Stiffness of concrete plate 1

The best you can do is adjust the E value or plate thickness to give yourself an equivalent stiffness.

Note: RISA also does not account for cracking of the concrete either. I would argue that cracking has a much larger effect on stiffness than the reinforcement. Now, for concrete members (or even walls), you can specify an I_cracked factor to modify the stiffness due to cracking. But, that's not available for plate / shell elements that are used for modeling slabs.

I did a quick hand calculation of an RC beam and showed that stiffness effects due to rebar can be greater than the stiffness of the gross concrete section in bending, so it certainly seems that the modulus of elasticity of the concrete shell material should be increased to account for this as you suggested. Of course, an RC section is not isotropic and will have varying stiffnesses in different directions. Does RISA typically recommend modifying the modulus of elasticity to account for bending effects only, since that will often be the dominant action of concrete shells (usually slabs or irregularly shaped walls).

Edit: The cracked MOI should be easier to take care of since shell thickness can be taken from the top of the section to bottom rebar for a shell element supporting monotonic loads.

Mega -

I don't know that RISA has a recommendation one way or the other on how to handle this. I will say that, during my time working for RISA, we did not generally get many requests for considering reinforcement in the stiffness of beams / columns / walls or slabs. My belief is that in general structural analysis, most engineers are comfortable using the gross section (or some multiple of the gross section) for the equivalent moment of inertia of the concrete element.

Your point about the behavior of concrete slabs being non-isotropic is a good one. The plate element that RISA uses internally is sometimes modified to be orthotropic when it is used in wall elements. But, that is all automated. And, the program never gives the user the ability to unlock orthtropic behavior for plates.

Obviously, if these items are important to you, then you could send a message to the RISA folks and request the feature be added. Though, I have a feeling they would not give that request much priority.... at least not unless a lot of people are requesting it.

The stiffness reduction due to concrete cracking is far greater than the increase in stiffness due to the reinforcement, especially for typical lightly reinforced slabs.

If you really want to model the increased stiffness, increasing the E value would be the easiest way to do it, but I'd suggest double checking the cracked stiffness, which can be lower than the unreinforced uncracked short-term stiffness by a factor of 5 or more.

Doug Jenkins
Interactive Design Services