Bearing Capacity of a Pier and Pad Foundation.

[saurav.str]

Does skin friction of the pad come into play increasing bearing capacity? Can we add the skin friction provided the sides of the pad to the bearing capacity of the foundation?

 

[oldestguy]

NO. You can't predict what might be the situation in years to come.

 

[BridgeSmith]

Beyond the uncertainty with future excavations, there is the work by O'Neill and Reese for drilled shafts. Their research indicated that the mobilization of skin friction 'shearing' along the sides occurred at much smaller displacements than the end bearing. Therefore, when the full bearing capacity is reached, the side friction has already experienced too much movement to retain its full friction value. There's interaction equations from their work now included in the AASHTO bridge design spec, but we have not bothered to use them since the first time we went through the arduous calculations, only to get a result that gained us almost nothing (we were able to utilize about 5% of the skin friction with the full bearing and vice versa).

You're supposed to ignore skin friction near the bottom anyway (in the case of drilled shafts, 2 diameters), due to the 'downdrag' effect of the bearing load on the surrounding soil. I don't know about how your geotech info was presented, but what we get is adjusted based on the size of the footing to account for that effect.

In short, OG's response of a flat "NO" is correct, but for more reasons than just uncertainty about future conditions.

 

[JoshPlumSE]

We typically only use skin friction for drilled piers, not pad footings.

 

[EireChch]

OG is right..it would not be accepted engineering practice to consider it. Also have a think too about the interface friction. If, just if, you were to consider it...some pads are constructed without any formwork so concrete is cast directly adjacent to the soil. That would provide some degree of interaction. However, some pads are cast against timber formwork with backfilled after striking of formwork. This would likely provide less interaction as the material may not be properly compacted against the pad.