Micropile Connection for Fixity

[jwatso33]

FHWA gives guidance of a micropile attaining full head fixity at 24" of embedment. With a top plate attached and threaded bar/nut/ embedded into the pile, could the connection not be designed for fixity with a lesser pile cap embedment? Basically check that breakout on the bottom side of the plate/cap is sufficient to withstand the applied moment? Love to hear some thoughts.

 

[BridgeSmith]

Presumably, if the pile has a fixed connection to an adequately stiff embed that develops the full moment capacity of the pile through the connection and into the concrete, then it's essentially fixed.

I'm a bit surprised at how much embedment they're calling for. TXDOT did some full-scale testing of H-piles (HP12X53) embedded in a concrete abutment cap and the piles hinged (developed the full plastic moment capacity), without breaking out if the concrete, with 12" of embedment.

 

[jwatso33]

I agree. Another DOT ( I think Oregon) did one with micropiles and the deflections even at 6" embedment mimicked the expected fixed head deflections. Often times when dealing with piles in either a grade beam setting or diaphragm, it is not feasible to get that kind of embedment but it is desirable to have the deflections calculated as fixed head rather than pinned.

 

[BridgeSmith]

I think you might have that turned around, jwatso33. For a conservative design, I would calculate deflections using the pinned assumption, which would be the most flexible, and design for strength using the fixed head assumption, which would max out the moments.

 

[jwatso33]

For a conservative design I agree...but that is not always the case. Where trying to limit deflections, often the most economical method designing a fixed head connection even when it requires more embedment. From there you just have to deal with the moments with casing size/reinforcement. That is why I am curious about what connection could get that kind of result without such a great embedment into the cap.

 

[BridgeSmith]

I apparently misunderstood your statement, jwatso33. I agree that with adequate embedment (or other restraint), modeling the pile head as fixed is a reasonable assumption. Of course nothing is truly fully fixed, but you can get pretty close to it with direct embedment or a stout connection plate and anchors.