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# 2D model or 3D model

## 2D model or 3D model

(OP)
Hi, I posted this thread in foundation section but no help there. I think its pretty basic question about modeling so im posting it here too.

Heres the question - if we want to calculate contact pressures under foundation, which model is more accurate/better 2D or 3D? SEE ATTACHED FILE.

I think plane - 2 d model has too conservative results (because it doesnt count the contribution/effect of other foundation nearby?)
I like 3D model better. Is it OK? How do you guys calculate this?

### RE: 2D model or 3D model

I'd say that your 3D model would be be fine for design and surely more accurate for the distribution of any loads applied post-construction. Of course, it requires that some of your walls span as deep beams to accomplish a more uniform distribution and the walls would need to be designed for that purpose.

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.

### RE: 2D model or 3D model

With a difference of 40 kPa, I doubt you'll be struggling to justify the design. Unless the geotech has specified a bearing capacity of say 175 kPa... which is uncommon, usually they work in 50 kPa or 100 kPa increments.

Good little comparison exercise though.

### RE: 2D model or 3D model

(OP)
In this case the difference is only cca 40 kPa. But in some other cases (long walls)its much bigger.

### RE: 2D model or 3D model

You underestimate stresses below outer foundations, as you can see the difference between 2d and 3d model results.

In 3d model you have a rigid structure, so load distributes between all walls based on stiffness and load location. So basically load from inner walls gets distributed to outer foundations.

There is no stress distribution between foundations because of the soil - I think you where asking this. It is only distribution because of the structure itself, not soil.

### RE: 2D model or 3D model

(OP)
tnx for answer, so we all agree 3d model is better (more realistic) option?

### RE: 2D model or 3D model

It's realistic if you design the walls and strip footing to be stiff enough to distribute the loads uniformly.

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