Shoring with 1-level tie back

[HanStrulo]

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Happy New year everyone!

I have a somewhat basic question.

I am new to shoring (my background is petroleum engineering). I am trying to find the best way to size the piles and tie backs for a 1 level tie back shoring.

What I am using so far is Supportit. I used fixed earth condition and size for the piles for the combined max moment on the pile with the max vertical tie back load.

My question is: the max moment on supportit is always below the bottom of excavation and it is usually very high (compared to hand calculations). Should I only size for the max moment above the bottom of excavation? If I consider my pile as a simply supported beam (one support on the tie back level and one support on the bottom of excavation), I get lower moments. is that approach realistic?

I would love to see an example if anyone could provide it.

Thank you for any advice.

 

[PEinc]

I do not have or use Supportit. I do not understand how Supportit's maximum moment is "always below the bottom of excavation." Are you using a triangular, rectangular, or trapezoidal earth pressure distribution for the one-tier, tiedback wall? Before using any sheeting design program, you should knw how to design these walls using hand calculations and you should be very familiar with several design manuals that cover cantilevered, anchored, and braces walls. You should also know exactly how a computer program "designs" the wall (i.e., what are its design assumptions, safety factors, earth pressure coefficients, program limitations, etc). You should be able to verify its design results with hand calculations. If your hand calculations are correct and they do not match the Supportit results, maybe you should not use Supportit?

http://www.PeirceEngineering.com

 

[HanStrulo]

Thank you very much for the answer.

For you first point: I always use triangular for the 1-tie back level walls.
2. The design example i use is from Pile buck, they have a really good example of 1-level tie back with fixed earth condition (page 166, example problem page 170). This method seem to agree with the supportit results. However, the moments it gives are still pretty high. Any other design examples you suggest i take a look at?

My main issue is that I believe the moments i get from the fixed earth method are pretty high. (I compared the sections i use for the piles with the sections i see in other drawings for the same conditions and they always seem higher). Any suggestions for an example are very appreciated.

Thank you again.

 

[PEinc]

1. Nowadays, it seems that most people (who have newer design references than the US Steel Sheet Pile Design Manual, now Pile Puck, and who specialize in design and construction of these walls) use trapezoidal pressure distribution rather than triangular or even rectangular. Even AASHTO and FHWA use trapezoidal loading for one-tier walls. USS and Pile Buck are, in my opinion, outdated references.
2. Fixed earth design gives less moment that free earth design but gives longer embedment.
I suggest that you review on-line US State Design Manuals or FHWA Design Manuals for additional information.
EDIT: Another reason for getting higher bending moment is putting a safety factor on the passive earth pressure coefficient rather than just increasing the embedment depth by at least 20% beyond that depth needed for equilibrium when checking overturning and sliding.


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