Reporting lateral capacities of piles

[Okiryu]

Hi,

How do you normally present in your reports your recommendations for lateral capacity for piles? I am planning to prepare a table indicating PILE SIZE - LATERAL LOAD - DISPLACEMENT AT PILE HEAD. Do I missing other information? Perhaps Maximum Moments?

Also, what are the typical allowable values for pile head displacements?

 

[humanengr]

Consider adding the following:
1)Depth of Pile Head Below Existing Grade.
2)Maximum Vertical Load Applied at Pile Head (In case interaction of vertical and lateral loads
is critical for soil or pile material strength).
3) Indicate whether the lateral load is Ultimate or Allowable Capacity
4) Indicate if Allowable Capacity applies to Sustained Loads or Transient/Short Term Loads.
5) It is good practice to include Maximum Moment and depth to maximum moment, to ensure
the pile material design is adequate for the given moments.
6) It is also good practice to indicate Pile Length. This is to ensure the pile
embedment depth extends at least down to the depth of zero lateral force and zero Moment.
7) I would also include the Safety Factors used for Sustained and Transient Loads. Although
these may not be needed directly by the Foundation Engineer, I think it is good practice to be
aware of these factors, not to mention to have them stated for the record.

 

[Okiryu]

Thanks for the input. Some comments:

1. I think that depth of pile head depends on the size of the pile cap and grade beams, this has to be determined by the structural.
2. The lateral load that I am using in my analysis is related to the base shear force that is used for seismic analysis. So it is a percentage of the vertical load applied to the column. I am assuming that sustained lateral load is very small as my structure is quite symmetric and not so tall (6m).

Any ideas about max. allowable deflection? My structure is a metal building and I am getting around 10mm deflection and just wondering if that is acceptable for that type of structures.

 

[FixedEarth]

On a smaller job you can pick one diameter size and embedment depth. On larger jobs, you can report several diameters & embedment depths. As mentioned earlier, you should clarify if you used service level loads or factored loads, their magnitude and Free Head or Fixed Head condition.

This is how I worded my recent job- some designers like it since all they have to do is check that these capacities are not exceeded:

"We have analyzed an 18 inch diameter (B) drilled pier embedded 15 ft below existing grades. Each pier can sustain a downward load of 25 kips, an uplift load of 12 kips, a lateral load of 6 kips and moment load of 35 kip-ft. All of the above loads are at service level. The structural engineer shall proportion drilled pier spacing such that the above geotechnical capacities are not exceeded. This is based on a free headed pier condition. Minimum pier spacing shall be 2.5B on center and maximum pier spacing shall not exceed 6B on center. If more support capacity is needed, this firm should be contacted for additional analysis involving a 2 or 2.5 ft diameter drilled pier. The above capacities assume axial & lateral eccentricity less than B/8. In addition, the drilled pier shall be reinforced with a minimum longitudinal steel of 1% due to expansive soils and transverse reinforcement shall be per ACI 318-11, Sec. 7.10 provisions."

If we are expecting torsional moment or negative skin friction, we include additional sentences.

http://www.soilstructure.com/

 

[Okiryu]

Thanks FixedEarth, that language is very helpful.

If you provide recommendations for service loads, it means that the structural will have to use their own factor of safety to affect your recommended capacities? Do you recommended also what FS can be used?

Thanks again.

 

[humanengr]

"Depth of pile head (top of pile) Below Existing Grade" was added since the piles capacity and deflection will vary depending on depth.

10mm (3/8") top of pile lateral deflection sounds reasonable for a metal building. However, since that is a serviceability issue, it depends what type of service is required of the building. Its possible more stringent deflection criteria exist if the building is supporting sensitive equipment, materials, cladding, etc.

Regarding comment by "FixedEarth" and your reply; Service Loads are used to check pile capacity with respect to Soil,
or the soil-pile interface. I wouldn't want to comment on Safety Factors in that case. It should come from Geotech.

The structural engineer, still may use strength (LRFD) for pile structural design, by simply applying the appropriately factored Load Combinations, such as ASCE 7, or IBC LRFD Load combinations. I mention U.S. based codes - don't know your location.

 

[FixedEarth]

The geotechnical capacities I gave earlier are allowable loads. In my region, the foundation designer will use factors for various load combinations. For example, 1.2 for dead load, 1.6 for live load, etc. This is because the reinforcement design for the drilled shaft is based on ultimate design of concrete. As mentioned by humanengr, I don't report what geotechnical F.S. I used. Typically, I use 2.0 factor of safety on the skin friction, 2.5 factor of safety on end bearing (if the pier terminates in dense stratum)and 1.33 on ultimate passive resistance. But these are already included in the 25 kip axial downward load, 12 kip uplift, etc.

In your case, you will likely have higher than normal uplift and shear loads. Therefore you may end up with rigid short pier (length/dia.< 10.) Also you will likely have smaller end columns and larger side piers. So it may make sense to have two or three drilled pier diameters. Since you will have larger shear loads than a "normal" building, it makes sense to check lateral deflection & lateral soil pressure in the upper 3B or 4B. You could have a case where your deflection is 15 mm but you exceed the allowable lateral soil pressures in the upper 3B of embedment length.

http://www.soilstructure.com/

 

[Okiryu]

Thanks for your replies. I was thinking that structurals normally use non-factored loads for foundation design. I mean, non-factored loads to calculate the size of the footing for example. But they use factored loads to calculate the reinforcement of the footing.

