Deflection of foundation. 3

[JoanWill]

Hi guys, I have questions regarding the deflection of foundation that I hope you guys can help me.
When we design say pad footing we do load run down, load combination (DL+LL gravity in my region) and then design the footing accordingly. I don't see much consideration is given to the deflection of footing. Is it because when we design the footing we are using the allowable bearing capacity instead of ultimate one? If so, does it mean that what founding material (either clay or rock or whatever) is doesn't matter as long as the strength design works fine under allowable bearing capacity? (This doesn't sound right to me)
When it comes to pile foundation, we provide the loading (DL and LL) to the contractor. Does the contractor simply stop when these target loading is reached? If so, what consideration of deflection is given here? (The allowable and ultimate scenario does not apply here)
Thank you.

 

[SlideRuleEra]

1) When we design say pad footing... I don't see much consideration is given to the deflection of footing.

2) When it comes to pile foundation... Does the contractor simply stop when these target loading is reached?

1) A well designed footing will have negligible deflection. To do this, the footing's moment of inertia (I) needs to be "large". Most cost effective way to get a "large" I is to make the footing thicker (relative to it's horizontal dimensions). A "thin" footing will deflect, causing non-uniform soil loading, which should be avoided. A "thick" footing (no significant deflection) provides essentially uniform soil loading over it's entire contact area. See my sketch:

2) Assuming you mean driven piling and the "contractor" is the pile driver. No, a competent pile driver does not stop based on design loading. Pile driving is as much "art" as it is "science". The pile is driven until the pile "stops" significant movement (for point bearing piling)...
or driving resistance (blow count) is adequate for the load, including a generous safety factor (for friction piling).
A properly driven piling will have predictable deflection under design load... and that deflection should be very "low".

http://www.SlideRuleEra.net

 

[Ron]

Nice explanation, SRE.

 

[dauwerda]

SRE hit on the topic of the concrete footing itself deflecting, it sounds like the OP is also concerned about the soil the foundation is bearing on deflecting - or settling.

This is where following the recommendations of a geotechnical engineer are paramount. The geotech will typically give allowable bearing capacities of soil and in the report specify what the expected maximum settlement is. I often see this quantified as 1", if the project requires something less than this, the geotech should be made aware and they will specify the required foundation types and bearing capacities to achieve the wanted results (poor soils often require some type of deep foundation).

If all foundations for the structure settle the same distance, there typically aren't any issues. It is differential settlement that can wreak havoc on a structure, this should absolutely be a consideration in the foundation/structure design.

 

[SteelPE]

I once worked on a project where we design a pile foundation based upon the information given to us by the Geotech. If I remember correctly, they stated that the piles were capable of supporting 15 tons (they were wood piles in terrible soil). Then the pile contractor came along and they said they could increase the capacity to 30 tons (this was a while ago so this might not be 100% accurate). They hired their own geotech to verify the increase. Part of the increase process required submission of the pile cap loads to the geotech for review.

Then the pile driver began installing the piles..... and we ended up with piles that were 1'+ away from where they were designed to be (some were 1'-6" away). When we went back to "fine tune the numbers" based upon the 30 ton capacity piles we found out that the geotech reduced the pile capacity in each group based upon the loading information we provided them earlier in the project (if we had a 3 pile group with 75 tons on them they reduced the pile capacity to 25 tons/pile). It was a f*****g mess.

That was the last time I ever took a pile job.

So I guess I take slight exception to the statement:

2) Assuming you mean driven piling and the "contractor" is the pile driver. No, a competent pile driver does not stop based on design loading. Pile driving is as much "art" as it is "science". The pile is driven until the pile "stops" significant movement (for point bearing piling)...

 

[r13]

From structural engineering point of view, I would like to change your first question to: "Provided that foundation elements met strength requirements, and the bearing stress was within allowable, then, what deflection would do on foundations and what are the concerns?". I would like to hear your answer.

From geotechnical engineering point of view, when bearing stress is kept below/at allowable, well, the soil ain't broken, the settlement will be kept within tolerance set by general practice on the use of structure, and the type of foundation system, or local code requirement.

I don't quite understand your second question, so no comment.

 

[JoanWill]

Thanks for your replies guys.
Q1. Sorry I didn't ask properly. My concern was the soil settling instead of the deflection of the footing itself.
Q2. Yes, I meant driven piling. The only work left for me is to provide the loading to the piling company or pile driver. So I was concerned about how they could make sure the settling would be okay. If the settling is ensured like what SRE said, by driving the pile until the pile "stops" significant movement, then why does the standard say "earthquake serviceability actions need not be taken into account" considering the piles will be driven to reach (above) the target loading caused by earthquake and no significant movement anyway? In another way, how can they consider the settling for gravity loads only when loading caused by earthquake is greater than the DL+LL?

 

[SlideRuleEra]

The only work left for me is to provide the loading to the piling company or pile driver.

