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# Civil Engineering/Sign convention and Shell Output in ETABS

Question
Dear Sir,
Regarding shell contours in ETABS, we can get values of M11, M12, M22, F11, V13 etc and so on!

1) I have found that if we want to take Fmax, Mmax, or Vmax (or min) on enevelopes of many combinations then ETABS will show zero (0) values. Do you have any idea why is it so?

2) For reinforcement design what values actually we use? Do we use F11, F22, M11, M22, I mean the direct stress or moment values or we use Fmax, Mmax and Vmax, which means the principal stresses and moments and shears?

I think to check shear for a shell (one way) we would plot Vmax contours (max transverse shear) and check with concrete shear capacity rather than to compare V13 and V23 individually? If thats true then we should also check Mmax and Mmin and Fmax and Fmin on each load combination separately without including them in any envelope. Is this approach okay? or we will still use only the direct stress or moment value?

Many thanks!

Hi Waseem,

1. It must be a modelling issue, as using shell and with proper meshing I do not get zero values for Fmax, Mmax, Vmax, etc

2. For rebar design you should use M11, M22, and V13 and V23, and just use the stresses to check that they are within capacity of the concrete grade chosen.

Yes, you can either look at Vmax (which reports max for V13 and V23) or look at each individually. In the same way, you can either look at Mmax and Mmin (which are envelopes of M11 and M22), or look at M11 and M22 separately. F11 and F22 are, as you know, forces per unit length acting at mid surface of the shell element, so if you divide that by the thickness you will get the mid-level (average)stresses.

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