Expert: Dan Griffin - 6/10/2008

Question
What are the relative trade-off's, max span, performance and price for three types of beams: 2x12 double header, 2x12 fitch beam, 3.5"x12" LVL?

I would like to install a support beam to replace a 2x4 @ 16"oc wall to faciliate the installation of picture windows/butted glass where a 22' long 8' tall wall currently exists.  It is a first floor wall, supporting 2x12 @ 16"oc second floor joists, but not the roofing structure which bears on an independent beam.  My preference would be to span the entire distance, but if price is prohibitive I could break it up into smaller headers each supported by a post on a solid foundation.

Thanks

Answer
I started to answer this assuming it was an ineterior load bearing wall because you said the roof was not bearing.  As I re-read it, I guess you are on an exterior wall because of the windows.  You are way beyond a doubled 2x12 header.  YOu will be into steel or one of the manmade materials.  A double 2x16 with a 1/2" steel plate (flitch) starts to get close.

If it is an exterior wall, you need to work with a structural engineer.  You will be removing a huge portion of shear bracing and there will need to be major design work to compensate.  I suspect you will be looking at a steel moment frame and the steel depth will be getting fairly deep to carry its own weight and the joist load.  The joist load will be about 50#/SF over half of the contributing floor footage.  If the  span from an interior bearing wall to this outside wall is 16 feet wide, then 1/2 or 8' x 24' x 50# = would give you a 9600# uniformly distributed load on the header and a little over 400# per foot of beam.  For deflection with windows involved I would use l/480 (.55, just over a 1/2 ") or even L/600 (.44, just under a 1/2")

Dion, if this an interior wall you will need to measure the square footage of floor bearing on this wall.  Half the distance from this wall to the outside wall each direction.  Unless you have something unusual going on (water bed, pool table, grand piano), you should be able to use 40#/SF live load and 10#/SF dead load to compute the load that is bearing on the wall or proposed header.  If you are in an identified seismic area the interior load bearing wall may also be a shear wall and will again demand an alternate solution.

Here is some good reading about sizing headers:
http://www.umass.edu/bmatwt/publications/articles/sizing_engineered_beams_header...