Size of beam to hold sagging ceiling?

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1OLDREMODELER
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I need some advice, please:
I am remodelling a one story, gable roof, wood construction, cabin built in the 50's. The ceiling has sagged about 1-3/4" across a 17 foot unsupported span at the center of the cabin.
I would like to know what size of a steel or aluminum beam you would recommend to keep the ceiling from sagging any further. Some limiting factors include:
1) Unfortunately the ceiling is low (about 92") to begin with, and
2) There is a nice view that I would like to remain unobstructed, hence the beam would need to span 17 feet with posts at the ends only - none in the middle.
What other info do you need to help me out?
Thanks,
Dave
 
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1OLDREMODELER: Could you give us the cross-sectional dimensions, and material, of your current 5.2 m (17 ft) sagging beam?
 
Here is a handy span calculator if you were to go with wood:

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Mixed+Southern+Pine&size=2x12&grade=No.+1&member=Ceiling+Joists&deflectionlimit=L%2F360&spacing=24&wet=No&incised=No&liveload=20&snowload=-1&deadload=10&submit=Calculate+Maximum+Horizontal+Span
 
NVN,
THAT'S THE PROBLEM - THERE IS NO BEAM THERE NOW.
Dave
 
Fred-
Do you have a calculator for beams?
Thanks,
Dave
 
How many joists do you have running perpendicular to this proposed beam, and what are the joist cross-sectional dimensions, material, and length (width of room)? I am assuming these joists are the bottom, horizontal member of trusses, right? How many trusses (not including the two trusses at the room walls)? Is there a floor surface on top of these joists? Or is there only a ceiling attached below them? How and where do you propose to attach the steel I-beam to the joists?
 
NVN - All good questions - I will have to check when I am there this weekend. It is a one story, so just insulated attic space above the ceiling - no floor.
The main floor living area is basically one open room (approx. 16 ft x 24 ft - the 24 ft runs N-S) with the exception of a small bedroom (7 ft x 7 ft) in the NW corner. So the 2x4 walls in that corner do add some support for that area, but the rest of the ceiling is unsupported from below. It would be easy (and not very obtrusive) to put a post against the south wall, and another at south side of the door frame to the bedroom, hence the 17 foot dimension for the beam running from post to post N-S across the center of the ceiling to prevent it from sagging any further.
(I learned today that I may be able to get a "surplus" 4" x 2-3/4" stainless steel I-beam cheap - wouldn't that be more than sufficient on top of 4" x 8" posts?)
Dave
 
1OLDREMODELER: Assuming an applied load on this I-beam of w = 5.243 N/mm, a 102 x 70 mm (S4 x 9.5) I-beam appears inadequate.

Assuming mild steel (instead of stainless steel) and w = 5.243 N/mm, it currently appears the first, second, and third best I-beam choices, in that order, would probably be:
(1) 131 x 128 mm (W5 x 19),
(2) 160 x 102 mm (W6 x 16),
(3) 152 x 152 mm (W6 x 15).​

Let us know the cross section name and exact dimensions (including flange thickness and web thickness), and material, of the I-beam(s) available to you. If you want to use stainless steel, it is slightly weaker than mild steel, and would be a slightly different list than shown above.

The I-beam needs to have good lateral support at each end of the I-beam, and also preferably at the one-third span points, to prevent any lateral movement of the I-beam ends and column tops. You would also need to find out if your foundation can handle the point load below each column.
 
You might want to find out what the local building code requires. Building codes take into account roof loads that are common to the area (e.g. snow). A local construction contractor might be able to help you.