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When to Ignore End Bearing for driven H pile
2

When to Ignore End Bearing for driven H pile

When to Ignore End Bearing for driven H pile

(OP)
Is there any guidance/reference to determine when end bearing of H-Pile should be ignored? Recently I found some driven H-pile design that ignored end bearing at all though the soil is dense sand. As a result the pile length becomes higher. Appreciate if you share your experience. Thanks

RE: When to Ignore End Bearing for driven H pile

Quote:

Is there any guidance/reference to determine when end bearing of H-Pile should be ignored?

Yes, Sections 3 and 4 of "Bethlehem Steel H Piles", which can be found on this page of my website.

H pile are "non-displacement" piling with "large" surface area (for skin friction) and "small" cross sectional area (for point bearing).

Another time to ignore point bearing is when piles are used in soil that has underground voids (such as solution voids in sedimentary limestone). It's possible that pile driving stopped just inches above an undetected void. If significant loading is applied to the pile point, the void could collapse causing the pile to settle. We had to apply this principle for design/construction of an electric generating station.

RE: When to Ignore End Bearing for driven H pile

Unless I'm missing something here, I've always considered the driving resistance as a measure of pile capacity and have never separated the friction from the bearing. My experience has been that it's the foundation type selected that determines the economy and that once selected, the foundation cost is relatively insensitive to slight variations in loading... my $.02. I don't design foundations too tightly, either.

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

-Dik

RE: When to Ignore End Bearing for driven H pile

How about "always"!

RE: When to Ignore End Bearing for driven H pile

'always' to what?

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

-Dik

RE: When to Ignore End Bearing for driven H pile

Quote (Shakta

Is there any guidance/reference to determine when end bearing of H-Pile should be ignored? Recently I found some driven H-pile design that ignored end bearing at all though the soil is dense sand. As a result the pile length becomes higher. Appreciate if you share your experience. Thanks)


The perimeter calculation and assumptions are always a question for H-Piles. When you calculate the perimeter with the assumption of plugged , the perimeter would be less ( gross perimeter of H-pile , so one will get low skin friction, high end bearing) . When you calculate the perimeter with the assumption of unplugged , the perimeter would be more ( interior web and flange surfaces H-pile also considered ) , so one will get high skin friction, low end bearing ...After static test , still the contribution of end bearing to the total resistance is unknown..

IMO , the best practice, do both analyses (unplugged and plugged) and use the lower value for design.

EDIT: I have a problem with with this forum.. I have added the print screen. The letter size is less than 5 and hardly readable.This problem is only at ENG-TIPS forum.. Any suggestions?





RE: When to Ignore End Bearing for driven H pile

If you like wasting other peoples money, ignore it.

RE: When to Ignore End Bearing for driven H pile

I was always taught to include end bearing for H-piles. I don’t see the point of excluding it. What would be the concern?

RE: When to Ignore End Bearing for driven H pile

Also "alwasys" to what


I am with MTN on this, I always consider end bearing in pile capacity. For H piles, the soil plugs in the flanges. I consider the area of steel section plus the plug area.

One thing to think about though is mobilisaiton rates. Full end bearing is mobilised at approx 10% of diameter. Full shaft is mobilised at approx 1%.

10% of diameter would lead to excessive settlement, hence why many people (not everyone) only consider a portion of end bearing capacity. I was thought to consider 20% and have generally stuck with the approach.

The flip side of this is that we are generally so conservative in our factors of safety, parameters etc that when you do a load test you almost always have well above the required capacity. Based on this reason my boss often forces me to consider full end bearing capacity. He is reducing some of the conservatism by considering full end bearing capacity. Funny way to do it but he gets a lot of repeat clients. He tends to live on edge but contractors love him for it...

RE: When to Ignore End Bearing for driven H pile

EireChch, what sort of site verification do you do when forced to take the full end bearing? Australia has a varying capacity reduction factor that depends on whether you do eg static test, PDA, or just theoretical calc. There's quite a benefit to doing a few PDAs.

