Maximum allowable span for a double 2 by 12 header

[djsdaddy]

I am helping my friend remove the lousy upstairs that is above his garage. We want to put one header that will span from his house to the end of the garage, this is 24 feet. There will be a walk in door next to the house, and then a 16 foot garage door. So technically the "beam" will only have to span the 16 foot garage door. We are going to be using trusses to construct the roof. I have been looking for span tables to tell me if the double structural grade 2 by 12 header will span the distance. Unfortunately I don't understand the ones that I have found. I understand that LVL's will span this distance without a problem, but these are very expensive compared to structural grade lumber. I would appreciate any help I can get.

 

[boo1]

While you description was vague, the following may help;
see

http://www.southernpine.com/designtools.htm

for design vaLUES

Like any structural design situation, the design of beams is an iterative process that relies on the making of assumptions and then the checking of those assumptions.

The design process for beamscan be shortened if one is able to correctly predict the controlling mode. The following simple rule of thumb may help you decide which mode; bending, shear, or deflection; controls.
If l/d < 10: shear controls
if 13 < l/d < 25: bending controls
if l/d > 30: deflection controls.

STEPS
1. Develop Design Loads; live load 30 or 40 psf, dead load 10 psf, wind xx, snow xx ect
2. Calculate Fb
3. Trial section (2-2x12) S=M/Fb'
4. Check bending
5. Check shear
6. Check Deflection
7. Design bearing and connections