Calculating No. of Piles

[imsengr]

Hi everyone,

I've got a question about calculating the number of piles for an isolated spread footing, or for any footing, for that matter. It seems simple enough, and in the end analysis, the answer as to the number of piles to use might not differ much at all one way or the other, in part because the load capacity of the pile will overcome any small differences in the loading. But please give me your advice and comments.

OK, in determing the number of piles required for a footing, do I just use the sum of the all the basic loads acting on that footing, i.e. summing up the dead load, the live loads, live roof loads, snow load, wind loads, seismic loads, etc. and divide this sum with the pile capacity? I know that service loads are used to size the footing and determine the number of loads. Is the sum of the loads that I just did, the "service load"?

Or do I plug these loads into the load combinations, such as U=1.4(D+F), U=1.2(D+F+T) + 1.6(L+H)+0.5(Lr or S os R), U=1.2d+1.6(Lr or S or R) + (1.0L or 0.8W), etc. and get the largest load, and use this load and divide by the pile capacity to get the number of piles required.

Your suggestions and response on which one do I use, and why.

Thanks, y'all and have a great weekend.

 

[Qshake]

Typically you use just the service loads and not the factored loads.

To actually design the footing for shear/bending if that is the intent, then factored loads are used but not to determine the number of piles as this was already done with service loads.

Clear as mud?!

Regards,
Qshake

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[JAE]

....and yes, you use the required load combinations from the applicable building code....

You do not simply add them ALL up.

 

[Geoquery]

You have missed another point. Resistance capacity of the pile, which is different at different loading combination. Consult AASHTO design guide.

 

[MrFurleyEIT]

Part 1: The responses from Qshake and JAE seem to conflict. Can we clarify this, please?

I would think that you need to find all the basic loads and plug them into the load combinations provided in ACI, get the most critical sum, divide this by the pile carrying capacity and get the number of piles. Is this correct? This is what JAE is saying, and this is what I believe to be correct. Agree?

Qshake seems to be saying to add up ALL the loads and not worry about load combinations at all, and use this total load to divide by the pile carrying capacity. One of my colleagues seem to say this, and I don't agree with this.

Part 2. Let's say that the resistance capacity of the pile is neglected because the soil is pretty bad. If you need to add the resistance capacity of the pile, what is the best way to do this.

Appreciate your feedback, please.

 

[WillisV]

JAE and Qshake's comments do not conflict. Some people think that any time a load combination is mentioned that it must have to do with ultimate loads. This is not correct. There are load combinations for both service design and ultimate strength design. JAE was simply referring to using the correct Service Load combinations (for example D+0.75L+0.75W)as opposed to just "Adding Them All Up."

Qshake's additional comment was that after the number of piles are chosen via service load analysis the actual pile cap design is determined using ultimate strength load combinations and analysis.

 

[JAE]

Yes...I'd agree with what WillisV says.

 

[HP14X89]

In your original posting, you mention wind load, so I presume you have lateral loads. In my opinion, simply dividing the axial service loads by the allowable capacity (ASD design approach), the method you mention in your original posting, is only applicable for preliminary pile group evaluation. [For LRFD design this would be the maximum factored load divided by the factored resistance]. For a laterally loaded pile group, the resulting axial load, and bending moments in each pile will be different, and should be evaluated.

In my opinion, for a laterally loaded pile group, once the preliminary group is developed, the loading conditions on each of the piles should be evaluated using either one of two methods. The first is using the rigid-cap model/theory, where all loads, axial, lateral (x & y), and moments (x & y), are applied to the group, assuming a rigid, non-yielding, cap. In using the rigid-cap model, one must first determine the net loads at the centroid of the group, then, determine the stiffness of the entire pile group, and the lateral response of the soil due to lateral pile loading (p-y analysis, with multipliers for row effects). This approach is outlined in detail in the LRFD for Highway Bridge Substructures and Earth Retaining Course Manual (NHI Course No. 130082A) Publication No. FHWA-NHI-05-094. I am not sure if this publication is still available, as FHWA was revising the course. The rigid-cap model approach requires multiple p-y analyses using different multipliers for row effects, and can be quite cumbersome. I belief the approach is also discussed in the COM624B users manual.

