FRP and Concrete Design (AS3600)

[asixth]

Hi guys

I have been given the task of designing with and developing design procedures with externally bonded FRP reinforcement. This will be an on-going thing that I am responsible for and hopefully it will provide me with a great Career Episode that EngAust recognises. I have a few questions about this that I would like to ask so if anyone can help with the answer to one of the questions than please do so.

Q1. Does anyone know of what requirements I need to satisfy when designing externally bonded FRP reinforcement? I have a document called HB305-2008 "Design Handbook for RC Structures Retrofitted with FRP and Metal Plates: Beams and Slabs". This document was put out by code committee BD-101 "Retrofitting by Plating". Does anyone know how well this comittee ties in with AS3600 and whether any future revisons will be made to co-ordinate these two comittees?

Q2. What would be the industry best practice document for designing externally bonded FRP concrete structures? The documents that I have currently are:

1. TR55 "Design Guidance for Strengthening Concrete Structures Using Fibre Composite Materials" by the Concrete Society (European). This has been the best document that I have used to date with good interaction with British Standards however I have come across some points in the book that I don't agree with from a engineering viewpoint.

2. Bulletin 14 "Externally Bonded FRP Reinforcement for RC Structures" by CEB-FIP (European). Again this is another very technical document that I am going to have to get familar with. It is also the basis of the Sika Engineering Software.

3. HB305-2008 that I mentioned above.

I was planning on going down to my old University tomorrow before work and getting a copy of ACI440.2 hoping that it will prove to be a more helpful document. But again, if anyone can provide a reference of the documents that they are using for their design it will be very much appreciated.

Q3. I am putting together a document that can calculate the increase in flexural capacity that is attained by providing FRP reinforcement. Looking through Technical Report 55 by the Concrete Society, it is incorporating material reduction factors to all materails in a similar way that AS3600 would provided a reduction factor of 0.8 to both the conrete and steel stress provided that steel yielding failure occurs first.

For my design (multilevel carpark, stressed, 2.5kPa live load), I want to strengthen the section for flexure and I am going to ignore any flexural capacity that is obtained by any tensile steel (relying solely on the FRP). I have found in one of the documents that the strains of the FRP should never get within 45% of their rupture strains. The rupture strain is 0.017, for design I will never allow the strain in the FRP to exceed 0.00765. If I say the ultimate strain in the compressive concrete block is 0.003 (AS3600 Cl 8.1.2.1.d) and I reduce the uniform concrete stress from 0.85*fc (AS3600 Cl 8.1.2.2) to whatever is required to balance the FRP tension, this stress reduction would be akin to providing a material reduction factor set-out in Table 2.3, only much more severe (0.3 opposed to 0.8). Please look at the digram I have provided an comment on the validity of my approach.

I know this only calculates the increase in flexural capacity and I will need to look at the re-distribution of stresses (and therefore the increase in flexural strains) and increase in strains in the FRP that are a result from the concrete creeping. As well as the transverse stresses along the epoxy interface and debonding of the FRP which may govern the design over the FRP strains.

Any comments will be appreciated.

 

[rapt]

Asixth,

Looks like the same list of references I used as a basis for the logic for FRP design included in the latest version of RAPT!

There seem to be a lot of unresolved areas of FRP design at the moment. Bond and development length logic is still very undefined.

We have handled the different material factor problem by including the option of different material factors and different capacity reduction factors for each reinforcing material, basically making the capacity reduction factor version work like a material factor logic.

Why would you ignore the other tensile reinfoircement? It is there, is in tension and is contributing and will not fracture before the FRP sheeting in most cases.

You cannot assume that the concrete strain goes from 0 for FRP design. You need to consider the strain in the concrete at which the FRP is attached!

 

[csd72]

rapt makes a very good point! Unless you are going to jack the beam up to zero stress then the reinforcement is already taking the dead load.

I used to work with a guy who was pretty hot with FRP so I will see what reference he used.

As one of my previous bosses once said to me - this is real engineering! You have to really understand the basis of the codes you use so you can compare this to the factors of safety in the references.

As for the EngAust career episodes, I used oridinary projects and just picked ones that showed the necessary points (one of them covered 4 points). If you want any more help on theis then just ask.