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Soldier Pile Design (Sx vs Zx)

Soldier Pile Design (Sx vs Zx)

Soldier Pile Design (Sx vs Zx)

Hello everyone, hope I can get your input on the following question.

Here is the background:

I am designing a cantilever soldier pile using a pipe section. Normally I spec out a micropile sizes available from Nucor Skyline (see link: https://www.nucorskyline.com/File%20Library/Docume...). From their product sheet you can see that each size has their available section modulus Sx which I use to pick a pipe size. I have never had an issue with using Sx to design my pipe solider piles, but for this one particular project an issue came up. In this project the design demands a section modulus of about 36 in^3. The pipe section that I must use in this particular project is the 9.625 inch diameter by .5 inch thickness, which has a section modulus of about 31 in^3. After bringing this up to my superiors, his conclusion was that the section is okay since the Zx value (41 in^3) is greater than the demand section modulus. This was the first time we did something like this (using Zx over Sx). In previous times we would just choose a bigger section or switch to a brace system, but for unknow reasons we need to make the pile size work. I was hesitant to accept this exception with out really understanding the logic behind it. The reason given to me would not convince me so I sought out to do some research on my own. I began by checking the flexural formula for a round HSS section in chapter F of the steel manual. F8 states that a round HSS section has to be check for two limit states: Yielding and Local Buckling. Local bucking does not apply if the section is compact, therefore only yielding is the limit state. Per the yielding equation, Zx is used. I get this section specifically says for "ROUND HSS", but this is pretty much a pipe. This is the closest thing I have found that allows me to use Zx instead of Sx. The solider pile design is done in ASD and the only safety factors is on the embedment. The pipe strength is 80 ksi steel.

1) Is using Zx correct in this situation? if so...
2) Why do pipe suppliers (in this case Nucor Skyline) provide the Sx values? are they just being conservative?

As stated above, I have always used Sx to design my pipe soldier piles. If I can use Zx in the design than for sure I cant get more strength for any pipe size so long as it meets the "compact" requirements of chapter F in steel manual.

Thank you all for the help.

RE: Soldier Pile Design (Sx vs Zx)

I usually use Zx because sections are stockier and not prone to local buckling. The use of Zx is less conservative, but the stocky section will develop the moment.

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RE: Soldier Pile Design (Sx vs Zx)

I recommend using Zx, per AISC Chapter F8. The allowable moment is (Zx)(Fy)/omega, where omega = 1.67.

Sx is not used for determining the capacity where yielding controls. It does come into play where local buckling is an issue.


RE: Soldier Pile Design (Sx vs Zx)

I say it depends on how much deflection of is allowed for the wall. If it's compact, it won't buckle, but if the moment exceeds the moment at first yield (Fy*S), the section begins to deform plastically, and may bend excessively, especially if the demand is calculated using active soil pressure or the retained material is clay.

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RE: Soldier Pile Design (Sx vs Zx)

I use Sx for soldier beam design. Remember that, if you are using flush-jointed pipe sections, the pipe strength will be reduced significantly (perhaps 50% to 60% less) at each threaded joint. You will need to check the stresses at each reduced strength location, threaded joint. If you need to add some strength, you can add a small beam or a smaller diameter pipe inside the pipe soldier beam and then fill it with grout.


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