CSA S806-12 (Canadian FRP-Reinforced Concrete): Factored Resistance

[Settingsun]

Hi all,
I'm starting to look at FRP-reinforced concrete and the specified standard is CSA S806-12. I don't understand how the factored bending resistance is intended to be calculated if a concrete stress-strain curve is used per clause 8.4.1.6.

In clause 6.5.1, I think the first paragraph says that (generally) an overall capacity reduction factor is applied to the nominal (best estimate) section strength. The second paragraph then says that partial material factors are to be used instead where specified. However, I could alternatively read both paragraphs as saying that partial material factors are used, but with the second paragraph drawing attention to the additional phi.m factor from CSA A23.3.

Clause 8.4.1.5 (rectangular stress block) has the phi.c factor built into the concrete stress, so is a partial material factor. The rectangular stress block is however optional ("may" be used). Clause 8.4.1.6 says that concrete stress shall be based on stress-strain curves that are representative of the concrete used. Again not clear to me whether "based on" means to use curves that are accurate (which would rule out partial material factors, therefore implying an overall factor), or if it means that a partial material factor should be applied to the accurate curve.

On balance, I lean towards partial material factors being the intention and I understand that the Canadian code for steel-reinforced concrete uses this, but the wording is just unclear to me and I have a nagging doubt that the very-different behaviour of FRP might have warranted a different approach.

 

[TehMightyEngineer]

I'm not familiar with CSA S806 but if you're using fiberglass rebar I could run one of your sections through a spreadsheet I made for ACI 440 and we can compare final strengths and see if you're close. If you're using FRP sheets or wrap I haven't got any US tools setup for that.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Settingsun]

Thanks for the offer, TME. I am using reinforcing bar.

I've also done some more reading and partial material factors are the way to go. Clause 6.5.3.2 sets it out: "The factored concrete resistance used in checking ultimate limit states shall be taken as phi.c*f'c..." Though it could stand to be fleshed out similar to Eurocode 2 IMO. It also has some clumsy (wrong) wording around an exception for certain precast elements whereas the corresponding clause in the steel-reinforced code A23.3 is worded correctly.

So the wording in the first paragraph of 6.5.1 of S806 is misleading:
"The factored resistance of a member, its cross-sections, and its connections shall be taken as the resistance calculated in accordance with the requirements and assumptions of this Standard, multiplied by the appropriate material resistance factors."

Again however, A23.3 is worded unambiguously:
"The factored resistance of a member, its cross-sections, and its connections shall be taken as the resistance calculated as specified in this Standard, using the material resistance factors specified in Clauses 8.4.2 and 8.4.3."

Not sure why you wouldn't just copy the wording from the established code; bit embarrassing to change it for no good reason and get it wrong twice.