Another rafter span question.

Started by RickD, March 04, 2014, 02:16:08 AM

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RickD

Having difficulty figuring out minimum rafter size to span a 16' addition. I keep getting lost in deflection and live loads and all the important stuff.

Will be a shed type roof, almost considered flat, only 2:12. Plan on having a sloped ceiling made by enclosing rafters, meaning no joist spanning room.

Actual span will be 15' 8"
No snow load
Central Texas

Just wondering what kinda rafter I am dealing with, or maybe I will have to by a beam to span perpendicular to rafters halfway to peak to support half the span. (Don't know what that rafter support beam would be called)

Thanks for any help.


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UK4X4

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

Find out what wood you can actually get locally...plug in your numbers

2 x12 number 3 on 16centers gives me 16'10" with 30 live and 10 dead

I went high on the live, just because flat roofs always seem to attract things walking on then and would hold a lot of snow if it ever snowed where you are.

Span tables too are another way of getting the information
http://www.awc.org/technical/spantables/

2 x 10 number 2 on 16 centers gives me 17' with 20 and 10, with more roof like weights


RickD

Thanks! I was getting close to the same results but was hoping for someone more knowledgeable with the span calculator to confirm it.
Looks like 2x10 rafters of southern pine will be what I use.

Does roof decking and/or ceiling material add any strength?

Question on deflection. Is there a calculator that you can input the size lumber/span and it computes the deflection/stiffness of a given scenario?


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UK4X4


Forte software free from

http://www.woodbywy.com/products/software-services/forte/

Its a lot more technical, but the report provides actual deflection in " V's target.

I bugged Don for months getting my head arround the calcs, he was extremely helpfull

Then did them manually and compared to the programs results till I got the answers the same...!

Still not an expert but I used the program outputs along with my own dwgs to backup my house design and they were all accepted by the permit office

Don_P

Your design load would be the minimum code live load of 20 psf + 10 psf dead load to account for the self weight of the materials.

If the rafters are spaced 16" on center then they support a "tributary width" halfway to the rafter on each side of it, so 8" on either side of the rafter, or a trib width of 16", or easier 1.33'.

Multiply trib width, 1.33' X 16' span = 21.28 tributary area X (20+10 psf load)=638.4 lbs

Go to the awc spancalc and enter southern pine, 2x10, #2 eff 6/1/13, rafters live load, 24" centers, l/240.
Span should be 16'6"

Then scroll just below that and you'll see a box with the adjusted design values.
Enter those into this calc;
http://www.timbertoolbox.com/Calcs/beamcalc.htm
The E value entry is shortened to 1.4 in my calc
The load is what we just figured above, the span is the actual horizontal clear span inside the room. Enter actual 2x10 dimensions of 1.5 and 9.25
Click "show result" and this will give the total deflection at full design load.
For typical deflection calcs you only figure deflection based on live load so you can refigure the live load from the description above enter that, click "show result" and see the live load deflection... at full design load.

You can also refigure with any other uniform load scenario and see what the deflection would be.

For grins you can alter the E value upward or downward so see what other stiffnesses would do to the deflection. The highest stiffness wood based material commonly used is an LVL with an E of 2.0 typically.

Have fun playing "what if"


RickD

Thank you both, this is just the info I was hoping to get.

I noticed you used 24" centers and this gives a span of 16'6". I am gonna go with    16" centers this will allow a span of like 21'.  This should be plenty strong for my barely 16' span.

Now I'm gonna play with the calculators all night here at work and possibly open a whole new can of worms. I might be back with more "what ifs"


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Don_P

Doh  d*, I got as far as the trib calcs sentence and went away for several hours then came back and wrote the rest... in between I switched from 16" centers above to 2' centers below... don't make that mistake in the real world!