Pile Uplift
Pile Uplift
(OP)
I am working on the design of a steel framed 4 storey office building founded on piles. I have quite significant uplift loads at the columns of the braced bays. The geotechnical engineer has provided an uplift resistance of 20% of the pile compressive resistance. With 245mm dia. steel piles this works out to a factored uplift resistance of 360kN/81 kips (Limit States design used for foundation design in Canada).
Does this seem reasonable?
Any suggestions to combat the uplift and reduce required # of piles. At all braced bay locations the # of piles is based on resisting the uplift.
I've used the resultant of the (EQ uplift less Dead load) / pile uplift resistance to determine the # of piles.
Does this seem reasonable?
Any suggestions to combat the uplift and reduce required # of piles. At all braced bay locations the # of piles is based on resisting the uplift.
I've used the resultant of the (EQ uplift less Dead load) / pile uplift resistance to determine the # of piles.





RE: Pile Uplift
To directly answer your question, we would need much more information on the length of the piles, type of soil, etc.
RE: Pile Uplift
RE: Pile Uplift
RE: Pile Uplift
RE: Pile Uplift
Thanks for the help.
RE: Pile Uplift
RE: Pile Uplift
As suggested by civilperson I will span grade beams between the columns to resist the excess pile uplift that cannot be taken by # of piles I have at the uplift locations. It means quite substantial grade beam reinf. at these locations but is still cheaper than adding add'l piles to resist the uplift.
RE: Pile Uplift
RE: Pile Uplift
We are in a fairly high seismic area and the weight of the building and floor heigths are quite high. Seismic loads and uplift seem in line. Note the new 2005 NBCC (Canada) results in higher seismic loads in this case compared with previous NBCC versions.
RE: Pile Uplift
RE: Pile Uplift
Bear in mind, if the N-value is proportinatly greater then 25, then the capacity would be greater as well. . . .
What it all boils down to is we don't have sufficient information to really help you. But I don't mind guessing.
f-d
¡papá gordo ain’t no madre flaca!
RE: Pile Uplift
RE: Pile Uplift
N-Values of the Gravelly sand till layer (layer starts at 7m depth) range from 18 to 66 (Most values measured around 40 with only 1 at 18).
I will look into the helical screw pile alternative with geotech engineer. It will come down to $$ as always.
While I have the help of some geotech experts:
The geotech has provided a lateral resistance for vertically installed piles of 9 kips. I will then pick up the remaining base shear with battered piles and ignore any soil/gradebeam friction to resist base shear (0-3m soil has N-value of approx. 4)althoug I am sure there is a lot or resistance there. I generally locate the battered piles at the corners of the building but In this case I will likey need several addtional battered piles along the perimeter columns in line with the braced bays. Is this typically how you handle base shear with piles? I've seen some structural drawings with no battered pile at all. On another pile job we did, I asked these questions to the geotech and he said he didn't get questions regarding the lateral resistance of piles much.
Thanks in advance
RE: Pile Uplift
RE: Pile Uplift
RE: Pile Uplift
If you just asked for a report without discussing the scope of project in detail, what you have is what you will normally get. If you need specifics then the scope changes. I would not want to say that the geotech needs changing since this practice also come from reputable companies.
In any event, very often the structural and geotech need to work together in flushing out issues that may not be fully known at the investigation stage. For this, one has to pay for the required information or pay the Contractor. Sometimes, the latter is preferred since it is easier to justify despite that thousands could be saved.
Sorry to go into this but sometimes clients etc often forget this aspect when they solicit geotechnical work, which is often expected to be cheap for such work as described. I would venture to say that overall with drilling your investigation and report was under 5 grand, perhaps 4.5. Half of which perhaps was drilling.
Not sure of your case but something doesn't seem correct.
RE: Pile Uplift
The unit installation cost is likely to be higher than piles, but at the same time you are getting a much higher capacity foundation element(especially in tension) that allows you to reduce the number of piles. Not sure about the Canadian practice, but in the US west coast micro piles are often designed and installed by the Contractor based on the subsurface data in geotech report, and perfomance criteria specified by the Structural enginner.
Here's a reference for micro piles:
http://isddc.dot.gov/OLPFiles/FHWA/009966.pdf
RE: Pile Uplift
The comment about attributed to the geotech about "...not getting many questions about lateral resistence..." is mainly what led to my comment. If the area is in a high siesmic zone, as cframer has indicated; then a good geotech should have all kinds of questions about lateral load.
RE: Pile Uplift
Does your geotech think that the gravelly sand till might stay open if you use drilled shafts instead - with a slight belling at the bottom? (better if a clayey silt till - but you might not need much of a bell). Or, with a drilled shaft, you can get the bearing you need but could also consider use of rock-embedded tie downs drilled down through the bottom of the pile. Just some thoughts of other possible pile systems that you might want to consider. Drilled shafts would also be better (larger dia) to counter your lateral requirements.
RE: Pile Uplift
Factored pile capacities for various sizes.
Pile downdrag loads
Earthquake design data (Site classification for seismic site response - OBC 2006)
Pile uplift resistance (for uplift at braced bays)
Lateral resistance of vertical piles and allowable vertical and lateral load capacities of battered piles (various size and batter options).
Pile testing recommendations
Protections against frost
Minimim separation distances from adjacent buildings for piling operations
Foundation drainage
Slab on grade construction
To clarify the statement about "...not getting many questions about lateral resistence...". This was a geotech in another city and I had asked if he could provide a lateral resistance value of the vertical piles so that I could determine the number and location of my battered piles. Sorry for the confusion.
I wanted to know if this is typically how you handle the base shear. Are other considerations made such as grade beam/soil friction accounted for. Even with no battered piles (as I've seen some designs), the structure must still have significant lateral resistance to base shear?
Regarding the embedment depth of 7m, we've done several piling jobs in the area and have had no previous problems.
Thanks to all for the help and suggestions!