FRP Column Buckling

[TDC84]

I'm analyzing a beam-column according to ASCE's Standard for Pultruded FRP Structures. The material has a flexural modulus of ~4,300 ksi and a compressive modulus of ~2,100 ksi. Given the mechanics of buckling (axial load + lateral deflection -> bending -> more deflection ->more bending) it seems like the flexural modulus should govern buckling. The standard specifies that compressive modulus should be used. Does anyone have insight on why compressive modulus is used?

 

[JoelTXCive]

I believe the flexural modulus is used when the FRP can only fail in tension. For example, you have standard beam with FRP bars at the bottom of the beam operating as the tension element.

For a beam column, the moment creates a tension side and compression side. Therefore some of the FRP will be in compression.

 

[TDC84]

This is not a reinforced concrete beam with fiberglass rebar. The full section is FRP.

The question is that since Euler buckling is caused by a positive feedback loop involving increased bending, why the bending modulus isn't the specified modulus used in design.

I have an inkling that for most commercial FRP sections there is only a small difference in E_compression and E_flexur, so it wouldn't really matter. However, in my case it's a factor of 2. So, I'd like to get a better handle on why E_compression is used.

Hope this explanation is more clear.