Pultruded Fiberglass Wide Flange Beam for Use as Mast Compression Post

New member here but longtime reader of this forum. Did a few searches but didn't come up with anything directly addressing this.
I have an existing mild steel square tube compression post 140mm x 140mm x 3mm wall with 10mm thick plates at top and bottom. The bottom is set directly in the bilge and is badly corroded approximately 30cm from the bottom. I wish to replace the post with a wide flange beam of either aluminum or pultruded fiberglass sized appropriately and with gusseted plates at top and bottom. In addition, I would raise the bearing surface from the bottom of the bilge approximately 20cm. The length of the post in this circumstance would be 1.93m.
Is a wide flange beam structural shape to be avoided? I've done some google searches and haven't found many examples of this shape being used for this application but I like the idea of using this shape, as it permits full inspection and allows for easier routing of wiring.
Is a pultruded fiberglass column to be avoided? This option would allow tabbing at top, bottom and also the sides of the post to the main bulkhead to add stability to the post. The post would also be non conductive and corrosion would not be an issue. In the case of aluminum, the effect of welding at the interface between the web/flanges/gussets and top and bottom plates will likely weaken the material in these areas. Using aluminum will also require additional measures to combat corrosion.
Look forward to reading any input on this and want to express my appreciation for all the insight I've gained reading posts on this forum!

 

What section of fiberglass are you planing on using? If it is structural, there should be published data for structural properties.

 

I doubt you will wish to do it once you get the final dimensions for a wide flanged beam that matches the existing square hollow section.
Have a new steel post made, galvanized and painted. Changing a post every 20 years is acceptable boat maintenance after all.
The real question here is why you don't have a dry bilge to begin with?

 

First question is who has a totally dry bilge?

Have you given thoughts to constructing your own I-beam?...perhaps slight different materials for the 3 components of that I-beam? ...some carbon fibers for the tension forces in the bottom flange, a better fiber for the compression forces in the top beam, etc

 

There is published data.
My preference is to stay close to the existing footprint of the bearing plates as originally designed. Existing plates are 160mm x 160mm. There is room to increase the size of these plates up to 210mm x 210mm.

If I were to duplicate the size of existing top and bottom bearing plates, easily sourced off the shelf options would be:
Pultruded 6" x 6" x 3/8" wide flange shape
Aluminum 5" x 5" x .313" H- beam 6061-T6 American Standard
Aluminum 6" x 5.938" x .250" H- beam 6061-T6 American Standard

Increasing the size of bearing plates:
Pultruded 8" x 8" x 3/8" wide flange shape
Aluminum 8" x 7.938" x .313" H-beam 6061-T6 American Standard

Unfortunately, I have not found available information on the mast. It is an older Francespar mast long out of production:
The profile of the mast is a symmetrical oval/elliptical shape 230mm x 150mm x 5mm wall thickness x 16.5m length
Double spreader
10mm forestay, backstay, cap shrouds, aft lowers, baby stay
8mm intermediate shrouds, staysail stay

The boat is a 14m "cutter rigged" Jeanneau Sun kiss 45
Approx. 102 sq. m upwind sail area, 182 sq. m downwind sail area
11,500 kg displacement, 4100 kg ballast

 

One side of the existing post had zero drainage in the bilge area. In addition, the bottom 30cm of the post was glasssed in. At some point there was water intrusion and nowhere for water to go. Also a concealed area mostly sealed up.

The beam is intended to be installed in a vertical orientation, in compression.

 

Thank you for the input. Steel is definitely an option as well.

One side of the existing post had zero drainage in the bilge area. In addition, the bottom 30cm of the post was glasssed in. At some point there was water intrusion and nowhere for water to go. Also a concealed area mostly sealed up.

 

I see some example calculations are in order. 140x140x3 square tube, L=2.13m, E=210GPa, Iy=503cm4, Pcr=9191.53kN, fixed ends. We need to match Pcr with aluminium, but E=72kN, we can't modify L, so Iy must increase, in this case to 1467cm4. Converted to imperial for american use = 35.2in4. The only Al I-beam that I can find over this Iy minimum is I 12x14.3 (depth x lbs/ft), that's 12in depth, 7in wide flange, 0.61in thick flanges and 0.31in thick web. The needed beam will weigh 99.8lbs, that's 37lbs more then the steel HSS it replaces.
I don't have any data for fiberglass beams, but you can usually get it from the manufacturer.

Don't use my calculations as gospel, if for example your Al alloy has E=68GPa the I12×14,3 beam isn't sufficient anymore. I did it just to show the problem, wich comes from the fact that I-beams have different moments on the two axes, and one is significantly lower. That's why most compression posts are round or square.
If your post was glassed in, cut it away, dress the platform, change the bolts and install a stainless or galvanized post with proper drainage. The alternatives are Al or carbon tubes, sized to match the existing steel one. Lastly, let's not forget that your model ended production in 1988, the original "bad" installation had a service life of minimum 35 years. Give it modern paint and drainage and it will last another 40+ even without galvanizing.

 

Thank you for the generous reply! One reason for using the h-beam shape I did not mention is that bolting of the post top and bottom would be possible if duplicating the size of the existing top and bottom load bearing surfaces. The existing post is not bolted, only tabbed in at top and bottom. I would need to add additional area to the plates at the top and bottom to accommodate the bolts.
Please forgive my ignorance but may I ask what formulae is used above to determine Pcr? Pcr=critical buckling load or maximum axial load prior to buckling?

 

Euler's critical load - Wikipedia https://en.m.wikipedia.org/wiki/Euler%27s_critical_load You can find online calculators for this, Iy of standard sections is published, just have to comb through the tables, E is material dependent.

For the bolts, glass in a thick fiberglass plate 2in wider then the beam, make matching metal endplates for the beam, bolt together. At the lower end, the flange can overhang the existing pedestal (what remains after you remove the existing steel), it doesn't affect the load path. At the top you can bolt directly trough the deck, no need for glasswork if you don't want to.

Really paranoid people don't use drainage holes at all, they weld the endcaps on, drill a small hole and and spray cosmolene (or other rust inhibitors) inside, then weld or epoxy the hole shut. On the outside the endplates and pedestals are beveled so that any water coming down the column can easily run off. If wiring has to go trough the beam, a tube is installed with its ends properly sealed to it. Endplates are bedded to the boat with flexible goo, but only after thickened epoxy made sure to mate the pieces perfectly. It's a lot of work, and the benefits only show after the vessels life expectancy has expired anyway, so most yards can't be bothered to do it.
Anyway, your post probably died from water getting between the glass and the steel, it's not a problem that can't be solved a little easier if you like to replicate the factory solution.

 

Several of the online calculators I've used have opted to give results using the Johnson formula, given the input data for the existing post. The result for critical load seems significantly less than using the pure Euler formula and in contrast to your result. Is it inappropriate to use the Johnson formula as long as it's used to evaluate both the existing post and replacement?

 

Euler is used for slender columns, like what you have, which fail in buckling. If the post is against a bulkhead, a center support would allow a smaller section post.