huangyhg

shallow foundation vs shallow foundation !
#1. is it a good idea to build a two story house with cmu walls and wood 2nd floor and roof trusses on a wall footings that are cast monolythicly with the 4" concrete slab on grade? ( similar to a thickened slab and/or edge )
or
#2. is it better to use the other option which is to cast wall footings first start the stem walls and then cast the slab on grade with premolded joint filler material all around?
please state why you would go with either one of the above options.
your thoughts are appreciated.
v2
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if the wall footer is cast monolithically with the slab on grade, differential settlements may cause the slab to crack.
for a house, i would not hesitate to use monolithically placed slab with footing granted the footing is adequate for the soil bearing pressure. i would use reabrs and extend them into the slab. this will reinforce the small section of the slab should any settlement takes place.
the monolithic slab-footing has been used very successfully in florida. i have an old dos program that designs the footing and the bar size and length into the slab.
good luck.
i found some good information on thickened slabs just by searching on google for "thickened concrete slab" there are links to several building departments. it seems to be a common item for building departments to decide whether they will approve this detail. it seems prudent to not use a thickened slab under individual column loads or for an addition greater than one story.
check out this spec from denver colorado:
when i design the thickened footing, i assume it is a grade beam on elastic foundation. i usually make it 16 inches deep and 24 inches wide (minimum!) and of course the steel reinforcement in top and bottom. i use #3 hoops equally spaced.
butler foundation manual has an excellent dissertation about this type of footings.
i agree that the use must be left to local engineer. this is always prudent.
depth of the footing foundation is usually determined by the soils investigation. if a shallow footing is adequate, i usually go with monolithic, sometimes the soils characteristics will determine a footing depth of say 24 inches or deeper, in this case a stem wall is a better solution.
the project will consist of more than 150 repeated buildings. the footprint of a typical buildings is 150 feet by 52 feet. these buildings are townhouses with a mix of one and two story units. there are concrete columns at the end or midle of reinforced c.m.u. walls that transfer loads from concrete beam above on the second level.
i may sound repeating myself. the big question is this:
is it common to to cast the the first floor slab on grade monolithic with all the footings in one poor with no control joints? this project is in dade-county florida.
thank you
v2
if i read your last post correctly, building has a reinforced concrete frame supporting the second floor and roof, so the footing in question will only carry the weight of one story of cmu.
in southern states, where frost heave is not an issue, a thickened slab should be a fine option for a lightly loaded wall. however, if left unreinforced, taro is correct in stating that the slab will crack at the edge of the thickened part.
i'm assuming that this crack, and the potential differential movement would be unacceptable to the homeowner, therefore, i would recomend providing dowels from the thickened portion into the main slab. that being said, the slab itself should have a minimum reinforcement, else you run the risk of a crack developing wherever the bars end.

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