combination of connections: new slab + existing slab
combination of connections: new slab + existing slab
(OP)
What is your opinion on using rebars + steel angle (130 x 75 x 10 mm) anchored to the existing slab for connection: new - existing slab
Im asking this because Im little uncomfortable relying on rebars only.
Rebars are designed on shear (concrete cover above rebars are approx. 50 mm),
Steel angle + anchors are desinged on tension/pullout forces (from bending moment) + shear forces.
Both rebars + steel angle would be designed on full force.
What do you think?

Im asking this because Im little uncomfortable relying on rebars only.
Rebars are designed on shear (concrete cover above rebars are approx. 50 mm),
Steel angle + anchors are desinged on tension/pullout forces (from bending moment) + shear forces.
Both rebars + steel angle would be designed on full force.
What do you think?







RE: combination of connections: new slab + existing slab
Check to see how many rebar dowels are required, but consider that number as just a theoretical minimum. Install as many rebar as will reasonably fit along the existing slab - based on calculations of optimum rebar spacing. Center the rebar (vertically) in the existing 200 mm thick slab.
Reasons for these suggestions are as follows:
1. Each hole drilled into the existing slab is an adventure... will existing rebar be encountered (making that hole unusable)?
... will that hole be aligned (perpendicular to the edge of the existing slab) to allow that dowel to develop it's full load capacity?
... will the existing concrete crack, or otherwise be compromised by drilling (degrading the performance of that dowel)?
Hopefully, the number and placement of usable dowels will exceed the the calculated minimum number of dowels required.
2. Centering the dowels vertically is for essentially the same reasons as given above. That is, to allow a wide tolerance for field construction variation from the design.
Again, this is just my opinion, retrofit design is very different from new design. Existing conditions have absolute control of what can be done. The designer has to work within those constraints, be conservative, and make maximum allowances for field variations.
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RE: combination of connections: new slab + existing slab
Dik
RE: combination of connections: new slab + existing slab
Well, design vertical shear force on rebars are around 30 kN/m, which is not a lot. If I choose 200 mm spacing between rebars, that is: 30 kN/m x 0,20 m = 6 kN. based on that M8 rebar would be sufficient.
Based on that there is no need for big diameter, but I think its better to choose bigger diameter of rebar - there is also horizontal shear acting on this anchors in case of lateral forces as earthquake (its hard to determine actual shear forces)and there also may be some issues with deflections at a spot of anchoring in case of small rebars. I was thinking about choosing rebars M14 at 200 mm distance between anchors. Its much more than it is requared, but i think rebars at 200 mm spacing are still acceptable as far as field work/drilling go.
M25 rebars seems like an overkill in my opinion, but i dont have much experience so... perhaps M20 at 250 mm spaing would be better than what I suggested (M14/200 mm). I dont know what spacing between rebars is acceptable (normal) in situation like this.
RE: combination of connections: new slab + existing slab
Selecting the size rebar and spacing is a compromise.
1. Existing slab thickness is a major factor - need to keep the diameter of the drilled hole from being too large. Don't want to make Swiss cheese out of the concrete.
2. For the same reason as above, don't want the dowels spacing too close together.
3. However, don't want dowel spacing too far apart... may have to "omit" a planned dowel every now an then because of interference with existing rebar, etc. Don't want that "gap" to the next dowel to be any greater than necessary.
4. As dowel size goes up, both edge distance and dowel spacing become more of a concern.
Taking all this, plus your calculations on dowel loading, I consider your proposed M14 rebar at 200 mm, to be reasonable... but M16 at 200 mm, more conservative, and still reasonable. But it is truly an engineering judgement decision.
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RE: combination of connections: new slab + existing slab
Thanks SlideRuleEra!
RE: combination of connections: new slab + existing slab
I'm hoping you can find (or make) space within the finishes to place anchors over the vertical height of the beam. Maybe you could even make this like a top mount hanger and have a leg that sits atop the concrete beam.
RE: combination of connections: new slab + existing slab
- Mechanically, I think that you're proposed detail is pretty much ideal. And experience has taught me that many engineers would consider that the preferred detail. Your shear transfer mechanism is robust and the shear is delivered well above the bottom of the beam stirrups.
