wooden spar strength compared to aluminum [Archive]

chrisk

08-20-2004, 04:04 PM

Can anyone point me to some info on how to compare aluminum tube strength to hollow wooden spar strength?

Essentially, I've got a 2" diamteter by 1/4" thick aluminum tube and I'm wondering how to get the same strength using a hollow wood spar potentially sheathed in fiberglass. What type of wood, what diameter and thickness and how many wraps of what weight of fiberglass cloth would be equivalent to the aluminum tube.

Any references on how to figure that out is really what I am looking for as I've got a couple of different tube sizes I am looking at swapping out. This is not for a mast or a boom, just a load bearing bracket.

Chris Kottaridis (chrisk@quietwind.net)

mmd

08-21-2004, 12:59 AM

Oh, dear. One of "those" questions again! :eek: :D

The answer is.... (wait for it) .... It depends! :rolleyes:

It depends on what alloy of aluminum your pipe is.
It depends on whether you want the wood spar to be the same OD as the aluminum pipe.
It depends on what type of wood you plan to use.
It depends on what type of 'glass fabric you plan to use.
It depends on the type of epoxy you plan to use.
It depends on the weave orientation in relation to the spar axis you lay up.
It depends on whether the spar is loaded axially or transversally.
If loaded transversally, it depends on whether it is a point load or evenly distributed load.
It depends on whether the load is static or dynamic.

Tell me more about how and what you want to use the stick for, and I'll try to be more of a help than a pain in the butt. In the meantime, Google "BeamBoy", download and install the program, and noodle about with it to get a grip on the parameters needed to define your question. An introductory book on properties of materials would be good, too. Any bookstore that sells Schaum's books (high school and college fast-track softcover textbooks, usually $12 - $20 each) will have a copy.

Basically, what you need to do is determine the sectional modulus of the pipe and the bending moment imparted by the load. From this you calculate the stress in the extreme fibre of the pipe. With this information in hand, you revise the bending moment equations to enable you to calculate the required diameter so that the extreme fiber stress is less than the allowable tensile strength of the fibres per square inch that are oriented parallel to the spar axis (or the effective fibres per suare inch, if the fibres are laid up on the bias) and the allowable shear stress parallel to the grain of the wood you are planning to use, and less than the shear strength of the epoxy-to-wood bond.

Please don't ask about stress risers in the structure. ;)