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SteelPE (Structural) (OP)
10 May 10 9:57
Unfortunately I have not had much experience when dealing with pile foundation systems.  Currently I am faced with designing my first pile foundation system.  The building is a 30,000 square foot warehouse a portion of which will have a mezzanine.  The slab live load is 250psf and the mezzanine live load is 125 psf.  The piles are to be founded at least 4'-0" below grade for frost protection.

I have reviewed the geotechnical engineer's recommendations on what type of piles to use (he gives 2 options steel and wood).  However, in his report he does not give an uplift capacity or a lateral load capacity of each type of pile.  I am wondering if this is normal.

Currently I am resisting the lateral forces in my building using concentrically braced frames.  Some of which have large uplift loads.  I can offset some of these loads by shifting my pile locations around pushing more slab dead load to the braced columns.  Therefore, uplift capacity may not be that big of an issue.  However, I am a little concerned when it comes to the lateral forces in the building.  I should be able to rely on passive resistance to resist the lateral forces but shouldn't I need a lateral pile capacity in order to develop this resistance (a force couple between the slab and the pile)?
 
TDAA (Geotechnical)
10 May 10 16:09
Ask for uplift and lateral capacities.  While you are at it, if you think you may have piles within 6 diameters, ask for the recommendations for load reductions due to closely spaced piles.

Be sure to ask for what you want.  The lateral pile capacity is more of a soil-structure issue (more so than a foundation sitting on soil) that is based on the stiffness of the pile, as well as the soil.  If your office has L-pile, ask for the soil input parameters, if it does not have the software, ask your coworkers how they normally calculate pile capacities in the area.
oneintheeye (Structural)
10 May 10 16:51
we generally calculate the loads in the piles and inform the piling contractor (and designer) what loads we require. Of course they see the si before hand and give us manageable figures and this is UK so may be different where you are. We can then ask for a bit more if needed. Generally it will be a case of adding more reinforcement possibly for higher lateral or uplitft loads, provided the ground is suitable. If you are using a flat slab construction (as opposed to ground beams connecting piles) then you can pretty much share the lateral load throughout the piles in the system. i.e. if you have a rectangular slab with 20 piles and the bracing imposes 10t lateral load you can share that load througout the 20 piles.  
BigH (Geotechnical)
10 May 10 20:58
4 ft long piles?
SteelPE (Structural) (OP)
11 May 10 8:13
Ahh yes, I made a mistake.  The piles must begin below frost elevation which is 4'-0" below grade.  Sorry for the mistake.

I had a chance to talk to the geotechnical engineer yesterday.  He said that I could figure the lateral load capacity as 10% of the overall capacity.  If needed he would do a L pile analysis (what ever that is) to get me the specific lateral load capacity (I'm sure if I wanted to know I could figure it out).  He also stated that the steel piles were virtually out of the question because of their cost.  Basically they needed to be 100ft + in length and they were going to cost way too much money for only double the strength of the wooden piles.

So now I am back to wood piles.  We have figured that we will need about 700 wood piles.  Initially I thought this number was astronomical but the geotechnical engineer didn't seem to flinch when I gave him that estimate.

Thanks for your help guys.
 
Zambo (Civil/Environmental)
11 May 10 8:56
I've only ever used timber piles about 6m long, for piles longer than that either driven steel, driven precast concrete or concrete bored piles. For your purposes I would have expected driven precast concrete piles possibly 450mm a 450mm (or even 300 x 300mm) square section or possibly 600mm diameter spun tubular piles.
TDAA (Geotechnical)
11 May 10 9:57
SteelPE

Look at http://www.ensoftinc.com/ for information on LPile.  It is a fairly widly used program, although I must admit, we do not own it.  I am familiar with it though to the point of providing input values for it.  It sounds like it is beyond what you need for this project, but I think it is good to know what is out there.
SteelPE (Structural) (OP)
11 May 10 10:02
Remember, I am the structural engineer who is designing the pile foundation system.  I am taking the information provided by the geotechnical engineer and using it to develop my design.  I have no idea how to design a pile using the geotechnical data provided.  I just using the allowable loads provided in the geotechnical report to design my foundation.

