Rafter Size/Spacing

[Jackhammer]

I need some professional advice.. What is the difference of the structural integrity in using 2'X4' vs 2'X6' for rafters with wall studs on 16'oc? That is when the 2'X4' rafters are placed on 16''oc vs the 2'X6' being placed on 24"oc. I may be completely off base here but it would seem that 2'X4's being installed directly above the wall studs would provide more stability then 2'X6's being placed on 24"oc. In either case I realize the weight is being transferred to the top plate and then distributed to the wall studs. It just seems if the rafters were placed directly above the studs the transferring of weight would be more uniform.

[giddonah]

More uniform in what sense? If they were in line with the studs, the top plate would only be in compression at those points. If they're off center, then the top plate will be in tension and compression as the rafter pushes down on a span. The wall will still be carrying virtually the same weight though (varying only in the total weight of the rafters). If the top plate were too thin, then the rafters would sag between the studs, but if the top plate isn't going anywhere, there's not much difference since the studs will still experience the same compressive forces.

Others might see reasons for doing it either way, but to me, the forces involved look like either way is fine.

[Jackhammer]

Giddonah, by uniformity I meant that the load of the roof would be transferred to the top plate then directly to the wall stud beneath it.
If the studs were staggered the load would be transferred to the top plate then to the nearest wall stud. Could this mean that the load transfer might not be similar on each wall stud?
Just a thought..

[Jackhammer]

Are there any other thoughts or suggestions from the pros??

[Joe Bartok]

I'm still "in training" as far as the engineering math goes, but I think giddonah's got it. There's no real advantage one way or the other. The top plate is a beam, the wall studs are the supports, and any forces are eventually transferred to the studs.
If your studs are unequally spaced, then they will receive unequal loads, but the sum of the loads (equal to the total roof load) remains the same and is eventually transferred to "ground". If the spacing is severe, it's possible for an individual member to receive enough load to cause buckling or failure, but this kind of spacing would hardly be a normal building practice. :shock:
Anyone have an engineering program to test some numbers?

[Joe Bartok]

Here's a page resurrected from my notebook. Not your studs specifically, but you can set up a similar calculation on a related program.
Unequal spacing, unequal loads.
The actual calcs were executed on Sizer 97b.

[giddonah]

I'm not saying that's not cool, but I bet it's *more* cool when there's some audio to go with it :wink: .

[Joe Bartok]

You mean everyone else is adding AUDIO to their images? Have I been missing out on something all this time?

[giddonah]

lol. I was just thinking how I would like to have heard the lecture that went along with that.

Where does the 600 lb/ft come from? Factored for computer? Why measure the length that each stud supports and not the weight? Does the 10' mean you can move the studs another 4' apart? Why 3200lbs vs 6000lbs? I also am having trouble getting to the conclusion about the perlins. I imagined them running horizontally, are they the dashed red lines?

Notes are much more meaningful when you were there to take them.

[Jackhammer]

Hey guys, I'm so cornfused now I don't know the difference between a stud and a hanger. I guess I should have listened a little closer in school.

[Joe Bartok]

There isn't much of a lecture.
The beam length and loads are convenient pieces of fiction. You could enter any length, any load. Factored means that this particular program automatically increased all dead loads by 25 percent. So, if you want 100 plf actual load, factor by .8 and enter 80.
Beam length times plf gives the total load. Note the reaction forces at the supports returned by the program, do a bit of division, and, voila! The dashed red lines show how many feet is carried by the rafter. Given this information, you can calculate an actual plf for the rafters.
I don't have access to the program, hence the scan, and my suggestion in an earlier post Anyone have an engineering program to test some numbers?.
The purlins referenced are in an actual roof (still standing as far as I know), and not shown on the drawing. Just ignore the reference to them.

[Jackhammer]

Joe, thanks for the info

[Rich]

Look on www.toolbase.org for Optimum Value Engineering learning. I've actually done a small 700sf addition using the methods of OVE. Here's the direct link to the paper - I purchased the whole manual several years ago.
http://www.toolbase.org/tertiaryT.asp?TrackID=&CategoryID=1153&DocumentID=2021

[Joe Bartok]

Rich, thanks for the link.
Giddonah and learning: apologies for the short "explanation" of the math yesterday. It seemed the gollywoggles were active on the Internet or this bulletin board and I was having a hard time posting a response. And to put the calculations in context and explain them properly, the subject deviates from the original question. So I've posted the rest of the story in the Engineering Issues Forum.
lol. I was just thinking how I would like to have heard the lecture that went along with that. :)

[Tom R]

I didn't have time to check out the links referenced above, - - but I would say theoretically that if the rafters and studs share equal spacing, and are 'in-line' with one another, a 'single' top plate would suffice.

If the 'spacings' are different, - - or if they're 'out-of-line' with each other (by any more than 1 1/2"), - - a 'double' top plate would be needed.

Professionally, - - we just use a double top plate all the time.

[Jackhammer]

Thanks for all the info, you have been a big help