Modeling question in etabs, What Etabs does to a shell element to make it "ribbed" ?

[GJoeRMK]

Greeting,
What Etabs does to a slab shell element to make it "ribbed" ?

I found an article that claims that a ribbed shell elements mimics the orthotropic stiffness of the element if you select this type, but not idea what are the exact modifiers values

Ribbed and Waffle objects: These objects can be used to model ribbed and waffle slabs. The generated finite elements have an orthotropic stiffness corresponding to the geometric data defined in the cross-section properties. The weight and mass of these objects are calculated excluding any lost formwork, so in these cases, the user must define mass and weight modifiers to account for the action of these non-structural elements. The design and detailing of these slabs also consider the geometric data of the cross-section.

I should be able to make a simple slab shell element and modify so I get as the same response as I would get using a shell ribbed element

 

[DynamicsMX]

I have noticed that, in a stiffness matrix perspective, the software only transforms the ribbed transversal section into an equivalent solid slab (based on the inertia). The design process should use the width and depth of the ribbed section, but how it "trims the section" is a black box. So what I use to do is extract the shear and moment, and I design the ribs with my own spreadsheets.

Typically, I apply a strip at every rib and provide a width equivalent to the separation O.C.


Hope it helps you.

 

[JoshPlumSE]

I'm not 100% certain. But, my belief is that the program selectively modifies the stiffness matrix to increased the bending stiffness in the direction of the ribs. Effectively making this a "orthotropic" plate element.

Then when the program gets the force / moment data from the analysis it merely uses those forces for a design / code check based on the what the user entered for the ribs....

I believe that there was once a problem with taking the analysis results and projecting it to extreme fiber stress plots for these elements. I know I saw an "enhancement" in the last six months or so that was meant to improve / correct this behavior in ETABS and SAFE... which are the programs I'm more familiar with.

From the CSI Knowledge Base / Wiki. This is about user specified "stiffness modifiers". But, my belief is that this is essentially what the program is doing behind the scenes when you tell it you're using a ribbed slab. I'm just not sure if other stiffnesses (i.e. shear, axial, or such) are modified as well.

How does the software apply stiffness modifiers to shell objects?​

Answer: When stiffness modifiers are applied, the corresponding terms of the shell stiffness matrix are modified. This is comparable to modifying, for a given direction, the Young's Modulus or the shear moduli of the material. The structural stiffness matrix is then assembled, and global equilibrium equations are solved to generate deflections. These displacements and rotations then relate to strain fields which produce internal member forces and stresses.