huangyhg

shear wall on two caissons vs. on strip footting
we are desinging shear wall that will be supported on two caissons at the ends. my question is : how different this wall is from the one supported on a strip footing in terms of detailing steel bars?
thanks for the help
my guess would be that you would have to put all the tension steel at the ends of the wall instead of being able to take advantage of the vertical shear steel (if vu<0.5phivc) that is distributed throughout the wall. the reason i would say this is that you are going to have the otm resisted by a true couple imparted by the caissons, rather than a linearly varying soil pressure, as you would have under a wall footing.
i just assume this wall is only spanning 10 ft so it will take the moment from linear load on the top of the wall. you just have to make sure it can take lateral pressure. if this is a basement wall then calculate it if it was pinned at the bottom.
we have done this many times on mid-rise concrete buildings (14 stories), but instead of caissons, the chord of the wall was centerd over pile caps. your shear wall chords should be desinged to resist all of the axial load due to gravity and lateral. don't forget to check buckling of the chord due to the compressive load. typically we would have around 6-#6 bars with ties in each chord. and the shear wall reinforcing would be standard design. also, if you have doors or other openings near the bottom of the wall, be sure to check to see if you need additional steel in the bottom of the wall (which is essentially a deep beam).
both can work...
dik
you also need to get the shear out of the wall into the caissons. with strip footing this could be done with shear friction dowels from the wall into the footing, here it's all got to come out at the ends.
thanks all for the inputs.
this is one of a number of walls we have in a 6-storey building located in a high seismic region. one wall is about 10m long and has about 65000 kn.m moment at its base. we have designed the wall so that all the bars, concentrated and distributed, to contribute in the resisting strength.
it's not uncommon for use in these areas for buildings > 20 storeys (almost non-existant seismic area). the reactions on the wall are determined and the caisson/foundation wall/beam interface is designed. we use caissons for high loads because bedrock is down about 40'. where bearing is a problem, the top of the caisson can be nibbled to provide a longer bearing length. in addition steel dowels can be used to increase the load transfer. the wall is generally designed as a deep beam with arching action included.
dik

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