Concentrated load check …
Okay, now let’s do a (free, online) check of the concentrated load situation. Note that I am looking at the concentrated load separately from the uniform 20 psf live load … yeah!, the two loads are not taken to act together … one or the other, but at the end of the day, any structural member needs to be able to resist the `worst’ (worst-case) condition.
Let’s go over to another favorite webpage of mine. I don’t know if I have met this guy or not. Maybe, or maybe we have only passed each other in the halls of some meeting somewhere. Anyway, here’s the website:
I’m going to have to `trick’ the program to give me a single load at midspan (midspan gives me the worst `bending’ condition for a concentrated load). Looking at Fig. 1, plan (b), the framing drops a single load at midspan, delivering a uniform load from the shaded area. If our rafter is 12 feet long, and they are spaced 2 ft apart, the shaded area is ½ of 12 ft, times 2 times ½ of 2 ft, equals 12 square feet. I want to model a 300-lb concentrated load, so, to use the program, I’ll spread this out of the shaded area … 300 lb / 12 sq ft = 25 psf.
Span 12 ft
Spacing 24 in.
no (wet service)
live load 25 psf
dead load 5 psf
Grade No. 2
allow live load deflection 180
allow dead load deflection 180
Let’s look …
for b) (load at midspan) we see … Mmax 900 lb-ft … yeah, that’s what we had in the hand calcs!
Vmax = 150 lb … we haven’t talked about `V’ yet (shear) … but, basically, a concentrated load of 300 lb will tend to shear the beam with ½ of 300 lb = 150 lb toward each support.
And it gives us 2 x 6 (No. 2).
(I wish I didn’t have to trick the program to give me a concentrated load … generally the simplest solution to any problem is the best … once we start getting fancy, more complicated, or try playing tricks, we’re asking for trouble.)
Now, before we leave the page, let’s put in 20 psf live (construction) load, and 0 dead load, we get the same M = 720 lb-ft as our hand calcs. Good. If we add 5 psf dead load, we should get the same thing as AWC did … No. 2, No. 1, and SS 2×6. Good.
We’ll talk about shear, bearing, lateral support … in other posts.
(c) 2021, Jeff R. Filler