Alternately, the loading condition on each pile can be determined using either GROUP (Ensoft Inc.) or FB Pier (BSI). Once the loads and moments are determined for each of the piles, the demand/capacity ratio should be checked (i.e. interaction equation). This is different depending on method of design, either ASD or LRFD.

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[mendinho]

Unfortunately, in engineering practice all around the world, we need to use two different philosophies of calculation.

In structural analysis, we use the limit state approach, this is

Ultimate stress (for concrete, a factor beetween 1.5 and 1.6 is enough) > Design Resistance (with factors that range from 1 to 1.5)

In my opinion geothecnics are one step behind structural analysis. The admissible stress theory is quite common (specially amongst old engineers), this is

Admissible stress (with factors from 2 to 3) > Real stress (No factors or sevice combination factors)

At least the eurocodes are based only on the limit state theory.

 

[BigH]

mendinho - so we are behind, eh? Maybe that is because the coefficient of variations of real soils range from 10 to 40% on various soil properties and not 1 to 2% as with steel and concrete. We do/and have taken into account service combination factors - usually, you will utilized a SF = 3 for dead and sustained live loads and permit in increase of 33% on bearing for transient wind loads, etc. Secondly, where to you get your "factors" (as you indicated in "Design reistance" for ultimate stress paragraph? I have noted that any such factors given in geotechnical sense are based on calibrating to the "admissible stress" theory so that both will give basically the same answers. Thirdly, do the factors for LRFD take into account the different types of clay minerals that are present in a deposit? i.e., the use of one set of factors for illitic clays, another for montmorillonitic clays and yet another for kaolinitic clays? Fourthly, it is seldom the ultimate situation that ever governs - but it is nearly always the serviceability limits that determine safe allowable bearing pressures. Does the ultimate stress approach address this up front - not really; it stresses the ultimate stress approach firstly, then suggests that serviceability be taken into account and, hey wait a minute, why do I spend so much time on ultimate when it doesn't have a real effect on my conclusion? Mmmmm. Soils are not 'man made' - suggest Terzaghi's observation of many years ago be reviewed. Sorry all for my - and hasn't the Eurocodes done a bit of changing in their direction on the geotechnical issue since the original proposals were made?

 

[mendinho]

Cool down, BigH. I apologise to you for my latest controversial comment. As you said, soils properties are difficult to characterise. In fact, in the past concrete reinforcement was done with admissible stresses.

It seems the same but the allowable stress theory is deterministic and the limit state theory introduces some probabilistic ideas. The uncertainty resides not only in the material side but in the loads also.

Because the geothecnical engineering methods are getting better, the EC-7 proposes analysis of the foundations under the limit state combinations and a design resistance of the soil. The design resistance is the characteristic resistance from the statistical adjustment of soil surveying divided by a coefficient that depends on the number of survey points.

I am quite sure that future models will be dominated by randomness.

 

[Mccoy]

mendinho,
are you writing from a European country (Portugal, Spain)?

Have you started designing by the EC7 yet?

In my country (Italy) structural professionals tend to be at the same level of geotechs, since probabilistic analysis so far has been limited to the achademical realm. Everyone is behind, in that sense!

A few remarks about BigH's points:

COV's in soils are sometimes large, although now they are pretty much well known, at least as far as the main strenght parameters are concerned. And, if COV is known a priori, then the effect of variability is somehow reduced as classical statistics teaches.

Sometimes LRFD is fit to ASD, other times LRFD is calibrated to existing databases (a number of real-size load tests in the specific conditions, i.e.: skin resistance for drilled shafts in sand...)- See the NCHRP report 507 downloadable on line:

http://trb.org/publications/nchrp/nchrp_rpt_507.pdf.

