huangyhg

suggestions requested on reducing shear of footings.
hi all.
i am investigating a concrete frame structure and have discovered the footings are undersized excessively, more than 3 times overstressed. i am working up ways to fix the problem. i can deal with bearing and bending fairly easily, but am having some questions about how to strengthen the footing for both one-way and two-way shear.
i have 16"x16" concrete columns bearing on a continuous strip footing at approx 18' centers. the footing is 4' wide and 16" deep, with a single layer of steel at mid-height. the site is sandy soils with an allowable 2000 psf bearing, and assumed k=50 pci (corrected for footing size/shape). i am analysing the footing as a beam on elastic foundation, by hand, enercalc, and staad 2003.
i have a one-way shear overstress of about 3.5 times allowable. so far no signs of failure are evident, except for some isolated cracking of cmu walls, but i am going to recommend doing a field investigation and repair of the footings.
the obvious solution to a shear failure is make the footing thicker, and wider for a one-way shear. however, just making it thicker won't necessary help if i can't get the shear to transfer into the new concrete poured on top of the existing footing.
can i get this shear transfer if i epoxy in staples along the top of the footing? do i need to epoxy dowel into the columns as well?
any other suggestions are much appreciated. thanks in advance.
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i think you are on the right track - topping the 4' wide footing sounds like an economical solution. my first thought was also to bolt twin steel channels into the sides of the columns, with the channels running parallel with the continuous footing. these would help in transferring load from the column to the new upper concrete.
the staples into the lower footing would transfer the horizontal shear and increase your effective d.
the channels would act like shear-heads (similar to channels used in two-way flat slabs).
shear friction can be used to provide conjoint work of what new and what existent. epoxy can truly be used to provide anchor in the existing side. and you need to provide some way of passing the now permissible shear to the column, it may use also epoxy embeded anchors or other kind of mechanical interlock, such a inverted dowel anchored in some way.
can you roughen the column and cast a concrete 'sleeve' around it using bonding agents and having sufficient ties in the sleeve to prevent it from bursting. the bottom of the sleeve can bear on new concrete to increase the depth and help with flexural requirements.
dik: i was thinking something similar, but the top of foundation is apparantly only 8" below the surface. i am trying not to exceed the existing surface elevation, but i may not have any choice. i pretty much think i will add 8" to the thickness of the footing, with extra top rebar, and make tie into the sides and make it 4' wider or so. then if i have to, i will sleeve the columns, 4-6" on all sides, with some dowels epoxied into the column and new ties.
if i don't sleeve the column, i am going to use something like what jae suggested to transfer some of the column load into the new concrete topping.
of course, all this assumes that the footings are in good condition. if i find, once we excavate and expose them, that teh footings are full of shear cracks or other damage, the problem becomes more complicated. it may come down to replacing the worst areas and repairing the others. that is something i am sure the owner won't like hearing. but you get what you pay for.
we originally bid the design for this structure, and along with all but one of the other design/build teams, we were over twice the owner's budget. one team met the budget and was selected. and they gave the owner crap.
if there are any owners reading this: think before you hire someone whose bid is less than half that of all the other competition.
first an errata, i meant inverted corbel where put dowel for passing the shear to the column.
then, on your good advice to the owners. in fact some legislation here for some specific kind of contracts mandate to disregard the bids from bidders that stray more than a percentage (i think to re
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i wish we used a european-style bidding process here in the u.s. i really like throwing out the high and low bids, then picking the bid closest to the average of the remaining (complete) bids. it seems to me that we would have fewer fights with contractors about "extras" and "hidden conditions." and the contractors would be paid fairly.
i've done some retrofit work, so maybe there's a stregthening possibility there too. you say you have a cmu wall. can you consider making the wall and the footing composite?
if there's room for a deep bond beam and you can make a continuous tie across the columns, perhaps grout the block...
you have not specified the column load.
the first problem is to transfer the column load to the footing - so check the "punching" or peripheral shear around the base of the column. if this is excessive, then some form of remedial work is required.
the second problem is the soil bearing pressure - equivalent to 100 kpa (go metric usa) which is pretty gunky material - appears to be made ground or the sandy material contains peat. if the sand is clean, reasonable and dense you could (subject to a geotechnical report) increase the allowable bearing pressure by 50% or better.
i would tend towards towards: 1) locally "overpin" the footing by increasing it's depth both sides of the column; 2) installing top reinforcement; and, 3) retrofitting a peripheral shear reinforcement system such as "studrail" which, in australia, is obtained from reid construction systems or caa anacon.
you could could also consider drilling "jetcrete" type piles through the footing on both sides of the column. "jetcrete" piles (varying in diamenter from 600 mm to 800 mm, in my experience)can be readily installed in areas of restricted headroom. the hole through the footing needs only to be 150 mm diameter (6")and can be installed 250 mm (10") away from the column. jetcrete piles are installed in australia by frankipile p/l which is a division of the keller company who should also be in the usa.
good luck
i'm not sure of the feasibility in your situation but i have prestressed some footing in the past for additional shear capacity. they were isolated though and not strip footings. however, they were for an existing structure so that much is encouraging. overall the price wasn't outrageous either, i'll have to look it up.

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