Resultant Outside Middle Third of Tower Crane Footing
Resultant Outside Middle Third of Tower Crane Footing
(OP)
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
*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





RE: Resultant Outside Middle Third of Tower Crane Footing
DaveAtkins
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
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
RE: Resultant Outside Middle Third of Tower Crane Footing
Is it feasible to put in some bored piers or rock anchors? With such a big footing, some deep assistance should be considered.
RE: Resultant Outside Middle Third of Tower Crane Footing
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
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
f-d
¡papá gordo ain’t no madre flaca!
RE: Resultant Outside Middle Third of Tower Crane Footing
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?
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
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.
RE: Resultant Outside Middle Third of Tower Crane Footing
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
RE: Resultant Outside Middle Third of Tower Crane Footing
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.
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
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
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
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
RE: Resultant Outside Middle Third of Tower Crane Footing
RE: Resultant Outside Middle Third of Tower Crane Footing
The geometry and stress distribution is quite simple if you assume P/A + Mc/I stress distribution. It is relating this to the allowable bearing capacity. I consulted the rock column engineer and he told me that the allowable bearing capacity (he is providing) is based on a uniform load distribution.
The general convention is to compute your toe stress and check it against the allowable bearing capacity. I just dug a little deeper into the soil structure interaction because I thought I was stepping outside of conventional analysis methods (by letting the resultant fall outside the middle third of footing)
This text (Bowles 5th Ed.) uses a more in depth approach that doesn't assume the soil is always linear-elastic and that stress redistribution takes place.
I would scan in the text and post it but don't want to violate copy rights ;)
cldea8