Helical Screw Piles 4

[dik]

Does anyone have an easy solution to determine the weld required or the weld capacity for attaching the helix to the 'pipe' portion of the screw pile? and designing the thickness of the helix required? Can anyone recommend a paper or article on this?


Dik

 

[XR250]

Dik,

Why are you reinventing the wheel?

 

[Retrograde]

I would imagine that the thickness of the helix is based on trial and error - and that the weld is sized to achieve a full strength connection.

 

[atrizzy]

Assuming not much in the way of normal force is put on the helix during installation (probably a fair assumption), I'd take the load on each helix, assume it's distributed evenly over the entirety, take, say, 1/4 of the helix (like a pie slice) for simplicity, calculate the shear and moment arm, thus moment, on the welded connection, and design the connection and helix base metal thickness to resist the shear and moment at the pipe.

Seems pretty simple, or what am I missing?

 

[dik]

I've been asked to check the design of some... can you answer the question rather than ask another one? Currently looking at it as a strip based on soil bearing pressure and treating the strip as a cantilever from the shaft... really conservative... just wondering what the better way would be other than a FEM model.

Dik

 

[atrizzy]

dik, I'm curious. What makes you think the bearing pressure based approach is really conservative?

 

[HTURKAK]

I think it is worth to look the past thread,
Helical plate design

thread507-442660

 

[dik]

Bad choice of words on my part... it the treatment of the spiral design being a 'flat' plate and not a spiral. Thanks...


Dik

 

[dik]

The info by Jiang deals with an annular plate and does not reflect the strength of the spiral. Thanks.

Dik

 

[CANPRO]

I think stiffness comes into play here as well. If your helix plate deforms too much you'll lose some bearing capacity.

Seems to me like this is something that you would end up testing to verify.

The link HTURKAK has another link in it that could answer your question. Look way down on page C-4 (page 387 of the PDF).

 

[HTURKAK]

The info by Jiang deals with an annular plate and does not reflect the strength of the spiral. Thanks.

I just wanted to draw your attention to the respond of Bobby46 (Structural) rather than The info by Jiang . at the same thread

thread507-442660: Helical plate design


Bobby46 (Structural)1 Aug 18 19:18
Most manufacturers publish the individual helix plate capacity for their anchors. Are you designing your own helix plates?

https://www.masongradyllc.com/wp-content/uploads/2017/09/DesignManualForEngineers.pdf

Hubbell Power Systems (A.B. Chance) Manual

https://www.masongradyllc.com/wp-content/uploads/2...

 

[Brad805]

This does not seem an easy task. We have installed piles with 1/2" helix into clays, but if you have too many cobbles I we have needed to use 3/4". I am not sure how one will calculate that reality. We use torque specs up to 80,000ft*lb. Bearing pressure will have little to do with the spec.

 

[dik]

Thanks... I caught that and already had a copy of Chance booklet.

Dik

 

[dik]

I've been asked to check the design provided by others.

Dik

 

[EngineeringEric]

Are you talking about the lead section to the extension? these splices are done by manufacturers all the time, usually a a pair of bolts for 30-kip ultimate piles. my experience using the torque correlation method is that torque equates to bearing.... so whatever your ultimate load is, correlates to maximum toque, then ad 1.5 for over torque... then boom got the weld around a circle.

But maybe i'm missing what force you are evaluating

EDIT i now see that you mean weld the plate to the pipe!!! I think my anwser still applies, but i would take the torque and apply it over the actual length of the plate. this installation force is probably greater than the 'bearing' force of shifting the pile weight into the immediate soil due to the three helix gross length

 

[dik]

I've used them for supporting six 400,000 lb transformers on a 3' slab... and still holding out OK. The torque is the 'magic' number. It seems for a given soil, there is a very close relationship between torque and load capacity. The ones I'm looking at are 16" helixes and supposed to support 25K... really scratching my head on this (not really... visually doesn't work)... but if you have 15ksf bearing capacity, I want to know how thick the helixes are and what sort of weld...

Dik

 

[Phil1934]

https://docplayer.net/37439195-Ultimate-strength-of-steel-screw-piles-in-sand.html

does strip analysis
Malik wrote a paper that refers back to Roark flat plate 2L "Effect of helix bending on load settlement behavior of screw pile" It's on internet and analyzes full circle but NO movement so 75% of thickness more applicable
Weld is std. formula, welds top and bottom. If you want to get exact 3" 90-atan(3" pitch/2.88*PI)=72*
ASD for 2.88" OD 0.3*70 KSI*0.25*0.707*2*(1+0.5*sin(72)^1.5*2.88*PI=98K
For 1-1/2" square 90-atan(3"/6")=63* and two 1/4" welds is 63K, etc.
If you want to get even more exact you can calculate the length of the spiral around the shaft instead of the horizontal distance.