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partial rebar development
4

partial rebar development

partial rebar development

(OP)
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?

RE: partial rebar development

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.

RE: partial rebar development

Why not hook the top bar?

RE: partial rebar development

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.

RE: partial rebar development

(OP)
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.

RE: partial rebar development

(OP)
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.

RE: partial rebar development

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

RE: partial rebar development

(OP)
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.

RE: partial rebar development

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?

RE: partial rebar development

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".  

RE: partial rebar development

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

RE: partial rebar development

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?

RE: partial rebar development

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

RE: partial rebar development

(OP)
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.

RE: partial rebar development

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.

RE: partial rebar development

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.

RE: partial rebar development

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.

RE: partial rebar development

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.

RE: partial rebar development

I'm with csd72 - grind and butt weld.

RE: partial rebar development

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:

Quote:

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

RE: partial rebar development

jmiec:

The original post mentioned rebar welded to the bent plate, not a stud welded to the bent plate, thus the load path I described.

As for the fillet weld loaded perpendicular to its axis, imagine a vertical fillet weld, with a load in plane at 90 degrees to the weld line.  Or rotate the whole thing sideways, and you have a vertical load and a horizontal weld line.  This weld is 50% stronger than if the load is parallel to the weld.

For confirmation, look at Table 8-6 in the 3rd edition LRFD manual, page 8-58.  For the case with two welds, a=0, and for any value of k, the strength is 1.392*1.5*2 welds=4.176 (the tabulated value is 4.18).  (This would get multiplied by D and L for the total strength...)

As for the load being in the plane of the weld, I think of the load acting in a circular line around the circumference of the bar.  This load is in the plane of the weld, so the equation would apply.  This analogy would be similar to the special case shown in Table 8-5, page 8-52, for a pair of vertical welds loaded out of plane.  This special case can be used as long as there is solid material between the two welds.  The round bar certainly qualifies as solid material in the case of the round weld.

RE: partial rebar development

nutte-

You're right.  A shear vertical shear force would act in the plane of the weld.  However, the weld in that plane makes the shape of a circle, or a donut.  For a shear force in the plane of the weld, little or none of the force is perpendicular to the axis of the weld.  Equation A-J2-1 would have to be integrated around the circle.

RE: partial rebar development

(OP)
I agree that the shear load can be taken into the slab via the headed stud.  I guess if I were going to do that with the hooked rebar, I would just need to make sure the slab can take teh bending moment at sections that it would have little rebar development as unreinforced concrete, concrete?

nutte-
When I was saying that I didn't need to develop the full bar strength with the weld, I wasn't talking about shear from bent plate to rebar.  I was talking about actual rebar development from the maximum moment section.  The available length for development was 1'-6", and I needed 2'-5".  I took advantage of the 1'-6" and was counting on the weld for the remaining value.  That was before I thought about your view of the shear into the concrete via the headed stud (or welded rebar).

Our General Notes just say to use a special electrode for welding rebar (I believe it is E90xx).  Is this an acceptable alternate to the A706 rebar that jike mentions or should our General Notes be changed?

RE: partial rebar development

A706 is a low-alloy rebar that is suitable for welding. A615 bars tend to develop cracks and are generally unreliable for welding.  From what I've heard you can weld the 615 bars but special welding procedures are required.

Note that in chapter 6 of the PCI Design Handbook 6e you can find tables for welding rebar.  

RE: partial rebar development

I agree with jmiec with using headed studs and a hooked bar at the end, you don't have to weld rebar to the plate. I usually think of welding rebar to plates when there is direct tension on a plate. For your case the shear goes from the studs into the concrete then the moment is created from your cantilever. You need to check that the length of rebar plus hook develops the rebar at the cantilever side from the point of maximum moment and then vice versa for the rebar on the other side of the beam, and that's it.

A straight length > 12" + standard hook for #4 bar should be adequate to develop the bar for typical cover, normal weight aggregate, and 3 to 4000 psi concrete.

RE: partial rebar development

Quote:

I guess if I were going to do that with the hooked rebar, I would just need to make sure the slab can take teh bending moment at sections that it would have little rebar development as unreinforced concrete, concrete?
If you take the end of the hooks as close to the end of the slab while keeping the required edge cover you shouldn't have to worry about unreinforced concrete taking the moment.   

RE: partial rebar development

Use Nelson Deformed bars welded to the bent plate. They are welded with a gun similar to headed studs. The weld will develop the full strength of the bar. 5/8" diameter bars come in lengths up to 30". Nelson is a subsidiary of TRW and is located in Lorain,Oh.

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