Precast Inverted Tee Beam - Tie Detailing for Offset Ledges

[KootK]

1) A lot of my precast engineering competition will detail their offset ledges as shown below, as shown in black. Fabricators like it.

2) The usual detail creates a condition at the right side, as described below, that troubles me a bit.

3) One alternative is shown in blue below.

4) For what it's worth, we're not using the outer tie system for shear or torsion.

5) For what it's worth, based on PCI manual provisions, ledges seem to be treated with a lesser degree of detailing care/paranoia than proper corbels owing to their, generally, much reduced demand relative to corbels. So I may be attempting to apply corbel logic somewhere where it in undeserved.

Thoughts? This nightmare scenario plays out in my mind where something goes wrong and the engineer working with opposing counsel says something like "dude, you basically detailed the ties like a water-slide for the loads?!?"

 

[rapt]

Kootk,

I much prefer your Blue solution for exactly the reason you have shown. I do not see the offset lap as a problem. Australian code increases the lap standard lap length by 1.5 times the offset. Though I would like cog/hook on the other end.

You do not show the width of the seat. It looks insufficient also. The load bearing area should be fully within the "corbel" bar.

 

[KootK]

Thanks for the feedback Rapt.

You do not show the width of the seat. It looks insufficient also. The load bearing area should be fully within the "corbel" bar.

For commonly used ledge geometries in my market, that is pretty much impossible to accomplish. It seems to go back to the issue that I mentioned in the original post that it is my impression that a continuous ledge seems to be held to a lower standard than a discrete corbel. And, to a degree, that makes sense to me.

 

[TehMightyEngineer]

I'm going to say from a constructability standpoint your offset lap splice will be easier to fabricate and install.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[KootK]

I'm going to say from a constructability standpoint your offset lap splice will be easier to fabricate and install.

You would think that but my precaster actually made a point of requesting a closed stirrup for stability of the cage during fabrication.

 

[rapt]

Interesting that "the "market" is allowed to trump (not the one south of you) design logic!

 

[KootK]

Interesting that "the "market" is allowed to trump (not the one south of you) design logic!

Puhlease... if there's a practicing engineer out there, anywhere, that doubts that market forces influence technical decisions, I'd love to hear from them. I've got a heap of major precaster catalogs all showing 5" ledges or less. As such:

- Me introducing 8" ledges isn't going to end well for me and;

- Anecdotal evidence would seem to support that 5" is not a problem in practice.

 

[TehMightyEngineer]

You would think that but my precaster actually made a point of requesting a closed stirrup for stability of the cage during fabrication.

I guess, but there seems like far easier ways to ensure this. The sloped bend needs to be precise otherwise it won't fit the longitudinal shelf bars. Plus for cage stability the could just wire tie the horizontals of the split lap splice bars to the verticals of the interior hoops. Either way, I suspect they're not actually saving themselves money with the sloped stirrup and it's clearly not the best structurally.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[KootK]

Precise bending ought not be a problem as the stirrups come pre-fabricated. I'm compelled to view this from an evolutionary perspective. We're replicating the work of some very large, local precasters that have been at it 50+ years. The odds favor their knowing their craft over my having invented a better mouse trap I think. I simply need to reconcile my technical concerns.

 

[dhengr]

Koot:
At the very least, I would cast (form out) some .75 – 1” chamfers at the upper outer corners of the two ledges, just to keep any loading or beating/abuse away from the very outer edge. Could you make up some light “L” shaped rebar cages like your “3BB1”, out of #3’s or #4 bars, with hooks at the two ends? The one size fits both sides and has a couple longitudinal bars at the two outer corners. These cages, in 10 or 20’ lengths, are just stuffed-n-tied down and into place after the rest of the rebar is done. That’s better than nothing.

Alternatively, what would happen if you took your single, closed stirrup cage, with the sloped top, and changed it in one way? Slope the top down, from the left longi. bar, to a longi. bar on the outside of the cage, and near/at the middle width of the beam. Then run the top bar horiz. over to the right longi. bar, with min. bar cover.

 

[bones206]

Intuitively, I can visualize the waterslide effect of the sloped bars that you're worried about. But if it works in practice, it's probably because the actual local bearing forces aren't high enough to cause that local shear failure. Can you estimate that local shear/bearing capacity based on plain concrete? If your demand/capacity ratio is pretty low I wouldn't worry about the sloped bar effect coming into play. Plus, the rebar waterslide only occurs discretely at the stirrup spacing, with a swath of plain concrete in between. The rebar nubs probably provide a decent amount of tangential resistance as well.

The outside bend of your blue bar might actually have the same waterslide effect, but with the shear failure plane pushed higher and breaking off a smaller chunk of edge.

 

[TehMightyEngineer]

The odds favor their knowing their craft over my having invented a better mouse trap I think.

You underestimate how much they likely internalize "we've always done it that way". While it's true that learning new things or changing their standard will cause a brief loss of efficiency or what not this is such a small change and likely benefits them in the long run if I had to guess.

Either way, I've also always found that it's easier to go with the grain than against.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Settingsun]

How much of the tie actually intercepts the shear plane once bend radius is taken into account? If not much, you could make it weaker by introducing a crack-starter into the ledge.

Even if that's not the case, isn't this failure mode still a potential issue in square beams with horizontal stirrup legs anyway? With the stirrup not resisting?

 

[Agent666]

Not sure if this is any help, discusses many issues/solutions for support corbels and presents a method based on strut and tie to size the reinforcement.

In your particular situation I'd put a square stirrup under the first (right) corbel, then add a further bar like your blue one for the left one, probably with a 180 degree hook through to the right side of the beam to develop it properly as presumably won't have sufficient development length as drawn.