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Author Topic: Which is stronger?  (Read 608 times)

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Offline PAmizerman

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Which is stronger?
« on: October 11, 2018, 09:33:16 PM »
Is a 2x12 or a 3x10 stronger as a rafter?
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Offline POSTON WIDEHEAD

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Re: Which is stronger?
« Reply #1 on: October 11, 2018, 09:39:35 PM »
How long of a span?
I would say the 2x12
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Offline Magicman

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Re: Which is stronger?
« Reply #2 on: October 11, 2018, 09:56:20 PM »
I am sure that someone will speak up that has the formula, but personally I would spike together two factory 2X10's (3") to take advantage of the changing grain patterns/slopes.  Whatever I used whether 10's or a 12" they would be grade sawn "jacket" boards.
 
"Magic has a Brand New Sticker" but he has been busy getting ready for Colorado and has not had time to install it yet.  ;D
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Offline CabinCreations

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Re: Which is stronger?
« Reply #3 on: October 11, 2018, 10:14:44 PM »
Based on the second moment of area (moment of inertia), the 2x12 would be a stiffer rafter.

For future reference, the most simple way to compare beam strength is to use the equation b*h^3/12 - where b = the thickness of the beam/rafter and h = the height of the beam/rafter.

Offline PAmizerman

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Re: Which is stronger?
« Reply #4 on: October 11, 2018, 10:21:02 PM »
20' span 
Thinking 2' centers and a 5/12 pitch with steel roofing
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Offline PAmizerman

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Re: Which is stronger?
« Reply #5 on: October 11, 2018, 10:22:09 PM »
I'm gonna price trusses tomorrow. It may be cheaper
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Offline Magicman

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Re: Which is stronger?
« Reply #6 on: October 11, 2018, 10:56:34 PM »
I figured that the formula would disprove my notion but as mentioned, trusses trump single stick construction.  ;D
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Offline Brad_bb

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Re: Which is stronger?
« Reply #7 on: October 12, 2018, 12:23:09 AM »
Don't forget the proper order of operations (example below):
2x12

2 =b
12=h

I = b*h^3/12
=(2*12^3)/12
=(2*12*12*12)/12
=(3456)/12
=288
Anything someone can design, I can sure figure out how to fix!
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Offline Greyman

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Re: Which is stronger?
« Reply #8 on: October 12, 2018, 12:53:10 AM »
Actually the correct comparison is the section modulus: S = MOI / c, where c is the distance from the centroid to the edge and MOI = bh^3/12 as noted above.

2x12:  S = (2 x 12^3 / 12) / 6 = 48

3x10:  S = (3 x 10^3 / 12) / 5 = 50

Therefore the 3x10 is slightly stronger.  The stress due to bending is f = M / S

Being an aerospace structural engineer comes in handy once in a while...  lol

Offline Don P

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Re: Which is stronger?
« Reply #9 on: October 12, 2018, 06:20:04 AM »
Which is "stronger" is an interesting question. Often it isn't strength that controls but deflection, stiffness. We do allow twice the deflection in a roof member as compared to a floor.
 
CabinCreations and Brad are describing stiffness where Greyman is describing bending strength. Both are answers to the question.
What Magicman was saying is also correct, you get a 15% bending strength increase by nailing 2 or more pieces together because of distribution of defects, deflection doesn't change.  


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