Shear wall foundation on piles

[dcStrucEng]

I'm in the preliminary design phase of a 10-story reinforced concrete building on the east coast (US). The Geotech report came back and the conclusion was that the soil is quite poor. Due to this, the building will only have one below-grade level. A deep foundation system of square concrete piles was recommended. The report when on the state: "The lateral capacity of the piles is expected to be poor."

I appreciate the geotech's acknowledgment of the piles' lateral capacity, however I need to be able to transfer the lateral forces through the foundation and into the ground somehow. Don't I need to go back to the geotech for a more detailed value of the piles' lateral capacity? How else am I supposed to transfer the lateral forces?

It's all well and good to design shear walls to take the lateral forces, but if those forces don't have a load path into the ground, what good are they?

Thanks, in advance for any comments/assistance.

 

[JAE]

I'd think your below-grade walls would utilize passive pressure of the surrounding earth. You would need to get the passive pressure information from the geotech. I'd ignore lateral piling stiffness totally.

 

[msquared48]

Use batter pile if you have to. It's done all the time.

The only constraint may be pile too close to the property line in which case interior, or setback batter pile would be necessary.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

You must supply the Geotech with preliminary load information if you want her real involvement, otherwise you will get similar vague estimates.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dcStrucEng]

Thanks for the responses. I have some follow-up questions/comments:

JAE - If I utilize the passive pressure of the earth against the below grade foundation walls as my lateral force resisting system, then my lateral forces will need a way to get to the exterior walls. If, for example, my shear walls are at the interior of the building, then the lateral forces will need to transfer from the shear walls, through the first floor concrete floor slab (at grade) to the exterior foundation walls. But the lateral load isn't going to know to makes its way directly to the exterior foundation walls and bypass any interior columns it sees along the way. In this situation, I feel like much of the lateral load will get transferred to any interior columns along its way to the exterior foundation wall, most of which are only meant (and designed) for gravity loads. Do you agree?

m48 - If I go with batter piles (property line concerns shouldn't be a problem), then I assume they only need to go under pilecaps which support the shear walls. Is there any reason to put batter piles under any of the gravity columns?

pgreen - I agree about supplying load information to the geotech, this is something that we always do. But we tend to only supply the geotech with preliminary gravity loads, not lateral loads. Do you give the geotech lateral loads too?

Thanks.

 

[mmillerpe]

Depending on the seismic class you may need to provide grade beams between pile caps. If you do, you may be able to use passive pressure on the side of these to resist the the loads.

You mentioned that there was one level below grade. I would consider transferring the load from any interior shear walls at the first elevated slab level (not the slab on grade) to the foundation walls.

 

[msquared48]

dcStrucEng:

If there are grade beams in line with the shear walls that can transfer lateral load like a drag strut to another pile, then you could add one there. Otherwise at the shear wall.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

dcStrucEng
I'm not sure I understand fully your comment: [blue]but the lateral load isn't going to know to makes its way directly to the exterior foundation walls and bypass any interior columns it sees along the way[/blue].

Interior columns have relatively LESS stiffness than shearwalls in your building. The framing system of your building should be designed such that load from your building gets dragged into shearwalls that take the load down to the lower level of your building.

Once there, the system should be designed to drag that load into adjoining basement walls (via floor diaphragm, collectors, etc.) where such walls are perpendicular to the direction of loading. They then press against the soil - passive pressure.

Interior columns are like wet noodles compared to the stiff floors and walls of your lower level.

 

[slickdeals]

mmiller:
Correct me if I am wrong, but I don't think you can use passive pressure in seismic conditions.

 

[JAE]

What else is there? In reality, it is a sort of active pressure that is bringing the load into the building anyway.

The static method just brings the load backwards through its load path to the source of the load - which is the earth against the building.

 

[mmillerpe]

Slickdeals: I haven't had to deal with designs in high seismic regions, so there may be instances where you may not be able to use passive pressure, such as where liquification may occur. But other than that I am not aware of any restrictions of using passive pressure to resist seismic loading. As JAE mentioned, how else do you deal with the lateral loads, especially in shallow foundation situations? There is passive pressure, friction (which depending on gravity loads cannot be relied on in all conditions). For deep foundations you can use the lateral capacity of the piles or batter piles, and it seems that you cannot get a good lateral pile capacity these days.

The geotechnical engineer should be able to provide recommendations for lateral capacity of the soil if the soil is capable.

Additionally, IBC 2009 tablulates presumptive lateral load-bearing capacities for different soil types (Table 1806.2) Additionally, under Section 1806.1 allow for 1/3 increase of these values where "used with the alternative basic load combinations of Section 1605.3.2 that include wind or earthquake loads." This statment leads me to believe that passive pressures (or a percentage thereof) can be used to resist the earthquake loads.

 

[FixedEarth]

dc..;

You are 100% correct. You need to transmit all loads to the ground and you need to know how much deflection the soil can take. You have to return to the geotechnical and say for example, How much lateral capacity is available from a 16 inch square driven concrete pile and embedded 55 feet in the ground, if we limit the lateral deflection at the ground surface to 3/4 inch for example?

You may need to give the typical axial, lateral and moment loadings for single and group piles. Also you need to mention if you are in a free head condition or fixed head condition.

We provide charts for say 16, 18 and 24 inch piles and give 1 graph for vertical capacity, another for lateral capacity and we also give efficiency in pile groups. Sometimes we include settlement calculations. This information is routinely provided by the geotechnical firm.

 

[tdstructural]

I live/work on the west coast and deal with seismic design on a regular bais and yes you can use passive pressure to resist seismic forces.

 

[abusementpark]

JAE,

Do you assume that the basement wall to which to the shear walls are distributing the lateral load is rigid enough to spread the load out in uniform passive pressure?

Doesn't the diaphragm need to be transferring the lateral load near the center of wall bearing area?

 

[JAE]

abusementpark,

I would think it would all depend on the configuration of the basement walls, the shearwalls, etc. It's all about load path following, proportionately, the stiffness.