Micropile, bending allowable stress 1

[AllStructure]

Hi everyone,
I am designing a foundation consisting of micropiles (10" OD steel casing, filled w/ grout). Since my design assumes a fixed-head, i have to design the micropile for max fixed head moment. I am having difficulty to find what is the allowable bending stress specifically for micropiles. I know that AISC ASD allows 0.66Fy (lateraly supported "column"). Please include any references with your response.

Thank you

 

[J1D]

This sounds a bit new to me.

We usually specify micropiles based on the local geotechnical report, where vertical and lateral load resistances are given. The report gives the bending capacity as well sometimes, but it is very small.

Apparently, you're considering the casing steel for the bending. I don't know how you consider the moment transfer at the interface between the tube and the pilecap. Do you have reliable anchorage?

If your foundation is supported by a group of micropiles, why not just consider they are pinned?

 

[AllStructure]

Thank you for your response JiD,

The reason why it was considered a fixed head connection was that more latteral capacity can be achieved from the micropiles. Micropiles are emeded 20" in pile cap and i have checked that there is enough embedement to transfer the moment. What i am having trouble with is qualifying the casing for a full axial and moment load. If i could use higher allowable than 0.5Fy i would be in a better shape.
To clarify: geotechnical capacity of micropiles is adequate, structural capacity is not that satisfactory.

 

[J1D]

If that's the case can you use thicker casing pipe? Some casing steel has higher yield strength. Did you try AISC LRFD (as you design the structural steel)? Did you consider the grout to take the compression as well?

In addition, is battered piling feasible for your case?

 

[AllStructure]

J1D, all of your comments are valid, but the casing width is set, and batter piles are not a feasible option becouse the axial loads are very high, and geotechnical capacity of micropiles would be not good enough. I also considered the grout to take some commpression but it is only 10% of total compression load, and it does not help to much.
AISC LRFD is mostly for building members (above surface), which is not the case for micropiles. Micropiles could be demaged during driving or rusted after some time, and keep in mind that inspection and accessebility are very limited in the case of foundation.

 

[Donrolla]

when I design for grouted steel shaft piles with lateral loads or moments, I use the composite properties of the transfored section and the design loading conditions in LPILE to determine the moment at an acceptable deflection... then check to be sure that the maximum moment does not exceed the yield moment of the pipe shaft. I also check using the ultimate loading conditions and check that the max moment is less than the plastic moment of the pipe shaft. Of course you also have to take into account the combined bending and axial stresses (P/A + M/S) and be sure that you do not exceed .66Fy