huangyhg

existing wall footing modification
i am working on a job that has an existing 8" masonry wall sitting on a monolithic 16"x10" footing. the wall is about 25' tall braced at 24'. the wall is currently non-load bearing.
the owner wants to add 3 stories to the building and continue up from the existing masonry with new masonry. the wall will be load bearing in the new levels. i have checked the wall, and the existing masonry wall looks like it can take the additional load, but the footing does not have a chance of working.
the contractor wants to keep the wall, but remove the current slab-on-grade and replace with a mat foundation between the exisiting monolithic foundation for all the new columns we are adding for the new structure.
how can i modify the wall foundation to make it work?
the allowable bearing is 2500psf
the new wall load is about 6 k/ft

16" wide footing will not be working for 25' high wall.is it interior partition wall? your statement "existing masonry wall looks like it can take the additional load" worries me. additional 6000 #/ft is quite for this slender wall. you have the grouting and rebars data available for this wall? if yea and wall is ok than add spread footing both sides and do underpinning. if wall is also not ok, take top course off, add bond beam 12 feet approx, cut a slot in wall under bond beam and add conc column and footing below.
sure it can take additional load, but have you checked the lateral yet? 25 foot is pretty high and you are telling me it was not load bearing wall? how much reinforcement does it have?
never, but never question engineer's judgement
the wall has sufficent reinforcing according to existing drawings. the owner is also adding an intermediate level at about half-way up the wall which will also brace it as well.
dgkhan i am not familar with underpinning, what is it?
underpinning is a method used to modify and strengthen the foundation of a wall or column. the type of underpinning will depend upon the type of soil and how stable it is. for example in stable clay, it maybe possible to excavate under the footing in short alternating segments and then build a wider footing under that segment and then come back and do alternating segments. this can be very risky and expensive but can be done. another idea is to underpin with mini-piles or helical anchors.
before i would explore these underpinning options, it might be beneficial to do some additional in-situ soil testing to see how conservative the 2500 psf is and see if can be increased.
jike, do you think it is possible to do helicals on cmu? you expect the wall to span from one helical to another?
never, but never question engineer's judgement
the helical piles is definately an option if you can get access to both sides with a backhoe.
the standard off the shelf helical pile is usually good up to 50kips (depending on soil).
check out
but if you use helical you are treating the cmu as a beam spanning between the piers right? is that really possible? i assume the wall does not have a lot of horiz. reinforcement on it.
never, but never question engineer's judgement
to use a wall that is currently not loadbearing to support 3 additional stories sounds like wishful thinking to me. if you are supporting the new floors on new columns on a mat foundation, just provide columns near the external wall.
i would expect that hokie66's suggestion would be far cheaper than underpinning the wall.
you better check the wall per pca slender wall recommendations for p + m at midheight to include the wall dead load and any eccentricities. somehow, it just doesn't feel right to me based opn what i have designed in the past, especially if the wall has any openings.
i would consider underpinning with 4" diameter pipe pile, installing a lateral pilecap under the existing footing, with one pile inside the building and one pile outside, all spaced per the bearing capacity recommendations of a geotech.
mike mccann
mmc engineering

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