Raft Foundations Design

[TwelveRox_12]

Hey guys, so my ex-colleague designed a raft foundation using software (Note that this software unable to convert drop panels to strip), and so it has drop panel for every column (for the purpose of punching shear), and obviously there are different depth for the drop panel that depends on the loading of the column. My superior told me to change the individual drop panel into several strips of drop panel so to reduce the depth of the panels. Now then I have converted the drop panels into strips, I would like to ask how do I determine the new depth of the strips? As the strips were based on the drop panels, there are various depths, and my superior intended to standardize the depth (there are some strips with 800mm and some 500mm, superior said that the difference were too much to change everything to 800mm) to make it easier for the contractor. Hope to get some answers from all of you.

Thanks.

P/S : Picture for reference. (The slab is 150mm thick)

Best regards,
-TR

 

[atrizzy]

This should be done as you would for any suspended slab. I would suggest a deflection criteria as a first check, then ensure each column has adequate punching shear resistance and that the strips are OK for flexure and beam shear.

 

[rapt]

If the drop panel depths were based on punching shear requirements, that will not change, so all are now 800 to make it easier to build.

I do not believe you can justify a 150mm slab depth if it is a raft slab. Sounds far too thin to me.

 

[dik]

I would have thought that a 300 or 400 slab would have been used with 800 drops. I would have thought with the formwork reuse that drops would be preferable to 'beams'.

Dik

 

[rapt]

dik,

I assume these drop panels/strips are below the slab, and the ground is the formwork!

 

[dik]

The drawings should have shown the sloped soil as hidden lines... reinforcing might be a little tricky... best to have spread footings?

Dik

 

[KootK]

At 150 thick, I'd say that this IS spread footings with an integral slab on grade.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

Agreed

Dik