Using Steel and FRP rebar in the same structure 1

[blou1]

Good day everyone,
I am working on a project, in which the design was a 10" concrete wall, with 60ksi steel rebar verticals and horizontals.
doing an as-built for the project, and we discover that due to material shortages and costs, the contractor took it upon themselves to substitute all the horizontal rebar for the structure (temperature and shrinkage reinforcement) with FRP rebar... in the floor and the walls.

so this is what happened. now I am being asked to determine after the fact if it meets code, and what remedies can be done to correct if need be. But I am not quite sure where to tackle this.
Floor was intended to be 60ksi steel #4's @ 18" OCEW, and was replaced with FRP #3's @ 18" OCEW.
Horizontal reinforcement in the walls was intended to be 60ksi steel #4's @ 10" OC, and was replaced with FRP #3's @ 18" OC.
Vertical reinforcement in the walls was intended and placed as 60ksi steel #5's @ 10" OC.
Corner splices were done with #4's 60ksi steel corners lapping with the horizontal wall FRP #3's.

where do I start/stop analyzing this?

Thanks,
Blou

 

[phamENG]

Start at the beginning and stop at the end. Analyze the entire thing. And charge a retainer equal to 150% of what you think it'll take. This is probably going to be a nightmare.

 

[canwesteng]

First stop - review the concrete for cracking after curing (assuming this has been cast already). Next would be to check the minimums in ACI 440 for t+s bar and skim the code for anything else that might apply. You are fortunate that the bar wasn't the main flexural or tension bar. The floor (assuming slab on grade?) is probably the easiest to check. I'm not aware of a code or recommendations for mix and match rebar like this, but assuming this is a bearing wall and not a shear wall I don't see a big issue.

 

[wrantler]

Before you point too hard at the contractor I would ask if they can prove out their bar substitution. They may have gotten the FRP supplier to redesign the wall using their bars...this would make life easy.

FRP comes in a few grades. My local supplier sells 45GPa,50GPa,60GPa.

 

[KootK]

For T&S, I don't think that you need to do anything other than verify that the strength and stiffness of the substituted bars meets or exceeds that of the required T&S steel bars. That would be an easy out if it works.

First stop - review the concrete for cracking after curing (assuming this has been cast already).

Further to that statement, if shrinkage has substantially occurred and objectionable cracking has not materialized, one could argue that, whatever was provided for T&S, it was enough. The point of T&S reinforcing is performance, and the quantity is not inviolate if performance can be achieved. ICF structures are routinely built with much less that code T&S reinforcing using similar logic.

If this is one of those "the owner must have what they paid for" things, then have a conversation with the owner to see how they want to proceed.

 

[bugbus]

It's not a simple area-for-area substitute for crack control reinforcement since the FRP stuff is about 1/3 as stiff as steel and will probably give you far wider cracks.

If all the reinforcement were substituted for FRP, you could probably tolerate the wider cracks on the basis that there is no longer a corrosion risk. But you vertical bars are still steel.

 

[EngDM]

FRP also has different strain tendencies you'd want to look into. I'll see about posting any University notes I have (assuming they aren't copywrited). If not I can look to see some relevant sections. IIRC the deflection calculations for FRP is a nightmare. If the FRP was only used in the walls and slab (hopefully on grade) then you might have lucked out.