Under eurocode, wouldn't you be doing limit state, so reduced end bearing for serviceability but ~full for ultimate (taking account of capacity factors)?

RE: When to Ignore End Bearing for driven H pile

We do static tests which just confirm capacity, no separation between skin and end bearing capacity. CAPWAP and Bi-directional load tests can be used to separate the two. We work with some large diamter piles greater than 1m in dia so static tests are impractical and too expensive so we do a lot of BDLT.

We typically do a pile design and specify that we consider full skin and end bearing capacity. Consultants/clients dont challenge on the amount of end bearing we consider.

Then we do a load test find we have adequate capacity. But we dont see fully mobilised end bearing for the main reason that piles arent failed, even using a load 2.5 times allowable.

We find that contractors dont care what end bearing or skin friction we get, just as long as the sum of the two is above what they need. Our pile capacities are less conservative than other consultants in the market which is good for our business.

We often find that we are way too conservative in our skin friction calculations. For example, the critical depth recommendation limits skin friction, however we often get skin frictions that are 2-3 times above those calculated using critical depth assumption. This cant purely be down to our parameters being too conservative as if we increased those it couldn't lead to such an increase in capacity. See below, we did 2 no BDLTs on large diameter piles. The 1.5m dia test didnt come near failure however we pushed the 2.5m diameter futher a got higher skin frictions.

RE: When to Ignore End Bearing for driven H pile

Quote (EireChch)

10% of diameter would lead to excessive settlement, hence why many people (not everyone) only consider a portion of end bearing capacity. I was thought to consider 20% and have generally stuck with the approach.

Sure, if you're applying a load near the ultimate capacity of the pile. Usually my steel piles are either long or end bearing on rock. The skin friction on the long piles bearing in sands are typically close to half of the pile capacity. The allowable pile capacity is at best 1/2 of the ultimate. So if the pile only sees 1/2 of the ultimate then it's not using much of the tip resistance or possibly any at all.

I feel like ignoring end bearing is too conservative.

RE: When to Ignore End Bearing for driven H pile

MTN - I dont disagree with you

(edit, I removed the ! to make it sound more like "yes I agree" and not sound like "I AM NOT DISAGREEING WITH YOU" smile

RE: When to Ignore End Bearing for driven H pile

@dik....always as in end bearing is typically irrelevant in H-piles...they don't displace much so little or no accumulation of anything below that helps bearing

RE: When to Ignore End Bearing for driven H pile

Thanks, Ron... that was my understanding, too. I can't see the inclusion of this having a real financial impact on the cost for foundations. To reiterate, I've found that foundation costs are relatively insensitive to minor changes in loading... unless you end up with site issues.

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

-Dik

RE: When to Ignore End Bearing for driven H pile

EireChch, do you get to increase your design capacity by doing load tests? Eg we could increase the capacity reduction factor by ~25% (0.60 approx to 0.75 approx) by doing static load tests on 2% of piles. This can be taken at the design stage without doing test piles, if you're confident enough in the ultimate capacity calculation.

RE: When to Ignore End Bearing for driven H pile

Since the OP's question is about "dense sand", I agree with ignoring H-pile tip bearing for that soil (so does the Bethlehem Steel reference I provided).

However there are other soil conditions (such as deep layer of hard rock) where H-pile tip bearing can be essentially "infinite"...



"What Goes Down Comes Up - Sometimes" Bengt H. Fellenius

RE: When to Ignore End Bearing for driven H pile

SRE,

I’m missing your reasoning for ignoring tip resistance in dense sand. I haven’t thoroughly read the whole manual you posted but under Section 4 - Friction piles - sub section - h-piles in sand, gravel, sand and gravel it states:

“H-pile is sufficient to develop a high intensity of compressive stress both at the point and along the sides of the pile, with correspondingly high values for point resistance and skin friction.”

I didn’t see anything in section 3 either. Can you clarify what your reasoning is?

RE: When to Ignore End Bearing for driven H pile

Quote (H-pile tip bearing can be essentially "infinite"...)


I prefer the term 'really big', myself.

