Resultant Outside Middle Third of Tower Crane Footing 2

[cldea8]

I am designing a tower crane footing for the construction of a parking garage. As expected, I have a large moment on this footing. The resultant falls outside the middle third of my 18'x18'x5' thick footing. I know this is generally undesirable, however, I performed (and passed) the following checks:

*My allowable bearing capacity (on Geopiers) is 5000psf

*Overturning about the toe yields a Factor of Safety > 1.5

*M/P yields an eccentricity of ~5ft which is > L/6 (outside middle third).

*Because it is outside the middle third, I used the following bearing equation from Bowles 5th Ed.(zero stress for a distance then triangular load on bottom of footing): q = 2P/(3B(B/2-e))< qa (have seen this on several threads too)

*This resulted in a toe pressure less than 5ksf allowable.

*Flexure, Flexural Shear, 1&2 way Shear, Min Ast Satisfied


If I keep my resultant within the middle third, the footing grows to 23'x23' (which is about $4000 more in concrete).

Is this procedure acceptable? As expected summing moments about the resultant gives me a Factor of Safety of one. I would assume my factor of safety is equal to that of the bearing capacity (2-3)?

This text reads that there is no way to determine qa for a triangular load and that it is "prescribed" by the geotechnical engineer. However, some texts show triangular distribution when the resultant is within the kern.

Have I checked everything? Do I have instabilities? Is this procedure acceptable?

Thanks,

cld

 

[DaveAtkins]

I think what you have done is correct.

DaveAtkins

 

[csd72]

looks fine to me.

 

[cldea8]

Thank you for the responses. I appreciate your confidence

There is a line in the text of Bowles 5th Ed. Section 8-10.2 "Eccentricity Out of the Middle 1/3 of a Footing" that states "...Note carefully that qa is estimated by the geotechnical engineer. There is no current method to compute the allowable bearing pressure for a triangular presure distribution..."

The stone column engineer is providing a bearing capacity of 5000 psf. Should I request a statement that OKs this capacity for a triangular distribution?

cld

 

[hokie66]

You should check the load at 45 degrees. I think the overturning and soil pressure will be worse in that direction.

Is it feasible to put in some bored piers or rock anchors? With such a big footing, some deep assistance should be considered.

 

[cldea8]

I would think that overturning about the corner wouldn't control as the diagonal is a longer dimension. I have not seen any examples that check this case? How do you resolve the soil stress at the corner where there is a three dimensional triangular load?

At this point the contractor would rather use more concrete than install tensile elements. Therefore, I am trying to give them the most cost effictive chunck of concrete that will withstand overturning (and allow me to sleep tonight )

Thanks for the input.

cld

 

[hokie66]

The dimension to the corner is longer, but the width at the corner is zero. I don't remember exactly how I approached it, but I would tend to use an ultimate soil pressure approach, find the triangle size which provides the required overturning resistance, and check the stability.

 

[cldea8]

I used Risa Footing and placed 70% of the moment in each direction (to get a resultant at 45 deg.). I did get a footing that is 9" wider. Bearing controlled. Over turning was plenty OK. Thanks for the tip.

 

[hokie66]

Yep. The 70% factor accounts for the load at 45 degrees. You are welcome.

 

[fattdad]

did you include the reaction from any depth of burial on your eccentricity calculation? (The depth of burial can help lessen the amount of eccentricity.) Otherwise, it appears you've done your job.

f-d

¡papá gordo ain’t no madre flaca!

 

[DRC1]

The concern with working outside the middle third is that as the overturning gets greater, the eccentricity increases and the engaged portion of the footing gets smaller. The result is that toe stress increases rapidly for modest changes in moment, as for a sudden wind load. The other question is what is the allowable bearing pressure and how was it computed. For overturning, sometimes the toe pressure is limited to the allowable bearing, others use 125 to 140% of allowable on the theory that you would get some redistribution of the stress and that the envelope is probably not triagular anyway. I have not seen any studies that quantify this, so I stick with using allowable bearing at the toe. How that value was conmputed is important also. The highest stress will be at the footing contact. If that soil near the footing yields the strucure will have som rotation. IS the bearing value is kind of an average based on a few N values near the footing or or is it for the weakest layer? Most bearing values for foundations are based on an allowable 1" of settlement. If that is so, what does that do to your calculations?
I don't advocate spending the owners money unecessiarily. However for temporary work maximum loads can be difficult to predict.Temporary work ussualy have operating loads much closer to design values than permenant work. So the question is, when you visit the site and see the crane swinging a big column out 50-60 feet and a slight wind is starting to pick up, are you going to be glad you saved someone $4,000 on a multi million dollar project?

 

[GeoPaveTraffic]

I concurr with DRC1, partically the last sentence.

