Will Blanchard wrote:
I had a question about detailing a hollowcore slab diaphragm that I
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
The first calc Vn =3D AvfFyM references 'shear friction reinforcement'
following page 168, Fig R11.7.4 shows a bar at 45 degrees or so across
'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
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.