I'm trying to install an electric hoist (70 pounds) to the wood beam at the ceiling I'd like to know how much weight this structure can stand.
I attach an image to give a better idea of my project.
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You didn’t answer if the beam is 4x15 or 6x15.
A 4x15 that is 33’ long will support about 4,500 lbs. placed near the midpoint of the beam. A 6x15 that is 33’ long will support about 6,000 lbs. placed near the midpoint of the beam.
If the beam already supports the roof, then (depending on snow loads, type of roofing, etc.) and depending on how far it is to the next wall (I’m guessing about 12’ on each side of the beam) then the beam is already supporting about 30 lbs. per square foot (live load and dead load) times 12’ (6’ on each side) = 360 lbs. per linear foot along the beam. If the beam is 33’ long , then 360plf x 33’ = 11,880 lbs. it is already supporting.
This shows the beam is over stressed or just decorative.
Edit: If the beam is 6x15 and no snow, then the existing total load on the beam is about 10psf x 12’ x 33’ = 3960 lbs. If the maximum allowable is about 6,000 lbs., then there is about 2,000 lbs. the beam could support. (Verify that my guess of about 12’ from each side of the beam to the adjacent wall on each side of the beam.)
So, if my assumptions about the distance to the adjacent wall is correct, then you could add a small load (hoist) to the beam.
Load capacity of a wooden beam is based on material (type & condition eg. moisture content, temperature, density), size & length.
For eg. in the given table below for some manufacturer:
max beam span for Southern yellow pine on 2x6 beam size is 7' for a deck width of 4' to support 10 lb dead weight.
Note: Perimeter beams carry half the weight for the same given size.