Continuous reinforcement in HollowCore keyways

[byrneoin]

Hi Folks,
Have a floor (roof) slab diaphragm comprised of Hollowcore planks spanning onto precast beams,
5Bays of planks (onto 4 internal beams)

Where planks are supported onto the internal beams we have rebar in 2no. cores in each plank (2dia.16 bars) joining adjacent planks together (over the beams / tied in with protruding reinforcement from the top of the beams / all then concreted in)
Planks are tied into perimeter beams with U-bar stirrups coming out from 2no. cores in each plank - wrapped around lacers/longitudinal steel forming beams

We are placing no reinforcement in keyways, but they will be grouted up (nominal longitudinal shear transfer via grout to concrete)
We have no structural topping screed. Just a light-weight non-structural topping for roof substrate

Question - should we be placing continuous rebar in the keyways for diaphragm action?

 

[byrneoin]

Hey thanks, by keyways I mean the longitudinal joint between adjacent hollowcore panels

 

[530_3D]

"Precast concrete components may be fabricated with grout keys and connected by grouting the joint. For components connected by grout keys, a conservative value of 80psi can be used for the design shear strength of the grouted key. If necessary, reinforcement placed as shown in Fig. 4.8.3 can be used to transfer the shear. This steel is designed by the shear-friction principles discussed in Chapter 5." - PCI Manual 7th edition page 4-55

Edit: I wasn't able to find any untopped hollowcore details since we are in seismic country. Apologies

 

[byrneoin]

Thanks for the reference @530_3D much appreciated.

 

[KootK]

Question - should we be placing continuous rebar in the keyways for diaphragm action?

What aspect of diaphragm action would you be intending to use those bars for? Obviously, they would not do anything for longitudinal shear transfer at the joints between planks. If you have dowels coming out of the beams and grouted into the keyways at the ends, those can be used for to transfer shear perpendicular to the planks at the plank ends. That said, if you've got rebar in the cores that performs the same function, I see no reason whey you would have to use keyway dowels in addition to that.

 

[byrneoin]

@KootK Totally agree with you. This has been the subject of debate with an external engineer who is insistent on their inclusion ....without justification "its a standard detail". I too don't see the need for them from a diaphragm action perspective, nor from a progressive collapse detailing requirement.

 

[jayrod12]

If you have other rebar, I'm of the same opinion as Koot. That being said, dowels from beams into the keyway is a common typical detail as they're easier to install and grout than directly into the cores.

 

[lexpatrie]

I feel I've seen that on drawings but perhaps not for a roof.

As a side note, does one typically (in say non-seismic, 1970s era construction, northern US) see the gaps between the planks grouted or otherwise sealed on a roof system?

I haven't been able to find any fire rated assemblies that show that grout but intuitively I think you need it to reduce heat flow and pass the 1 hour test for temp rise on the non-fire side of the assembly (top).