Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
(OP)
A 50+ year old 2 storey below-grade parking garage (under a superstructure tower), is in need of structural repairs to the floor of the intermediate level parking floor. This floor is a shallow (7½" deep) waffle slab with ribs at 2 foot centres each way. In some cases, the repair requires floor thru-slab removal for a significant length parallel and adjacent to the supporting exterior wall. The existing floor-to-wall dowels are only #3@24". The factored reaction at the wall is a maximum of 2100 pds per foot, or 4200 pounds per waffle rib (the waffle ribs are at 2 feet centres and span 23'-8"± clear)
My questions relate to what should be done to reconnect to the wall, when placing the new concrete in these locations. To illustrate the the questions, I have attached 3 diagrams.
Questions:
Option 1 is to remove the existing slab flush to the interior face of the wall and then to intermittently chip 4" into the wall to create "teeth" where the new slab will extend into the wall and be supported. This is illustrated in accompanying drawing S208.
Questions:
1. In zone "A" (see attached drawing S208) where the new concrete stops at the supporting wall face, there is insufficient development length from the construction joint at the face of the wall to the existing bar end within the wall to develop any significant force in the existing rebar (see attached sketch SK-1) and therefore zone "A" of the floor cannot transfer its reaction to the wall in zone "A". All the load in zone "A" must find its way to zone "B" and be taken out onto the wall in zone "B"?
2. Since the load from zone "A" applies its load to the slab in zone "B" so close to the supporting wall, the effect (shear and flexure) on the Zone B waffle ribs is almost negligible.
3. Do you agree that "toothing" as shown on the attached is the way to do this?
4. Do you agree that 4" bearing of the teeth into the wall is adequate, or should it be 6"? i.e. the existing waffle slab bears only 4" onto the supporting wall.
5. Should we specify that the wall be chipped out for 2" above the top of the new floor teeth and then dry pack grouted with a non-shrink grout to ensure that the wall concrete is tight to the top of the new floor concrete, so that the full thickness of the wall is active in resisting the negative moment in the wall caused by the lateral earth pressure on the wall. See detail A" on attached drawing S209. In other words, I am concerned that if we do not dry pack above the slab, then the new slab cannot be relied upon to avoid a gap between its top and the wall above.
6. If the wall is laterally braced above and below the floor that is being removed, I suppose that the floor can be chipped out into the wall to a depth of 4" for the entire length of the floor repair and no toothing required. I would still question whether only 4" bearing is enough, given the sparseness of bars connecting the floor to the wall (#3@24).






RE: Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
Unless you require waffle slab moment fixity at the wall, I'd be inclined to do this:
1) Chip out a localized bearing pocket for each rib, at each rib. Not sure how buildable that is at 24" o/c but you could run that buy your contractor.
2) Drill and epoxy some bottom steel starter dowels into the wall at the back of the pocket.
Just go with whatever your bearing calcs suggest. If you chip out a continuous bearing ledge, I'd keep the width of the ledge to a minimum. If you chip in too far, you'll risk inducing a shear failure in the wall at the joint.
I get what you're saying but I'd be inclined not to worry about it. The shrinkage should be relatively small and should close up quickly under serious moment. It's not as though they would have done this with the original construction and the scenario would have been similar. The upper walls would have been poured well before the slab shrinkage was substantially complete.
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: Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
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: Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
Second, determine what loads need to be carried between the wall and the floor system (you mention the floor vertical reaction load. Are there any bracing, diaphragm, shear, etc... loads to be concerned with? OK, we assume there isn't and we'll move on).
Now, shear transfer into the existing wall really should pick up the joists and not just the slab. I'm really resistant to hanging the joists from just the slab. I'd think drilling and epoxy anchoring a bar through the bottom of the joist and into the wall at an angle would get you what you need (ex. #3 at 45 degs) and put your shear inside of the joist shear reinforcement.
As for bearing, if you can develop a bar into the wall, all you want is a roughened surface between the floor and the wall. If you go deeper, I'm concerned about damage to the bearing wall. If you can't develop a bar into the wall...well, you need to. Use smaller bars. Even for a bearing pocket, you need the top of the pocket or the floor to be developed back into the wall (don't do pockets).
RE: Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
RE: Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
To Kootk -
Thanks as always for your time and thoughtful comments.
Re: the dry-pack grout on top of the slab- I felt that if they chipped the pocket into the wall such that its vertical dimension turned out to be a little more than the slab thickness, there would be left a gap above the slab that would not be filled when the slab was poured. That was the main concern. Shrinkage of the slab is also a bit of a concern. In the original construction, the walls and the slab shrink about the same amount in our Toronto climate-- actually the wall will shrink more slowly than the slab because the wall is thicker than the slab. It is after the winter heating system is turned on that the concrete drying and shrinking is accelerated.
Re: the wall shear - that might be a concern if the wall were not temporarily braced from just above the floor (diagonally to the floor above), and just below the floor (diagonally down the to the slab-on-grade). Also, the negative moment reinforcement in the wall should provide considerable shear-friction strength in the wall at this location, although I have not checked that.
Re: bottom drilled-in dowels. That may be a way to do it, but I have shown top drilled in bars, as these can develop shear friction. Anything wrong with that? It seems to me that either top or bottom dowels would be ok.
To Teguci
Thank you for your comments. My fault for forgetting to attach the drawings.
Re: Hanging the waffle rib from the 2.5" slab - The edge of the floor is a solid slab the same thickness as the waffle stem + slab. The width of this solid edge depends on which side of the building it is on, but it is not less than 15". So this should give some substantial thick slab to transfer the rib reaction to the adjacent rib.
RE: Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
I think that it's still a concern as the temporary bracing will eventually be removed and all of the shear will be transferred back to the joint.
The trouble is that you need both shear friction and diagonal tension shear at the joint to work. It's not the case that, if one or the other works, you're okay. If you turn your detail 90 degrees such that the wall is the slab and vice versa, I think you'll see why I'm concerned. If you decide to chip an extra 2" into the wall, I'd definitely spend a few minutes checking one way shear there.
1) You'll have direct bearing here so shear friction ought not be an issue.
2) For me it all comes down to the story your telling at this connection. Is it a simple support for the waffle slab or are you trying to develop a real moment connection at the wall? For a waffle slab, I'd expect a simple, pinned connection. And I believe that means that you somehow need to satisfy code provisions for the anchorage of your bottom steel at supports. Hence my recommendation for the bottom dowels.
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: Bearing of repaired reinforced slab on basement reinforced concrete exterior wall
Examine the bracing design carefully. If the braces are not preloaded, the axial shortening of whatever bracing members are used may create problems with the analysis that you have done of the wall. Keep in mind that the bracing will induce upward (or downward) forces into the intact slabs. How much horizontal deflection of the wall at the slab level can be tolerated? You do not provide any information on the plan extent of the required demolition. BTW only one drawing (S209) was posted.
Ralph
Structures Consulting
Northeast USA