Pad foundation effective bearing area and foundation depth

[TWfea]

Hi all, I have a couple of questions regarding the effective bearing area, and foundation depth for pad foundations.

1. With regard to the critical depth of the pad foundation. I have concluded the following, and was wondering if this seems reasonable, or are there other methods that can be applied.

• For the bending moment, if the required steel area (number and diameter of re-bar) is reasonable, i.e. doesn't require re-bar every few centimeters
• The shear and punching shear stresses are not exceeded.

2. Effective bearing area

• By projecting the column area at a 45deg angle down to the bottom face of the foundation, we get an effective bearing pressure, I have seen it suggested that this area that should be used to determine the bearing pressure. But if we take a raft foundation as an example this is not the case. So my question is:
  • What are the criteria that should be met before we can assume that the pressure at the bottom of the foundation is distributed uniformly over the entire base.

I would be very grateful to hear your comments and feedback on the points above, and even a short reply would be much appreciated!

 

[EireChch]

Regarding the 45 deg projection area down being the new foundation area - i think that could be a requirement of Eurocode 7. However, it doesn't make sense, if you have a rigid pad then there is no reason why the bearing pressure would be distributed over the full or nearly full pad area.

In practice I think most geotech engineers just assume an infinitely rigid pad with the column load is distributed evenly over the entire pad area. It is probably inaccurate but pales in the comparison to most of our other inaccurate assumptions in a settlement calculation.

 

[TWfea]

Thanks EireChch. I agree with your comment, and it all seems to stem from the fact that the foundation will be more rigid than the soil that it is built on.

It is good to hear the view point of others, so thanks again for your contribution.