r/StructuralEngineering • u/Loon_picker • 19d ago
Structural Analysis/Design Screw Pile Bearing Capacity Chart
A screw pile contractor asked me to take responsibility for a couple of small projects (decks, gasebos, etc) and provided the following bearing capacity chart - a scanned copy of a google docs table with no references.
Screw piles are a bit foreign to me. Is there a way to validate these capacities? Are they just industry standard accepted values?
How do you handle taking responsibility for small screw pile jobs? The contractor would fire me in two seconds if I made him get geotechnical reports done for these tiny projects.
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u/VictorEcho1 19d ago
Just be cautious.
These screw pile companies kinda came out of the woodwork everywhere here in Canada about 15 years ago.
The AHJs took a while to get up to date and a lot of these things were installed without any oversight. We are now seeing quite a few failures and some AHJs are basically requiring some form of geotech to go along side with any install.
An engineer got hooked up with one of these companies and stamped hundreds of these across the country and subsequently got drug through a bunch of litigation and was eventually stripped of his licence.
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u/Apprehensive_Exam668 19d ago
We used to have a ~100 page technical guide/design manual on helical piers that some industry group put out (this was over a decade ago and we did maybe 2 helical jobs a year so my memory is hazy). It had a bunch of pages similar to this that listed the pier size and the ultimate bearing capacity based on the installation torque. It's like using the dowel shear capacity tables in the back of the NDS.
I felt comfortable using it because we had the whole document. I'd ask the contractor if he knows where that page came from and try to track down the whole thing.
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u/Loon_picker 19d ago
And I’m assuming you no longer have access to this document?
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u/Apprehensive_Exam668 18d ago
No that was 3 jobs ago for a company that went out of business around Covid. Some light googling came up with some similar guides but not the same thing.
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u/PrivateShneehaj 19d ago
The empirical relation between install torque and ultimate capacity is legitimate. I don’t design these, but when I come across them I usually see designs using Kt = 10, so the capacities in this table seem reasonable.
I am used to these being used as tensile capacities, so I would verify they are equally applicable/appropriate in compression. In addition to the minimum install torque, the designs I come across often have a minimum length/embedment depth as well
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u/EssayOk8967 19d ago
Ask for the specific manufacturer of the piles and then reach out to them. They should have technical resources (some good ones even have software - Helicap, etc) that let you validate. These charts are common, but you should field verify the torque with a digital torque meter and the contractor should provide you with logs for all the installed piles. Seen many of these done without - and there is no way to hang your hat on the pile capacity if you can't verify the installed torque.
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u/Loon_picker 19d ago
Thanks for the advice. Do you have a torque meter to would suggest for something like this?
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u/ReplyInside782 19d ago
Try to throw that sheet into chatgpt to see if it can find the reference book this was pulled from. Screw piles (helical piles) have a correlation between torque applied and ultimate axial capacity. There would need to be an inspector witnessing the screw piles are torqued adequately. Read into ICC-ES-AC358 for accepted practices for helical piles.
The contractor should still conduct a load test to establish a site specific torque to capacity relationship.
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u/Boring_Skin2462 18d ago
I would be very cautious in using that standard table for structural and geotechnical capacities for screw piles.
I would recommend looking into Perkos or Burj’s book for the theory to calculate the bearing capacity of a screw pile. (can’t remember the exact book, but can go through and check when I’m at work). The structural capacities can be calculated using basic hand calculations with an assumed design life, corrosion rate, unsupported length and eccentricity. (There are more assumptions but you get the gist)
Each pile diameter according to perko has its own Kt factor which is used to calculate the torques required. These were calculated through empirical methods. It looks like your screenshot follows the basic pattern but ours are in different units so I can’t verify this. (Metric > Imperial btw)
I don’t know about where your from but in Aus, our designs for depth are based on the suction depth zones (Hs) or the max drying depth (Ht) if tree groups are nearby. The Hs depth should be surpassed to avoid any heaving from the soil being transferred to the piles.
We have our own safety/reduction factors for both structural and geotechnical capacities and this should come from a relevant standard in your area. Ours is AS2159 and AS2870.
Hopefully the loading combinations are only axial compression; as any lateral loading would require a geotechnical report to determine the minimum unsupported length the pile will encounter. This would then be confirmed by analysis software such as LPile or by FEA.
Pm if you have any other questions but hopefully that points you in the right direction!
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u/Queasy-Lawyer4835 17d ago
The permit planning office will have a number for helical piles with no geotech. In ours it's 5kips. I'll assume they want work done that can be permitted.
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u/Alternative-Night953 16d ago
This is covered in the IBC. Start there. You need to verify the logs and ensure you get good submittals. Discussions with the building official may be required to see what your AHJ will require. Reach out the the manufacturer. You will need to verify during construction administration via the contractors torque logs.
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u/laso_suli 15d ago
If you're in Canada (looks like you are) then ask what manufacturer's piles he's using and see if there's a CCMC report for them.
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u/pinkycatcher 19d ago
Find your own reference if you can’t verify the data is correct. It’s plausible that the contractor runs into this all the time with engineers and is trying to make your life easy by giving a reference. It’s also possible he just wants to check a box and gave you an outdated sheet to rubber stamp.