Torsion in precast header piece

[JStructsteel]

So I have a dilemma on a precast lintel over a window. Opening is 11'-0". Section is shown in attachment. Precast supports the brick load, but not the CMU (they will shore precast while constructing CMU beam as shown). Bearing at each end is only on the CMU, so piece is truncated down as shown by red line for bearing.

My issue is the torsion. I dont want to worry about torsion. To negate that, I would need the bearing thickness to be almost 24" tall. creates a "L' shape beam.

My other thoughts are to have some dowels along the piece to be worked into the CMU beam when its built, to negate the torsion. I figure I could have grouping at the end near the bearing that would just resolve the torsion.

Thoughts?

 

[JStructsteel]

Any one have any thoughts on this?

 

[phamENG]

I don't see how you can get away with not resolving the torsion. Even if you can get away from torsion from the brick, you still have wind and snow/live load acting on it. (If you're not assuming live load, you should - somebody, someday will put their foot on that thing.)

Feels like you need starter bars coming out of the top of it that develop into the wall above. They could use ribbed sleeves and grout or an epoxy, or have it shipped with the bars already started. Development may be tough, though.

 

[JStructsteel]

Thanks phamENG.

As you can see, the EOR is calling for holes thru the precast to place bars to make the CMU wall steel continuous. But my client also is telling me that it wont carry the CMU load, they will place the precast, shore it, and then build that 32" CMU beam. I still see it carrying some weight. That wall will deflect.

I agree with development, I will need bars to resist the torsion other than than the wall bars thru it (no way they get developed).

Im working on the design now for Torsion, once I get a design done, might share and see what folks think.

 

[phamENG]

I'm thinking more about torsion and global stability of the lintel. That thing needs to be locked into the wall along its length, not just at the ends. That'll alleviate internal torsion and turn it into bending in the precast.

 

[JStructsteel]

I agree with that, but the EOR and precaster do not want to do that the way it sounds. I will present the idea back to them.

 

[KootK]

But my client also is telling me that it wont carry the CMU load, they will place the precast, shore it, and then build that 32" CMU beam.

That doesn't sound too far fetched to me. That 32" CMU beam ought to be vastly stiffer than the lintel. I like the dowels into the supported wall above along the length of the beam. With that in play, the 32" CMU beam will probably be holding up the lintel.

 

[KootK]

A mechanism like this might be possible. 12" block is a fair bit to work with.

 

[KootK]

Surely there will be some kind of doweling between the CMU and the lintel just for out of plane shear transfer.

 

[Brad805]

What is being proposed seems more complicated than it needs to be. Unless they have a design gap between the sill and the masonry, I do not follow how this does not wind up being a composite section. The precaster can easily supply the sill with threaded inserts or form savers, and the sill would become part of the bond beam.

 

[JStructsteel]

I have proposed making it a composite section, so the piece bends out from the wall only. We will see what they say.
Thanks all. Always a complication on a project between trades.