Ash Seats - How Strong?

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Ash Seats - How Strong?

Post by ealger » Mon Oct 24, 2005 11:33 pm

I'm not an expert in strength of materials, and thus disclaim the fitness of these calculations for any purpose :eyebrows

With this formula: S = ((PL)/4) / ((b(hxh))/6)
Solve for the load P to fail a beam where:
S = stress in psi
P = force in pounds
L = span of beam in inches
b = the base of the beam (width of the seat arm) in inches
h = the height of the beam ( height of the arm) in inches

Assume that white ash has a strength of 12,700 psi. (tables vary from 7000 to 15000 psi!)
Assume the width to be 1.5"
Assume the height to be 7/8"
Assume the arm length to be 30" between suspension bolts.
The load calculation is 324 lbs.

Increasing the width to 1.75" increases the load to failure of 432 lbs.

This assumes a load to be in the center of the arm as when a person was to stand on it in the center. (An abnormal act in a canoe!)

This is all well and good until we drill a bunch of holes in which to weave cane!
Now if we substitute white oak with a strength of 15,200 psi, we get a higher load bearing capibility.

The strength of the material depends how dry, or green, the material is as well as its grain etc. These calculations consider a static load. An appropiate safety margin should be considered for dynamic loading!

Ed Alger

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Post by BillB » Tue Oct 25, 2005 4:45 am

Ealger and Riverrats calculations and Riverrats real life experience all support a decision to make the new seat. I'll probably go with the 1-1/4" ash and fiberglass the underside. Thanks to all for your input. Bill.

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Fancy Pansy...

Post by Woodchuck » Tue Oct 25, 2005 9:21 am

I would laminate the seat rails and then bevel the underside to a vee shape without going to a point. That way you would save the appearance issue from the front or rear and have the majority of the increased load carrying capacity. Maybe even a aluminum T bar epoxied underneath... Just thinking...
Joe "Woodchuck" Gledhill
Garden City, MI

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