Expert: Dan Griffin - 4/17/2008

Question
I am spanning a 2nd floor area of 24' from wall to wall (with 2x8's, 16" o.c.) and a support area of 12' from wall to center girder(consisting of (3)2x12's) that run perpendicular under floor framing, supported every 13' by 4" steel columns. Total building length is 40' in length, that has wood trusses that span over the building width of 25' and 1' overhang each side.  The question is if I am going to have 50 people on that floor area, do I need to double up the 2x8's(sister them) to handle floor loads or is the 2x8's @ 16"o.c. resting on outside wall and inside girder ledge sufficient span support?

Answer
I don't have my code books at home.  I am not sure what the code required minimum live load is for assembly usage.  The issue is ruled by usage, and the 50 people in a small room sounds like one of the assembly usages.

Residential usage is 40#/SF
Office usage is 60#/SF
Education hallways are 100#/SF

YOur 2x8s, using #2 yellow pine @ 16" are fine at 40psf live load
At 60psf, maximum span is 11-3, the 2x8s are fine at 12"o.c.
At 100psf, you just can't use 2x8s.

Contact your AHJ for your required usage.  A good span calculator is here:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
Here is a span table:
http://www.awc.org/pdf/WSDD/C5.pdf