How can I determine or lookup the load ratings or the load failure point for
a fir plank that is 2 (1.5)" x 12 (11)" x 40"? The plank is oriented widest
side up and level and the load is against the flat side while being
supported with 1.5" ledges on each side.
thank for any tips
Try one of these:
If I read your post correctly you have a 2x12 being used as a shelf.
If this is correct, probably none of thes will give you what you want
to know but may point you in the right direction.
The Sagulator I think was a good indication. I plugged in the values for
Douglas Fir, used 600lbs load (3x the expected), load concentrated in
center (vs distributed)......and it came up with a 0.1 inch deflection. The
sag limit of L/360 is 0.11. So that sounds like it should be plenty strong
Thanks again for the great input!
Point load or distributed load?
GIven the ineffective orientation it may be that the deflection of the
will be unacceptable long before it breaks, unless you are intentionlly
using it as a spring. If it is loaded to failure, it will fail in
near the middle of the lower surface, uness a point load crushes
fibers in the upper sruface sufficiently to precipitate compressive
Regardless, it is a simple rectangular beam. You can look up formulas
online for calculating the maximum tensile stres at the extereme fiber
abd the deflection at the center.
Is it construction lumber? 'Fir' is ambiguous. It can be hem-fir or
You can look up the minimum strength in bending for hem-fir online,
as well as the modulus of rigidity (Young's modulus).
Then use the formulas for a rectangular beam to determine how much
it will deflect under the intended load and your factor of safety.
If it is Douglas Fir, then use the minimum values for the species group
Doug Fir or Larch.
If you know the particular species, then use the minimum values for
It is doubtful you will find useful values in any look-up table as no
one uses beams in that orientation for obvious reasons. One
wonders if you should not reconsider what you are doing--will
this be a ramp?
Now, if you want to know what time is is you start by scribing a circle
on the ground out where it will be indirect sunlight for most of the
2x12 doug fir is plenty strong. A 40" length of it should be able to hold a
few hundred pounds without breaking.
Compressive strength is high too. It would have to be a particularly nasty
point load to crush it enough to make it fail.
Or are you just trying to scare him?
Agreed, though it is not clear that this is doug fir. It may be
Regardless, it will bend a lot before it breaks.
It would be about 70 times stronger and 40 times stiffer used on edge.
(back of an envelope calculation late a night), right?
Yes, though we have no idea how it will be loaded or how much.
We don't know the application.
We do know that to determine a factor of safety we compare
the load that will cause failure to the design load, right?
To do that, you need to know how it would fail, right?
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.