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Does concrete topping reinforce the floor diaphragm for wood frame structure?

Does concrete topping reinforce the floor diaphragm for wood frame structure?

Does concrete topping reinforce the floor diaphragm for wood frame structure?

Hi everyone,

In my area, it is common practice to have 1.5" concrete topping (25 MPa) on the floor sheathing (for radiation heat purpose). The concrete topping is usually plain or with 6" wire mesh. Normally when we look at the floor diaphragm capacity, we only look at the panel thickness and nailing pattern. I am just wondering why not include some contribution of the concrete topping (concrete bonding strength is pretty high), which will make the floor much more rigid than just nails and sheathing.

The reason I am asking this question is that when I practice the design of single family house, a lot of time I have to deal with shearwall offset horizontally between upper floor and lower floor. A reference book stated 4' offset is acceptable but sometimes it is still not enough. I am wondering if it is reasonable to consider the concrete topping contribution to the floor rigidity and assume the floor will distribute the shear force from the upper floor shearwall to the nearest shear walls on the lower floor, when the offset is more than 4' and less than say 10'.

Also wondering if it is possible to treat the concrete topping as drag struts.

Thanks for your thoughts.


RE: Does concrete topping reinforce the floor diaphragm for wood frame structure?

I would be inclined not to for the following reasons:

1. Its only 1.5 inch thick and presumably laced with plastic heating pipes? I suspect the ply sheeting has better capacity and a clearer load path.

2. I would question the actual bond since the sheeting in reality will be covered in sawdust etc when they pour the concrete. I would particularly question the long term effectiveness when the concrete’s cracked and debonded after years of expansion/contraction and differential rates of movement between the concrete and timber.

Not sure what you guys call it, but here that stuff is referred to as a non structural screed and for good reason in my view.

You could look at creating some type of dowel shear key connection to get the materials acting compositely.. but I’m not aware of much design guidance in that regard.

RE: Does concrete topping reinforce the floor diaphragm for wood frame structure?

If it's not composite, then no. With composite section you have shear connectors. If concrete is thick enough, it will be much stiffer compared to timber diaphragm. But you need shear connectors between timber and concrete.

In Europe there are a lot of projects with composite mass timber panels and concrete (like this). I never had a chance to design something like that. I wonder how you control concrete cracking if you pour whole slab at once since concrete is restrained at the bottom, plus one way timber panels can shrink/swell quite a lot.

RE: Does concrete topping reinforce the floor diaphragm for wood frame structure?

If we're talking about standard, north american, residential practice, then I'd say that it's definitely not composite for the purpose that you have in mind (moving lateral diaphragm loads around). Consider:

1) Your topping probably does not pass beneath your stud walls.

2) Any drag strut forces that you put into your topping will have to be moved both into and out of the topping and into the surrounding structure somehow. Unless you make special provisions for connections that would allow that, you'll not have it.

That said, I don't doubt that the presence of the topping would stiffen your diaphragm up some. I almost find that more of a nuisance than anything as it makes me feel more obligated to bracket my design with rigid/flexible assumptions.

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