Design for Shear V13, V23 or Vmax of Shell 2

[jackyyau]

I am designing a circular slab (with numerous openings, of course) fix-supported on a cylinder shaft.

At the slab-wall interface, I am confused which shear force (V13, V23, Vmax or combination of V13 and V23) should be used for design. Anyone can calrify? (the slab thickness cannot be changed at this stage)

For moments, I put in perpendicular bars to resist M11 and M22, respectively. Any other forces to design for?

 

[zer]

Hi. Check out the sign conventions in SAP help.

For shear design I think you should use V13 and V23 (or better, the maximum value from each).

For moments, add up +-M12 (twisting moment) to M11 and M22 to obtain the design moment for each direction.

I think the best and most correct way though is to use "Section cuts" (see help) which gives integrated forces/moments.

 

[jackyyau]

Does anyone have the Chapter 26 (as refered in help file) Manual discussing section cuts? Where can i get a copy?

For design moments, can we simply add up M11+|M12| from M11 and M12 resultant diagrams?

Could zer or someone help.

ugoveri (Structural) 2 Nov 04 13:29
When drawing section cuts for example aplied to a slab, something that at first case seemed to solve the problem of slabs, it was quite a desillusion, since the diagrams shows with the right shape, but you only have access to the integration of the total forces on the left and right sides of the section cut.

Isn't there a way of showing the same diagram, like if it was a beam, in which you could get points along that line?

As far as I've seen section cuts only integrate forces, so it doesn't make it easy to get the combined mx = mxx + |mxy| or my = myy + |mxy| moments and or other forces.

I always wondered why don't they put that option on SAP2000. It's because they have to justify SAFE?

Thank ugoveri
for this valuable post!


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stressed (Civil/Environme) 11 Nov 04 16:26
If you mesh you shells, SAP2000 integrates not only forces, but moments are integrated as well. A section cut combines forces and moments at a point, not along the length of an element.

I may be mistaken, but what you seem to be looking for is not a section cut, but a moment diagram for a shell element, kind of like the moment diagram results available for frame objects. If so, i agree with you that this would be a useful feature to have. In the meantime, you could divide/mesh your shell elements such that you can get results where you want them

Thank stressed
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ugoveri (Structural) 11 Nov 04 19:42
Well basically it was 2 questions actually...

If I just draw a section cut in one portion of the desired shells, of course i will have the integration i'm pretendind (for example if you want to have the negative moment at the support)...

But yes basically what you've written is completeley right and i completely agree...

What I am kind of complaining is that after they had the work to put the section cut avaiable, although they already got the integration (very good functionality), it wouldn't be to difficult to have the diagram like if it was a beam. It already appears qualitatively, but it shouldn't be too difficult to have values in it

The other complementary function, is the diagram that you can get coloured in the plane that could also be combined, for moments or forces with the crossed term m12 or f12 for example... So that way you would get the needed design forces and moments

I already sent them an email an they say it would be avaiable in the next versions of SAP 9... Let's wait and see...

 

[zer]

The answer to your first question is yes. Add M11 +- |M12|.

As for the section cuts, which version do you use?

 

[jackyyau]

We are using SAP2000 nonlinear version 8.1.2.

I still don't understand why the design moment is the sum of M11 and M12.

How about the following:
the reinforcement bars are arranged parallel to local 1-axis and 2-axis, do we still are add M12?

I still don't understand the twisting moment, M12. What is its effect on the slab? The manual says "M12: twisting moment per unit length acting at the midsurface of the element on the positive and negative faces about the 1-axis"
How can the slab be twisted (=rotate?) about 1-axis, is this indeed M11 or a component of M11?

Or it should be about 3-axis, i.e. the shell remains in plane (say 1-2 plan) and rotate about 3-axis?

Thanks very much for your advice.

 

[jackyyau]

the following lecture notes explin all about designing shell/plate structures based on FEA results:

http://www.gtstrudl.gatech.edu/gtst...sentations/2003/GTSUG2003_Tips&Techniques.ppt

Thanks

 

[zer]

jack, it is indeed a little hard to see where M12 comes from.

If you imagine a rectangular simply supported slab and divide it into strips (beams), you will notice that the middle strip deflects more than a parallel strip thats closer to the edge.

Well, this "middle" beam causes torque on the parallel beam which has less deflection. Same happens in the other direction.

Therefore in addition to every elements pure bending moment, there is a transfer of torque from adjacent elements which tend to deflect more.

Hope this helped.