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*SEFP Consistent Design*
*Pile Design*
*Doc No: 10-00-CD-0005*
*Date: Nov 21, 2017*
 

This article is intended to cover design of piles using Ultimate Limit State (ULS) method. The use of ULS method is fairly new for geotechnical design (last decade). The method is being used in multiple countries now (Canada, Australia etc). The following items shall be discussed:

  1. Overview
  2. Geotechnical Design of Piles (Compression Loads, Tension Loads and Lateral Loads)
  3. Structural Design of Piles (Covering both Concrete and Steel)
  4. Connection of Pile with the foundation (Covering both Concrete and Steel)
  5. Pile Group Settlement
  6. Things to consider

 

1. Overview

Piles provide a suitable load path to transfer super-structure loads to foundation where shallow foundation are not suitable - this can be due to a number of reasons like existing space constraints or suitable soil strata is not present immediately below structure. Other uses can be to meet design requirements like to have reduced settlement etc.

This article shall cover the use of straight shaft cast-in-place concrete piles and straight shaft driven steel pipe piles. There are a number of additional piles types like belled concrete piles, precast concrete piles, screw / helical steel piles etc but the discussion to choose a suitable pile type is not in the intended scope of this article. The article is intended  to discuss design requirements for straight shaft piles only (both concrete and steel) . The aforementioned topic about pile selection is a very diverse subject and requires a separate discussion on its own.

Before I get into the nitty and gritty of pile design, it is important to highlight that as a structural engineer working on pile design, there are a number of parameters that you would require from the geotechnical engineer. Generally, these parameters are provided in the project geotechnical report. Based on those parameters, the geotechnical design of piles is performed first followed by structural design of pile. The next section talks about  the geotechnical design of piles.

2. Geotechnical Design of Piles.

Geotechnical design of pile means sizing of pile. This includes determining the following two geometric properties of piles:

1) Diameter or radius

2) Length

Straight shaft piles embeded in soil derive their capacity from two sources. The first one is the skin friction along the pile length and the second one is the end bearing. In order to complete the geotechnical design of piles or in simple words to "size up the piles", you will need skin friction values for different soil strata through which the pile would penetrate or lie and the bearing capacity of the layer in which pile would terminate.

This information is provided by the geotechnical engineer in the project geotechnical report. Generally, they would provide a table showing skin friction values of each soil layer for both tensile and compressive loads along with end bearing values of each layer. In addition to this, for areas susceptible to frost loading, the geotechnical engineer would also provide ad-freeze and frost heave forces. You can't design a pile without knowing what these values are. So this is something that you need from a geotechnical engineer. 

Once you have received the project geotechnical report with all the required information, you need to start  sizing the piles. The easiest way to do it is to create an excel sheet and do preliminary calculations for different standard diameters like 200mm, 324 mm, 406mm, 460mm, 508mm, 610mm, 762mm and 914mm. The geotechnical report shall also provide recommendations if certain top soil layers need to be ignored or not.

Example Problem:

From your structural analysis, the maximum factored compressive load is 100 kN. and maximum factored tensile load is 50 kN. You need to size a pile (do geotechnical design) to meet that applied load. Sizing piles for geotechnical capacities is simple. Here is the formula for capacity of pile based on skin friction only (ignoring end bearing for simplicity):

ULS Geotechnical Pile Axial Capacity: Pi * Pile Diameter * Total Embedment Length of Pile * Skin Friction Value * Resistance Factor

Where,

Pi= 3.14

Pile Diameter = 2* Radius

Total Embedment Length of Pile = Pile Embedment Length - Frost Depth

Skin Friction Values = See geotechnical for values

Resistance Factor = 0.4 for compression and 0.3 for tension.

For, the above problems, lets assume Skin Friction values of 80 kPa for both tension and compression and initial pile size (diameter) of 324 mm, Frost Depth of 3000 mm.

For total length of 10m (lets assume a starting length), Total Embedment Length of Pile = 10m - 3m = 7m (Total Length - Frost Depth)

ULS Geotechnical Pile Compressive Capacity= 3.14 * (0.324m) * 7m * 80 kPa * 0.4 = 228 kN > 100 kN Okay.

