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Composite section for soldier pile wall

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Panars

Geotechnical
Apr 18, 2005
298
Does anyone use the composite section for the part of the soldier pile encased in concrete? We are using H-piles and W-sections placed in a drilled hole and filled with concrete to stabilize landslides in Ohio. The designs I have seen typically use the section modulus of the steel only and neglect the concrete. It seems a shame to totally neglect all that expensive concrete.

In my research I have found the following info. AASHTO basically refers to the AISC for concrete-encased beams. The 9th edition of the AISC (the green book) says that if the section is totally encased in concrete then shear studs are not required, and one can design based on 0.6Fy and the composite section, or 0.76Fy and just the steel section.

I have not found any design examples of concrete-encased beams or any recent journal articles that are entirely appropriate. Most deal with beam-columns which must include longitudinal and transverse reinforcement in the concrete in addition to the steel section.

Peter Narsavage
Columbus, Ohio
 
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I have always used just the steel section.

To use the concrete, you would have to determine if the concrete on the tension side has cracked and the depth of the crack. It is likely in your situation that the concrete would crack to near the center of the section. You could then determine the composit EI of the steel and concrete section and use that in the analysis.

As for what various codes would allow, I don't know. I have always based that type of analysis on the amount of allowable deflection and limited my stresses to about 0.8Fy.
 
I can calculate the composite section modulus based on the concrete cracking to the neutral axis. It's a long calculation, but not particularly difficult, and I have put it into a spreadsheet.

Theoretically, I think this is a valid approach. However, I recognize that there are more uncertainties with constructing a soldier pile than with constructing a concrete-encased beam for, say, a bridge superstructure or a building.

Peter Narsavage
Columbus, Ohio
 
I would not reccomend i for seeral reasons. 1. There is no tension steel so you are asuming ft = .1f'c which may or may not be true. 2. strength of concrete is ussual not high and consolidation is generally self weight and is not confirmed. 3. To lower drilling costs, the hole is close to the diameter across the corners of the proposed H-pile, leaving little cover at these high strss areas. 4. Nor is placement of the pile relative to the center of the casing perfect. Generally t is simpler and more reliable to simply use an adequate steel section.
 
DRC1-

Thanks for your comments. I was hoping people would raise concerns that I had not thought about, but I think I can address these ones. I would agree with your 4th point that it is simpler and more conservative to simply use an adequate steel section. The problem is that sometimes the steel section required is very large (W40 beams) and our retaining walls are very long. Therefore, the steel makes them very expensive. Regarding your other points: 1. There is tension steel. More than half of the wideflange is below the neutral axis and is in tension. 2. We are using 3500 psi concrete and while it is true that the only consolidation is self weight, that is also true of drilled shafts where we count on the full design strength of the concrete even if it is not vibrated. How is this any different? 3. We make sure that there is at least 2 inches of cover over the corners of the H-pile or W-section.

Peter Narsavage
Columbus, Ohio
 
Panars,

W40????? As in 40 inches deep? These are monstrous soldier beams. Are you designing high cantilevered walls? Aren't you using tieback anchors? How do you assure 2 inches of cover along the beams? Sounds like a very expensive wall.

Are you chipping out 3500 psi concrete to install lagging between the soldier beams? If so, that's very expensive.

FHWA's Engineering Circular No. 4 states that "Lean-mix concrete should be used to backfill the pre-drilled hole for the soldier beam from the elevation of the base of the excavation to the ground line. Lean-mix concrete can be easily removed to allow for lagging installation. Lean-mix concrete may be used to backfill the pre-drilled hole from the bottom of the wall element to the base of the excavation. As an alternative to lean mix concrete, controlled low strength material (CLSM) or "flowable fill" may be used. When using either lean-mix concrete or CLSM, the contract specifications should require that the compressive strength of the material be a minimum of 0.35 MPa" (51 psi).

I agree with DRC1.
 
PEinc-
Yes, those are 40 inch deep beams. I forget the exact weight now, but it was around 200 lbs/foot. They are designed to stabilize a deep landslide. I don't remember why, but tie backs were not an option (I'm not designing these myself, just advising another engineer). The design cover is 6 inches, so that we will have at least 2 inches. The original design was done as a conventionally reinforced drilled shaft, but this is being provided as an option to the contractor. We fill the hole with concrete only to the proposed bottom of lagging.

Thanks anyway for the quote from FHWA, but I am familiar with the FHWA publications (I work for Ohio DOT). It's interesting that you included that particular quote, as I have used that as an argument that if we are designing the system to rely only on the steel, then we are wasting money on providing 3,500 psi concrete. We might as well just backfill with lean mix concrete. In the long run, that may be the compromise solution.

Thanks for your opinions.
 
You are wasting money using 3500 psi concrete. Also, you should fill the drill hole entirely with the lean mix concrete to assure full contact of the soldier beams to the dirt behind. You don't want the dirt (hillside? landslide?) to have to move over to the soldier beams to achieve its support. Are you lagging between the soldier beams? If so, you will not get any arching action if there is no dirt in contact with the soldier beams. Also, how much movement (deflection) must the soldier beams undergo in order to pick up their design load? Probably more movement than you want the hillside to see. I think you are kidding yourself if you think you can accurately locate a long W40 soldier in a drill hole, pour concrete around it, and get the proper encasement (unless you case the drill holes and also use some type of soldier beam centralizer).

Unless a wall is very low, tiebacks are the most efficient way to support the slide. I would question the other engineer why he can't use tieback anchors.
 
PEInc,

The way I read it, there is no lagging. The soldier beams are providing the entire lateral resistance (I imagine that the tips of the beams are designed to be embedded in a relatively stiff and immobile stratum).

As some background, OH DOT presented research they have sponsored on using drilled shafts to stabilize slopes at a conference for consultants earlier this year. That particular researcher (Dr. Liang of the University of Akron) will also be presenting at the DFI Conference in Chicago next month. His paper is available for download from the ODOT website. Let me know if you want the link.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.
 
jdonville,

Yes, I would like the link to the OHDOT paper if you have it. Thank you.
 
I don't believe the final report has not been prepared yet.
The title of the research project is "Field Instrumentation, Monitoring of Drilled Shafts for Landslide Stabilization and Development of Pertinent Design Methods"

You can find other research reports on the ODOT website here:

Jeff, if you can find the paper on the website, please let me know.
 
PEInc and Panars,

Here is a link to the paper that I am referring to. No doubt Dr. Liang will present the results of any emerging field tests or other research that may confirm or contradict these ideas.

It is worth noting that some consultants expressed reservations about some of the conclusions reached by Dr. Liang with regard to the magnitude of the forces advocated by his proposed approach (Dr. Liang's methodology, as I understand it - but I have not looked too deply into this, yields design loads that can be significantly less than predicted by other more "mainstream" approaches).

It is not a huge conceptual leap to go from drilled shafts to drilled soldier pile beams - as long as the economics and technical soundness are justified. As I have no intimate knowledge of the particular project Panars is reviewing, I will not comment further on the appropriateness of the application of the soldier beams.


A copy of the title and abstract follow:

Drilled Shaft Foundations for Noise Barrier Walls & Slope Stabilization

The objectives of this research are to develop a methodology that would allow the use of the SPT (Standard Penetration Test) results for analysis and design of the laterally loaded drilled shafts and to develop a methodology for designing the drilled shafts to stabilize the unstable slopes and embankments.

I hope this provides useful fodder for the discussion at hand.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.
 
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