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Hollowcore Slab Diaphragm Connection Detailing

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Will Blanchard wrote:
"Hello all,
I had a question about detailing a hollowcore slab diaphragm that I was
hoping to get some help with. I've asked around here, looked through =
and the old PCI manual that we have here and I've yet to find a clear
When you are designing these things for lateral transfer, my =
is what typically happens is the contractor grouts the shear key =
the HC slabs and the friction occurring between the adjacent panels =
to transfer the shear through the diaphragm. According to the
manufacturer's website the detailing is to be in accordance with ACI 318
Section 11.7.
The first calc Vn =3D AvfFyM references 'shear friction reinforcement' =
and the
following page 168, Fig R11.7.4 shows a bar at 45 degrees or so across a
'crack' which in this case I assume to be our shear key. My question is =
exactly does one get a bar into a pre-cast slab with such small =
There are a couple of ways to look at this.  Basically, you need to ensure the plank are sufficiently clamped together to maintain friction on the grouted key.  Laterally, a 4 foot wide plank is really stiff.  Can you put an equivalent amount of rebar across the ends of the plank to ensure they can't separate?  Maybe you have a 4 or 6 inch space along the ends of the plank, formed up and stuffed full of bar.  Block most of the cores 6 or 8 inches in; leave maybe 2 feet of the end couple cores, extend hooks into them, fill everything with grout.
If the plank are supprted on a steel beam, life is even easier.  Weld them all to the beam, both ends.  End of problem.  This works fine on a small roof.
To be honest, one of my bosses at a previous employer (one for whom I had and have a lot of respect) told me not to worry about it, invoking the advanced technical argument "where's it going to go?"  There's something to that idea; those plank are pretty difficult to move.  The plank on this particular job were welded to large steel beams, so his point was well taken.
I agree with you that topping is often a waste of money on a roof.  However, if it's a visible roof (i.e. sloped, without a parapet) you want to make sure all the plank are flush.  I did a job with a standing seam metal roof over plank once (not my idea) and the roofers had a very difficult time because the plank weren't flush, due to variations in camber.  Put a note on your drawing requiring threaded rods with large plate washers to be run vertically through the joints at midspan (at least) to pull all the planks flush before the keys are grouted.  Remove them after the grout has set.
The whole idea of topping on precast plank is something I'm not very comfortable with.  At past jobs, if we had to use it (which you do at floors), we considered it non-structural, as we never were confident of the bond.  We just used it as a levelling course.
A few thoughts.
Mike Hemstad, P.E.
Minneapolis, Minnesota