Building Homes or Extensions/bed loft weight limit
I live in a heritage building with very narrow rooms and high ceilings. The dimensions of the room are 8'4" x 14'.I would like to build a partial platform/loft in the bedroom over top of the murphy bed that is set up on one wall. Closed, the "Wellington" murphy bed looks like a big wardrobe cabinet. The frame is 3/4" solid wood, bolted to the wall. The dimensions of the murphy bed are 16" deep by 8 feet long by 70.5 inches tall. So the surface area on the top is 10.66 sqft.
I plan to make the platform span the width of the room, using for the main pillars the murphy bed cabinet as one support on one wall and two bookshelves (solid wood 16"d x 2'w) on the other wall as the other pillars. On the tops of the murphy bed frame and the 2 bookcases I will first make a 2 x 4 frame that will span the 8 feet along the wall, and two feet wide. I will add a support nailed to the wall between the two bookshelves. On top of this I will screw or nail 2x4s which will span the room 8 feet 4". I will then lay 2 4x8' 1/2" plywood sheets over the 2x4s. I will affix 12" plywood sides to the platform. The area of the platform will be 8 x 8'4",(66.3 sqft) and on each side the platform will be supported by the 16" wide cabinets; the span across the middle wall to wall will be 68 inches, or 5.7'.
What is the weight limit of this platform?
Donna, this sounds interesting. I don't know that I would depend on the bookcase or the bed case as true load bearing members. I would prefer suggesting pinning your 2x4 framing to the walls.
If you make the framing members from 2x6 and assume the 8-4 span, you could easily carry 80 P/SF which would allow 5000 lbs. overhead. The span tables really don't like doing the loft in 2x4, but it does look like you could make 20 P/SF which would allow about 1300 pounds.
I don't quite understand what the 12" plywood sides do, but they could increase the load bearing capacity of the outer members tremendously if glued and adequately fastened.
I hope this gives you the information that you need.