partial rebar development 4

[Lion06]

I have a situation where the stair guy is attaching a stringer to a bent plate at edge of slab where the slab is cantilevered out past the beam 1'-6".
This was not the intent of the original design and the slab isn't poured yet. I am detailing some rebar in the top of the slab(attached to the bent plate at edge of slab) to take this moment back into the slab.
I can't get a fillet weld around a #4 bar to develop the bar unless it is like a 1/2" weld (that's not going to work). I ended up counting on the length I have from edge of bent plate to centerline of beam (1'-6") to partially develop the rebar and am only counting on the weld to the bent plate for the remaining portion.
Does anyone see any problem with this?

 

[UcfSE]

Why can't you weld all around the #4 bar? You should only need a 5/16 weld around to develop it, at grade 60.

 

[miecz]

Why not hook the top bar?

 

[nutte]

If the load is in the stringer, then in the bent plate, all of it would need to go into the rebar, not a portion of it.

 

[Lion06]

jmeic-
I could hook the top bar, but the load needs a way to get into the slab. The stringer is attached to the bent plate, not the slab itself and there are no studs from bent plate into slab.

UcfSE-
How do you get 5/16"? A #4 = 0.2 in^2 * 60ksi = 12K. The length of fillet weld will be pi*d = 3.14*0.5 = 1.37". A fillet weld (using LRFD) is good for 1.392K/in/16th in.
I take 12k/(1.37*1.392) and get a required fillet weld of 6.3 16ths --> 7/16" fillet weld.
Am I doing that wrong?
I am welding the rebar all the way around, but given what I showed above, I couldn't get it to work with the weld only.

 

[Lion06]

nutte-
I know all of the load has to go into the rebar, not a portion of it. I don't have to take all of it into the rebar at the weld, though, do I?
It needs to be developed at point of max moment, not at the end of the cantilever.

 

[miecz]

Oh, missed that part. OK, so why not anchor bolt the bent plate to the slab?

 

[Lion06]

what exactly do you mean by anchor bolt the bent plate to the slab?
If it is what I am imagining (bolting through the bent plate and having a nut on the end of the bolt into the slab), it seems like it would need to be a relatively long bolt to develop the rebar. Also, I don't think the architect wants to see a nut on the outside of this bent plate as it is exposed below.
I thought about just adding studs on the bent plate, but I have the same problem with developing that into the rebar if it is just hooked.

 

[lkjh345]

Can you just use a #5 or #6 bar to get enough diameter to use a 5/16" weld and then only count on the bar as if it was a #4?

 

[miecz]

I think the anchor, stud or bolt, only needs to carry the force into the concrete (Appendix D), not develop the strength of the rebar. By the way, I think your length of fillet weld is 1.57".

 

[csd72]

Just get the thing ground to a cone and butt welded to the plate - end of story.

 

[PMR06]

This seems like the stair designers problem. Can you suggest he concoct a connection that extends his stringer 1'-6" to the beam?

If not, maybe look at Lenton weldable couplers?

 

[miecz]

Accounting for the size of the weld, isn't your weld length greater than pi*0.50"?

 

[Lion06]

lkjh345-
Good idea.

jmeic-
I think I would need to develop the rebar with the stud. Just to think about an extreme case. If I get a a 3" headed stud to work and the hooked bar comes to 1.5" of edge of plate such that there is 1.5" overlap of hooked bar and headed stud, I don't think I would be comfortable saying that the headed stud is getting the load into the rebar.
I'm not sure Appendix D would apply here, since I am not taking a tensile load into the conrete. I am putting a shear into the conrete that is developing a moment about a different point.
Good catch on the 1.57" circumference. I don't know how I came up with 1.37" unless I just looked at it wrong.

 

[UcfSE]

You need to be consistent with applying the resistance factors. You applied the 0.75 to the weld strength but didn't use 0.9 for the rebar. The circumference is about 1.57 inches, not 1.37.

12*0.9 = 10.8

10.8/(1.57*1.392)=4.94 Use 5/16.

 

[miecz]

Appendix D applies to shear loads in unreinforced concrete. If you can prove that your stud can transfer the shear to the concrete (maybe you can't, depending on edge distance), then the concrete transfers it to the hooked bar. How's it going to fail?

On the weld length, if you add a little to account for the weld itself, then you can get the size down to 5/16", as UsfSE said way back in the beginning.

 

[nutte]

Maybe I don't understand the problem correctly. If the load is in the bent plate, it is only getting to the rebar via the weld, so the would need to take the full load. You're talking about developing the bar at the other end, but don't forget how the load gets into the rebar in the first place.

Also, keep in mind that a fillet weld loaded perpendicular to its axis has an additional 50% capacity, so your 1.392 becomes 1.392*1.5=2.088. For the extra 50% capacity I'm referring to, look in LRFD 3rd edition, Appendix J, equation A-J2-1, for the 1+.5(sin theta)^1.5 term.

 

[jike]

You have to use A706 rebar if you are going to weld! This is way too much trouble!

Two possible solutions:

1) Headed studs on bent plate to transfer shear load to concrete and hooked rebar of size and spacing to meet the development length.

2) As PRM06 suggested, mark up stair drawings to have stair stringers connect directly to beam.

 

[Tomfh]

I'm with csd72 - grind and butt weld.

 

[miecz]

nutte-

I see the path of the force as: bent plate to stud to concrete to rebar/concrete. I don't think you need a direct connection from the bent plate to the rebar. Don't precasters frequently use this kind of connection for shear loads?

Also, I don't believe Appendix J, equation A-J2-1 applies to a fillet weld loaded perpendicular to it's axis. Paragraph (a) reads:

For a linear weld group loaded in-plane through the center of gravity, the design strength is...