What size beam for ceiling/attic floor


Old 08-30-14, 05:41 PM
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What size beam for ceiling/attic floor

Inside an existing large 30X32 foot metal building, I want to install a 23X21 foot self standing multipurpose room.
I do not want a post in the middle of the room so I can install a Billiard table.
I am looking at a single beam the length of the room with 10' and 11' joists on each side made of Douglas Fir 2X8s on 16" centers.
The walls will be made of 2X4s on 16" centers with a double top plate on top that the joists will line up with and rest above the studs.
I intend to use the area above the room for lite storage by placing plywood on top and insulating between the room's ceiling panels and plywood flooring. I estimate the dead weight for Joists and plywood at 1,600 pounds plus the weight of the beam chosen.
My question, what material and dimensions should the 23' beam be made up of to support the weight? If made of 2X12s bolted together, how many? A steel beam?
Any other suggestions?

Thank you.
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Old 08-30-14, 07:00 PM
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I would suggest you use engineered floor trusses for the entire span. Just like the floor in this thread.


You could also look into engineered i-joists.
Old 08-30-14, 09:14 PM
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About 22 years ago, I designed and built an addition for a pool table in my then-house. My plan required removing an entire load-bearing exterior wall, and replacing it with a built-up beam, hidden in the ceiling, and supported by square steel columns at the ends. My number-crunching came up with a triple 2 x 12 (glued and bolted) for spanning 19' total. Although my span was shorter than yours, I had exterior roof (and snow) loads factoring into mine, so your final beam design might be similar.

You would do well to hire a local engineer to work up the numbers and a plan for you. If for no other reason than to have documentation for when you need to sell the place, and buyers question the engineering that went into the room addition.
Old 08-30-14, 09:36 PM
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From what I'm seeing from the TJI span tables (based on L/360), the lightest grade of 11 7/8" TJI could span a little over 22' if placed on 12" centers. or a 14" TJI could span 21' if placed on 16" centers. I'd forget the beam, especially for the light load you are working with and probably get the appropriate series of TJI's for your span. JMO.

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