Plate principal moments

[Anonymous]

Hi all,

I must design a RC slab and I've set up a spreadsheet according to BS8110. Then, in order to check results and gain more accuracy I've set up the global model and looked at the panels moments. Unfortunately I cannot work out the difference between Mxx, Myy, Mxy and M11, M22, M12.

To try to understand better I've set up a simple test model, attached. In this I see that Mxx=M11 and Myy=M22 (or Mxx=M22 and Myy=M11) but the mixed index moments are totally different - and it seems to me that Mxy is the right set. Why there is such a difference? And what the x-y and 1-2 reference systems represents? Which is the local? And how do I show the 1-2 system?

Thank you very much

Davide

[Rafacascudo]

Mxx , Myy and Mxy are moments in the directions you set

M1,M2 and M12 are the principal moments . They don´t depend on the direction you set .

[Anonymous]

Dear Rafacascudo, thank you very much for your reply.

I didn't know I could set the local reference system and I used the automatic one, which I guess is the panel local reference system.

And I agree, moments with 1,2 pedices SHOULD be principal moments. Problem is that they are different from what one would expect. For example, have you had a look at the model attached? In that model x,y local system and 1,2 principal system are the same. Nevertheless, bending moments are the same but twisting moments are totally different. How come? What's happening? If 1,2 is the principal system there's something wrong.

Thanks 

Davide

[Pawel.Pulak]

Principal moments M1, M2 are from definition moments in such perpendicular directions that the twisting moment, associated with them, is zero.

M(1-2) is the maximum twisting moment, which is 45 degrees from principal directions  - the relation described by Mohr's circle. As concerns its value M(1-2)=(M1-M2)/2

In Robot it is also possible to graphically display the "crosses" of principal moments or stresses in the centroids of finite elements - this way the direction, magnitude (length of arms) and sign (color - by default blue for positive and yellow for negative) of principal moments is visible. See the screen capture below (without map it would be even better visible):

In your first post you have mentioned that you want to use these values to calculate reinforcement. In Robot you can also acceess Wood&Armer moments, useful when calculating reinforcement:

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