Skip to main content
ST_105338451-Visitor
1-Visitor
Dec 20, 2022 12:05 PM UTC
Solved

Multiple Units - Calculation Error

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

ST_10533845Answer
1-Visitor
Dec 20, 2022 9:47 PM UTC

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

View in thread

6 replies

LucMeekes23-Emerald V
23-Emerald V
Dec 20, 2022 5:28 PM UTC

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

 

Translate
Werner_E
Werner_E25-Diamond I
25-Diamond I
Dec 20, 2022 6:26 PM UTC
@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.

 

 

Translate
ST_105338451-VisitorAuthor
1-Visitor
Dec 20, 2022 9:28 PM UTC

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

 

I'll post the file.

 

123.PNG456.PNG

Translate
Werner_E
Werner_E25-Diamond I
25-Diamond I
Dec 20, 2022 6:17 PM UTC

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

Werner_E_0-1671561966850.png

 

Translate
ST_105338451-VisitorAuthor
1-Visitor
Dec 20, 2022 9:34 PM UTC

The though was that the formulas would  be unit aware. You seem to be using different Mathcad version. I have prime 5.0.

 

I've attached the file.

Translate
ST_105338451-VisitorAuthorAnswer
1-Visitor
Dec 20, 2022 9:47 PM UTC

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

Translate
Terms & ConditionsAccessibility statement