combination of connections: new slab + existing slab 6

[straus12]

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?

 

[SlideRuleEra]

IMHO, Don't do it, the detail is too complicated for the limited space (200 mm, vertically).

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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[dik]

Probably 25M or #8 for dowels... I like large dowels... Also need a joint perhaps at the top interface with rod and caulk if moisture is an issue. Recently used Hilti TZ anchor with rebar welded to the angle to hold things together...

Dik

 

[straus12]

Great post, thanks for elaboration, I appreciate time and effort you put in reply.

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.

 

[SlideRuleEra]

straus12 - Concur with Dik, large dowels are preferred.

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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[straus12]

Reconstructions are always a challenge. We learn every day...
Thanks SlideRuleEra!

 

[kipfoot]

I agree with your original notion to be uncomfortable relying on the dowels. There has been some relevant discussion on this site which you'll find searching 'shear friction.'

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.

 

[KootK]

- I think that you're right to be concerned in this particular application. The inability to guarantee moment on the joint makes traditional shear friction dubious. And, treated as a pure dowel, the top spalling is an issue as you've identified. Lastly, because you're dealing with an upturned beam, the issue of where you deliver the shear on the beam is of some importance. Kind of like the hanger reinforcement phenomenon. The higher you deliver the slab shear into the beam, the less the chance that a wedge of concrete will just shear off the bottom of that beam.

- 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.

 

[dik]

SRE... your post was much better than mine... and gave you a star for it... I really didn't deserve the one I got. I don't care about stars, but, I care about fairness...

Dik

 

[dik]

The attached was used for some serious loading on a recent project. The rebar hook was only to hold the ledger angle in place. New 8" slab reinforcing went to the the L section and Hilti's secured the ledger to a wall.
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

 

[hokie66]

I agree with SRE. Dowels located centrally to carry the shear make much better sense than steel bolted to the slab. Both solutions would be just in shear, and the concrete bearing under the dowels is the controlling factor. Make sure the dowels are deformed bars, as you don't want them slipping.

 

[KootK]

@dik: slick detail. When I was considering OP's detail, I thought to myself "kinda resembles what I might do in precast". Your detail even more so.

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.

 

[dik]

KootK
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 draftsman draftsperson (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

 

[Yt.]

Dear Kootk, I'm not sure about the suggested detail, the sloped bar might have some issues. At some point the bar will be governed by shear/tension interaction, weakening it but it depends on the load. The horizontal component seems more like a near edge anchor, i'm not sure if a proper cone would develop. Also his extraction cone is overlapping the traction bars' ones.

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

 

[KootK]

At some point the bar will be governed by shear/tension interaction, weakening it but it depends on the load.

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.

 

[straus12]

I need some help with understanding dik's detail which I like the most I think. Can someone please tell me what @400 O/C means? that spacing between ancgors are 400 mm?
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?

 

[dik]

400 o/c... by default without units, the units are assumed to be inches or millimeters stipulated in the drawing notes. This was a metric project and the 400 is 400mm or roughly 16" o/c.

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

 

[straus12]

Thanks again!
That was very helpful.

 

[KootK]

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.

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.

Also I think dik's detail is better and easier to istall?

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.

 

[straus12]

One more thing, English is not my primary language, so I'd like to know what is a ledger?

The rebar hook was only to hold the ledger angle in place.