structural ref: glu-lam beam

i'm looking for a practical reference on framing/structural engineering. probably a book with a good overview as well as practical equations/tables. but maybe online too. not sure. here's why. i'm done with adding mass to my walls, now moving to the ceiling. i calculated on the heavy side the new roof will be 7lbs/sqft. the plan was to sister up the old 2x4 framing with 2x6s. i found a calculator for perpendicular load on standard sized members and for the spans i have 2x6's should work fine. maybe even 2x4s. but this framing feeds into a large (5.5" x 9.5") glu-lam that spans 23'. glu-lam looks to be 7 layers. 5 middle layers look like 2x6, top and bottom appear to be plywood. i have no idea what the max load is for this thing. i also have no idea how to calculate it's share of the roof load. confused? attached pics and sketchup should help. oh, left out some of the wall framing...it's really there. actual 2x4s, 24OC. ideas? thanks k, my skp file is too big. tricks to get filesize down? i'm guessing make everything that's reused a component? ah..purge unused!

I was told a rough ruling for the span of a support beam in inches. "halve the distance and add 2." so if the span is 10 feet - it's 5 +2 = 7" x 2" beam. if the span is 20 feet - it's 10" + 2" = 12" x 2" beam.

OK..... Load distribution is not that difficult. in your example - take one 1/2 of the total roof load - and attribue that to the beam. In other words - when ever you have a horizontal (or close to horizontal) member under load - one 1/2 of it's load is distributed to each end of the member...... (except for high pitch rafters). So if you stick a beam in the middle - it is 1/2 of the span on each side of the beam that transfers load to the beam. Once you have that you take the total length of the beam - multiply it by the total span attributable to the beam - that's your total square foot load the beam is carrying. Multiply that times the required live and dead loads to arrive at the existing total load for the beam - then add in (in exactly the same manner) the new load you are going to add to the beam. Please remember to add in the load of the lumber itself when calculating the total dead loads. As far as good publications for this type of construction - that depends on how comfortable you are with math...... The LFRD for Manufactured Wood Construction is an excellent manual - this coupled with Allowable Stress Design (ASD) Manual for Engineered Wood Construction (2001) include with this the National Design Specification® (NDS ®) for Wood Construction (2001 Edition) and its Supplement: Design Values for Wood Construction. These bring together all of the required elements for design of wood structures in one comprehensive package. They include design information and examples for structural lumber, structural glued-laminated timber, structural-use panels, timber poles and piles, structural connections, wood I-joists, metal-plate-connected wood trusses, structural composite lumber, and pre-engineered metal connectors. Over 900 pages. This can all be bought as a 4 book set for 150 USD from the American Wood Council. Here's a link: http://www.forestprod.org/awc/index.html It's Publication #T101-05 Rod

john, so by that formula. 23ft span / 2 = 11.5" x2" ... a 2x12. but there has to be more to it then that. i mean that must assume certain loads. seems like that would be most useful when framing an entire ceiling with residential loads...what are they? 10 dead/40 live. rod, let me see if i understand. i take the total roof load (drywall, framing, huge beam itself) and divide by 2. for me that's around 2,600lbs. so my beam is carrying that. then i want to find out what area is on the bearing side of the beam. for me that's around 23'x5.5"...or about 10.5 sqft. i guess you could also do this as a load over linear feet for the beam but i don't know how beams are rated. so 248lbs/sqft or 113lbs/lin foot if that is the case...well yes it is straight forward. thanks. i'm not super strong in math but i went well past calc in school. diff equations and vector calc. but my recall of that is worse then the alien abduction i had 3 years ago. those look like serious publications. perhaps too much for my current level... 1000 pages. the last time i payed $150 for a book was...never. you should have seen me belly ache buying books in school. however i may still have to cave and get it for specs on this beam. i guess another option would be to try and measure stress/deflection on the beam currently and calculate what the new load would add to that. i read something about deflection as a percentage of span...maybe on the AWC site. thanks both of you. good stuff.

john, so by that formula. 23ft span / 2 = 11.5" x2" ... a 2x12.

No - 1/2 x 23' = 11.5" PLUS 2" = 13.5" x 2" beam. It's just a rough formula to get a rough idea of what you'll need. Rod's is the more accurate method.