Punching Shear reinforcement for raft foundation slab.

[Refaatfaraj]

According to (EC2 – Clause 6.4.5) punching shear reinforcement can be used to resist the two-way shear in FLAT SLABS around the columns.
But I am wondering to know if the shear reinforcement can be used in RAFT FOUNDATION SLAB (Uniform thickness).

Note: the capture attached shows how does the punching shear reinforcement distribute around the columns according to EC2 .

Please , I would like to prefer if you support your response with a reference .

Any Guidance will be appreciated

 

[JoshPlumSE]

There is no reason why you couldn't do it for slabs as well. However, it's not generally needed as foundation slabs can easily be increased in thickness compared to elevated slabs. Also, you can subtract the bearing pressure on the soil below from the punching shear demand of the column.

 

[Refaatfaraj]

Mr . JoshPlumSE

Thanks a lot for your response.

Generally, the drop panel for flat slabs and pedestal for raft foundation is the common solution to increase the concrete section capacity.
But, for some reasons and time construction saving we are trying to avoid increasing the raft foundation thickness (Pedestal).
So, other solution could be using the shear reinforcement since the raft foundation behavior is similar to the flat slab behavior. But I am not sure about is solution.

 

[r13]

The added cost of reinforcement and labor can easily out weigh the cost of adding a few inches of concrete. You can deepen the slab around the column though. However, it complicate the earth works.

 

[HTURKAK]

The use of punching shear reinforcement for RAFT FOUNDATION SLAB is not a good idea.. What is the reason for slim raft?...The thickness of foundation slab should be selected to provide fixed support to the columns and walls and the selected thickness normally should not require punching reinforcement. If rigidity of mat is not required ,( may be for single storey warehouse etc..) it is better to choose thickened slab under columns rather than punching reinforcement.

 

[Refaatfaraj]

Dear HTURKAK

Many thanks for your guidance.

(The use of punching shear reinforcement for RAFT FOUNDATION SLAB is not a good idea? )

Do you have any reference or code precaution which mentioned that the using of punching shear reinforcement is not recommended / allowed for raft foundation slab ?

 

[KootK]

I'd be pretty shocked if you actually found an extensive reference specifically on punching shear reinforcement for raft foundations. You may have to settle for simply whatever codes and standards govern elevated slab design in your jurisdiction. Here's a little something.

 

[Refaatfaraj]

Dear KootK

Many thanks for your guidance and really very appreciated.

Actually , our case that the raft foundation slab thickness (2000mm) and there is no possibility to increase the raft slab thickness at the column locations snice the basement height level is very limited . So, we have used the shear reinforcement to increase the raft foundation capacity as mentioned in your attached reference -- (standee bars) to resist the beam / punching shear .

 

[HTURKAK]

I could not see the reason why you can not increase the slab thickness... The mat thick . could be thicker where necessary keeping the top level of mat constant. There are a lot of examples and one is ; ( Froum bldg complex Frankfurt ). This snap is taken from J.A.HEMSLEY

You provided 10 mm dia links with 125 mm dist. both ways ..If this is true, i will suggest to ask the services of an experienced structural engineer rather than looking for an answer from a web forum.

 

[r13]

Another way is to use higher strength concrete (a 11% increase from 4 ksi to 5 ksi).

 

[Refaatfaraj]

Dear HTURKAK

Many thanks for your response

1.I think your capture shows ( piled raft foundation ) and the raft thickness has been increased at the pile locations to resist the two way shear from two sides of foundation ( top &bottom) .
Whiles our issue regarding to raft foundation supported on soil . my doubts was about using reinforcement to resist the shear forces .

2.(If this is true, I will suggest to ask the services of an experienced structural engineer rather than looking for an answer from a web forum.)
Yes it is true based on analysis and design results which govern by the structure geometry and the output of geotechnical study .

Thank you for all of you for your useful contributions

 

[HTURKAK]

Dear Refaatfaraj (Structural)(OP),


The thickness increase is not due to piled foundation. The same concept is used for rafts directly supported on soil. Here is another picture... May be you have seen Al Faisaliah Tower Riyadh , 40 storey bldg.

10 mm links can not provide support to top reinforcement. You should use chair with increased dia or better use steel profile . I will suggest you increase the thickness under the cores or choose higher strength conc. ( as proposed retired 13) rather than providing punching reinforcement.