Integrated Buildings & Structures

Hello,
I have a project with micro piles (i.e. 7.5" diameter hollow steel tube filled with concrete and with one reinforcing bar in the middle).  I want to calculate how long the steel tube needs to be so that it can transfer the entire compressive load applied on top of the steel tube to the concrete which continues below the steel tube.  AISC Manual Eq. 16 I6-6 (Rn=0.25*π*D^2*Cin^2Fin^2) for the available bond strength between the steel and the concrete uses the cross sectional area of the entire micro pile section for the calculation since D= outside diameter of round HSS, in. (page 16.1-96 of Manual).  This, however, does not take into account the actual surface area in contact between the concrete and the steel.  Is there a mistake in this equation?

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Fernando Martinez EI, A.M.ASCE
Douglas Wood Associates
Coral Gables FL
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AASHTO LRFD Bridge Design Specifications Sections 10.5.3.5 and 10.9 has specific guidance for design of Micropile.  The length of your casing should be based on (1) lateral load requirement and/or (2) buckling requirement if the casing is founded in weak soil.  If you want to dig deeper into this subject, see FHWA NHI-05-039 (Micropile Design and Construction) Reference Manual / Chapter 5.

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Dustin
Engineer
Bellevue WA
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Piles ^{are rarely designed to take up loads like columns in order to transfer it to the base! They are designed to transfer loads by friction to the sides. So the length and circumference are important elements for computation of loads to be transferred per unit area. You may refer to a pile design hand book to find out the loads transferred by surface friction per unit area. I suppose buckling of your pile may not be an issue!}

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Peter Singh M.ASCE
Managing Director
Kingdom Designers
Birganj
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A micropile is a small diameter pile, and the necessary bonds with the underlying soil <g class="gr_ gr_41 gr-alert gr_gramm gr_inline_cards gr_run_anim Grammar multiReplace" id="41" data-gr-id="41">is</g> through contacts with the circumferential surface area. If in the design according to his <g class="gr_ gr_59 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="59" data-gr-id="59">explaination</g> the <g class="gr_ gr_60 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="60" data-gr-id="60">cross sectional</g> area is used is an assumption that the are approximately equal.

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Olusegun Afolabi P.E., M.ASCE
University of Lagos
Lagos
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Original Message:
Sent: 05-04-2017 02:50
From: Peter Singh
Subject: Project with Micro Piles

Piles ^{are rarely designed to take up loads like columns in order to transfer it to the base! They are designed to transfer loads by friction to the sides. So the length and circumference are important elements for computation of loads to be transferred per unit area. You may refer to a pile design hand book to find out the loads transferred by surface friction per unit area. I suppose buckling of your pile may not be an issue!}

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Peter Singh M.ASCE
Managing Director
Kingdom Designers
Birganj
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The first thing you must know is what are the soil conditions.  The type of soil will dictate how much friction can be developed along the pile.  That calculation is best done by a geotechnical engineer and it would be noted in the geotechnical report.  Also, the depth to bedrock has to be know because if it is shallow you may not be able to develop the friction pile.  In that case a caisson to rock would be better suited.  If this is new construction, why do you want to use micro piles?  Other pile types would be better suited.  If you are doing an indoor retrofit and are restricted in the size of the equipment, you may consider auger cast pile.  They come in 5 foot section and larger diameters than micro piles.  In any case, if you do not have a geotechnical report that gives you options, I strongly advise you to get one.

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Joseph Buongiorno P.E., M.ASCE, Life Member
Owner
Hopatcong NJ
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Original Message:
Sent: 05-04-2017 02:50
From: Peter Singh
Subject: Project with Micro Piles

Piles ^{are rarely designed to take up loads like columns in order to transfer it to the base! They are designed to transfer loads by friction to the sides. So the length and circumference are important elements for computation of loads to be transferred per unit area. You may refer to a pile design hand book to find out the loads transferred by surface friction per unit area. I suppose buckling of your pile may not be an issue!}

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Peter Singh M.ASCE
Managing Director
Kingdom Designers
Birganj
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It depends to the specification requirements of the structural engineer. If the steel casing is part of the caisson or just a temporary. If temporary casing, you need to wait till the concrete has set and you can pull it out. Temporary steel casing is just a protection from soil mixing to the concrete. This is usually a standard length of 20'. Hope I answered your question.

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Phil Gonzalez C.Eng, Aff.M.ASCE
Project Manager/Estimator
Pernix Group
Lombard IL
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Looking at the commentary for this section (the pages with gray trim), equation I6-6 seems to show the equation for circular area by coincidence.

The engaged perimeter is assumed to be one-quarter of the circumference (0.25 pi D) assuming the member giving load to the filled steel member frames in from only one side. The engaged transfer length of load is assumed to be twice the diameter in each direction (above and/or below) (Cin D)

Therefore the surface area engaged in load transfer is shown as (0.25 pi D)(Cin D) = 0.25 pi D^2 Cin.

The commentary states that you are allowed to increase the amount of the circumference engaged in load transfer when the applied load comes in from multiple sides or some configuration other than the basic assumed one.
For a micropile, I believe you could use the full circumference, and a value of 2 for Cin (the pile is only below the point of load application, not above).

Hope this helps! It's always good to visualize the load path for your structure.

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Walter Kaniecki A.M.ASCE
Transportation Engineer
Penfield NY
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