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I'm looking to build a treehouse and wanting to ensure the support beams can carry a sufficient load over an 11-ft span. Based on page 78 (aka 83) of https://awc.org/wp-content/uploads/2021/12/AWC-WSDD1986-ViewOnly-0301.pdf, it seems like each 4x6 could carry between 962 and 2138 lbs of an evenly distributed load; however, I don't know what value of Fb to use for southern yellow pine so don't know which column applies.

I considered just assuming the weakest possible value but don't want to use more beams than needed as that increases the weight.

Here's the specific beam which is planned to be used https://www.homedepot.com/p/4-in-x-6-in-x-16-ft-2-Pressure-Treated-Ground-Contact-Southern-Pine-Timber-128619/100016690

There will be perpendicular joists which rest on top of these for the treehouse platform / structure (10x10 enclosed), but this is in reference to the base support beams.

Based on http://www.southernpine.com/app/uploads/TABLES_1-2_USE.pdf it appears that Fb could range from 575 to 2400, so do I perhaps need to go in to the store and see if the label on the beam says what grade it is and then go back to this table and/or contact the manufacturer? Or if not, how should I approach this?

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    $\begingroup$ I'm a tad rusty on my wood code, but if I recall correctly there are various species as well as grades of lumber that will give you the various material properties that you will need to design with. S-P-F (Spruce-Pine-Fir) comes in at least 3 grades: Select Structural, No 1, and No 2. I doubt Select Structural will be carried at your local Home Depot but I could be wrong. Since you do not want to design based on the worst case you will need to find out the grade of the wood being sold so you know what parameters to use. Alternatively, design based on each grade and see if its difference $\endgroup$
    – Forward Ed
    Jul 31 at 7:01

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There is a code method to determine the allowable bending stress. Published values of Fb should be adjusted by appropriate adjustment factors.

You can find all of this in the latest editions of the NDS and the NDS Supplement. Free download from AWC.

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