Burial Depth of foundation

[Zee]

Hello everyone,

I have asked this question at many forums but didn't get any satisfactory and conclusive reply. A 5 story (59' including mumty) reinforced concrete structure needs to be constructed in a seismic zone 3. The structure is rectangular in shape with it's dimensions being 60' by 180'. To increase the dynamic stiffness of the structure, two expansion joints have been provided at equal distances so the structure finally ends up getting into 3 equal parts i.e 60' by 60'. The seismic stresses are approximately 10% of the dead load stresses. 
According to the geotechnical report, the construction site mostly contains gravels with less than 10% of the fine material. However, there is a small patch of rocky soil as well. The bearing capacity of soil is 2T/SFT at 3ft depth. Based on these details, I have few queries;

1. The size of foundation in this case is not a problem since the soil has a very good bearing capacity. Keeping in view the minimum cost of excavation, sliding and overturning what should be the minimum depth of foundation below natural surface level?

2. There is 20' retaining wall located 10' away from the structure which retains the same soil. Will it transfer any lateral thrust to the foundation of the structure?

3. The rocky soil covers a very small area, is there any need to provide anchoring to the foundation??? I suspect uplift in that region.

 

 

   

[Badar (BAZ)]

2 hours ago, Zeshan said:

1. The size of foundation in this case is not a problem since the soil has a very good bearing capacity. Keeping in view the minimum cost of excavation, sliding and overturning what should be the minimum depth of foundation below natural surface level?   

As it is a reinforced concrete structure, the sliding should not be a concern due to the self-weight of the structure; same goes for overturning moment. The consideration of the frost penetration will govern the depth, if applicable. IF there is SOG, it can also govern the minimum possible depth of the footing.

2 hours ago, Zeshan said:

2. There is 20' retaining wall located 10' away from the structure which retains the same soil. Will it transfer any lateral thrust to the foundation of the structure?

Why would you ask that? How can a retaining wall transfer any thrust to the foundation?

2 hours ago, Zeshan said:

A 5 story (59' including mumty) reinforced concrete structure needs to be constructed in a seismic zone 3. The structure is rectangular in shape with it's dimensions being 60' by 180'. To increase the dynamic stiffness of the structure, two expansion joints have been provided at equal distances so the structure finally ends up getting into 3 equal parts i.e 60' by 60'. The seismic stresses are approximately 10% of the dead load stresses. 

There is no need to split the structure of this size in three portions.  What is dynamic stiffness? I have not come across this term before in relation to the structural design of a building.

[Zee]

Thank you for your reply.

Dynamic stiffness refers to the ratio of dynamic force and dynamic displacement. I look at the dynamic characteristics only when a structure needs to be constructed in severe seismic zone. Looking at the overall global behavior of the structure (without any split), it has two sides, flexible (short side in this case) and stiff (long side in this case). Even a uni-directional excitation in longitudinal direction yields around 15% absolute maximum and 20% absolute minimum displacement in transverse direction mainly because of stiffness and mass eccentricities. Considering first 3 modes with 3rd mode being dominant in torsion has developed height wise variation in the deflection response of the structure in this particular case.       

Before the architectural planning, I reminded the architect about one expansion joint in order to have a reasonable aspect ratio of the structure considering the seismicity of the region. But the architect himself planned the structure with 3 portions. Anyways, you are right and I agree to your point on this.

Regarding the thrust on foundation, I'm not sure that's why I have asked it here. Found this statement in the geotechnical report.

[UmarMakhzumi]

I would also add that you need to place the foundation on competent strata so that settlements are within acceptable limit. Settlement governs foundation burial depth too at times.

Thanks.

 

[Zee]

There is a small patch of rock. I suspect uplift in that region. It can't be checked for settlement in that region. Am I right???

[UmarMakhzumi]

Why would be there uplift due to patch of rock? Or do you mean everything would settle except for the location where that rocks is?

Thanks.