footing is too small for residential addition1

huangyhg

footing is too small for residential addition!
i have a situation where my client wants to add an entire second floor to his existing home. the home is a single story rambler built in the 70's with a 16" wide footing.
per irc2000, the footing will have to be at least 19" wide. when i apply the plf loading from the new second floor, my calc's say the footing needs to be 24" wide.
how do we add the additional 8" (16" + 8" = 24") to the existing footing?
i'm not familiar with your irc2000 code.
however, australian standard as 2870-1996 residential slabs and footings has standard, "deemed-to-comply" designs, for various types of construction and soil.  these tend to be somewhat conservative designs for use by builders and other non-professionals in the residential construction industry so that they don't have to consult a professional engineer for each new dwelling.  importantly this code also allows for design in accordance with engineering principles to modify the deemed-to-comply designs.
if irc2000 has a similar provision, perhaps you or your client can commission some geotechnical testwork and then have a professional engineer do calculations to determine the actual width of footing required, without some of the conservatism inherent in deemed-to-comply designs.
some questions.
is the home slab on grade or with basement or crawl space?
are there competent sub contractors in your area?
do you know what the reinforcing is in the existing footing?
i have had some success with doweleing bars into the sides of existing footings.  i have also had some success with pouring the requiered width footing under the existing footing.  both situations were very limited in size or length of footing required.  maybe kind of pricey for residential construction.
if the reinforcing in the existing footing is known, think of a way to make the footing a multi span beam.  reinforcing will need to be verified because of the reverse moment forces.
i have handled a similar situation by pouring a new footing under the old and doing it in small sections (say 4' or so) so that the existing footing can be safely undermined.  however, i would take a look at helical steel piers which bracket on to the existing footing and provide you with a deep pier solution.  (atlas piers or a.b. chance)  these may save enough labor to make their cost worthwhile.
is the footing too small to carry the loading because the concrete stress is too high or because the soil bearing stress is too high?  the answer determines the fix!
sperlingpe's provides two possible solutions. chance anchors could also be used at a approximately 6' spacing (depending on loading) with a support angle up under the existing footing.
i would suggest the same as mikee55. i dont know enough about the frost depths and whether the wall is a basement wall or not but you could also consider shoring or needling the wall in sections, removing some of the bottom of the wall and pouring a new footing on top of the exisitng footing. the old footing just becomes filler. you would probably have to pour mass concrete to the bottom of the exisitng footing for the 4 inches on either side. this is the best way to have a monolithic footing that you can be sure of its behavior, design, and construction. i have used similar solutions on commercial buildings(both wall and column footings. i dont know if it would be woth it for a small residential project.
dbuzz:
1. i could not find any provisions provided by irc2000 for what you are talking about.
2. my client does not want to pay a geotechnical engineer at this time. i have tried to explain to him that he should verify the soil. in lieu of a geotechnical report, i am using 1500psf for the soil bearing pressure. the soil is probably 2500psf or better in the real world.
sperlingpe:
1. the home has a basement with 8鈥?0鈥?cmu walls.
2. i have to assume there are competent concrete contractors in the washington dc metro area.
3. the existing footing was poured over 30 years ago. i do not know if any rebar was installed nor do i know the compressive strength of the concrete mix.
mikee55:
pouring a new footing under the existing does not seem to be the answer. i have been exploring underpinning with helical piers as you suggested. however, i do not completely understand this method. it appears that i could simply consider the footings as grade beams (simple concrete compressive strength of 2500psi) and calculate the spacing of the new helical piers based on the new footing spans. for example, i could start with 6鈥?0鈥?grade beams which would yield the piers @ 6鈥?0鈥?on center. would this approach be correct?
jheidt2543:
as mentioned above, the exact footing specifications are unknown. however, when i calculated the exiting footing (16鈥? to carry the new 2nd floor, i determined that a 24鈥?wide footing would be required based on 1500psf soil bearing pressure.
jjeng2:
1. the frost depth is 30鈥?
2. what exactly is needling?
many thanks to all of you and merry christmas
i think the solution in your picture looks good. i've used it before.  i would make the additional footing wider so that there will be room to get a drill in there to drill and epoxy rebar into it.  you can design the epoxied rebar for the bending and shear and a bonding agent to the concrete would be a good idea.
if you put piers under the footing to make it have multiple spans, the existing footing reinforcement may give you a problem since most residential footings don't have top reinforcement to handle the negative bending moment over the new piers.
if you look at the cost of these retrofits and the cost of a geotech report you might find that the report is cheaper if it allows you to avoid making changes to the existing footing.  of course that doesn't help you much if you have to have the report to know if it will help you.
needling is a method used to shore walls in this type of situation. you would punch small beams, usually steel w's, through the wall and shore the beams so you could work on the wall or foundation under it.
it there is an 8' besement as i understand you need to be careful of lateral soil loads throughout any shoring process.
i almost alwalys use 3000psf soil if i dont know what the soil is. i leave a big factor of safety in the calcs though.
with a basement, you could work from the inside.
it is also possible to do some soil sampling near the footings from the inside.  this will required some concrete to be removed to ahnd auger some soil samples.
considering the cost of underpinning or adding section to the footing, having some soil sampling/testing done may be prudent.  contact some local soils engineers.  either operation to gain more capacity will be expensive.  excavating around the house, saw cutting concrete in the basement, replacing concrete, recompacting excavated soil, and the actual repair will be a fairly expensive and time consuming effort for residential construction.
a few more comments:
1.  don't forget that to install the additional footing width as shown in your sketch you have to cut and patch the floor slab on the inside of the wall -- added cost!
2.  the ~$1,000 for a geotech report would be cheap compared to the costs associated with a guess of 1,500 lbs soil bearing capacity.
3.  soil bearing capacity is not the only worry. i've seen some sites where the clay soil can bear 1,500 psf, but settle 1"-2" in a year.  the geotech report would resolve that question mark.
4.  if your final solution is to increase the footing width per your sketch, don't forget rebar dowels @ 12" - 16" on center to tie the new concrete to the old.