Steel Design to Structural Fiberglass 1

I would be interested to know the answer too. However I am sceptical about the full substitution of steel sections by fibreglass sections in 2003.

You do get good strength from fibreglass or even carbon laminates but they must use resin as a binder. Resin can't stand heat! In fact the method of verifying the fibreglass content is to heat the sample in an over to a set temperature and weigh the fibre that left behind. People need to think of the fire risk if fibreglass sections are the main structural components in a building. From memory the loss of ignition test temperature is about 600 degree C.

Resin is a plastic and that means it does not have a large elastic working range like steel. The elastic modulus must correspond to the stress range otherwise it is pointless.

Perhaps the showstopper is the method of connection. In order to ensure a full stress distribution, say a moment connection, all fibreglass products I know of have to be joined together by a wet lay-up process (using resin and fibreglass mats). Bolts and welding don't do much for them.

The strength of a fibreglass product is primarily dependent on the percentage and orientation of the fibreglass in the resin matrix and any design code must address this feature.

I always recommend people to think fibreglass products as reinforced concrete, with the concrete and reinforcing bars replaced by resin and fibreglass strands (or filaments).

Fibreglass (and carbon fibre) products have been used selectively in many outdoor structural applications because of its high strength and chemical resistance. I will be surprised if it has got as far as a direct substitute for steel in a building.

Where there is no fire risk I can see a future for people bonding fibreglass (or carbon fibre) locally to steel sections to increase the load carrying capacities. After all we are doing it everyday with our bridges, aren't we?

The design of the bonded fibreglass to steel (or to concrete) follows the same fundamental structural assumptions as reinforcing steel in concrete, except the section may not be circular and the a new set of elastic modulus and Poisson ratio (direction sensitive!) has to be adopted.

auxblade,

My experience designing with fiberglass elements, both hand-layon and structural extruded shapes, is limited to mechanical equipment as tanks, chimneys, etc.

The degradation of the mechanical properties of the fiberglass with temperature and exposure to the environment need to be factored in the design. These disadvantages make its use questionable as a main structural element in buildings. Does fiberglass pass the Code fire protection requirements? Could the member be replaced easily if needed?

Going back to your question. Yes the section properties can be used to calculate stresses and deflection. The big question there is what are the allowable stresses, the failure mode, and the modulus of elasticity. Both of them are reduced considerably when the temperature rises, and the material become brittle when the temperature drops close to freezing.

There is a design manual available from Extren with design information about allowable loads, safety factors, connections, etc. You can get it at: