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?

 

[jimstructures]

The angle you show being attached to the existing building is acting as a ledger to support the new slab. In new wood frame construction a ledger is attached to the existing building to support the new framing which will be attached to the ledger and the existing building. The ledger provides a new "ledge" to support the new framing. You can also use a ledger on new framing to help support the new joists.

Hope this helps.

Jim,

 

[straus12]

It helped! tnx

 

[greznik91]

I have been reading this thread and I find it interesting.

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.

 

[KootK]

It was said that it holds the ledger angle in place, but I dont see how?

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.

 

[dik]

KootK... and that the slab was fairly heavily loaded, I wanted to make sure there were no bond issues with the bottom reinforcing as well as provide a positive connection for the Hilti anchor.

Dik

 

[KootK]

Curious: wall support condition in your case Dik?

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.

 

[Yt.]

Hi kookt, sorry for the late answer. I'm dealing with a deadline so i haven't much time for reading the post.



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.

 

[KootK]

Hi kookt, sorry for the late answer. I'm dealing with a deadline so i haven't much time for reading the post.

No worries. If you're a structural engineer and you're not up against a tough deadline, you're probably unemployed.

Maybe you are thinking in a trussed action between vertical shear and two relative sloped tension dowels

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.

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

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.