laterally loaded piles

[gagne73]

Hey everyone,

I am being asked to design a pile for the lightpost at a football stadium, and I have done extremely minimal pile design. It will be a cast-in-place friction pile. I dont really know where to begin with the lateral loads. I know how to do the reinforcing part of it, however I don't understand how to determine my necessary diameter to minimize the lateral deflection. I have a geotech report with the skin frictions and such. Any tips on how to go about this, or a good resource of where to look would be really appreciated. I have scanned through here but didnt seem to find what I'm looking for.

Thanks

 

[Lion06]

Can't you use the embedded pole foundation equation in IBC? See 1805.7 in IBC '06.

 

[gagne73]

Im assuming that is International building code? I am in Canada and we don't have that in our ofice. we have the NBCC (National Building Code of Canada) any chacne you know where to look in there? I will take a look to see if i find anything.

 

[azcats]

Have the light pole people provided the lateral loads and deflection criteria?

If the pile stays relatively shallow and you have consistent, reasonable soils, I'd probably feel comfortable using the code design and adding 10-20% to my moment at grade for pile reinforcing design. If not, I'd find someone (possibly the geotech) to run an LPile analysis for me. Depending on the loads and soils, the moments can sometimes increase fairly significantly below grade or you can get relatively large deflections.

 

[KootK]

We don't have a similar provision in our code Gagne. You might want to check out the IBC formula though. It's handy. There's also a MathCAD sheet available from the Florida department of transportation that I've found to be a useful reference. It utilizes the modified Broms method. It's located here:

http://www.dot.state.fl.us/structures/proglib.shtm

The Canadian foundation manual also provide some information on the p-y method which is similar to the algorithms used in the L-Pile software mentioned above.

 

[Brad805]

If the geotech provides you a subgrade modulus you can model the pile with springs. It works out reasonably close to LPile analysis that are going to take a while to get out of your geotech.

Brad

 

[B16A2]

Completely depends on how tall of a lightpole. I've designed a few super tall lightpole foundations for football stadiums. What is the height on yours?

 

[a2mfk]

Its funny as I was reading about this I was thinking about Koot's reference. Its been a while since I used it but it was a good program, and the FDOT approves of it and they are strict. They use these to design large cantilevered traffic lights on steel tubes/pipe structures, so the pile has significant torsion and moment. Good place to start!

Oh, those use drilled shaft piers with rebar cages and CIP bolts, pretty heavy duty stuff.

 

[gagne73]

In my geotech report have the values for horizontal subgrade reaction (ks), can I use those as springs along the side of my pile to determine the deflection?

 

[gagne73]

B16A2, It is 70 ft tall and about 2000lbs. I am thinking a 36inch 20ft deep will be fine, however i need to check that size with the soil conditions, which is what I'm Struggling with.

 

[a2mfk]

The little pile design I have done, the geotech does the design of the soils portion because they have the specialized software and knowledge. I design the actual pile and give them the loads from the supported structure. But if you feel up to it and have a geotech report, I guess there is no reason you couldn't do it all if you have the right program since its a very iterative process.

 

[FixedEarth]

If you can get beam on elastic foundation equations for a free headed pile, you can enter your soil modulus values on each layer, your loads, and determine the shear, moment and deflection with depth. See if you can get a copy of a good Foundation Design text. The lateral deflection can be reduced faster by pile width than by pile embedment. Once you determine your critical length (when moment is zero), the rest is easier.

 

[Brad805]

gagne, using those as springs should give you a reasonable idea. I have compared these results with LPile results from the geotech we work with, and found this is reasonable.

You might want to discount a region near the top if you are concerned about any fill issues. Try a few different spring values and you will see the impact. The pile head rotation can amplify the deflection of the light post quite significantly.

brad

 

[B16A2]

If you use soil springs, make sure the report makes specific reference to this usage. Hard to tell if this report is just for the poles or if its part of a larger project. There are many times that the constants they give are not appropriate for this type of modeling.

I personally would have an L-pile analysis run in lieu of taking the extra liability.

Now you have to detail the base attachment. Remember, no grout and use the fhwa procedures.

 

[JoeBaseplate]

Gange, are you sure you want a pile or a drilled pier. I have always used drilled piers for light poles, flag poles and gazebo foundations. If you want to do a drilled pier design take a look @ Foundation Design-Principles & Practices by Donald P. Coduto. This book has design methods for both cohesive and non-cohesive soils.

Also, while trying to get a drilled pier design through the LA County I discovered that older CABC (based on UBC) also has a procedure for designing drilled piers.

What is your base shear and moment btw?

 

[hokie66]

Joe,
A drilled pier is a pile. Gagne said in his original post that the pile is to be cast in place.

 

[gagne73]

shear is 12 kn and moment is 240knm

 

[KootK]

@B16A2: I'm curious, why can't you use grout? Also, could you point me to the document containing the fhwa procedue? I'm not familiar with that.

Thanks,

Adam

 

[BigH]

Could I enquire as to the nature of the soils that will be supporting the post? Lots of discussion about programs, etc. but nothing has been said about the actual soil condition. Soft over stiff? stiff throughout? soft throughout? compact (medium dense) granular?

 

[ishvaaag]

And not to forget, if modeling with springs from horizontal modulus of subgrade reaction, state them as COMPRESSION ONLY springs.