Concrete stair, supported by isolated footing and existing precast wall

Hmmm... I don't know your ACI code, but I don't see why you wouldn't be able to develop a joint where you have enough clear cover. Alternatively, run the stair fully through and have the joints go through perpendicularly.

As an aside, do you have an integral landing in this staircase? If you do, be very careful... The joint at the nose of the landing (heel of the stairs to the landing) can have a shear issue and fail unexpectedly. I've posted a really great practical discussion of this on Eng-Tips before.

I don't know about your suggested boundary conditions. It could be pinned at the top with a roller support at the bottom. You may need an intermediate landing as suggested by CELinOttawa.

BA

Sorry i wasn't very clear, essentially i have the stairs and the landing bearing to the precast wall. Also from my analysis i get a high moment and as Cel mentioned high shear almost equivalent to that at the support. I am not too concern with shear since i have reinforcement going over the top and bot of the 10" thick slab

I thought the stair was supported at the low end by a footing and at the high end by a wall. I think a sketch is needed to clarify the geometry of the stair, wall and footing.

BA

This is the detail

Not convinced I'd use a pin-fixed condition there.

Anyhow, I don't see why that connection wouldn't be permitted. Though I dislike it very much. We encounter a similar wall / slab condition when we repair intermediate slabs and most of the time we use a stiffened angle iron at the perimeter underneath the slab. We generally cast it in a haunch (for fireproofing) monolithically with the slab, but in your case you could do it first and create a true roller up there!



CWB (W47.1) Div 1 Fabricator
Temporary Works Design

Thanks for the advise, i had thought about an angle connected to the wall and just have the landing sit on it. The reaction there is some 3000 lbs/ft which should be nothing to handle.

Marc R said:

I am designing a stair going up to a mezzanine, its gotta be supported by an isolated footing and a precast wall. Had to do the calcs by hands since there was no software available to do so so technically my support conditions used are fixed at the bottom level and pinned up top. My problem is I get a reaction up top bearing against the precast wall and the engineer working on it had it connected with #5 epoxy dowels into the wall 4".

Click to expand...

I do not agree with your statement in red. The bottom level support is not fixed and is not pinned, but is somewhere between a pin and roller. Your top support is not pinned, as the precast wall will deflect under lateral load. There will be a horizontal reaction at each support, but it is hard to say how much. At the bottom, you have passive resistance of the soil and possibly a grade slab which will shrink in time.

You could design it two ways and reinforce for the worst of the two conditions:
1. Assume reaction at top support is horizontal, like a ladder leaning against a frictionless wall.
2. Assume reactions at each support is vertical, with horizontal force of zero.

Neither of the above assumptions is realistic, but, of the two, my inclination would be to go with 2.

The drawing provided is not dimensioned, but there are 17 risers and 16 treads. If the treads are each 10", the horizontal dimension between extreme risers is 160". Assuming the drawing is drawn to scale, the overall span of the cranked slab is about 27', a very long span for a slab. Dowels into the precast wall is not advised. If the foundation is wide enough, it would be better to building a wall under the slab, immediately adjacent to the existing wall. Otherwise, it may be better to provide a support a few feet from the precast wall, giving a two-span condition with much lighter load going to the existing wall.

BA

BAretired, you bring up a very solid point about the span of the slab being too long. My original thoughts on this was to put a wall with a footing right under the joint landing/stairs. The reason why I used a fixed pin connection its because of the sloped stairs being continuous seemed reasonable that a moment would develop at the footing and assuming no sway at the wall with the original dowels at 4" i thought a pinned connection would approximate the real conditions there because i have a horizontal and vertical load at the wall slab intersection. I also ended up running number 5 bars top and bot at the joint, because my numbers show a high shear/moment develops there. That's why the idea of an angle to connect at the wall sounded great to me, leaving 1/2" between angle and slab i would take care of the vertical reaction and allow some movement at the joint as well