Cantilevered Mid-Landing
Cantilevered Mid-Landing
(OP)
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
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






RE: Cantilevered Mid-Landing
RE: Cantilevered Mid-Landing
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?
RE: Cantilevered Mid-Landing
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.
RE: Cantilevered Mid-Landing
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.
RE: Cantilevered Mid-Landing
RE: Cantilevered Mid-Landing
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.
RE: Cantilevered Mid-Landing
RE: Cantilevered Mid-Landing
(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.
RE: Cantilevered Mid-Landing
RE: Cantilevered Mid-Landing
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.
RE: Cantilevered Mid-Landing
My preference of option 1 over 2 is because the wall will be poured about 2 weeks before the landing, therfore shrinkage of the landing will creat a cold joint at the interfaces with wall, which means that the fixit line is at the back of the block-out of options 2 and the top bars have only about 4" of embedment before the hook. The shear key of option 1 is adequate to resist shear.
The other factors that I should have mentioned are:
1- the wall is a shear wall,
2- the building is in high seismic region.
RE: Cantilevered Mid-Landing
DaveAtkins
RE: Cantilevered Mid-Landing
RE: Cantilevered Mid-Landing
Best regards,
BA
RE: Cantilevered Mid-Landing
If you are using f`c of 4000 psi max bar size would be #6 with 2" cover. You only need a standard hook on the bar. Anything longer is wasted material.
RE: Cantilevered Mid-Landing
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RE: Cantilevered Mid-Landing