Hollowcore Slab Ties in Topping Vs Grouted in HCS

The answer will be heavily dependent on standard practice in your area. In North America, where I've practiced, these connections can be as modest as:

a) Keyway dowels into the beam ledge below.

b) Waste strand grouted into the keyways or broken cores.

Seismic jurisdictions such as New Zealand tend to approach this situation in a much more rigorous fashion (and good on them for that). This is a solid, free document that discusses their concerns and the approaches that they commonly employ to address them.

Thanks Kootk, I have actually came across this link earlier and for sure having the topping in addition to the grouted keys would provide a more robust solution but i have came across PCI design manual where they clearly say you can design the topping as your diaphragm and ignore completely the hollow-core slabs, yet most of the details found were still preferring having the ties in the cores.

The contractor`s claim is why do we need to do both?

I am having some heavy chillers above the hollow-core slabs and claiming that we would need extra redundancy to do both topping and grouted keys but wanted to get some backup from design standards specially on HCS supporting heavy equipment's but seems to be lacking in research.

At a minimum we provide these bars at 4-feet (each grout key) regardless of whether the hollow-core is the diaphragm, or the topping. Something about having zero connection between the plank and the supporting structure gives me the willies.

We fasten wood joists, steel joists etc, to the supporting structure, even when they aren't participating in the lateral system directly.