eccentric loads on edge strip footing that are supported by screw piles

[JasonS_001]

Can anyone please give advice on the analysis and design (detailling etc) on this eccentric loads applied on the edge beams that are supported by the screw piles? Thanks

 

[phamENG]

Screw piles generally have no lateral/torsional capacity to speak of as they are comprised of very small steel tubes. You can get them with larger HSS tubes, but they come and a significant premium.

For this case, you should probably either move your grade beam below grade and center it under the wall (best option) or, if you can't do that, design your slab as a diaphragm to resolve the moment and design your grade beam for the resulting torsion.

 

[BAretired]

Talk with your local supplier and see if there is a typical detail to provide for eccentricity. Some screw pile suppliers provide a flat plate on top of the pile, but the pile must be capable of resisting the applied moment.

BA

 

[JasonS_001]

Thank you.

Do you mean move the screw pile under the center of the wall? Thats what I was thinking but I wonder there will not be enough cover or room for screw pile to footing. Also, I have some small loads from the slab on grade as well. So it will be hard to find a perfect central location.

Can you please elaborate on this. I don't full get it. Thank you.

Is 150kN*0.125m = 18.75kNm torsion something rational for the screw pile to take?

 

[dik]

Why a footing? Can you extend the wall down 300mm?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JasonS_001]

If I dont use footing how do I connect the screw pile to? And may I ask where 300mm comes from?

 

[dik]

You use headed studs and cast the wall on top of the screw piles... done that lots of times... also with grade beams.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

Is 150kN*0.125m = 18.75kNm torsion something rational for the screw pile to take?

It isn't torsion. It is bending moment applied to the top of pile due to load eccentricity. 18.75kN-m is perfectly rational, but why not move the pile over to avoid or reduce the eccentricity?

BA

 

[KootK]

1) I did a helical pile job in a high seismic, crap soils environment earlier this year and it very nearly drove me mad.

2) In many regions, these things can be ridiculously slender. Mine were 1.5" square. Seriously... the decimal point is in the right spot.

3) Per the blurb shown below, in some situations the code allows your to ignore the eccentricity. To make a go of it, you do have to bring your piles a lot closer to being in line with your wall load though.

4) As the other guys have intimated, it's very common for the piles to be embedded within a much narrower, wall-ish element that serves as a grade beam. Something in the 8"-12" thickness range perhaps. Going that route saves a concrete pour and reduces the eccentricity. Like you, I've worried about providing adequate tolerances in the grade beam for pile misplacement. Don't. The piling contractors can locate the piles within very tight tolerances.

5) A slick solution for more "commercial" applications is to stagger the piles as mentioned in the blurb below. That way, the piles themselves can form a stiff load path for resolving the eccentricity. Try this on a residential wood frame project, however, and your corpse will likely wind up cast into the foundations some place.

NOTE: the code blurb below in a US thing, not Australian. You may need to source and Antipode here to help you sort out local requirements.

 

[Brad805]

"The piling contractors can locate the piles within very tight tolerances." This depends heavily upon the equipment used and is not always the case.

If you watch the head of the installation equipment below as they push the pile into the ground you will see how badly it wanders. How much it wanders depends 100% on the operator skills. We have put in 100's of screw piles for bins of all different sizes, and dealt with all sorts of problems due to alignment problems.

 

[KootK]

This depends heavily upon the equipment used and is not always the case.

Noted. I was really speaking to the smaller scale applications like the 1.5", residential one that I mentioned in my previous post. Whether or not that's OP's case, only he knows. For these smaller scale applications, the contractor's ability to make tight tolerances was both:

1) Critical to my being able to reconcile myself with the detailing expected in the region where the project was located and;

2) A surprise to me. Being based in western Canada, like you, I'm more used to helical piles like the ones shown in your photo: pipe sections of a meaningful diameter.

 

[dik]

If you put a 'spike' on the end of a screwpile, it doesn't wander very much... made an oversight on some large 12" shaft ones a few years back and cut the 12" shaft on an angle to provide the 'point'... the bevelled cut wandered a lot... by using a spike this is next to nil. This was not an issue for the large slab, but with the gradebeams it made a real difference and I had to make a 'fix'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JasonS_001]

Thank you very much.