I assumed the question was how the pile driving contractor determines in the field, in real time when to stop driving a pile. Perhaps I misunderstood this question.

In my experience, the design engineer (geotechnical or structural) determines the pile bearing values needed for the project... not the piling company (who supplies the piling?), not the pile driver (who physically drives the pile).

FWIW and IMHO, the single biggest mistake a piling design engineer can make is to specify a pile tip elevation as the sole criteria for when to stop pile driving. Don't fall into that trap.

http://www.SlideRuleEra.net

 

[oldestguy]

My 2 cents. Yes settlement is of concern for either piles or footlings. However, generally my experience has found that with a reasonable safety factor against soil rupture, tolerable settlement results. Thus one can compute settlement expected, but the general experience has shown you usually do not have to do that computation.

 

[JStephen]

In many cases, as in SRE's "thin" footing, deflections will reduce moments and shears, so it is conservative to neglect the deflection.
You can introduce a lot of extra complexity into a problem but may not gain that much accuracy in the results, either.
I seem to recall that using elastic support under a slab gives distinctly different results from assuming a semi-infinite elastic solid, for example, so you don't go from "approximate solution" to "exact", you go from "approximate solution" to "different approximate solution".
And some of the deflection is time-dependent for a further complication.

 

[r13]

"earthquake serviceability actions need not be taken into account"

Again, I don't quite understand. Where you read this?

 

[r13]

JoanWill,

Let me try to answer your questions and concerns.

I don't see much consideration is given to the deflection of footing. Is it because when we design the footing we are using the allowable bearing capacity instead of ultimate one? If so, does it mean that what founding material (either clay or rock or whatever) is doesn't matter as long as the strength design works fine under allowable bearing capacity?

1) For a properly sized and designed footing (satisfies shear and bending requirements), the deflection will be very small (as expected for reinforced concrete members), and in no way affecting its performance on supporting structure above. You can verify the claim of "deflection is small" by perform a few simple calculations.

2) From geotechnical engineering view point, The allowable bearing stress is interpreted as the maximum perceived pressure to produce a certain amount of settlement. The perceived pressure is derived from the soil properties and the anticipated maximum load with a safety factor much greater than the factors used in ultimate strength design. And the "certain amount of settlement" is the generally accepted settlement for the type of structure, say 1" of settlement, or 0.5" differential settlement. Thus, the actual settlement will be less than 1", or 0.5", respectively, and that's why settlement is not a concern, since the maximum perceived pressure is kept within the so called "allowable bearing stress".

3) Type of founding material is important, as different type of soil/rock posses different properties, which affect the derivation of allowable bearing pressure as explained in paragraph above.

Does the contractor simply stop when these target loading is reached? If so, what consideration of deflection is given here?

The contractor can't call "stop", the call is to be made by the project geotechnical engineer, who in turn, is responsible for soil exploration, and determine the target resistance. The target resistance is reached when pile shows no movement, or very little advancement within certain below count, which is calculated depending on the type of pile driving equipment in use, and the soil properties. The target resistance is much larger than the maximum design load anticipated, the safety factor is huge, as mentioned by SRE. For the point bearing pile, the potential for further settlement is close to nil, as the soil near the pile tip has already been densified by the pile driving force.

However, pile shortening can occur, but it's a time and sustained load dependent phenomenon, rarely critical in plie applications. For lateral displacement of pile head, it is limited to a limited amount during pile design stage through the evaluation of pile-soil-pile cap interaction. Lastly, the lateral deflection of pile within span, it is avoided through carefully selection of pile size (steel), and adequate reinforcing (concrete), the deflection should be very small.

Finally,

why does the standard say "earthquake serviceability actions need not be taken into account" considering the piles will be driven to reach (above) the target loading caused by earthquake and no significant movement anyway? In another way, how can they consider the settling for gravity loads only when loading caused by earthquake is greater than the DL+LL?

I still couldn't make sense out of your question(s). It is quite difficult to understand the intent of a "standard" without reading the whole article. Does the standard has commentary? Which may help you to understand its intent, and applicability. Also, inputs from geotechnical engineer should be immensely helpful.

 

[JoanWill]

Thanks for your replies guys.

retired13, It is from the Australian piling standard. I have uploaded a pic as below. Again, as we are going to provide the loading caused by earthquake and pile will be driven until target loading is achieved and shows no movement, which means serviceability will be satisfied, so why does the code add clause like this or do I misunderstand anything here?

 

[r13]

JoanWill,

Is "earthquake serviceability actions" explained/defined anywhere in the standard? It is the key to answer your question. At this moment, I couldn't comment, as I am not familiar with Australian codes and standards, not to mention design philosophies. But I am quite interested in your finding(s)

 

[mrlm]

Coming out of left field here, is the reason it states that earthquake serviceability actions need not be considered because the building is not expected to remain serviceable in Australia after an earthquake unless it is importance level 4.
In AS1170.0 clause 2.3 there is no mention of serviceability loads to AS1170.4 unless designing in NZ