Bending strength of lumber (2x4, 3x4) I keep finding conflicting info.

[cepe01]

What is with this large variance in bending strength of Spruce-pine-fir. One table is from nelma.org Northeastern lumber manfacturering and the other is from the 2005 NDS.

Is there a standard/base bending strength/rule of thumb for bending strength of lumber? On alot of drawings I just see lumber called out as "2x4" or "3x4" with no specification to the species or grade of lumber, so there must be a base bending strength one can assume?

 

[dik]

The strength stipulated in the applicable code of the area is what governs. The quality of wood continues to decrease as trees used get younger and smaller.

Stipulated strength reflects this. Also lumber sizes have changed... for example, a 2x4 used to be a 2"x4"rough cut, dressed it used to be 1-5/8"x3-5/8", it was actually only rough cut to a slightly larger dimension. Now a 2x4 is 1-1/2"x3-1/2" or the metric equivalent. A 2x8 is actually 1-1/2"x7-1/4".

Dik

 

[Triangled]

Looks like nelma.org has factors C_r and C_F (at least) built in to the table values whereas NDS retains them outside of the tables for use in equations. Note 'repetitive' vs 'single' and also note widths limited. suggest reading all footnotes on all tables, especially those generated by manufacturer's associations.

 

[DBronson]

cepe01:

You're looking at two different grading rules for the species - Spruce-Pine-Fir and Spruce-Pine-Fir-South. Find out which type of material is stocked in your area, then use the appropriate table of values.

Regards,

DB

 

[msquared48]

In other words, the problem is not with the tables, but how you read and interpret them.

Mike McCann, PE, SE (WA)