Fb of Built up Beams 1

[DEW29]

I know that there are span tables available that take this into consideration, however I have some confusion as to how the allowable bending strees is derived for built-up beams. Is it just simply multiplying appropriate factor(s) to the tabulated Fb? For example, a Double 2x10 w/ 1/2" plywood flitch. Is the the Fb of one 2x10 adjusted or is there more to this?

Any info would be greatly appreciated.

 

[boo1]

F’b = Fb*CD*CM*Ct*CF*CV*Cfu*Cr*Cc*Cf
Where
Fb is allowable bending stress in NDS supplement.
CD is load duration factor, (see NDS Table 2.3.2 reproduced below)
CM is wet service factor, (use when moisture of timber is higher than 19%)
Ct is temperature factor, (when timber is used in temperature higher than 150°F)
CL is beam stability factor, (See below)
CF is size factor, (apply only to visually graded sawn lumber members, and to round timber bending members, not apply simultaneously with Cv for glued laminated timber)
CV is volume factor, (apply only to glued laminated timber bending member)
Cfu is flat use factor, (when 2”-4” timber is loaded at wide face)
Cr is repetitive member factor, (apply to dimension bending member 2”-4” thick)
Cc is curvature factor (apply to curved glued laminated bending member)
Cf is form factor. (for round or diamond section)

Consider the effects of :
repetitive member
lateral stability
size factor
volume factor

 

[RockEngineer]

If I have a 1/2" plywood between the two 2x10s I usually ignore the added section of the plywood and use only the 2x10 section properties and the factors listed by boo1. The plywood is usually to help get the proper dimensions to mate up with other members and doesn't add much strength. If you use 1/2" steel for the flitch plate then you definitely want to account for the strength of the steel.

 

[boo1]

Some times the plywood is just spacers (two inch strips) and some times it is framed solid.

 

[DEW29]

Thank You Boo1 and RockEngineer for your help.

I also enjoy reading your replies to previous threads. Both take the time to give a thorough response, full of info.

Thanks again