About 40 years back, my wife and I were looking for a new stove. We were approached by a salesman who informed us that the stove we were looking at had infinite temperature control. He was puzzled when I informed him that if I lived forever, I might not be able to appreciate that feature.

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

-Dik

RE: When to Ignore End Bearing for driven H pile

>@dik....always as in end bearing is typically irrelevant in H-piles...they don't displace much so little or no accumulation of anything below that helps bearing

The zero length pile designer:

  1. Don't include end bearing because under working load conditions little or no end bearing is mobilised
  2. The pile is now designed based on skin friction only
  3. under working load conditions for skin friction only, some portion of the skin friction will not be mobilised
  4. According to the 'ignore end bearing' or as I like to call them 'zero length pile designer' we should now eliminate the portion of the shaft where skin friction isnt being mobilised from our calculation according to teh same logic as 1.
  5. Repeat ad naseum, you now have a 0m long pile. Or an infinite length pile? Congrats. Maybe we should call it Shroedinger's pile: is it 0m long, or does it reach to the center of the earth?

RE: When to Ignore End Bearing for driven H pile

Get off the fence Geotechguy1, let us know what you really think.......pipe

RE: When to Ignore End Bearing for driven H pile

Haha Eirechch

RE: When to Ignore End Bearing for driven H pile

MTNClimber - I'm looking at factors that could influence point bearing in dense sand:

Cross sectional area of an HP is really small, say, 15.5 in2 (0.108 ft2) for an HP 12x53.

Not only is the area small, the geometry of the pile tip makes it a poor "footing"... for HP 12x53, essentially 36" x 0.435" ≈ 15.5 in2.

In dense sand, a properly driven HP is going to have a lot of surface area to develop skin friction. Relatively little load is going to even reach the pile tip. Section 3 of the Bethlehem book talks about how even end-bearing HP driven to rock fail before the tip is fully loaded (unintended skin friction at work).

As a bridge contractor, we would extract temporary HP when finished. To do this (in dense sand), the best way was to drive the the pile a few inches deeper to break skin friction. Then pulling the pile was easier (but not always a piece-of-cake). To me, this just confirms that (in dense sand) point bearing, while probably not zero, is minimal compared to skin friction.

In total, I consider the pile tip bearing of an HP in dense sand to be just "noise".

This is based only on my experience. I will defer to those who do include pile tip resistance since:
Design of piling is better defined now than it was when I was in the business.

Since I never worked for an engineering firm, never had a client who pushed for every ounce of pile capacity. For our in-house work at generating stations, other factors were more important than first cost.

RE: When to Ignore End Bearing for driven H pile

>As a bridge contractor, we would extract temporary HP when finished. To do this (in dense sand), the best way was to drive the the pile a few inches deeper to break skin friction. Then pulling the pile was easier (but not always a piece-of-cake). To me, this just confirms that (in dense sand) point bearing, while probably not zero, is minimal compared to skin friction.

We install the things by smashing them into the ground with a big hammer so I suppose that logic is consistent with never relying on end bearing



All we're really trying to do is end up at point A (or make sure if we are wrong we end up at B and not C).

RE: When to Ignore End Bearing for driven H pile

Quote (geotechguy1)

We install the things (temporary HP) by smashing them into the ground with a big hammer...

An example of the difference between a contractor and an engineer's viewpoint of the same project.
To a contractor, any temporary HP (or other steel shape) is worth recovering for (temporary) reuse on another job.

RE: When to Ignore End Bearing for driven H pile

Quote (Since I never worked for an engineering firm, never had a client who pushed for every ounce of pile capacity.)


A really bad way to save money, IMHO...pipe

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

-Dik

RE: When to Ignore End Bearing for driven H pile

Thanks for the clarification SRE.

PDA and static load tests with strain gauges data that I’ve witnessed and reviewed have shown tip resistance in sands can provide with substantial addition to the capacity. Even if the end bearing is worth 15% of the total capacity, that reduction in pile length can save a lot of money on projects with 100’s piles.

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