So, given a definition of failure condition, and given a stated reliability factor (2.5 or 3) we are confident that the probability of failure in those conditions will be reasonably low and acceptable in standard practice. In our case, according to table 29 of the NCHRP report, to skin resistance in a single drilled shaft calculated by the FHWA method, all soils, a resistance factor of 0.35 should be applied (pretty low!).

Lastly, here BigH is dead right, there is not enough soil variability in recommended factors tables to account for the specific soil conditions encountered in real life. All is thrown into the statistical cauldron. Eventually, when the size of data will increase, situation may improve.

EC7 goes at great lenght to examine the ultimate limit states, whereas serviceability (settlements) get a secondary role.

Yet, as far as piles are concerned, the EC7 design is a little different from shallow foundations design.
The "model pile procedure" allows to calculate a nominal resistance directly from SPT's or CPT's, then reduces it to characteristic and design resistance by appling an algorithm which allows for the number of profiles investigated (same concept is present in LRFD tables for static load tests where # load tests per site is an input parameter). That is, the more the profiles/load tests, the lesser the uncertainty, the less conservative the design.
Site variability is accounted for in the EC7 algorithm, whereas it is another input factor in LRFD.
The EC7 authors also suggest the use of a "model factor" which furtherly adjusts the final resistance according to real size failure models (load tests).

BigH, if you're able to gather some stuff (load tests, site investigations...) we might try and write something on LRFD / EC7 comparison in deep foundations.

 

[BigH]

Mendiho - no worries - am cool; just trying to stoke the fire a bit - but I am of the old school although I am trying to learn the new one - but see little real advantage at the present time. Try a simple slope stability problem. Using something as simple as Taylor's curves, determine a plot of undrained shear strength vs permissible height for a FS=1. Assume, then, you have an "average" value for the undrained shear strength of 70 kPa; but, a COV of 30%. See what affect there is on the height of the slope when you desire to have no less than 2% of the slopes fail. Use the same Taylor's curves for other FS (say 1.3, 1.5, etc). What would your FS be if you superimposed your statistical criteria - bet it is more than 1.5 which is normally used by geotechs. I know that we dumb down the values at several times during the process, but, I would rather base my design on my judgment and experience - and experience of those who I trust and look up to - than some statistical 'randomness'. At my age, all I have to do is get through the next 10 years . . .

McCoy - I knew you'd respond and appreciate your insightfulness. Contact you later.
to all!

 

[mendinho]

As far as I know, in Portugal, France and Spain (where I am), eurocodes are not compulsory but strongly recommended. The curious thing is that the UK - a country I love but usually considered a bit euroskeptic- is ready to substitute the British Standards by the eurocodes in 2010.

Provided that it is a good idea to compare different methods I always have an eye on the excellent US&Canada methods and I must confess that they (the Americans) are more practical than us (the Europeans).

In the end I agree with BigH: no code can substitute good engineering criteria and personal experience.

 

[Panars]

mendinho-

Interesting that you view the Americans as more practical, because I think we view the European foundation community as more advanced than us. It seems that the "new" foundation techniques we are just starting to get comfortable with came over from Europe and have been in use there a long time - like soil nailing and micropiles.

I sympathize with BigH about LRFD, but we start using it anyway this summer in Ohio, USA. We'll see how it goes. The reality will probably be that it is not as bad as some fear but not as good as we have been told.

 

[Qshake]

Reliabililty in Foundations is being moved into academics now and probably a few years ago (but I am just aware of this now) and so the variability in soil conditions are really being treated in a way that is more than tweaking FOS. I've recently been through the First Order Second Moment (FOSM) material.

Being of old school myself and seeing the FOSM I do wonder if this is really a good thing....more work, lower FOS, and arguable quantification of the probability of exceedance.

Regards,
Qshake

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