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Concrete-filled steel piles

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From: Abolhassan Astaneh-Asl
To: Mr. Kevin Below
Subject: Subject: Concrete-filled steel piles
Date: April 19, 2008
CC: Seaint(--nospam--at)seaint.org
BCC: Graduate Students in CE247-Design of Steel and Composite Structures course at UC Berkeley


Dear Mr. Below:
Thank you for your question and particularly your last light-hearted sentence on asking for "comments, opinions, diatribes, irony, satire or knowledge" . I provide my 2-cents comments below and hope that you will not place them in satire, irony or diatribe category. Any other category(ies) in your list will be just fine! Here are my comments:


Not knowing whether or not your steel pipe piles are going to be subjected to lateral loads at all or any seismic effects, I am providing the following comments assuming that there may be some lateral load on the piles causing bending and shear in them in addition to axial loads. I also assume that there is seismic effect.

Adding concrete inside the steel pipe piles will make them composite Concrete Filled Tube (CFT)section. The advantages compared to bare steel tubeare that the pile section will have larger axial load as well as bendingand to some lesser extent additional shear capacity . The shear capacitymay not be much since the concrete inside such a relatively small pipe is not reinforced. Also, there will be additional axial, bending and shearstiffness added because of not only presence of concrete but because of its composite interaction with the steel pipe. To establish additional strength and stiffness, the provisions of Part II of the AISC Specifications and the 13th Edition of the AISC Manual of Steel Construction would be what I will use. Also, there are plenty of design-oriented literature , in particular, proceedings of Composite Construction Conferences ( every four years or so) that can be very useful. ACI-318 also has valuable information on design of composite columns.

Adding concrete inside also improves local buckling condition of the CFT pile since the steel tube cannot buckle in an s-shape with half of since wave going in and half coming out . In CFTs, local buckling can happen primarily by steel tube buckling out (the concrete inside braces it and prevents its buckling inward. That is why local buckling limits for D/t ratio in CFTs (Part II of the AISC Spec.) is higher than the D/t ratio limitsfor bare steel pipe given in Part I of the AISC Spec. Corrosion behavior of inner surface of a CFT is also expected to be better (with less corrosion) than the similar but bare steel pipe pile filled with moist or saturated soil or just water. the reason is continued supply of oxygen is needed for corrosion to continue to occur. If you fill the pile with concrete, even if the whole pipe is in the water and the concrete inside is saturated, the flow of water inside the pores of concrete will be much slower than the flow of oxygen in saturated soil or water. These items have been observed in submerged members of steel offshore platforms made of steel pipes.

If seismic loads are involved, addition of the concrete inside the steel pipe pile will increase the ductility, specially in bending, and significantly improve energy dissipation capability as well as low cycle fatiguelife. Again, local buckling under seismic load also will improve for CFTcompared to hollow or soil/water filled pipes.

Now, having said all of this, I feel that your piles will be driven into the soft soil with their end open. If this is the case, then there will be soft soil inside the pile after it is driven. in this case, the geo-grouting is the best and may be only solution to add strength and stiffness to these piles. Geo-grouting from what I understand is pushing grout under pressure into the soil to make it stronger, stiffer and more stable, but I am not a "geo" guy and hope others will correct me if I made incorrect statement here. If the piles are driven using pile tips, then there will be no soil inside the pile and it can be filled with concrete.

Last, looks to me a combination of geo-grouting of the lower parts and concrete-filling of the upper parts near foundations and may be a better and more economical solution here since these are friction piles , as you stated, and the bottom portions of the piles do not have as much axial load as the top (the load is continuously being shed to the soil.) If you had direct end bearing hollow piles I would think filling it throughout with concrete would make more sense.

Hope my comments are somewhat helpful.

Sincerely,

Abolhassan Astaneh-Asl, Ph.D., P.E.
(www.ce.berkeley.edu/~astaneh) and (www.astaneh.net)
Professor and Consultant on Structural Engineering, Earthquake
Engineering and Protection of Buildings and Bridges against Blast and Impact.


---------------------------

From: "Kevin Below" <kbofoz(--nospam--at)gmail.com>
To: seaint(--nospam--at)seaint.org
Subject: Concrete-filled steel piles



For a project on very soft soil, subject to liquefaction, we will be going
with friction pile foundations, which will also serve the geothermal tubes.
Piles will be 8" dia. steel tubes.
What are the structural advantages of filling them with concrete ?  The
geothermal people need to fill the tube with something that is a good
conductor, such as their geo-grout (I don't know what that is exactly, just
that it remains liquid) or concrete, or even water.

Thanks in advance for comments, opinions, diatribe, irony, satire or
knowledge.



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