huangyhg

when to go down to the frost depth
i always hear that the footings have to be placed below the frost depth but does that apply to any footing? for a job where the local code indicates frost depth of 42", all exterior bldg footings are placed at that depth but when it comes to the exterior stairs, they only go down 18" only!!
the steel stairs that come down from the second floor rest on footings that are monolithically poured with the slab on grade. the footing is 18" deep and 4' wide. the 6" slab is also 4' wide.
two questions:
1.how is shallower footing depth justified?
2.should the footings be replaced with deeper ones?
any help is appreciated.

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the shallower distance may be justified since the steel stairs can move if the seasonal movement is acceptable and the connection to the structure built built below the "frost" level does not have additional loads imposed due to any frost heving.
the prudent way in the beginning would have been to use sonotubes or similar methods to resist or eliminate the frost heaving.
for such a small area, going to special well drained soils and analysis would usually not be practical.
if you are talking about an 8 story exterior stair, that in one thing, but if is much less that is a different concern.
dick
normally, you would want (and are required by building code) to have footings below frost.
as dick mentioned, if the structure is flexible enough to accomodate heave, perhaps there will be no damage. the soil type and ground water will have a significant affect upon the amount of heave.
for a small outside stairs, it might not cause a problem.
the steel frame stairs have limestone treads and go up two flights. wouldn't any type of movement be bad force the connections. i have noticed some uneven pavers at the bottom of the stairs.
i am thinking that if during the replacement of the stairs it is observed that the footing is broken off from the slab to replace with deeper ones that go to frost depth. any thoughts
certainly if damage has been already observed you should take mesures to avoid its repetition.
you may take the whole outfit in cantilever from the main structure or make a braced (enough) foundation at such a depth as to ensure frost doesn't affect the proper behaviour of the foundation. in this case you may elect to prevent friction in such small foundation by encasing the foundation in polyuretan or so.
you may also choose to accommodate heave; i studied my career in a college where the access stair -over 2 m in difference of levels- spanned a soil made of debris and the building founded in sound soil starting more than 18 m below on piles. two simple box beams made the thing work and it should have some movement from settlemet but not much. not heave by frost but movement anyway. threads were hardwood buttoned to the stair beams, and damage if any was not visible.

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