Raft foundation with an edge beam

[mar2805]

Hello folks.
I need some advice / opinion
We are are going to be supervising an construction of medium size residential building.
Dsigner has defined and raft foundation 20cm thick with and edge beam thats 30cm in width and 50 cm in total height.
The raft slab is reinfroced in top and bottom layer and the edge beam also.

I did a simple hand calculation on pressure contacts beneathe the edge beam thats 30cm wide.
A line load thats coming from the bearing wall, directly above the edge beam is around 100kN/m.
This is resulting with and contact pressure of an 333,33kN/m2 (100kN/m divided by width 0,30m)

I know this is pure simplification but allowable contact pressure is 200kN/m2.
Shouldnt the beam width be wider?!
I understand that this is an raft but there is going to be a lot od high pressure concentration under the edge beam, since the bearing wall is directly above it. Wont this result with an soil failure?

 

[HTURKAK]

The beam width at least 500 mm will be better and should be haunched . What about the frost depth? I written slab on ground foundation and searched the web and found the following picture which depicts insulation detail however useful to see the ratios.

 

[mar2805]

I agree with you on edge beam width....Should be checked against allowable bearing pressure.
In this case it should be minimum 50cm.
As for your question on frost....we dont have any snow....but the edge beam will be -50cm bellow the top surface.

 

[r13]

Note that the designer might have used FEM to determine the pressure against the soil, thus validated the narrower edge beam dimension - FEM usually produces a lower pressure than approximation by hand cal.

 

[mar2805]

Designer provider an area support (spring support) beneath the slab but no line support (spring support) beneath the edge beam.
I found this to be wrong.
His contact pressures beneath the slab are very close to the allowable contact pressure, but not above.

 

[r13]

It is odd that not provide springs under the edge beam that support load directly above it, and share some floor load. He needs to double check his design.

 

[BUGGAR]

And the exterior grade should be lower than the interior.