Expert: Dan Griffin - 4/23/2009

Question
Hello I am trying to get information on what size structural steel tubing is needed to replace a 2x8 wooden joist spanning 12' in the design of a deck. My dimensions are 14' x 16' with a 2' cantilever on the 14' side. Can you help me?

Answer
Jake, there are far too many variables to give you a simple answer.  I am not a structural engineer, and this answer needs one.

If this is a commercial deck, it will require specific loading allowances, without my code book I would guess at 100 lbs/SF.  It makes a great deal of difference on how this deck is being carried, how close together the joists are, what is carrying the joists.  Corrosion protection becomes important also.

Here are a few places to get a bit smarter.  Southern pine would require 2x8 for 50 lb. loading.  Dietrich steel joists would require 7.25/18 gauge for the same.  No savings on dimension.  Steel comes in many weights and sizes and each has different properties.

A wood span calculator:
http://www.awc.org/calculators/span/reversecalc/reversecalc.asp

Dietrich steel stud calculator:
http://www.dietrichindustries.com/OnlineSpecs/cdconversion/trjoistadv.asp

A beam calculator:
http://www.toolbase.org/Design-Construction-Guides/Exterior-Walls/flitch-plate-b...

I hope this helps.  A call to a local steel supplier or fabricator might help, but most will suggest working with an engineer.