Joints in Concrete Framed Stair
Joints in Concrete Framed Stair
(OP)
I'm working on a concrete framed stair. It is open on the exterior long side, so I've designed the stair to span the long direction between the exterior wall of the building and an intermediate landing beam (stringer and landings together make up the span). I know there are going to be construction joints and am planning calling for the stair to be poured in (3) L's with all three landings being pour continuous each integral with one of the connecting stringers.
Another engineer in my office liked the idea of the stringer to landing joints being horizontal so that the stringer sits on top of the landings below (see attached PDF). What kind of gets away from me in this detail is what to do with the shear. If the cold joint were vertical right where the stringer meets the landing I could easily comprehend what to do with the shear and design for shear-friction transfer. It looks better to me as a horizontal joint and their is more contact between surfaces, but I don't really know how to design the shear interface (Its kind of like having a horizontal joint in a beam). I think their may be a way to analyze this condition by transposing forces at angles, but this seems like a common condition so I'd like to get a consensus on best method for detailing/designing/locating the joint.
As a side note, the stair is replacing an existing semi-exposed exterior stair at the corner of a building that has aged/deteriorated to the point that it's a constant maintenance concern (Its 46 years old). I plan on using 4.5 ksi mix, 1-1/2 cover and penetrating sealer to mitigate corrosion/moisture damage.
Another engineer in my office liked the idea of the stringer to landing joints being horizontal so that the stringer sits on top of the landings below (see attached PDF). What kind of gets away from me in this detail is what to do with the shear. If the cold joint were vertical right where the stringer meets the landing I could easily comprehend what to do with the shear and design for shear-friction transfer. It looks better to me as a horizontal joint and their is more contact between surfaces, but I don't really know how to design the shear interface (Its kind of like having a horizontal joint in a beam). I think their may be a way to analyze this condition by transposing forces at angles, but this seems like a common condition so I'd like to get a consensus on best method for detailing/designing/locating the joint.
As a side note, the stair is replacing an existing semi-exposed exterior stair at the corner of a building that has aged/deteriorated to the point that it's a constant maintenance concern (Its 46 years old). I plan on using 4.5 ksi mix, 1-1/2 cover and penetrating sealer to mitigate corrosion/moisture damage.






RE: Joints in Concrete Framed Stair
RE: Joints in Concrete Framed Stair
So I don't have a clear picture of what is going on. I'm guessing that your stair stringers are upturned and form some part of the guard rail system. Like a bucket within which people walk. I'm also speculating that the intermediate landings cantilever off of the building somehow and support your stringers intermittently. If that's true than you should be fine and your shear issues would be:
1) horizontal interface shear for the stringer hogging moment at the landing if the landing concrete needs to be part of the stringer flexural compression block.
2) one way vertical shear checks and detailing in the landing slab or beams that cantilever out to pick up the stringers.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Joints in Concrete Framed Stair
RE: Joints in Concrete Framed Stair
As for shear with the horizontal joint, that is a tricky thing. I've not seen that specifically considered before in practice. I suspect that it's not a big deal as most of the vertical shear should get transferred across the joint in direct bearing. To look at it in detai, you'd probably want to investigate it as a closing concrete joint and employ a method like strut and tie. Of course, nobody's got that kind of time to spend in a stair unless it's a matter of personal academic interest. Often with stairs, the geometry is such that it's tough to even tell whether the joint is opening or closing under load. Detailing often covers both conditions, at least nominally, to be safe.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Joints in Concrete Framed Stair
BA