ULS Geotechnical Pile Tensile Capacity = 3.14 * (0.324m) * 7m * 80 kPa * 0.3 = 171 kN > 50 kN Okay.

The above problem shows you how to calculate the compressive and tensile capacities (also called the axial capacities) of the pile. For lateral capacity, you will need to know the modulus of sub grade information from the geotechnical engineer and use a software like LPILE to see the response against the lateral load. It is important to note that lateral deflection of pile is a service limit state meaning that it should be checked against unfactored loads.

Generally, for petrochemical and oil and gas industries, pile service loads are defined as a deflection limit that will depend upon the maximum allowable movement of pile considering an elastic response from soil as well as the maximum movement piping and its attachments can take. Here is a scenario explaining that. For example, your geotechnical engineer recommends a maximum lateral movement of pile to be limited to 6mm so that soil around pile stays elastic. The structure you are designing, has a wind load deflection of 12mm. The pipes and equipment plus their connections shall be designed for 6mm+12mm = 18mm movement of structure. You need to notify piping of this deflection limit and if they are okay, you are good. If they are not, you will have to stiffen up the structure  to lower the overall structure deflection and work with piping to see alternate routing for pipe. For pile design, you need to see what diameter pile shall have a capacity at 6mm lateral deflection greater than the applicable horizontal service load. To calculate pile capacity for different pile head movements, you will need to use LPILE or similar software.

LPILE shall provide you a graph that would show you that how much a pile would move under applied lateral load or moment. LPILE is very easy to operate. You can look at the program tutorials and work your way through. It will also provide you the analysis results for a pile embeded in soil with soil modelled as springs along the length. This analysis result is important and allows us to see what is the maximum moment and shear developed in pile due to applicable load and based on combined response of soil and pile interaction.

If you don't have LPILE, you can ask the geotechnical engineer, to provide you with pile lateral capacity graphs. In this case, you will need to  provide the geotechnical engineer with estimated pile sizes, estimated axial and lateral loads, pile head condition (Fixed or Pinned) upfront. The goetech engineer will run the LPILE for you and provide you the graphs that will show the maximum load a pile can take against different lateral displacement values and would also provide the maximum moment due to max lateral load. I have done this on a number of projects and this is standard industry practice. 

3. Structural Design of Piles.

After completing the geotechnical design of pile, the structural design of pile needs to be performed. In order to do that, you will need to know the maximum moment in pile due to the application of axial and lateral loads. As mentioned above, the easiest way is to use  LPILE output as it provides you with deformed shape of the pile along with the maximum moments and shears due to applied loads - the analysis of pile embedded in soil. Using LPILE analysis results, you can use beam-column capacity formulas to design a steel pile or column interaction diagram to design a concrete pile. Beam-Column capacity formulas vary with different codes so therefore I haven't included any example. For steel piles, corrosion allowance should be considered as per the code requirements. Generally its 1.5mm each exposed face so for pipe piles it will be 3mm considering exterior and interior face of the pile.

4. Connection of Pile with the foundation (Covering both Concrete and Steel)

The connection of pile and foundation / pile cap is extremely simple for concrete piles. All you need to do is to develop the bars from concrete pile in concrete foundation/ pile cap. For steel piles, similar concept is there, except for you need to weld rebars on top of cap plate.

Hope this article provides the much needed guidance on pile design. It is written for beginners and a lot of things have been kept simple. Your feedback is more than welcome. Please post any questions should you have.

5. Pile Group Settlement

Single pile or pile groups should always be check for settlement. Geotechnical consultant shall be contacted to get guidance on what method should be used. Methods like equivalent raft method or finite element analysis can be carried out to get settlement numbers.

6. Things to Consider

For pile group, group effects are generally provided by the geotechnical engineer that can be applied to pile group. The group effects are a function of pile diameter and centre to centre spacing. Pile capacities are reduced if they are spaced closely. For straight shaft piles, rule of thumb is to place them greater or equal centre to center distance of to 3 * diameter of pile. For lateral loads, pile capacities are reduced at 3 * diameter spacing and generally piles need to be spaced at 5 * diameter to have no lateral reduction. Also, straight shaft piles if placed too close might result in pile installation issues. Some piles already installed might heave up if other piles are being installed in close proximity. Impact of pile driving to existing structures should also be considered especially if there is sensitive instrumentation installed in close proximity.

