Ridge beam going in

Started by Mark.alan65, July 05, 2017, 08:43:05 AM

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Mark.alan65

Due to the construction of my rafters and rafter ties, I am adding a structural ridge beam. I need a little help calculating the beam size. The room span is 20 feet, and the house is 24 feet wide. I am in north Alabama so snow load is almost non existent. The roof is a 5/12 gable. I am looking at getting a LVL. From what I found a 3.5x14 would work, but I would like would like some of the more experienced guys to provide some guidance. This will not be a full vault, but I would like to remove some of the rafter ties, and raise the remaining ties up. The support post will run from the foundation up to the LVL.

Thanks,

Mark

Don_P

I'm using one of my beam calcs here;
http://www.timbertoolbox.com/Calcs/beamcalc.htm

For load the ridgebeam is supporting a 12' wide swath down the center of the roof for a beam span distance of 20'. The 6' on either side of that center 12' is supported by the walls the rafters bear on. So the tributary loaded area supported by the beam is 12' x 20' or 240 square feet. Minimum non snow design live load (wind) is 20 pounds per square foot + 10 psf dead load, the weight of the roof itself, for a total of 30 pounds per square foot. Multiply the 240 square feet of tributary area X 30 pounds per square foot and the load on the ridgebeam is 7200 lbs

Enter 240 inches for the beam span
Enter 3.5 for width and 14 for depth
From memory typical lvl design values are Fb 2800, E 2.0 Fv 280

Hit "Show result"

You'll see bending strength passes by a large margin as does shear these are strength checks and you are plenty strong enough, Deflection fails for a floor but actually passes for a roof at about 13/16" total deflection at full load. So yes, it works. Plug in a 16" depth and you'll notice the deflection drops to about 1/2", deflection is a serviceability rather than a strength check, your call.

The supplier of the beam will do the calcs for you free of charge on software from the lvl manufacturer, they can provide an engineers letter if the building inspector needs one as well, also free as part of the service of the sale.



Mark.alan65

Thank you Don_P.
I had run the numbers and wanted someone more experienced to make sure I was correct.
From my understanding by doing this, I don't need rafter ties. I do plan on having some but I would like them higher up, and not on every rafter. More like 1 every 5 feet or so.


Don_P

Correct, the rafter ties are decorative/redundant, but never a bad thing. Remember to run a strap over the tops of the rafters from one side to the other connecting the rafter pairs together and also remember that the rafters are basically hanging from the ridge and connect them appropriately. you can check shear numbers on a long framing angle and often get there cheaper than a sloped hanger.