Flat Plate Slab Roof: Sloped Cement Topping - Adhesion, Effect on Stiffness, Uplift

[engjg]

I have a flat plate conventionally reinforced concrete slab roof. Contractor wants to use a sand cement slopped topping with either a torch down or liquid applied roof covering.

I am pondering the following about this:

1. I can account for an average or max additional dead load for the varying thickness sloped topping in my design, but if this topping behaves "adhered" is it necessary to consider how will it affect my plate stiffness and design analysis?

2. Project has extreme wind uplift requirements qh ult = 60 psf (ASCE 7-16) regardless of what GCp or GCpi you select or whether you are looking at 0.6xD or not it seems to me that unless adhesion can be technically substantiated the need to specify a significant minimum thickness will exist?

 

[EZBuilding]

1. If you were to consider the adhered topping in your analysis, you would be the first person I have seen do that. It is common practice in my area to ignore any additional stiffness properties from this topping and design for solely for it's added superimposed weight.

2. I would recommend researching "NOA" or Notice of Acceptances which are required tested and engineered assemblies for enclosure elements in Miami-Dade County. You should find plenty of information on tested systems which meet and exceed the uplift requirements for your project.

 

[engjg]

Thanks EZBuilding

1. I thought I would get some responses like "you would be the first person I have seen do that" and I suspected not many would say it would be standard practice to consider, partly because I don't know how one would consider. I myself would not give it a second thought if the topping was a less dense material than the concrete slab or not directly adhered. I guess I can come to terms that the stiffness of my model (and therefore stress distribution) will likely not accurately represent reality, but just trying to think of anyway this my affect the design and detailing of my slab that would result in a design that was not conservative...

2. I already suggested as much but heard nothing but crickets from design team and contractor. But we are really not talking about much of a "proprietary assembly" of any sorts just sand cement on reinforced concrete with a cold joint. I am assuming this is a more fundamental adhesion question...? You think this would be covered in a Miami-Dade NOA as I would be interested in that?

 

[EZBuilding]

I am not speaking from certain knowledge, however, I would assume that most conventional roofing systems have an NOA.

 

[r13]

Since the concrete and the sand-cement toping have different engineering properties, the best you can do to include the stiff topping on your analysis model is to design it as composite roof system, and calculate the property accordingly. You also have to check the strength of the adhesive against the shear stress at the interface to ensure proper composite behavior. I won't do it.

 

[JAE]

If you don't want to consider composite action, use a bond breaker, or perhaps a vapor barrier type sheet between them.
I'm like EZBuilding, however, in that I wouldn't typically worry about it.

If you want to consider it composite, you would need to specify a very roughened surface, much like the precast folks do with hollow core planks and their composite toppings (See PCI).

 

[bookowski]

It is typical in my area to specify all pitched roofs as monolithic. Rebar stays flat and variation in thickness is not considered in design. We have general notes that it must be monolithic, max thickness < 2" from specified, and the stated thickness is at the low points. Better off to go this route if you can. My understanding is that some roofing systems require this as well to warranty the system.

 

[engjg]

bookowski - I considered this as well, but wouldn't excessive cover on top lead to potential crack width control concerns in negative moment regions? I suppose this is one of my concerns with a fully adhered (composite) behavior...maybe less of an issue with a real roof covering but not sure about the liquid applied? Also not sure if you work under ACI 318, but if it were a one way slab this may require a tight spacing of reinforcement. I note unless stress in the steel was low, ACI 318-14 Table 24.3.2 would require a maximum spacing of only 2.5" for a max cover 5 in assuming fs=(2/3)fy, and although it does not appear a code requirement for two way slab, ACI 318-14 still appears to point to this section for consideration in the two way slab commentary.