 

[DarthSoilsGuy]

i'm going to throw in a red flag to check out, b/c of the geopier issue. i haven't seen a geopier project that had decent soils to start with.

the last job i worked at with geopiers installed was a 5 multi-story (6 floors a piece maybe) dormitories. The soil bearing capacity given by the geopier engineer was 5000 psf. The surrounding soils were about 30-50 feet of dump fill that had been placed over the last 100 years. Anywhere on that site was sink-it-to-the-hilt probe rod soil once you scraped off the first couple of feet. i'm imagining setting a tower crane on those piers and on that soil, and it disturbs me a lot. the presence of the geopiers doesn't make the adjacent surficial soils stiffer. what makes me iffy about it is how the piers are communicated in terms of soil bearing pressures which i think is an oversimplification. if the geopiers were made of concrete or steel and touching beadrock, we wouldn't be talking about 3000, 4000, or 5000 psf bearing soils.

The location and design of your geopiers must consider the dynamic loading of the crane operation. i have never designed geopiers, but it is my hunch that they are better suited for permanent buildings where the dead loads are a proportionally larger and live loads are typically vertical and down.

 

[cldea8]

Thank you all for your input. Especially the experienced geotechs. Sometimes the soil structure interaction gets hazy for us structural guys (like me ).

I misspoke about the soil treatment. They are actually using stone columns. Although they are similar to geopiers, it is my understanding that they transfer load different. Not sure if that changes your opinion, DarthSoilsGuy.

After viewing your comments, along with some additional research, I decided to dig deeper and take a closer look at possible assumptions made by the other parties involved.

--I contacted the crane engineer to determine the maximum allowable rotation of the tower (out of vertical) in order to eliminate the risk of second order effects on the crane and foundation.

--I then translated this to an allowable settlement under the toe of the footing.

--I then contacted the stone column engineer and told him that I needed to limit the bearing capacity and SETTLEMENT to the determined value

--As it turned out, limiting the settlement to that required for crane stability (P-Delta) was a little more expensive than adding more concrete to the footing.

--I decided to use a 23x23 footing w/ the resultant in the middle third. Methods of analysis then allow for a uniform load distribution under the footing. The stress, settlement, and stability are a little more predictable. I am still not afraid of the other method...as it saves a considerable amount of material. Given the loading and soil situation for this crane...it seems to be a risk.


Anyway, I have a happy client, done my due diligence, and am going to sleep well tonight!

Thanks again,


cldea8

 

[DarthSoilsGuy]

vibrated stone columns and rammed aggregate piers are both similar enough to me, with regards to double-checking the design assumptions on the bearing capacity of the soils.

i've worked on a geotech forensic crane foundation failure study that was forensic. i put crane footings next to roller coaster design, but thats just me being unreasonable.

good follow-up on both the work and in our forum. most people ask and never tell what happened.

 

[civilperson]

Soil load will not be uniform but probably OK. Non-zero pressure on one edge increasing to a max on the other edge, with the max less than the limit.

 

[cldea8]

See Bowles 5th Ed. Section 8-10 and Figure 8-13 to understand my comment about method of analysis and uniform load distribution. The actual distribution is of course triangular (P/A + Mc/I for rigid members). However, stability can be assessed similar to a concrete strength design analysis (for stress distribution on soil). It basically relates the allowable bearing capacity to a triangular stress distribution (which he says later that there is no direct method for determining qa for a triangular load).

This analysis does not work when the resultant is outside the middle third. (See section 8-10.2) Bowles says the distribution should be analyzed as triangular (starting toward the interior of the footing) and that "..currently there is no method for determining the allowable bearing pressure for a triangular stress distribution on soil..."

It was this statement that started my questions concerning this 'outside the middle third' analysis.

cldea8

 

[civilperson]

If the load resultant is placed "e" away from the centroid, then the soil pressure resultant can be located at the same location beneath the foundation. Assuming a triangular soil distribution, one third of the triangle is the distance to the edge of the footing where the maximum soil pressure will be found and two thirds or the triangle will be back toward the center of the footing where the soil pressure goes to zero. Use the recommended maximum soil bearing for the edge of the foundation and thus all equilibrium requirements can be met without exceeding the soil pressure.

 

[cldea8]

Maybe a misunderstanding from my last few sentences...

You can determine the stress distribution caused by the footing (as a triangular load), however, there is no current method for determining the ALLOWABLE BEARING pressure at the toe for a soil loaded w/ a triangular distribution. That is why he relates it to a uniform load distribution (when resultant falls w/ in middle third).

cldea8

 

[hokie66]

But the load distribution is not uniform just because it falls within the kern. It is trapezoidal, thus varying, and I don't know why the determination of ALLOWABLE BEARING pressure would be so different for the two cases.