What about the small loads from the slab on grade, which is considered as suspended. Do I transfer the loads from slab to the wall instead?

I was consider there was not much room if I shift the screw pile to the central line of the wall (but dik and Kootk have pointed out, I won't concern anymore). I also have some loads from the slab as shown. Maybe a 300mm edge beam with screw pile located under the wall centrally is a good option.

Since you mention high seismic. I am wondering how the screw piles take the lateral loads because I believe they have weak shear capacity as well?

 

[phamENG]

Typical helical piles have negligible lateral resistance. If you need to resist lateral loads with them, then they need to be battered. Specialty helicals - the ones with 4" round HSS shafts and larger, do have some lateral capacity. But it won't be a lot.

 

[dik]

The ones I designed for the transformer pad had an unsupported length of about 10' if memory serves to accommodate cleaning out the Sorbweb containment pit. They were large and had several flights of helices. This was the initial concept; I don't recall the actual installation. Another issue, the torque was supposed to be recorded and this was not done. I don't have the drawings for the project so I cannot check. The bevel at the bottom is a bad idea for large tubes. They were not filled or sealed.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Brad805]

dik, if you watch the top of the pile as they start you can see why they wander or more correctly lean. The spike helps, but if you have a 25' long stick the head usually goes in a circle as they engage it into the ground. It is just the reality of the number of pivot points on an excavator. If you are using a boom truck the motion is more radial, but the operator still needs to pay attention to keep the pile plumb if it is a long pile. I have watched endless very good operators. We have tried pilot holes as well, and the results were only marginally better for our common pile lengths. All piles are being staked by legal surveyors, so that was ruled out long ago. I am sure these little piles being used in other areas go in straight given the dimensions and equipment used. I do not penny pinch on the foundations given my experiences with clients and their old building foundation problems. Koots project was far out of my comfort zone even knowing he would have justified the numbers.

From a lateral load perspective a pipe screw pile is similar to a driven pile. You can model with soil springs, latpile or allpile if you have soil data. One must rely upon the fact that the soil disturbed by the helix re-consolidates some time after the pile is screwed into the ground.

"the torque was supposed to be recorded and this was not done" That is our number one problem with screw piles. Some either do not mount the appropriate gauges to measure, or do not provide it. Some provide a reading of the hydraulic pressure since you can mount a gauge to measure that very cheaply. We require logs with the field data, and it can be difficult to get.

 

[phamENG]

Brad and dik - those are some big helical piles. My experience is with what KootK has described - the dinky little 1.5" square tubes or, if you're lucky, a 1.5" diameter round shaft. That is what is most common on residential projects. They do go in a lot straighter and with more accuracy because they are small and are typically installed in 4-6 foot segments with a rig on a Bobcat.

I'm not so sure about the reconsolidation, though. Have you done lateral load tests to prove that out?

 

[Brad805]

Pharm, I usually have a geotech on board. We have load tested some as well. I am in frost country, so nothing we put in is less than about 20'. I had to google koots piles to see that it was common. I had seen them advertised for decks or small back yard foundations, but not for the main building. We use the larger piles for residential as well.

 

[phamENG]

Haha. Yeah. I looked into using the big ones once, and the response from the local helical pile installer was "we can get those, but they're really expensive and they're really only for big buildings". So he lost the job and we drove piles instead. I'm in the swamp, so we vary a lot in depth. Sometimes we can get away with a 15' combined end bearing and friction pile, sometimes we need 100' end bearing piles to get to the bottom of an ancient stream bed with 90' of WOH material below a 4' "crust" on the surface. In those instances, we occasionally just do timber friction piles that I swear do more for the building by adding to its overall buoyancy.

Around here, helical piles are used for retrofits/repairs and sites with limited access. If you can get a rig in that can drive/vibrate a pile, that will be done before helicals are considered.

 

[Brad805]

Pharm, swamp. Eek. We had a recent project in some silts that made me cringe, but I cannot imagine swamp. Our common soil is a CH clay. The image I posted is a very common application in these parts. It is interesting to see the regional differences. My favorite pile is CIP. The kelly pole goes up and down perfectly plumb every time, and I get to see what my building is bearing on. I have grown to dislike screw piles over the years mainly due to the suppliers I have encountered.