Question for the engineer type folks

upnover

upnover

Grumpy, decrepit Old Man
Moderator
Joined
Mar 20, 2005
Location
Morganton NC
What is the difference in two stacked 2X3's vs one 2X6? PSI I assume??? All finished, not rough cut.
 
How are they oriented and loaded?
 
Stacked on top of each other to look like a single 2X6, sorry not clear on that. Used for roof trusses on a woodshed, slant or a lean to type roof
 
upnover said:
What is the difference in two stacked 2X3's vs one 2X6?
Click to expand...
A lot. The stiffness of a beam (In this case a roof truss) is related to the moment of inertia, which is calculated as I=bh^3, where "b" is the width (1.5") times the height "h" (5.5" or 2.5") cubed, which is h*h*h.
So for a 2x6=1.5*(5.5*5.5*5.5)=249.6
Whereas two 2x3s=(1.5*(2.5*2.5*2.5))*2=46.9

In other words, a 2x6 is about 5 times stiffer than two 2x3s. The stress values are of a similar factor, but you would need to calculate for the correct load and span to determine stress and max load.
 
jeepinmatt said:
A lot. The stiffness of a beam (In this case a roof truss) is related to the moment of inertia, which is calculated as I=bh^3, where "b" is the width (1.5") times the height "h" (5.5" or 2.5") cubed, which is h*h*h.
So for a 2x6=1.5*(5.5*5.5*5.5)=249.6
Whereas two 2x3s=(1.5*(2.5*2.5*2.5))*2=46.9

In other words, a 2x6 is about 5 times stiffer than two 2x3s. The stress values are of a similar factor, but you would need to calculate for the correct load and span to determine stress and max load.
Click to expand...

What he said, beat me too it





(I have no idea what he just said btw :laughing: )
 
BigClay said:
What he said, beat me too it





(I have no idea what he just said btw :laughing: )
Click to expand...
Yeah me too neither LOL. Two things I have found on this board in the decades I have been here, there are a lot of folks that know stuff, and, 2. A lot of folks that "think" they know stuff.
Matt, is one that knows stuff for sure. Talk about busting a mans bubble! LOL. It's already built, I used what I had, that's why the 2X3's, Just wanted something better than two trees with firewood stacked and blue tarps, that barely last a year. And, it looks better. Matt mentioned what I was already thinking, about the only thing that might be too much for it is a wet heavy snow, or (I just thought of it) a tree coming down on it.
So, it cost me nails for my gun and screws for the tin, and my time, which these days I have much of.
Thanks for every ones help.
 
upnover said:
Yeah me too neither LOL. Two things I have found on this board in the decades I have been here, there are a lot of folks that know stuff, and, 2. A lot of folks that "think" they know stuff.
Matt, is one that knows stuff for sure. Talk about busting a mans bubble! LOL. It's already built, I used what I had, that's why the 2X3's, Just wanted something better than two trees with firewood stacked and blue tarps, that barely last a year. And, it looks better. Matt mentioned what I was already thinking, about the only thing that might be too much for it is a wet heavy snow, or (I just thought of it) a tree coming down on it.
So, it cost me nails for my gun and screws for the tin, and my time, which these days I have much of.
Thanks for every ones help.
Click to expand...
Good talking to you Chip. For the price, I'd have done the same thing :D And a tree will happily blast through a couple 2x3's or a 2x6, it won't care.
 
jeepinmatt said:
A lot. The stiffness of a beam (In this case a roof truss) is related to the moment of inertia, which is calculated as I=bh^3, where "b" is the width (1.5") times the height "h" (5.5" or 2.5") cubed, which is h*h*h.
So for a 2x6=1.5*(5.5*5.5*5.5)=249.6
Whereas two 2x3s=(1.5*(2.5*2.5*2.5))*2=46.9

In other words, a 2x6 is about 5 times stiffer than two 2x3s. The stress values are of a similar factor, but you would need to calculate for the correct load and span to determine stress and max load.
Click to expand...

Just wondering.... if the 2x3s were bonded together with contrstuction adhesive, would you then be able to calculate as one 1.5x5" member....get 180ish? IF the glue joint is as strong as the wood.
 
kaiser715 said:
Just wondering.... if the 2x3s were bonded together with contrstuction adhesive, would you then be able to calculate as one 1.5x5" member....get 180ish? IF the glue joint is as strong as the wood.
Click to expand...
In simplified state, yes. Your neutral axis would then translate from the centerline of each board and instead be at the bonded joint, and you would have an effective height of 5" instead of 2.5". You also are introducing an area with a different shear strength and elasticity, and probably inconsistent bonding, so it wouldn't be quite that good, but it would be a heck of a lot better than just laying them on topic of each other. You could also take a few truss plates and drive them onto the boards, or sandwich with plywood to greatly increase the strength. That changes the way the forces act on the boards, especially the further they are separated. That's why 2x4 floor trusses look so dinky even though they are very strong. Separation=stiffness (that's what she said)
 
Back
Top