Deep and Shallow foundations 1

[Okiryu]

Hi, what can be the main considerations or concerns if you have half of the building supported on piles and the other half supported on shallow footings? There is a big difference in the depth of the bedrock (mudstone bedrock from 1.5m to 20m below ground surface). The building is quite rigid (1-story box-type building with concrete walls and roof). Site is in active seismic area with liquefaction potential. Piles are approx. 15m-20m long. An expansion joint will be needed in the boundary of both systems. Some local engineers advice me to install piles for the entire building, even at locations where the bedrock is shallow. In any case, lateral resistance and displacement may have large differences throughout the building...

 

[Canuck67]

Differential settlement between the area of the building on piles vs. the area of the building on shallow footings. From your description, it definitely sounds like a portion of the building must be on piles. Where the depth to bedrock is reasonably shallow, I would either place it on short piles, or excavate to bedrock and put your footings directly onto bedrock.

 

[KootK]

The usual concern is differential settlement between the piled and shallow foundations. The piles often tend to settle less. I've done this before myself however. You can either isolate the two structures and accommodate the expected differential movement or you can attempt to design a structure capable of bridging between the two areas of differential movement. Certainly, if your shallow foundations will be sitting on bedrock, I would be much less concerned that if there were sitting on more compressible media.

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.

 

[Okiryu]

Yes, the footings are planned directly on bedrock. I expect that since the bearing layer is the same for the entire building (mudstone bedrock), axial capacity and differential settlement may not be an issue. My concern is about lateral resistance. There is liquefaction potential which makes things harder. I will need to do some preliminary checks to see deflections differences between short and long piles.

 

[KootK]

If the buildings are tied together laterally at the foundation level, I would expect the portion of the building that is on longer piles to impose considerable drag on the portion of the building that is on shorter piles. It sounds as though that's your concern too. Much will depend on the owner's expectation after a serious seismic event. If the buildings need to remain serviceable post-liquifaction, there will definitely be some challenges.

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.

 

[Mccoy]

Okiryu, interesting issue. In the recent building code in Italy it has been forbidden to use a foundation which is partly shallow and partly deep, probably because of all the uncertainties mentioned above. In Okiryus case, we should either drill piles thru rock and thru soil or make some kind of soil improvement and substitution and adopt a shallow rigid foundation. Either solutions are not without problems.

I would concur anyhow that it is best, to treat the design as two separate designs: 1: building half on rock; 2: building half on soil, subject to different actions. Seismic action would be totally different in the two halves because of different site response, no amplification on rock, very likely amplification on soil, with unknown effects at the boundary. regulations usually ignore the unknown effects.

Let us know about the final decision, I'm interested in such peculiar cases.

http://www.mccoy.it

 

[Okiryu]

We are planning to install the foundation in bedrock. McCoy thanks for remind me about the site response against seismic. The different site responses will be reflected based on there will be 2 site seismic classifications. Not sure how the structural can consider acting lateral forces in this case. Does anybody have experience with a dual site seismic class for the same building? Is it common to do the building seismic design in 2 separate designs as McCoy suggested? Are any codes for these conditions?

 

[Mccoy]

it common to do the building seismic design in 2 separate designs as McCoy suggested? Are any codes for these conditions?

The Italian codes, drawing upon the European codes and probably the IUCB-ASCE codes (but I'm not sure here), say something about such an issue.

You may either:

1) use a single seismic action, the most severe one (simplest and not necessarily most conservative)
2) use a maximum relative displacement between two points, based on the soil maximum displacement in such points according to the different local site response (a formula is provided)
3) carry out two distinct analyses with two distinct sets of actions if the structure is divided into separate parts (that could be your case).

It is likely that the ASCE/SEI 7-10 have some specific reccomendations, I'm going to check the Eurocodes and these, since they are international codes they are pretty much authoritative and can be implemented if the national code has no specific reccomandations.

http://www.mccoy.it

 

[Okiryu]

Mccoy, as always, thanks for your input. For option 2, do you have the formula which you are referring to? Our preliminary approach is to use a single seismic action (use the worst case scenario seismic site class) and use piles for the entire building. We will havve very short and very long piles, with liquefaction affecting the long piles only. As you mentioned that there is some specific recommendations for these cases, I need to check ASCE/SEI 7-10. If you remember where can I find these recommendations in the code, please let me know.