huangyhg

restrained basement wall design
i know a lot of questions have been posted on this topic, but i still am unable to see a popular consensus:
i have a very long rectangular building with a full basement (10' ceiling). there are no intermediate walls perpendicular to the long direction. the floor trusses run parallel to the long dimension of the building, with steel beams spaced at about 25' o.c.
since the basement is backfilled on all sides, i would like to design the walls as restrained masonry walls, and the contractor would obviously like to avoid the extra expense of a cantilevered wall. the geo report specifies the at-rest pressure as 63 pcf, which gives a lateral reaction of roughly 1000 plf into the floor sheathing.
i cannot justify the load transfer at an interior sheathing panel joint. i am able to justify the load transfer through the bearing plate/ledger into the sheathing via blocking, but that's where i run into problems. the apa recommends a 1/8" gap between sheathing panels, which means that 1000 plf will have to be transferred across this joint by the sheathing nails/screws. i am also slightly concerned about plywood buckling under axial load, since the tabulated values in the nds assume all edges are restrained, whereas in my case the floor trusses are spaced at 24" o.c.
i have seen many designs where a 10' basement wall is designed as a restrained wall, and many engineers extend blocking several truss bays to get the load into the sheathing, using the ibc diaphragm shear values - which is not correct - this is not a diaphragm, rather an axially loaded plywood floor. nevertheless, this design has been employed on many local projects (phoenix, az), and seemingly without problems.
any suggestions on how to justify this? i would like to avoid putting blocking in all the way across the building, as this is not a realistic solution when shrinkage and workmanship are taken into account.
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no joy from me. i would either use a concrete floor or cantilever the walls.
well...hmmm...not sure that i'd ever use a cantilevered wall. i've seen a case where a large amount of earth backfill deflected a wood diaphragm laterally such that the building experienced quite a bit of cracking, deformation and distress.
but i don't agree fully with the description that the plywood is an axially loaded "
i believe the ibc has an exception for basement walls supported by timber diaphragms to be designed by a lower at-rest earth pressure. couldnt tell you where it is...but it might be in the soils chapter.
i recall pressures were 20 psf for sand, 30 for silt and 40 for clay. i think the charts went to 10'walls in the 2006 code. they also had a minimum building depth to length and a large increase in number of anchor bolts. i asked counties about these last two and they ignore.
with the pressure at 63 psf, i see this as full hydrostatic pressure - no drainage i would also think you will have problems with 10 feet of hydrostatic pressure on the basement slab, to include the overall uplift on the total structure.
personally, i would go with the cantilever wall - it would make all your other concerns with respect to the plywood floor "diaphragm" go bye-bye. however, i would use cip concrete, not masonry block.
mike mccann
mmc engineering
if you design it as a restrained wall, make sure you tell the contractor that he will need shoring, or he will have to wait to backfill the wall until the floor diaphragm is installed. if he cant wait, the cost of the shoring, may outweigh the added cost of designing as a cantilever.
as the op said, this is not a diaphragm in the sense that he is trying to span from end to end. he is just trying to strut across to the earth on the other side for resistance. he has a very long building. he wants to resist at rest pressure on one side with passive pressure on the other, and vice versa.
mike,
63 psf equivalent hydraulic pressure is what you would expect for at rest pressure with no water involved. if there is water, it would be higher, and then you would have to consider uplift.
azeng,
if you want to persist with the floor bracing the wall and don't want to use a concrete floor, i would not rely on the plywood for axial loading. i would decrease the beam spacings so they act as combined bending/compression members, with the top of the wall designed to span horizontally between the beams. and there is nothing wrong with doing the walls in reinforced concrete masonry. re
you're right hokie. nevertheless, i would still check where the maximum high water table is to see if there is ever any uplift on the slab.
to me, pressures that high means that the soil could be more porus, lowering the intergranular friction and allowing the water to infiltrate more quickly. i'd check it out.
mike mccann
mmc engineering
i think you could justify 1000 plf across the joint in the plywood. it might take a lot of nails. and i wouldn't worry about plywood buckling--the plywood is nailed to every truss, so its unbraced length should only be 16".
if you are still uncomfortable with that, you could design the diaphragm to span 25' between beams, and design the beams for the axial force. you would have two diaphragms in each 25' span, one taking the lateral pressure on one side, and the other taking the lateral pressure on the other side.
daveatkins
this project is in phoenix, az - hydrostatic pressure from water table height is of zero concern. the geotechnical report has stated what to use for the at-rest pressure, and that's what i will use... i'm not looking for a way to reduce it, or second guess the geotech's analysis.
"but i don't agree fully with the description that the plywood is an axially loaded "member". if you simply run blocking perpendicular to the exterior wall and extend it far enough so that the plywood-to-blocking nailing is adequate for the transfer of the top-of-wall lateral thrust, then you are ok for strength. the blocking can be spaced at close enough intervals to keep the length of the blocking to a reasonable amount."
jae, how is this not an axially loaded member? if you take a section of the floor at the middle of the building, your fbd will have 1000 plf on both sides (see attachment). as i stated, i have already designed the blocking so that the lateral thrust is adequately transfered into the sheathing. but as hokie stated, this is not a diaphragm!
imagine if this were a tunnel, and not a house, so there were no shear walls. the structure will still work because the two retaining walls are still resisting each other's lateral force via the floor, which is indeed an axially loaded "member". if the plywood was replaced by a series of steel pipes, i think you would agree that these were axially loaded

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