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Wood Column

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bpstruct

Structural
Joined
Apr 23, 2008
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137
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US
Can anyone tell me why there is so much difference in Fc allowable for lumber vs timber? Seems crazy to me that you only get about 1/3 of the allowable by switching from a 4x6 to a 6x6.
 
Just seems like quite the change for a nominal dimensional change. I understand the logic behind reducing the allowable stress based on size. But it seems more rational to have some kind of transition....not dividing by 3 for a 2-inch thickness change. Also, it seems that an 8x8 or 10x10 or whatever should have even lower allowable stress than the minimum considered timber (5x5).
 
Don't forget they are visually graded and M^2 is right. I often use 3-2x6's bolted or nailed together to get some good values??
 
Works for some of these as they are embedded in walls. I have one that is exposed (in a dining room), and I am just going to make it bigger.
 
The difference is because it is easier to say this is the type of lumber/timber we want to call #2 and test them for the design values to used. Then to determine what the visual requirements are required to get a set design values for each size/grade of lumber/timber.
So we have a #2 grade that is easy to visually apply, by the grading inspectors, to a group of lumber/timber members with a lot of design values that the engineers get to use. Rather than one set of design values for each grade, for the engineers, with a difference set of grading rules for each grade/size of the lumber/timber members, for the grading inspectors.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
 
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