- I only have two concerns with your proposed detail and neither is technical in nature. Firstly, that angle may need to be fireproofed. That's extra effort and unsightliness if the condition will be exposed. Secondly, if that's the only structural steel in the contract, then I'd prefer to omit it so that there can be no structural steel contract at all. These issues might lead me to omit the angle and go with a dowel only connection.
- Rather than utilizing shear friction, my preference would be to treat it as a true dowel connection. This is a bit annoying in north america as, to my knowledge, we don't have a sanctioned method for pure dowel action. Just adaptations of appendix D. European codes, and Hilti in general, deal with this more explicitly I think so that's the path I'd be inclined to take.
- I also like dowels centered within the slab depth for the reasons mentioned by others above.
- I'm less concerned that others with respect to the use of large diameter dowels in this situation. Whenever I've run the numbers on these things, even modest dowel diameters will quickly change the governing failure mode to concrete shear breakout which is largely unaffected by dowel diameter. And I feel that smaller diameter dowels tend to do less harm to the receiving substrate.
- Conceptually, it would be nice to have some top side dowels just to keep the joint closed up tight.
Paying homage to the points above, I've proposed a detail below as an alternate to some of the others that have been discussed thus far. Part of what I like about it is that it mirrors what you'd do in a new construction joint: provide top steel and run your bottom bars over top of the beam bottom bars. The detail would require the low dowels be installed prior to setting the form work in that area. Depending on your situation, that may or may not be a problem.
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: combination of connections: new slab + existing slab
Dik
RE: combination of connections: new slab + existing slab
Added: Should have noted that I really like the Hilti TZ anchors...I go out of my way looking for places to use them.
Dik
RE: combination of connections: new slab + existing slab
RE: combination of connections: new slab + existing slab
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: combination of connections: new slab + existing slab
I've done a lot of precast and it's similar to other details I've developed... about 30 years ago, I went over my bike handlebars and injured the nerves in my right hand; I don't sketch worth a sh*t... so, often develop my details on cad and then pass them on to a
draftsmandraftsperson (politically correct) for inclusion with project details. I can CAD faster than I can sketch, anyway. Often details are complicated or 'detailed', and by the time I explain what I want, check it over, and provide markups... it's usually faster if I just draw them.Dik
RE: combination of connections: new slab + existing slab
On the other hand, extraction test for anchors are made in non stressed samples, beams' bottom surely develops somes cracks so rest on regular extraction capacity may be unconservative
RE: combination of connections: new slab + existing slab
With a reasonable steepness to the dowel, I don't think you'd ever see appreciable shear across the dowel or a breakout mechanism in the concrete. As support for the method, I submit the following:
1) Were the bar developed on both sides of the joint, one could consider the dowel contribution per the inclined reinforcement shear friction provisions of ACI 318.
2) Hilti uses an analagous system for punching shear reinforcement which would be a case of vastly higher demand. And that's been validated by testing.
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: combination of connections: new slab + existing slab
Also does fi 22/200 means diameter/lenght?
So this HILTI HIT - Z anchors (i used them before) are the only conection between new slab and existing beam/slab right? So that means anchor is design based on shear and tension/pullout (because of a moment).
steel angle is a bearing seat for a new slab and stirrup hooks are welded to steel angle?
KootK - im not so sure about the detail you provided. Thank you of course, but i dont like that top rebar is so close to top edge of new slab. Also I think dik's detail is better and easier to istall?
RE: combination of connections: new slab + existing slab
Added: Only partially answered the question... the 22dia x length is the hole drilled into the wall. The Hilti anchors are the only connection and are mostly in shear. The spec'd anchor is excellent for tension, also, with the wedged shaped sides compressing the epoxy on pull-out. There is little tension because the support angle is well anchored to the slab and rotation is minimal (span was short).
Feel free to use the detail, just check it out first.
Dik
RE: combination of connections: new slab + existing slab
That was very helpful.
