huangyhg

cantilevered mid-landing
hi all,
we accepted the contractor's proposal to pour a stair mid-landing, which is designed as a cantilever off a 12" concrete wall, after the wall is poured. we are thinking of two options as shown on the attached sketch. which one you think is more structurally sound and if you have any other comments please feel free to add. thanks

i like option 1 better. option 2 looks like the edge of the wall where the cant'd slab bears would spall off pretty easily. if possible get them to put something on the wall form for the 10" where the cant'd slab will abut it to get better bond and resistance to slip along that plane.
thanks structuraleit, oh btw, are you still eit?
i agree with you, can you please comment on the development length of the top rebars into the wall for the two options given all possibilities?
you'll clearly have more length to develop the bars in option 1. depending what size bars you're using it may, or may not be, an issue.
if that detail is going to be provided on two sides (and part of the third), it might be a good idea to use roark to get an elastic moment in the slab. it is possible that it will see positive bending.
yes, i'm still an eit. i only have 2.5 years of experience.
definately use option 2!
assuming concrete likes to crack at re-entrant corners, then the 2x4 key will probably shear off over time. use option #2 but only a 1 1/2" deep key and you'll still have plenty of bearing area. only down side is the contractor will have to get the bottom of the block-out correctly placed or it may show below the slab. if too high, then you've got option #1 again.
either should work but i'd prefer option 2 with the reduction to 1 1/2" that mudflaps suggests.

for case 2, i think bearing is better to be made larger than 1 1/2", you can make a quick sketch, over the 1 1/2" lapping, there is a zone left with concrete anainst concrete only, cracks (at bottom side of slab within wall, and wall corner above the slab) could be a concern.
for case 1, the key could be modified to trapezoidal shape (with 35 deg chamfers), which is less likely to be sheared off, but much difficult to form.
i don't think it would be bearing so much as simply shear friction. the full depth key simply helps out a bit and keeps your joint clean.
i tend to agree with option 2, with a bearing as suggested
(1.5 to 2 inches). the difficulty will be in maintaining a
'uniform' ledge as the wall reinforcing will likely be in this zone (interference).
therefore, you may suggest to the contractor
styrofoam inserts, between any interfering wall bars to
ensure the bearing ledge. this also provides a 'dovetail'.
chamfers also generally make for a cleaner seam.
option 2.
jae:
you are correct in pointing out shear friction, the bearing stress at the interface produces downward force on the ledge. also, the wall end of the landing (as catilever) tends to rotate, the upper corner of the slab would push against the wall above. i agree 1 1/2" could be adequate for strength concerns (never done that but have seen others do so) however, given consideration to phenomenons mentioned above, i think the profile, reinforcing arrangement need to be closely looked at to avoid interference, as pointed out by beton1, and/or weak planes within the zone of interface. for 12" wall, i usually key in 3"-4" to further reduce stresses, and to allow for ease of detailing and construction.
one more thought, for deeper (into wall) key, the roof of the key way (niche) should be tilted slightly upward to avoid air been entrapped that would cause honeycombs to occur. the gap can be grouted, or sealed after the construction of the stair is completed.

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