Wood beams

[awa5114]

When we say 4x blocking or 6x beam, which dimension of the beam are we referring to? Is it the width or the depth? Also what if it is a flat beam?

 

[mike20793]

Wood members are called out width by depth, so 2x, 4x or 6x would indicate a width of 1.5, 3.5 or 5.5 inches, respectively. If it's flat, I show it in section or include a note after the callout indicating it is installed flat (which is common for most diaphragm blocking scenarios with floor trusses).

 

[jayrod12]

on flat is also noted as plank orientation by many an engineer.

 

[hokie66]

The answers above are correct for the US. In some other places, it is done the other way. In Australia, it is a "4 x 2", or actually "100 x 50".

 

[OldBldgGuy]

In Canada it's width by height, whether it's a joist, a beam, a door, a window, or whatever.
And just as an aside, my pet peeve is draftsmen who put 2"X4" (or whatever) on a drawing when indicating a given framing member. That means 2" by 4", when they mean a 2x4 which is a piece of wood that is nominally 2"x4" but measures something completely different.

 

[JAE]

hokie66, isn't that because Australia is upside down near the bottom of the world?



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[hokie66]

You might be right, JAE. But that depends on which end your consider up.

 

[LittleInch]

UK wood sizes are the same as Australia - a 6 x 3 is nominal 6 inches deep..

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[awa5114]

Thanks guys

 

[awa5114]

In the 1973 UBC (and all other historical versions of the UBC), Table No. 25-A-1 lists material properties for various wood grades. However, under size classification, instead of depth and width, it lists thickness and width. This is confusing. Which dimension is vertical and which is horizontal according to UBC?

 

[Sawbux]

Thickness is the narrow direction of the lumber. I don't have a copy of the old UBC around, but I believe the table you are referring to lists properties for lumber up to 4" inch thickness in one column, then posts and timbers, etc.

These are related to the visual grading rules, which dictate different timber grades judged by size of defects, slope of grain, splits or checks and other factors.

2 & 4 inch nominal lumber is typically kiln dried before grading, thicker lumber (say 6x6's) are not. This is due to physical difficulty of drying out thicker materials and economics. Depth-wise, a 2x4 and a 2x12 can both be kiln dried and can be grouped in the same grade as far as stresses.

 

[awa5114]

"I believe the table you are referring to lists properties for lumber up to 4" inch thickness in one column, then posts and timbers, etc."

Yes, we are referring to the same table. So let me get this straight:

2x4 according to NDS : 1.5" width, 3.5" depth
2x4 according to UBC: 1.5" thickness, 3.5" width

Thanks.

 

[wannabeSE]

More often than not, the thickness width and depth definition can be sorted out by looking at the dimension. For instance, when the UBC has a category with 2"-4" thick and 6" and wider; I think it is obvious that the 2-4" is the skinny dimension. Also, it is a safe bet that the design values are based on edgewise use and there is probably a modification factor for flatwise use.

Also note that the 2015 NDS Supplement uses similar terminology with some clarification. For instance, the size factor table on page 32 has one column labeled "Width (depth)" and another column labeled "Thickness (breadth)"

 

[allgoodnamestaken]

Just watch out - up to 2x6 you reduce the dimension by 1/2" (x4, 3.5, x6, 5.5). But once you get to 2x8 it's 3/4", so a 2x8 is typically 1.5"x7.25", x10 is 9.25, etc.