Calculation of Q12 (Principal Shear Force in panels), and shear reinforcement ?

[lars.lrso]

Hi,

I cant figure out how Q12 is calculated? And is it a correct way for showing the max shear force in a slab? (Calculations (need) of shear reinforcement in (foundations) slabs)

I have looked at following pages, but it does not make sense. (It works fine for M12 and N12 etc)

http://docs.autodesk.com/RSA/2012/ENU/filesROBOT/GUID-1279D686-429D-46F7-AE21-7E37F65C4D1-848.htm

http://docs.autodesk.com/RSA/2012/ENU/filesROBOT/GUID-BD23B817-BEE0-4DB2-97AF-2571B877F80-846.htm

 Lars

[Pawel.Pulak]

Q(1-2) shear force is calculated as the resultant vector from QXX and QYY shear forces (Q(1-2) = sqrt (QXX^2 + QYY^2))

The state of stresses in panels is considered as 2D state of stresses with out-of-plane QXX and QYY shear forces considered as "extra" components.

That is why N, M, s  are transformed as tensors and Q is transformed as vector.

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Regards,

[lars.lrso]

Hi Pawel,

Thanks, for the quick answer.

I figured out that Q12 is the resultant vector. However when i calculate Q12 myself the numbers does not match with Robot. See the pictures attached.

/Lars

[jclcereno]

I think QYY and QXX is not does not correlate to Q12...

Based from Robot Help:

Shear Forces: Qxx, Qyy. Adds columns for shear forces values Qxx or Qyy, respectively, for individual planar FEs in the local co-ordinate system as selected during structure analysis.

Shear Forces: Q12. If selected, it adds a column for maximum shear force values Q12 for individual planar FEs in the main coordinate system as selected during structure analysis.

[Tuctas]

What about this observation, any conclusion..?

[Jummybear]

Same question here...

[Pawel.Pulak]

The reason of no correlation to the formula in some nodes is in low order and low precision of Finite Element shape function for Q shear force.

It can be noticed even displaying QXX or QYY without smoothing - like on the screen capture below. In such case is can be noticed that it is a linear function across finite elements.

Because of high inaccuracy of this function after smoothing the results (averaging in appropriate node from results of all finite elements connecting in this node)  the relation that Q(1-2)=sqrt(QXX^2+QYY^2) is lost. The averaging is done independently for QXX, QYY and Q(1-2).

It is OK and the relation is kept for results in the centroids of finite elements or for results without smoothing - as on the screen capture below.

Best regards,

[Tuctas]

Very well explained Pawel. Thank you