What load would a 2" x 4' x 8' sheet of pink XPS foam wrapped in fiberglass using epoxy resin support if used as an 8' bridge?

Question

I need to build a light weight ramp roughly 4' wide by 8' long. I've built similar out of 1/2" plywood and 2x4s. That's plenty strong but much too heavy. If I wrapped a 4' x 8' sheet of 2" thick pink XPS foam in fiberglass using epoxy resin how strong would it be? Assume an 8' horizontal span. How many layers of fiberglass to support a 250 lb person walking across? Would vacuum bagging be necessary? I know it would be better but I don't have the equipment. This is definitely not life critical but I'd like it to hold up.

Answer 1

Amateur built aircraft have used for years the method of sculpting a wing shape from foam and bonding fiberglass to it. Typically, vacuum bagging is used to ensure complete penetration of the resin into the fabric. It also requires careful calculation of mix weight versus fabric volume, but this is to ensure minimum weight with maximum strength.

You can use a similar process and embed a spar or two within the structure which will help distribute the load over the entire panel. If you picture two two-by-four pieces of lumber of eight foot length, it's easy to expect that it will handle a few hundred pounds. Adding foam and fiberglass with good bonding to the wood is going to make for an even stronger panel. Keep in mind that a common two-by-four is not two inches thick. Some adjustment may be necessary, but I suspect that your panel thickness would be sufficiently strong at an inch and a half. I can stand on an eight foot pine plank with a bit of bounce, but if there was foam and 'glass, it would be quite stiff.

Answer 2

A foam board has almost zero tensile strength, but it would have sufficient compressive strength. For example Owens Corning Foamular NGX250 has a compressive strength of 25psi. Fiberglass has impressive tensile strength (3400MPa) and specific strength (1307 kN.m/Kg) link , the problem may be flexing. Even ¼ inch fiberglass panels bend very easily.

That said modern freezers are built out of foam panels with just a thin sheet metal outer. These are structural members and are often used to build freezers over 20 feet high, where they support the roof. (To make these the foam is expanded between the panels, so adheres to them completely)

You should not need more than 1/16 inch thickness of fiberglass if you ensure that it completely wraps the foam panel. The edges are essential!

As Tiger Guy mentions, you might need bracing. If the panels covered in fiberglass flex too much, what you could do is to add 3/4” by 2” plywood strips (or something similar) that are embedded in the fiberglass on the bottom of the panels, maybe 12” apart (the 2" standing vertical, not laying flat). These can be added if necessary, bonding them to the fiberglass sheet. Just remember that you need to roughen any fiberglass that has hardened before bonding new glass on top.

Answer 3

A 250lbs person's footstep landing at the end of a stride can be twice his weight. Its dynamic load factor can be many times the weight of the person.

Think of a heavyset man briskly walking on work boots! Or a woman in sharp pumps holding a heavy bag of groceries or an infant. Unless It is for your exclusive use and nobody else has access, the surface has to be thick and tough to take the impact.

Just a reminder, In California, the code for static load for decks or balconies is 100 psf.

Also, traffic near the edge of the ramp creates more bending moments. All these factors impact the design.