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?
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
RE: partial rebar development
RE: partial rebar development
RE: partial rebar development
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
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
RE: partial rebar development
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
RE: partial rebar development
RE: partial rebar development
RE: partial rebar development
If not, maybe look at Lenton weldable couplers?
RE: partial rebar development
RE: partial rebar development
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
12*0.9 = 10.8
10.8/(1.57*1.392)=4.94 Use 5/16.
RE: partial rebar development
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
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
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
RE: partial rebar development
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:
RE: partial rebar development
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
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
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
Note that in chapter 6 of the PCI Design Handbook 6e you can find tables for welding rebar.
RE: partial rebar development
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
RE: partial rebar development