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Continuous Slab Ledge to Exist CIP 2

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Brad805

Brad805

Structural
Oct 26, 2010
1,490
I was asked to offer an opinion on the suitability of the detail below. They are planning to build a tunnel adjacent to the existing building. The tunnel will have a slab over it that supports 900mm of soil. I was only asked to comment on the continuous slab ledge. I would prefer to see a 40mm bearing surface for the new slab, but they are not too interested in changing the detail. In general the numbers do seem to work, but reading Joel's thread I thought I should seek others opinion on the field drilling/epoxy anchorage.

WALL-LEDGE_cdlksm.png


Corrected sketch
 
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That seems like a clusterfuck of a detail

First - that orange floor on the left...is that supported on grade or is it spanning?

Now, in terms of the block on the right.
What's the thickness and reinforcement of the existing wall? Guessing it is 8" with a single layer of reo?
I hope that the new slab is very stiff or else there is the potential for the eccentricity of the slab reaction to generate a moment into the wall - can that resolve?
Even just resisting an idealised perfect interface shear of 51kN/m (why are we mixing units here?) is going to put a big tension perpendicular to the wall to generate the shear friction across the interface
Can that tension fully resolve out? What's the junction detail from orange slab to top of wall - can that restrain any movement of the wall?

This seems like a highly critical detail i.e. even 1 anchor failing could be a collapse - is there any ductility or redundancy in the failure mechanism?

Edit: just scrolled one notch further down the image and saw it is indeed an 8" / 200mm wall woopsie
So putting a 150mm anchor in may make it behind the existing reinforcement layer?
 
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What kind of traffic are we expecting on this thing? I also usually insist on a key for this situation. I feel that I have good reasoning for that but, if this is just landscaping, maybe that's not worth getting into.
 
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Irrespective of the corbel setup, can the wall take the demand above the corbel? There, I feel as though you'd be down to plain concrete in bending and an effective shear depth of 30 mm or whatever. You know, unless there are some outside dowels that we haven't seen yet.

This might be ameliorated by claiming the slab as a lateral restraint for the wall if that's an appropriate thing to do in this situation.
 
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As in precast construction, I'd also be tempted to get a bearing strip under the slab, both to absorb rotation and to move the bearing reaction closer to the wall.
 
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I second the bearing strip suggestion. At least that way the maths is more quantifiable

Epoxy anchors are a very hot topic here at the moment as the tension they generate perpendicular to the wall fundamentally doesn't resolve in 99% of designs as there is no transverse reinforcement to pickup the tie required for the strut-and-tie mechanism
We've had plenty of debates on this in the office and will still use epoxy for minor to moderate connections, until someone comes up with a better option, but are accepting that we need to rethink our detailing for critical connections
To me, this is a critical connection and it needs something better than just epoxy if possible
 
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I'm sure that it would be received as "extreme" but perhaps it would not be so terrible to cast a new, 6" wall adjacent to the existing one.
 
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It may seem extreme on paper but I think something of that ilk is appropriate really

A tunnel is a scary place to be and there needs to be complete confidence in the future of the supporting structure
There's no redundancy, ability to inspect, or ability to maintain in that detail as drawn
But if it fails it's going to dump a concrete slap and 900mm of soil on some poor bastard walking his dog
 
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I revised the sketch to show the slab/corbel connection proposed. They were not planning for a moment transfer at the joint.

There is no traffic atop the slab. I do not have the arch plans, so I am not sure why the soil is being added.

The orange region represents an OWSJ/deck/roofslab. Apparently there is a finished kitchen space beside this wall.

I too would have looked at a new wall beside the existing. I am not sure that would have been more work than forming the ledge and chipping the wall.

Some of the work is done. I am not sure what led to the request, and I put that into the bucket of not my business. The rebar I saw did not look appropriate in scale.

Three votes --> Bad plan.
 
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Their dowelled connection is going to somewhat restrain the top of the slab and add a secondary moment into the connection in addition to the primary moment from the eccentricity
Bet the epoxy anchors aren't designed for that!
 
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Can you use an HDG L4x4x1/2 with HDG Hilti HAS anchors and Hy 200 3V V3 adhesive with strap anchors/headed studs?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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I vote for a new wall, and would not approve the corbel as designed. It simply violates rules for corbel construction.
 
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Dik, I would look at something like that if it were my design. I will leave it to them to decide what is best.

The fundamental problem sums up as?
1. Primary ties not anchored.
2. Critical connection.
3. Epoxy creep (study attached).

I will assume they checked the existing wall.

I revised the model to include the 300x4400 slab with the dowels as suggested by Greenalleycat. The force in the primary corbel ties do increase.

WALL-LEDGE-2_otchtx.png

 
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Easiest and likely less costly... [pipe]

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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With Hilti Hit-Hy 200 V3, creep would not be an issue. I often use ASTM F1554 Gr 55 S1 in lieu of E55 Hilti HAS anchors. 6" embed would be lots and you can use 3/4" or 5/8" anchors at whatever spacing you need. In these environs if exterior, then shorter lengths of angle for thermal expamsion issues. Wrap the angle in blueskin for waterproofing. Blueskin adjacent to the wall clamped by the Hilti anchor and wrapped around the angle of the leg so the slab bears on it. Provide a reveal at the top for ethafoam and caulk... and a good weather resistant joint. [pipe]

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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Epoxying an angle onto the wall is fundamentally the same detail though
It doesn't resolve the underlying issues?
 
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@Brad805, are you the dowels you've modelled between the wall and the corbel, or the corbel and the slab?
I just did some hand calcs as a rough estimate and I get higher loads, though I have had to make a number of guesses that could throw me out

I also wouldn't be so sure about the wall being OK
I did some rough calcs and it could be on the line - though again, I had to take a number of guesses
 
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As long as the wall can take the eccentricity of the slab load; there should be no issue, unless I'm missing something... load is about 4K/ft. a little high, but doable... sorry I think in Imperial, only although I work in both. 3/4" anchors @12" o/c or maybe 8" assuming the load is factored. (I think 12 works) and conc strength > 3000 psi. A notch for the slab would be nice, but not essential... a new wall works, too... but much more costly.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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No worries, luckily I've worked on a few pre-metric (Dark Age) buildings here, so I can handle the inches and feet...kips and that are just gibberish to my brain though!

A concern that I have is the secondary moment that's induced from the dowels cast into the slab
Presumably for construction sequencing the corbel will be built, dowels cast out of the top, then the slab poured with the starters in it
This gives a fixed end capacity
As the soil is added the slab will be trying to deform and rotate at the ends but the dowels will restrain that, helped by the pile of dirt on top
This then passes a secondary moment into the wall in addition to the primary moment from the eccentricity of the reaction force from the wall
Rotation in the wall will lessen this effect, so it depends on how much this rotation relaxes the moment vs the moment capacity of the wall

 
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