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Mx/Zx or Mx/Zy

 
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learncivil13
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PostPosted: Fri May 17, 2013 5:58 pm    Post subject: Mx/Zx or Mx/Zy Reply with quote

Guys

While doing the Base pressure check for biaxial bending foundation design
the Formula shall be P/A+-Mx/zx+-My/Zy
or
P/A+-Mx/zy+-My/Zx

please clarifybecause it has been given different in different text books
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es_jayakumar
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PostPosted: Sat May 18, 2013 5:17 am    Post subject: Reply with quote

It is P/A+-Mx/Zx+-My/Zy,  where, Mx is the moment that tries to rotate the X axis and My is the moment that tries to rotate the Y axis (moments about the respective axes).

E S Jayakumar
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learncivil13
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PostPosted: Sat May 18, 2013 7:42 pm    Post subject: Reply with quote

Yes Sir,

I agree with you.

Please find here attached an extraction of text book
"Understanding the concept of Structural analysis and design by JANAK P.Parikh"

Which tells the other way.
Not only this if you refer Saudi Aramco standard also refers the same.



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es_jayakumar
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PostPosted: Sun May 19, 2013 1:06 am    Post subject: Reply with quote

Moment direction is usually considered in two methods. About an axis and in the direction (along) an axis. In STAAD, for a beam element (3D) in  a frame, the moment direction is considered in the first method (that is, Mx tries to rotate the X axis- right handed screw concept), while in a plate element (2D), the moment direction is considered along or in the direction of an axis (that is, Mx tries to rotate the Y axis). The latter method would cause confusion for a 3D element. ie, if we say that Mx acts in the direction of X axis, it may be trying to rotate either Y axis or Z axis. However, for a 2D element, there wont be such a confusion. Hence, the first method is superior to adopt in all cases.
In the referred book, the author might have adopted the second method for the direction of moments. The same is not clear from the text or drawings. You may refer some other standard books.
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es_jayakumar
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PostPosted: Sun May 19, 2013 7:33 am    Post subject: Reply with quote

You may refer to example 19-8 in the book : Reinforce Concrete Vol. I by H.J.Shah, Charotar Publishers. This exhaustive example on the design of an isolated footing clearly ratifies our conclusions.

E S Jayakumar
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