This is my first attempt at this type of design and someone will be reviewing my drawings and calculations.  Also, there may be a code review process which will give a second engineer a stab at what I have done (although it is not really his responsibility to check me).

Thanks for the information TDAA.
BigH (Geotechnical)
11 May 10 10:06
Not sure what your loads are - but timber piles I've used have been commonly up to 40ft (13m) in length.  In NJ, they were typically in the range of 30 to 35ft.  Could you please advise what the soil profile is?  Clay?  Sand? Why must the steel piles go to 100ft depth and you are considering wood piles, say, to 30 or so ft?  
oneintheeye (Structural)
11 May 10 10:10
L pile is a standard design progam. I agree SteelPE that you don't need to know much about it other than you design piles with it (and do you really need to know that even).
Not sure about timber piles either I have never used them but my immediate reaction is the same a zambos in that I would expect precast driven piles or bored. Typically here we approach a piling contractor who gets a design done.  
SteelPE (Structural) (OP)
11 May 10 10:22
In the geotechnical report the steel piles are discussed as being end bearing piles.  Test borings were terminated at 42 feet with one hole being advanced to 60 feet to see what was below.  The site consists of fill (dense sand, stiff clay and soft clay) to 13ft., organic deposits from 13 ft to 23 ft and then clay below (ranging in stiffness).  The clay layer continued to 60 feet with no end in sight.  Ground water levels were around 8' below existing grade.

I'm sure there are other factors that are going into the selection of the pile system that I am unaware of.  I care about the process but I am more worried about designing my system than I am about the pile type selection process.

I don't really want to butt into the geotechnical engineers area and begin disputing his results.  I wouldn't want the geotechnical engineer telling me how to frame/design my building.

Hope this helps clear some things up.
 
aeoliantexan (Geotechnical)
11 May 10 14:16
Is the warehouse really only 30,000 square feet?  If so, 700 piles is a lot of piles! I guessed the dead load at roughly 150 psf and multiplied 400 psf times 30,000; then divided by 700 and got less than 9 tons per pile.  Did I miss something?

I used to use a lot of timber piles in lengths up to about 50 feet for 25 to 40 tons allowable capacity each.  In stiff or very stiff clay the capacity would be based on an allowable skin friction times the bounding area.  We would talk with the structural engineer about the range of column loads, the minimum number of piles per column, and the maximum spacing along bearing walls and agree on one or more design pile capacities.  We would then recommend the minimum penetration into the bearing formation for piles of a given diameter and capacity. Usually the tip diameter is specified, and the average diameter must be estimated based on common natural tapers.  During construction we would observe and count blows to determine when the pile tip entered the bearing formation, as well as to watch for breakage and conformance to location and plumbness tolerances.  We would report unacceptable piles to the structural engineer, who would redesign the group using one or more added piles.  Your geotechnical engineer should be able to do just what I described.

I expect that the lateral capacity of timber piles in clay will be rather modest. Special details or a lot of penetration of the pile top into the cap is required to obtain a fixed-head condition, so a pinned head condition is usually assumed.  You may be able to get substantial lateral load resistance from passive pressure around the pile caps and grade beams.  If you need more than a nominal 2 to 3 tons lateral capacity per pile, try L-Pile.

A filled site with organic soil may undergo additional long-term settlement and subject the piles to downdrag, downward friction on the pile from the fill.  Ask the geotech if that is an issue.
SteelPE (Structural) (OP)
11 May 10 14:38
aeoliantexan

Downdrag has already been considered in the capacity of the piles.  This is why the capacity is so low.  Slab loading is 250psf plus the weight of the slab.  We are currently figuring a 10" slab which will add another 125psf to that weight plus the weight of grade beams and exterior walls.  Total building area is around 34,978 square feet.  I initially came up with 580 piles based upon total weight calculations but you have to figure that each pile cap will not require and exact number of piles.

The geotechnical is well aware of the number of required piles.  He is figuring that even with the large increase in the number of wood piles there will still be a savings of over $200,000.  All foundation loads were transferred to the geotechnical engineer before the design of the foundation system began.
 

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