finishing attic - need to span 30'

[mcollinstn]

I have a trayed ceiling that "intrudes" into the attic area. I plan to finish the attic, and to build a platform over the trayed area for a pool table and ping pong table. The platform over the trayed area will be 30' from loadbearing wall to loadbearing wall and 22' across in the perpendicular direction. Let's call the 30' direction "right left" and the 22' direction "front back".

I have room for a 30" tall truss against the "back" of the platform. I have room for (3) 18" trusses on 12" centers coming from the rear starting at 12" front from the 30" tall truss. Then I must skip trusses from the last one @ 3' from the rear until I can resume with (3) 18" trusses @ 19' from the rear, followed at 20' and 21', finishing with a 30" tall truss at 22'. As you can see, there will be a 16' open span front to back. I have room to cross brace the trusses against twist and can stabilize the support framing with diagonal kickers.

#1) What are the most cost effective trusses to support the loads I need on that 660sf platform?

#2) Assuming I can obtain suitable trusses, what should I use to "cross" the trusses to support the flooring (lumber, wood I beams, something else)? And what spacing C-C? Remember, there is a 16' open span across it..

In looking through 2x4 truss span charts, I can't find anything with "doubled-up" members. I see them in restaurants and wonder if they are TWICE as strong as single member trusses.

 

[boo1]

Consider adding wood "I-beams" or floor trusses spanning the 22' direction. Typically would be 12" or 14" deep and 24" o.c. with 3/4" decking glued and screwed to floor beams.
With a heavy pool table you may need to double layer the decking. A 40 or 50 PSF Live Load and limit deflection of L/480 should provide loading.

 

[mcollinstn]

Thanks, boo, but can't span the 22' direction. The two loadbearing walls are in the 30' direction.

 

[ERV]

I suggest that you take a drawing of what you intend to do to a local truss manufaturer and have them work out the engineering details for floor trusses. This seems pretty simple and straight forward. If you are not shure of the bearing capacity of the walls, be sure to have someone experienced in that field check that for you. Truss manufacturers can run the design for the trusses, however, it is up to others to insure adequate bearing. This includes checking the foundation bearing capacity.

Good Luck.

 

[boo1]

Typically a 30' span uses floor joist 24" oc. But a wood I-beam can be used (see the product engineering sheet, typ. 12" oc). The wood I-beams are easier to use in retrofits, since they can be easly bent in the weak axis. Consider both cost and installation.

ERV has a good idea of contacting a truss manufacture. They can provide you the size, end loading bracing and cost. Engineering details though are provided by others (PE).

The ends of the truss will tranfer the load to the wall. Addition to the bearing, consider the axial loading, and bending effects in the wall.

 

[CSEllc]

Boo1,

Be very careful with wood I-joists. Since they are "light" relative to mass, they have a tendency to be very bouncy. I saw you stated 12" to 14" I-joists on 22' span, Based on my experience, you should be in the 14" to 16" range to control bounce.

 

[boo1]

Typically the beams sizing are from the manufactures guide sheets. Not only bounce but vibration is a concern for all Residential Wood Framed Floors. I prefer floor trusses (we us a crane to install them) but in retrofit cases I would consider other options. Also the spacing for 30' tyically would be 16" oc.

Some thinks to conside selecting vibration reduction are:
1.Minimize the span-to-depth (length/depth) ratio. Aim for a ratio of 20 or less. Deeper floor framing means better performance.
2.Thicker sheathing improves floor performance. Attach by glue and screws rather than nails. Contact APA–The Engineered Wood Association at

http://www.apawood.org

for sheathing and attachment details.
3.Cross-bridging and blocking for I-joists and solid sawn joists only helps if it is carefully installed. If not, it may cause squeaks, which can contribute to the perception of bad floor performance.

 

[boo1]

In residential construction I have seen many cases that a product was percieved to be inferio, becouse of substandard installation, not the products design.