Beam Design Question

[Jambruins]

I am trying to figure out a beam size. The beam will be 7.75 feet long and supported on both ends by a wood column. Which method below is the correct way to size the beam?

Method 1

W= Roof weight (snow+dead=65 psf) + Floor weight (40LL+ 10 DL)
W = 65 psf x 7.75' + 50 psf x 7.75'
W = 891.25 lb/ft.

Method 2

Trusses spaced 24" o.c. and the span is 24'.
Each truss is supporting 65 psf x 2' x 24' = 3120 lbs.
Truss is supported on both ends so total load on one side is 1560 lbs.

Floor joists are 16" o.c. and span 12'.
Each floor joist is supporting 50 psf x (16/12)'x 12' = 800 lbs.
Floor joists are supported on both ends by a beam so the total load one side is 400 lbs.

So the beam is supporting 1560 lbs every 2' + 400 lbs every 1.33' = 1560/2 + 400/1.33 = 1088 lb/ft.

Which is the correct method to use? If method 2 is correct am I doing it right?

Thanks.

 

[Jambruins]

I realized I made a mistake in method 1 and it should be

W= Roof weight (snow+dead=65 psf) + Floor weight (40LL+ 10 DL)
W = 65 psf x 12' + 50 psf x 6'
W = 1080 lb/ft.

So I guess either way works and method 2 is really just an in depth version of method 1.

 

[MiketheEngineer]

If you do the math correctly - it doesn't really matter much - as you found out.

Sometimes - point loads give a slightly better answer - but not really worth the effort if everything is 2' oc or less.

 

[rscassar]

When analysing a beam which is subject to load by a series of point loads it is the engineers responsibility to decide whether it is okay to idealize the point loads as a uniformly distributed load or whether the point loads need to applied individually.

Generally because every assumption made in the analysis and design tends to lead towards the conservative side I feel comfortable with analyzing a beam like the one described subject to a uniformly distributed load.

Regards

 

[beton1]

Sometimes, a simplified method is a good check! You answered it.