Thanks.

 

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19 hours ago, Sohaib Akhtar said:

Great information... can you please  also share  information related to pile cap design , my case is a single pile with pile cap over it ....

Pile caps are designed as slabs. For the case where you have a big pedestal on a pile cap, your pedestal is designed as a column and pile cap is designed as a slab taking the loads from the superstructure through the pedestal to piles. You need to check moment, one way shear (perpendicular and diagonal directions) and two way shear for pile cap.

Thanks.

 

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@UmarMakhzumi

Dear Engr i would to disagree regarding your statement of design of Pile cap as a typical Slab.

  1. Normally pile cap behaves like a Deep Beam action, normal bending/flexural theory is not applicable to Pile Cap design. ACI and AASHTO recommends to use STRUT n Tie model for Pile Cap.
  2. However for ease of analysis and design, due to absence of expertise on STM, structural engineers design it the other way. 
  3. The behaviour of Pile cap supported on Pile follows Bolt Analogy.
  4. Furthermore, for modelling in FEM softwares, In Usual analysis of raft slab, you put soil springs under the raft, however for pile cap we ignore effect of soil and pile cap is supported only on piles.

           Thank You

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On ‎2017‎-‎12‎-‎10 at 7:53 AM, SALMAN CH said:

 

  1. Normally pile cap behaves like a Deep Beam action, normal bending/flexural theory is not applicable to Pile Cap design. ACI and AASHTO recommends to use STRUT n Tie model for Pile Cap.
  2. However for ease of analysis and design, due to absence of expertise on STM, structural engineers design it the other way. 

Salman Ch,

Thanks for your comment. Pile cap would only behave as a deep beam if distance between ratio of slab thickness to distance between piles is more than whatever is required by code for deep beam action. I live in Canada and here if the shear span to depth ratio is less than 2.5, deep beam action can be considered. If not, it is a normal slab. ACI has a different limit. So yes, it may be a deep beam or not, but same would be applicable to a slab if your columns are too close. Anyway, thanks for highlighting that as now the clarification may be useful for someone who isn't aware of this

On ‎2017‎-‎12‎-‎10 at 7:53 AM, SALMAN CH said:
  •  
  • Furthermore, for modelling in FEM softwares, In Usual analysis of raft slab, you put soil springs under the raft, however for pile cap we ignore effect of soil and pile cap is supported only on piles.

This is standard practice and there can be different reasons for not considering soil springs. For example, in North America, heave is a big problem and generally pile caps have a layer of void form below them. Void forms are compressible and analysis should never consider soil springs for that reason as there would never be a hybrid action in that scenario. Also in typical cases stiffness of pile group is >> than soil so it doesn't make sense to put soil springs.

Thanks.

 

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On 12/11/2017 at 7:00 AM, UmarMakhzumi said:

Pile cap would only behave as a deep beam if distance between ratio of slab thickness to distance between piles is more than whatever is required by code for deep beam action

Dear Engr @UmarMakhzumi I just want to add something in it, I feel in this quote you wrote”MORE than require by code “by mistake, in simple words when the distance between supporting reactions is less than the twice depth of the member. I would like to add the ACI and AASHTO references in this regard.

Thank You

 

 

 

ACI PILE CAP.png

AASHTO 5.6.png

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8 hours ago, SALMAN CH said:

Dear Engr @UmarMakhzumi I just want to add something in it, I feel in this quote you wrote”MORE than require by code “by mistake, in simple words when the distance between supporting reactions is less than the twice depth of the member. I would like to add the ACI and AASHTO references in this regard.

Thank You

 

 

 

ACI PILE CAP.png

AASHTO 5.6.png

I think you both are talking about the same thing. His statement is about ratio of slab thickness (h) to distance between piles (L) -> (h/L) being more than the code number and what you have referenced talks about distance between load points (L) to ratio of slab thickness (h) -> (L/h) which has to be less if the first statement is correct.

