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?

 

[nutte]

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.

 

[miecz]

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.

 

[Lion06]

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?

 

[UcfSE]

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.

 

[haynewp]

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.

 

[haynewp]

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.

 

[NITTANYRAY]

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.