Buckling of steel cased bottomdriven piles

[JamesDavey02]

I am currently in the process of designing a number of steel cased bottom driven piles, for Axial Loads of up to 500kN (@273mm diamter) and 350kN (@220mm diameter). The piles will be driven through alluvial silts (SPT N values 0-1), through a medium dense layer of sand at 18m, before end bearing in lower mudstone. I am reasonable happy that the piles can resist the axial loads, albeit with the use of 40N concrete.

My concern is that the piles will fail in buckling, I have calculated the critical buckling load using the Euler formula for piles with a fixed head, and pinned at the base (2.04*PI2*E*I/L2)using 35N concrete, and a 100year residual thickness of the casing of 1.0mm. The calcuated loads are a long way short of the design ultimate load of the piles(f.o.s of 3.0).

How do I calculate the area of reinforcement required to resist the bending moments induced within the pile as a result of buckling, in order to specify re-bar and links? Although my current thoughts are that their may be insufficient space within 220/273mm diameter casings to fit sufficient reinforcement!

All help appreciated!!

James

 

[Ron]

Are you using a composite I value or just considering either the concrete or the steel? Something seems off in your assessment. The axial load is not that high but you don't give the length and the thickness of the respective soil strata, so difficult to assess.

Are you considering the lateral resistance of the dense sand?

 

[JamesDavey02]

Thanks Ron,

Until now I have been calculating the I value of both the concrete (35N)and the steel casing (100yr residual thickness), and calculating Pcrit seperately for each, summing the values to give a Pcrit for the whole pile.

The Piles will be driven through 18m of Alluvial Silt, into 2.0m of dense sand, and end bearing at 21.0m on mudstone.

The axial load values are those presented by the client, I am designing piles to a factor of safety of 3.0, therefore I think I need Pcrit to equal 1500kN. The best I can come up with is circa 600kN.

I am assuming the piles will be laterally restrained at the base by the dense sands, and fixed in position at the head via the permanent works.

 

[hokie66]

A residual thickness of 1.0 mm is far too little, in my opinion. You need thicker walled pipe. And you can't reinforce piles that small.