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:
Point load or distributed load?
GIven the ineffective orientation it may be that the deflection of the beam 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 tension, starting near the middle of the lower surface, uness a point load crushes fibers in the upper sruface sufficiently to precipitate compressive failure.
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 Doug Fir.
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 that species.
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 day...
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 with margin.
Thanks again for the great input!
Fredfighter,
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 hem-fir. 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?
No, what gave you that idea?
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