Effective section stiffness coefficient of composite slabs

[member 1218881]

Hello, In linear analysis, I get the effective section stiffness of the composite floor made on steel trapezoidal sheet as follows. These values ​​are equal to the values ​​I got for reinforced concrete slabs. Are there recommended effective section stiffness values ​​for composite slabs? Do you think this approach is correct?

 

[271828]

From what you've provided, it's hard to say, but the approach doesn't look correct at face value.

If you're doing this in SAP2000, then the following is what you need to do:

1. Compute the slab transformed MOI for bending parallel to the ribs. It should be in units such as in.^4/ft or mm^4/m. It would probably be better to transform the deck steel to concrete.

2. Compute the slab MOI for bending perpendicular to the ribs. Same units as above. Only use the solid thickness of concrete between the ribs.

3. Under Define Area Sections, define a shell with E of concrete. The shell thickness should be the solid thickness used in #2. Use the M11 or M22 PM to assign the higher MOI for the direction parallel to the ribs. Also, use mass and weight PMs to result in the correct uniform mass and weight.

 

[member 1218881]

If you're doing this in SAP2000, then the following is what you need to do:

1. Compute the slab transformed MOI for bending parallel to the ribs. It should be in units such as in.^4/ft or mm^4/m. It would probably be better to transform the deck steel to concrete.

2. Compute the slab MOI for bending perpendicular to the ribs. Same units as above. Only use the solid thickness of concrete between the ribs.

3. Under Define Area Sections, define a shell with E of concrete. The shell thickness should be the solid thickness used in #2. Use the M11 or M22 PM to assign the higher MOI for the direction parallel to the ribs. Also, use mass and weight PMs to result in the correct uniform mass and weight.

I do this at etabs. What I understand from what you said is to turn the composite deck into a concrete shell. Etabs automatically designs composite steel beams and composite floors. What I want to ask is that we do not use stiffness modifiers in linear analysis of steel elements. (excluding second order effects - direct analysis) However, I think it is possible that there will be stiffness losses in composite floors during an earthquake. Concrete cracking etc. In this case, is there anything I can do without changing the definition of composite flooring? Has anyone ever designed composite flooring before?

In theory, wouldn't composite floors actually crack like reinforced concrete floors due to the concrete layer? Or should I act differently in the calculations due to the steel beams underneath?