Waffle slab design and anlaysis as equivalent solid slab

[sybie99]

When designing a waffle/coffer slab, can the following method be used or are there errors or flaws in the approach:

1. Determine the equivalent thickness of a solid slab, that is with the same section modulus.
2. Now analyse the slab as a solid slab.
3. Check deflections are okay
4. See that, where there are positive moments, the neutral axis falls within the flange. To do this see that the maximum positive moment is less than Mr = 0.45 x fcu x b x hf x (d - hf/2, which, if true, means you can use the rib spacing as beam width when designing rib reinforcement.
5. Use postive moments from flat slab FE analysis to calculate reinforcement needed/rib.
6. For negative moments see that these moments(i.e. hogging moments) fall within the solid area around columns or the solid column strips if these are provided.

If this method can be used, it means I can design coffer slabs where the column layout is irregular making it difficult to analyse by hand, by doing a normal FE Analysis of the equivalent solid slab.

Thanks

 

[rapt]

How can you check deflections of a cracked structure on FEM software? Even if you are able to allow for cracking in the FEM (unlikely), the way an equivalent thickness flat slab cracks and its effect on stiffness will be completely different to a waffle slab which is effectively a T section.

Also, torsional stiffnesses will be completely different.

Model it as a coffered slab!

 

[msquared48]

Why don't you analyze it as a two way beam system with fixed connections, letting the slab give stiffiness to the connections of the intersecting beams? I do not think that you would get any reasonable answers here with just a flat slab approach...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[homostructural]

Hi! I don't think your approach is correct.
Check this method to design ribbed slabs using only hand calculations.

All about Eurocodes:

http://www.DesignToEurocodes.com

 

[TXStructural]

The method is describe in the CRSI Design Handbook. Also, preliminary design can be made using

http://concept.crsi.org

(free but you have to provide your email address.) This will help establish the final design parameters and provide a sanity check to your design. It also lets you compare various slab systems. The final phase is in development now, to allow use of voided slabs (modern filigree systems).

If you are using one of the European proprietary waffle systems, you may need to use their design guide.