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FRP (plastic bridge)

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Stan Scholl and Christopher Wright wrote:

">It was easy- cement was a penny a pound and plastics were a dollar a 
>pound. Perhaps this simplistic example still has some value in 
>evaluating the future use of FRP.

It's a little bit over-simplistic--it implies that my Saturn would have 
been a more economical vehicle if the door panels were concrete." 

There's a little more to it than that.  Obviously, extra weight isn't
good in a car, but it usually helps a bridge.   I had a big problem with
some FRP beams about 10 years ago.  I personally will never again use
FRP in a situation where they have to carry significant dead load (or,
for that matter, where there's any way to use anything else, like

Some of the reasons to exercise caution in using FRP:

1. The pultruded beams have very little shear strength.  At least 10
years ago, the manufacturers weren't particularly forthcoming about this
little problem.  Maybe they are now.

2. The stuff creeps under significant dead load.

3. Deflection is almost always a problem, as modulus of elasticity is
roughly 10 percent that of steel.

4. Since it's so light, vibration is a concern.

5. Connections aren't as fail-safe or well understood as in steel beams,
but they tend to get detailed as though they were steel.

6. Ultraviolet degradation is a concern.

I know Chris was typing tongue-in-cheek, as it were, but I've never
considered FRP a suitable replacement for steel, concrete, and timber in
static structures.  It may be fine in things like cars, where weight
matters a lot more, they make a zillion of them all the same, they test
prototype after prototype, they sell better if they have curves in them,
and 8 years later we throw them away.

For what it's worth.

Mike Hemstad, P.E.
St. Paul, Minnesota

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