg x m/s2 and sigma = F/A or kgm/s2/m2)

Thank you for your time and interest to solve the problem.

Samuli

L

LucMeekes

23-Emerald IV

I can spot one error on your sheet: The 3rd word says "Bedingnung", that should be "Bedingung".

Please attach your worksheet, not just a picture.

It 'll help us to help you much better.

Success!
Luc

W

Werner_E

25-Diamond I

@LucMeekes wrote:

I can spot one error on your sheet: The 3rd word says "Bedingnung", that should be "Bedingung".

🙂

I was also a bit puzzled and wondered what language it should be. You seem to assume German, but then is should also read "Dynamische" instead of "Dynamisher" and "Grössenfaktor" should be changed to "Größenfaktor. 😉
But I suspect that Samuli Turunen isn't that much interested in fixing such spelling mistakes and my guess is that he is using empirical formulas from a text book which are not unit consistent. But because I am not willing to retype what he shows in the pdf to play around and find out, how the value he expects could be achieved, I'll wait for the worksheet being posted.

S

ST_10533845

Author

1-Visitor

*SIA-norm, vielen Dank für die grossartigen Punkte.

I'll post the file.

W

Werner_E

25-Diamond I

Always attach the worksheet itself and not just a pdf of it.

Are you sure that the formulas you are using are really unit aware.
They may be just old empirical formulas which expect input values to be in certain specific units only.

You have z.g=380 m and alpha.r=0.19. What do you think is the unit of the last factor in your formula -> z.g ^ alpha.r ? meter to the power of 0.19 ????

I am pretty sure that the formula is not unit aware or unit consistent.
I guess you will get the result you expect if you divide every variable by the unit the formula expects it to be in. Especially q.p0 which is obviously not expected to be in kN/m^2.

But I wonder where the 91.74 you mention would stem from because