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Monday, September 28, 2009

Vertical loads and GFRP

Researchers at the University of Miami are studying and testing the behavior of columns reinforced with glass fiber reinforced polymer bars, GFRP. According to the researchers, “The use of GFRP bars as compression reinforcement may be allowed when design is for vertical loads only."

Any academic study is very much welcomed when it is not precluded by a feasibility study that relates to financial exposures and actual applicability.

There are a few realities that should be considered:

  • No design for vertical loads only exists, or if it does it is purely scholastic.
  • Even in the academic world, when a laboratory test for vertical loads is performed, the eccentricity will exist and moments applied.
  • In the real world, columns loaded only with vertical forces simply do not exist.

The University of Miami's academic efforts are to be lauded, but do leave room for discussion of applicability. Where are they going with this?

2 comments:

  1. Reginald Fequiere, M. Sc., P.E.October 7, 2009 at 7:09 PM

    The benefit of using GRFP bars would the longevity of the structure. With GFRP bars there would be no concern of corrosion induced by environmental effects. The key here is to have GRFP bars that present some ductility.
    The ACI's design approach requires that elements be reinforced in such a way that would allow for significant deformation prior to collapse. This approach guarantees that people will see the signs of imminent failure and will have sufficient time to evacuate.
    The deformations mentioned above are only possible when a ductile material such as steel is used. Reinforcing steel is able to sustain significant deformations before breaking. This is possible due to the modulus of elasticity of the steel which is 29,000,000 Psi. In the case of GFRP bars the modulus is much lower thus not providing the benefit of plastic deformation afforded by steel.
    That is why they are not efficient in columns that are subject to axial and bending forces.
    GFRP has been used for flexural design purpose in the past but due to the ductility issue it's full capacity cannot be used, the deformations in the FRP have to limited and thus only a fraction the tensile capacity is considered.
    In summary as the technology for GFRP bars stand they are not economically competitive with steel. The only projects that would justify the use of such a material would be one in an extremely aggressive environment that requires a great deal of longevity. However there already exists a material that offers the longevity benefit and that present great ductility and its called MMFX steel. It is reinforcing that has been engineered to resist corrosion more effectively than regular steel and it has tensile capacity of 93 Ksi compared to the 60 Ksi of regular steel.
    Unless UM can make FRP more ductile it's main use will still remain that of retrofitting buildings and building stealth fighters.
    Please find attached article on FRP design it will put everything in perspective.
    Regards,
    Reginald Fequiere, M. Sc., P.E.

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