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@UmarMakhzumi.. @Ayesha...Thank You for your concern. Yes we r on the same page . However my interest was only to highlight the fact that pile cap design is not like slab, due to pile spacing criteria and typical patterns, it will behave like a deep beam in almost every structure. However i feel there is no proper technical inputs on this component of structure and engineers follows whatever practices are in their offices.

In this regard i would like to add little knowledge as per my experience , that unlike the normal slab, the critical sections for maximum FORCES will also be changed , it needs 5 to 6 special investigation as per the reference handbook mentioned in commentary of ACI. 

Regards

 

Edited by SALMAN CH

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      *Date: August 07, 2014*

      I am writing this article about a very important, but mostly neglected topic of flexibility of diaphragm. I used to assume that all reinforced concrete slabs can be treated as rigid diaphragms. But as it turns out, only the slab with span-to-depth (depth is length of slab in direction of lateral loads) ratio of less than 3 and without horizontal irregularity can be treated as rigid diaphragm. The more important thing is that the span-to-depth ratio and horizontal irregularity is not the only criteria and one other factor also needs to be kept in mind before assigning rigid diaphragm to concrete slabs in numerical model of building.

      Another important concept that I learned, and it was a moment of epiphany for me, is about TRANSFER diaphragms. I had posted a topic “Amplification Of Forces In Etabs” earlier in this forum but we were not able to reach at a satisfactory conclusion. Now, I have the answer to that query: Back Stay effect. Another article is required to explain it , and this concept is not discussed in this article. This article is about flexibility of diaphragm.

      Diaphragms are horizontal members of the lateral-force resisting system of building structures. Their function is to distribute inertial forces, generated at its own level, as well as other levels, to vertical members of lateral-force resisting system.

      One kind of diaphragm only distributes inertial forces generated at its own level. This kind of behaviour is observed in buildings where there is a continuity of vertical members of lateral-force resisting system: building should not have a setback or podium at lower levels, or below grade levels. The other kind of diaphragm, known as “Transfer diaphragm”, not only distributes inertial forces generated at its own level, but also re-distributes forces coming from upper levels. This type of behaviour is typical of a building having setback or podium at lower levels, or below grade levels. Transfer slabs can attract huge forces due to a behaviour dubbed as BACKSTAY EFFECT.

      Now, coming to the issue of flexibility of diaphragm. According to ASCE 7-10,

      In addition to considering aspect ratio and horizontal irregularity as a basis for assuming concrete slab as a rigid diaphragm, the relative stiffness of adjoining vertical lateral load resisting system. Buildings with shear walls at ends and flexible frames in between are the ones where the assumption of rigid diaphragm leads to underestimation of drifts and erroneous distribution of base shear in vertical as well as horizontal direction (1)(2)(3); shear forces in middle frames can be reduced to 23% if rigid diaphragm is assigned in the model (1) for buildings with this type of structural configuration.

      M. Moeini et al. (2008) (3) conducted a parametric study using numerical analysis and proposed formulae that predicts the error associated with assuming concrete slab as rigid diaphragm. They also concluded that for buildings, without shear walls, rigid diaphragm assumption is suitable for irregular buildings as well. But, for long and narrow buildings with shear walls at ends, the assumption of rigid diaphragm is not suitable.

      The objective of writing this article was to warn engineers about the tendency of blindly assigning rigid diaphragm to concrete slab in any type of building configuration. The result could be underestimation of forces as well as drifts.
      Nakashima, M., Huang, T., Lu, L-W. “ Effect of Diaphragm Flexibility on Seismic Response of Building Structures”, In proceedings of 8th world conference on earthquake engineering. San Luis Obispo, MSc Thesis , “ An Investigation of influence of diaphragm flexibility on building design through comparison of forced vibration testing and computational analysis”, 2010. M. Moeini, B. Rafzey, W.P. Howsen, “Investigation into the floor diaphragm flexibility in rectangular reinforced concrete buildings and error formulae”, In proceedings of 14th world conference on earthquake engineering.
      The article is not finalized and would be completed in coming weeks.
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