huangyhg

helical pile resistance
i am designing a foundation for a large arch building. the engineer who is designing the superstructure gave me reactions at the pedstals where the arches fall.
the horizontal loads are about 80k and in order to resist this horizontal load we are using battered helical piles. the soil is good, so the plan is to resist the vertical loads using a spread footing with the piles attached to resist the horizontal.
is this possible? i need a better good batter to resist the horizontal load, and i am assuming the vetical component of the piles is resisting some of the vertical load and the rest is resisted by the soil bearing.
the owner does not want to use a truly pile supported foundation because of cost. the geotech also says that we should not count on passive pressure, but the soil is good enough for shallow foundations.
does anyone have a good idea about how to model this?
is it possible to provide a tension tie across the building (buried in the ground) to resist the thrust imposed by the arches?
'i need a better good batter to resist the horizontal load, and i am assuming the vetical component of the piles is resisting some of the vertical load and the rest is resisted by the soil bearing. '
i would be carefull of this approach, as i think you will have a deflection compatability problem. the piles will be very stiff and intially attract most (if not all) of the vertical column load, where as little of the load will be taken by soil bearing as it is usually assumed that some settlement is required to develop the full bearing capacity of the soil.
i agree with steelpe that, if possible, a tension tie might be a good solution here.
jmho
that is the other option, but we are trying to get pricing for both. the span is 205', so the tension tie has to go a long way, and the short sides of the building will be a problem as well because we would have to go 320' to go from short side to short side.
i usually see "hairpins" used for this. a piece (or pieces) of rebar bent in a "v" around a connectoin point at the base of the arch, with both legs extending into the interior slab (assuming there is one). a mass of concrete could be used as a dead-man under the slab if you need more resistance.
i dont know why helical piles would not work also. i agree that they should only be there for horiz. thrust, not "some of the vertical load". load attracts to the stiffest element.
also, getting a steep enough batter to keep the upper stem of the helical pile from wanting to "bend and pull laterally through the upper layers of soil) might be practically difficult.
key,
i have seen the hairpin detail you are talking about. i have always been told that a hairpin would be used for small loads (<20k) anything greater and you would need to use a tie. i have also been told that they are very unreliable as they are rarely installed correctly. i try to avoid them whenever possible.
i'm not sure what others think but this is what i have been told.

what is the maximum batter you can get out of the piles? can you not angle the piles to represent the direction of the reactions. that way your piles are working axially only.
surely, though, the best bet is to use the tie under ground and then just to support the building with conventional footings. the fact that the ties go for a long way surely can't equate to the costs of piling?

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