huangyhg

residential basement columns
fellers,
i looked at 2 4" diameter basement columns in a new house and the screw portion which allows adjustability is at the bottom of the column on top of the basement concrete slab. my engineer friend thought that if the screw was in the bottom position then it must be cast within the slab or the screw portion must be at the top. does anyone know what the int'l residential code might state? the house is in ohio. the builder had just 1 hilti nail shot through the bottom base plate into the basement slab. obviously, we'll tell them to change that out to 4 tapcon anchors.
thanks

first of all, the correct way to install these is with the screw portion at the bottom--i commend your contractor.
second of all, your friend is correct--it would be better if the screw portion was cast into the slab.
but this can be justified. determine kl/r of the screw portion (it is only a few inches long, so it will be a reasonable number), then determine the allowable axial capacity based on that and the area of the screw portion.
i think there are papers out there regarding what to do when a column has two different kl/r values (in this case, the post itself and the screw portion), but a "quick and dirty" way to do this is assume the post sits on top of a cantilevering screw portion. use k = 2 for the screw portion.
daveatkins
i found some info in irc 2009. i'm not involved in residential design but it appears from my reading that your column might be undersized. you ought to check with your local building code officer. also, i attached an article that states "telescoping" screw columns do not meet code requirements for permanent supports.
2009 international residential code for one- and two-family dwellings:
r407.3 structural requirements. the columns shall be
restrained to prevent lateral displacement at the bottom end.
wood columns shall not be less in nominal size than 4 inches by 4 inches (102 mm by 102 mm). steel columns shall not be less than 3-inch-diameter (76 mm) schedule 40 pipe manufactured in accordance with astm a 53 grade b or approved equivalent.
exception: in seismic design categories a, b and c, columns
no more than 48 inches (1219 mm) in height on a pier
or footing are exempt from the bottom end lateral displacementrequirement within under-floor areas enclosed by a continuous foundation.
r502.9 fastening. floor framing shall be nailed in accordance with tabler602.3(1).where posts and beam or girder construction is used to support floor framing, positive connections shall be provided to ensure against uplift and lateral displacement.
top and bottom of the screwed post support needs to be anchored to the respective load path.
a friction mount (at the top or the bottom) isn't sufficient. not much siesmic loading expected in hio - until the next memphis/niagra falls/st lawrence river fault earthquake at least. 8<)
if your columns are part of the sfrs this is not a typical residential building.
the lateral force on residential columns (i'm assuming an interior basement column supporting floor above) is 0. i believe this is the reason for the exception in irc.

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