RE: combination of connections: new slab + existing slab
You're welcome. I don't believe that there's any need to worry about the horizontal dowels close to the top of the slab in my detail. The sloped dowels will deal with all of the shear and the top dowels should see tension only. And you want them to seem tension because that's what keeps the joint closed. Additionally, there's every reason to think that the normal shear friction mechanism probably will help out here, even if we don't want to rely on it. Might as well set it up to succeed I figure. I still favor the use of centrally placed horizontal dowels here as well. As long as they check out, I suspect that's your lowest cost alternative.
Better is a case specific value judgement that I'll leave in your hands. Dik's detail is easy to install and that's a nice feature of it. As I mentioned above, however, it would not be my choice here owing to fire proofing and the need to add structural steel to a project that may not yet have a structural steel contract. Fundamentally, dik's concept is the same as yours. That, particularly since you'll still want to install the top side dowels that you originally proposed for crack control and keeping any temperature and shrinkage effects from straining your bolts in tension. I like the rebar welded to the angle but, in all likelihood, those bars won't see any tension.
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: combination of connections: new slab + existing slab
RE: combination of connections: new slab + existing slab
Hope this helps.
Jim,
RE: combination of connections: new slab + existing slab
RE: combination of connections: new slab + existing slab
However... I do not understand what is the purpose of the rebar with hook that is welded on a steel angle. (Dik's detail)
It was said that it holds the ledger angle in place, but I dont see how? Doesnt anchors hold steel angle in place? Can someone please elaborate this? Thanks in advance.
RE: combination of connections: new slab + existing slab
I believe that it holds the angle to the new slab rather than to the beam (bolts do that). For me, the hooked bar accomplishes two things:
1) The slab bottom steel probably needs to be anchored over the support so that it's flexural capacity out paces tension demand. The hooked bar accomplishes that.
2) Temperature and shrinkage slab strains will tend to pull the slab away from the connection a bit. The hooked bar restrains that. Whether or not that's good is another question I suppose. I'd wager there's enough flexibility in the angle for that to be accommodated.
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: combination of connections: new slab + existing slab
Dik
RE: combination of connections: new slab + existing slab
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: combination of connections: new slab + existing slab
Maybe longitudinal shear is helping, but as i see it i'm still not sure if dowell will work only in tension. Maybe you are thinking in a trussed action between vertical shear and two relative sloped tension dowels, but if we expect some moment at the joint the lower side of the slab will have a compression zone around the left side of the dowel, in that case probably tension is not being developed. If we expect a pinned behaviour there may be some amount rotation that allows some tension in that dowel but it will tend to be very small, so in both cases right side of the dowel may be acting in combined tension/shear.
I haven't reviewed punching shear deeply yet, but i think it's a different situation, compression will helping at one side of the dowels and wall reaction and tension bar will be holding the reactions (vertical and horizontal respectively) of the tension upper side of dowels. If this is the case tension dowels won't be relaying in the extraction cone only.
RE: combination of connections: new slab + existing slab
No worries. If you're a structural engineer and you're not up against a tough deadline, you're probably unemployed.
I'm thinking of a number of things but, primarily, just the ACI provisions for inclined shear reinforcement as shown below. They deal with this just as I've proposed with the additional requirement of the bar being developed for fy each side of the joint. They assume the dowel to be wholly in tension and not at all in shear.
You've made some interesting points but, all said and done, I think that you can simplify your thinking down to this:
1) For vertical slip to occur at the joint, movement has to take place across the dowel.
2) The dowel could resist the slip via tension.
3) The dowel could resist the slip via shear.
4) Since #3 involves dowel flexure, #2 will be the stiffer load path by a considerable margin.
5) It's reasonable to assume that dowel tension resists all the load.
Sure, but then your dowel not seeing any tension is really your best case scenario because it means that your flexural compression block is developing so much compression that you can just rely on classic shear friction rather than the dowels. And in our hearts, that's really what we all suspect was meant to happen anyhow so the diagonals just become a belt and suspenders load path.
This is your project, not mine. I respect any engineer's right to reject a solution that they simply don't "feel". That said, my solution is based on well established theory and I, personally, stand by it without reservation.
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.