Pile Uplift

[cfarmer]

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.

 

[GeoPaveTraffic]

You should go back to the geotech with the actual uplift and downward loads for these columns and have them check the required number of piles. I suggest this for two reasons. First, the uplift capacity is effected by the number of piles in the group much more than downward capacity. Second, the 20% may have been just a "round" number kind of capacity and there may be additional capacity available.

To directly answer your question, we would need much more information on the length of the piles, type of soil, etc.

 

[cfarmer]

I agree with your statements and will investigate. Although, I'm not sure the owner's geotech has the expertise to provide more detailed info as you have suggested. If not, I suppose I am stuck with the value the geotech has provided unless the owner is willing to obtain other geotech services.

 

[GeoPaveTraffic]

No matter who provides them, you need to get someone who know what they are doing to review the conditions. Uplift failure could, generally at least, result in collapse of the structure. Not something to mess around with.

 

[civilperson]

Tie the column bases together with grade beams. Thus sharing the up and down loads along the beam.

 

[cfarmer]

That is, provide grade beams to span between the columns of the braced bay such that the uplift load is also resisted by the pile group at the column under compression.
Thanks for the help.

 

[Panars]

If the piles are end bearing in a hard soil (or rock) layer, the uplift capacity could very well be 20 percent of the compression capacity. In this case, it may be worthwhile to investigate another foundation type like drilled shafts.

 

[cfarmer]

Total pile length is approx. 15m with the last 7m into a gravelly sand till. Bedrock is at 25m.

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.

 

[VAD]

I think you need to review your calculations re uplift as well. Something seems odd for the structure in question.

 

[cfarmer]

Gross uplift loads at each column (2 braced bays on each side of building) are in the range of 500-600 kips factored. Less Dead Load from this value leaves a net uplift that needs to be resisted by piles and/or grade beams.

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.

 

[VAD]

Depending on the strength characteristics of your subsoils as estimated from your SPT N values, you may be able to use some helical screw piles within the piled foundation system and also the grade beam system. These piles can be maufactured with 10 inch shaft and helixes. The latter would assist greatly in resisting uplift. You may wish to look into this foundation type.

 

[fattdad]

You have 57 square feet of surface area along the shaft of the pile in contact with the glacial till. If the average blow count of the till is 25 and you use N/50 (in TSF) for your shaft resistance, you'd have an ultimate uplift capacity of 28.5 tons. I'd use 2/3rds of this to account for the less than favorable soil to steel interface friction. If you then apply a safety factor of 2 (i.e., if this is within a pile group), the design uplift would be about 9.5 tons. How do you like that number?

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!

 

[Donrolla]

I agree with VAD, using helical piles to resist uplift is a viable alternative, however no need to use 10 inch shaft, depending on soil conditions you may get a 70 kip capacity with 1.5 inch square shaft helicals

 

[cfarmer]

4 boreholes at this site:

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

 

[GeoPaveTraffic]

Sounds to me like you need to find a new geotechnical engineer.

 

[mitchelon]

Is your geotech engineer "old school?" No ofense to the old-schoolers out there. I am not an expert on piles and am also a relatively young engineer, but think that the 20% may have been commonly used in the past as a "guesstimate." Chin et al. (1973) noted that shaft resistance from uplift load tests tended to be 20% of the resulsts from compression test. I believe the argument goes on today.

 

[VAD]

I know that we have debated this question many times in the past in forums on this site but I am curious to find out what was the cost for the investigation and report and what was asked of the geotechnical engineer re assignment.

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.

 

[Mudcake]

Where uplift controls the design of building or bridge foundation, you may consider micro piles to resist both compression and tension loads. In granular till-like soils, it is not uncommon to achieve a shaft friction of 10 to 12 kips per lineal foot (or higher) of pile for an 8-inch diameter micro piles, especially with post grouting. A micro pile with a 20- to 25-foot embdement in sandy/gravelly till should yield about 200 kips of ultimate capacity or more, or 100 kip allowable if a FS=2 is applied. Use deeper piles if you need higher capacity, but need to check againt buckling. It can also be battered for lateral resistance.
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

 

[GeoPaveTraffic]

VAD, I agree with you. However, (assuming you are correct about the curcumstances) the geotech in question needs to tell the client that additional work is required to address the current design issues.

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.

 

[BigH]

Gravelly sand till - how difficult will it be to drive your steel piles embedment of 7m into? Might not be as easy as you think unless you use a very heavy hammer.
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.