Estimation of pile foundation forces for a basement foundation 2

[javengar]

Hi, I'm a young engineer working as an assitant on the design of a 8 stories building with 2 basement levels in a high seismic area (Latin America). The soil profile in the area is really strenght weak, so we have to use 40 meters (131 ft) piles to reach a competent stratum. I've read that the basal shear must be considered only until the top grade level, neglecting the mass of the basement levels. Based on this I have some questions:

1) What would the load path be?, as I kind of understand after watching the model on ETABS, que top grade slab will transfer almost all the shear forces to the basement walls, am I correct? So the pile foundation and pile cap for the interior frames or columns, would recieve low shear and moment forces, so the lateral design of piles wouldn't be a problem? (axial force would govern the desing and the number of piles I need per interior column foundation).

2) Talking about the piles that would be part of the foundation of the basement wall, these piles would recieve a high shear force since almost all the basal shear would reach them? I've read in Jack Moehle book, that since the basement walls would be confined by soil you may restaint the translation of the wall, resulting in a similar design as the interior frames with no major shear forces (since they are taken by the soil passive force).

So basically, my questions are regarding if the lateral loads that would reach the pile would be high or not.

Thank in advance for your answers (I'm not a natural english speaker so maybe I've made some mistakes on my text)

 

[Greenalleycat]

Your english is all good mate, don't stress

I think your question (1) is the most important thing here - what would the load path be?
It's not OK to just assume that the surface soil takes the load - you've already said that these soils aren't very good, so can they really resist the load?
How much displacement is required to mobilise the resisting passive pressure in these soils - what load does that put into the piles, especially if you consider P-Delta effects too?
If the soils are really soft and gravity loads are piled, then is it likely that the ground could settle (particularly in an earthquake) and you lose friction at the underside of the basement slab as the superstructure will not settle with the ground
The answers to these kinds of questions will influence your final question - how much load actually reaches the piles?

For this type of problem I'd expect a model that incorporates the soil profile into the model
This would commonly utilise springs representing the soil stiffnesses at different depths - the geotech should give this
The superstructure loads then pass into your foundation (basement walls etc) and into the piles
The load distribution to different elements will depend on the relative stiffness of basement walls, piles, and the soil at differing depths
Making your piles larger will make them stiffer which will attract more load to them etc etc