A gentleman asked me with all things being equal such as grade, loads and use. What size post of Western Red Cedar would it take to equal a 6x8 piece of Hem/Fir?

I don't think that can be answered as asked, a post of some specific dimension interchangeable for any situation of load and effective length. The effective length of the column versus the stiffness of the species and grade is a big variable that makes a simple blanket substitution impossible I think.

I've tried to work a stripped down example below. This would be a pin/pin connected column with only an axial load, a simple post. I worked the problem with a 6x8 hemfir post 10 feet long and maxed it out at about 22,000 lbs. You can back out and re-enter the desired dimensions or species to find a substitution and get a feel for the variables.

http://windyhilllogworks.com/Calcs/simplecolumn.htm

Hopefully this will help explain why a simple answer escapes me.

Hi Colin & Don,

Most timber sizes are driven by geometry and joint accomodation considerations. The simpler a frame becomes i.e. where no second path load sharing exists then the more critical timber sizing and maybe species selection becomes. Probably the main consideration for timber species selection is that of durability i.e resistance to both fungal decay and insect attack and in this respect Western Red Cedar is quite a good choice. It does however suffer from being much less dense than other timbers like Douglas Fir and hence is less strong. Like oak, WRC will cleave (rive) easily and hence it has poor horizontal sheer properties and also it is more than a little brittle so maybe a good choice for easily replaceable or decay vulnerable components like rafters, wall studs, sills, nailers, cladding, lath, shingles, etc. Its a bit soft, weak and splintery for use as flooring and not a good choice for heavily loaded unsupported beams.

To answer this question possibly one could look to nature and compare the size & girth of WRC versus DF as they grow side by side in the forest since both trees are subject to much the same conditions.

Regards

Ken Hume
p.s. edit's back !