I have to clarify this with the structural, but for example, in your experience, if I ask for column loads in order to check settlements, are these loads provided by the structural, factored or non-factored loads?

Also, FixedEarth made a good point, I just remembered that for metal buildings, the uplift is higher than for "normal" buildings. I would expect that the structural asks for uplift resistance. So, I will add that information in the report.

 

[FixedEarth]

Okiryu- You should ask for service level (unfactored) downward load and use that to compute your settlement.

http://www.soilstructure.com/

 

[Okiryu]

Thanks for the reply. Very helpful.

 

[structSU10]

Out of curiosity, when you state all the loads the drilled pier can resist, what kind of interaction can you assume? What if I had a pier that had 35 k*ft moment, 6 kip lateral, and 25 kip down force simultaneously? Is it ok per your statement, or should some interaction be considered, so the drilled pier is no good?

 

[FixedEarth]

The example I gave was based on a specific site with very dense granular soils near the surface & no groundwater table in the upper 20 ft. (so the analysis is only valid for that specific site). Having said that, yes, that pier can handle simultaneously the compression, tension, shear and moment loads mentioned earlier from a geotechnical view point. That is vertical settlement, lateral soil pressures & lateral deflection are acceptable. Obviously if you had 4 kip shear and 25 kip-ft, everything else being equal, the same pier would work.

When you do the reinforcement design, you can compute your Vu, Pu, Mu and then get your longitudinal & transverse reinforcement considering the P-M diagram. If we did your soils report and you had markedly different loading scheme then we will rerun the analysis and give you a new Vmax, Mmax, settlement, lateral deflection and lateral soil pressure. Then you factor those and do the reinforcement design.

There are situations when you have torsional moment, negative skin friction and large shear load all combined (Cantilever Sign or Metal Building Foundation for example). Then in those cases, if I am doing the foundation design, I check for Tu & Vu interaction, Pu & Mu interaction, after I satisfy the embedment depth and pier diameter is adequate from a soils point of view. It is iterative process. Sometimes anchor breakout may dictate the minimum drilled pier diameter required.

See attached references for more in depth study.

http://www.soilstructure.com/

 

[Okiryu]

FixedEarth, for max eccentricity, is that as per codes or do you calculate it? If you calculate it, what is the max moment that limit that eccentricity?

 

[FixedEarth]

Ok-The B/8 limit for shear load eccentricity is something I picked up a while ago from a source. If I find that book title, I will gladly post its here. What happens is when you have a shear load that is off center, it creates a torsional moment which is less favorable than a bending moment. The soil around the pier is twisted and nothing happens until you exceed the geotechnical torsional moment capacity. However, because there is an interaction between shear and torsional moment (Tu & Vu), you will exceed that structural limit before you exceed the shear strength of the soil. Broms, R.F. Scott and others did work on geotechnical torsional capacity of soils.

http://www.soilstructure.com/

 

[Okiryu]

FixedEarth, thanks again. I have not heard too much about torsional moment and interaction V-T for soils. Based on your post, the structural capacity is more critical than the geotechnical capacity for this type of loading, am I correct? I will try to look at Broms and Scott, so I can educate myself.

My question was related to the axial eccentricity. If you exceed B/8, there is an additional moment at the pile head. So, if there is a limit for eccentricity, there may be a limit for this additional moment. I was wondering about what the magnitude of this additional limit moment can be? Do I need to check the P-M interaction curve to determine it?

 

[FixedEarth]

My error. In that case, just calculate the moment due to the eccentricity and see your Max. moment induced to the pier, usually at 2B to 3B below the soil surface. The magnitude of Mmax a given pier can handle depends mostly on its pier diameter, f'c, stronger upper soil presence and pier length. So let us say, I have a Max. Moment of 50 kip-ft. Obviously a 12 inch diameter pier will deflect too much. May be a 16 inch diameter will deflect "o.k" but the upper 2 ft of soil may be stressed beyond passive resistance. Then if we try 21 inch diameter shaft, it may work just fine for lateral deflection & lateral pressures. So yes, the limit of max. moment is dictated mostly by your drilled shaft diameter.

Yes, you will need to pick longitudinal bars and lateral ties and check P-M curve. In my experience, once you satisfy geotechnical capacity, you can get P-M to work with 0.5% to 1.5% of steel.

The other interaction is shear & moment. If you have large axial eccentricity which results in additional Moment, you can have larger only "e" if you have no lateral load. But if you have good size shear load, you have less room for bending moment to keep the same pier size. Hope I answered your question.

http://www.soilstructure.com/

 

[Okiryu]

FixedEarth, thank you. Appreciate your time on this. May I ask one more question: my piles are steel piles, so no reinforcment. Does the P-M curve shall be used considering the factored loads calculated by the structural? I checked my recommended values (allowable axial and moment capacities) and I am under the P-M curve. So, I conclude that the recommended values are reasonable since it gives some room to the structural to use the P-M curve with his factored loads.

Thanks again !!

 

[FixedEarth]

You will have to get the tea today! For steel piles, filled with concrete or driven hollow with a soil plug, just do lateral load analysis using service level V and service level M. Then take your Mmax and factor that by 1.6 and call it Mu. Similarly take your vmax and factor it by 1.2 or 1.6 and call it Vu. Then check the pipe piles moment & shear capacities are not exceeded. By the way, others may have a different approach.

http://www.soilstructure.com/

 

[Okiryu]

This conversation was helpful. Thank you !!