Raft Foundation Design 2

[damo74]

I'm designing a raft foundation for Semi-D domestic dwellings. The contractor wants to use this form of construction, only if we can design the raft for a maximum thickness of 200mm. He does not want to use ground beams. It is only economical if we use mesh top and bottom and u-bars around the edges. The ground is good with an expected bearing capacity of 150kN/m2.I have analysed the raft structure using FEA and it will work theoretically. The maximum deflection is about 5mm at the corners. I'm worried about differential settlement and excessive cracking of the blockwork. Has anybody designed a raft foundation this thin without ground beams? Has anyone had any problems with such construction? All advice is welcome!

 

[Lutfi]

200mm (about 8 inches) doesn’t sound like much for a raft foundation!

Design the raft as required and ask the contractor to build it per design. Engineers should be in charge of the design not the contractor. It is your responsibility to produce safe and code compliant design.

Regards,


Lutfi

http://www.cdeco.com

 

[damo74]

Lutfi,

I agree. However, I'm only looking for some past experience from other engineers who may have designed such a crust raft. I'm not letting the contractor tell me what to do, only investigating other methods of construction!!!

 

[mubash]

As you stated that bearing capacity of soil is quite ok I do not think there will be any problem of differential settlement and cracking there of.Your raft will bridge any chance of settlement being continous and monolithic.Just imagine as if you are designing a slab or turn your raft upside down and you do not need beams in slab as being of short spans.I hope there will be no pocket of loose soil and you have got tested bearing capacity bore holes as required.

 

[DaveFlax]

Hollow areas caused by the curling at cracks and at the edges can be very detrimental, especially when the thickness is only 200mm. You may want to consider using shrinkage-compensating Type K cement to eliminate the curling problem.

 

[BigH]

You say that the bearing capacity is 150kPa - is this the ultimate bearing capacity or allowable bearing capacity? What is the allowable bearing pressure based on settlement? Why a slab foundation? For a house, footers on allowable bearing pressure of 150kPa should be routine.

 

[lexim]

Hi damo74,

Just to lend a hand here if what you are referring to is a slab raft without an beams for transfering load or stiffner.

I have designed a five storey office building as a beamless (flat) slab raft on a safebearng soil strength of 65kn/m2 but i had end walls acting as abutments for soil retension on all sides. I suppose you can analysetour structure likewise if your desin incorporates columns. But a note of warning though:
1. settlement expected over extended period of time.
2. Effect of setlement on the brckwalls.
3. The possibility of scour at the edges of slab( los of support)
4. Leakage and rising dmp problems if sab is below grade.
5. You need to seek advice from local engineers on existing same consruction.

You may require for my calclations on aileme@yahoo.com

Cheers