Lateral Stability of built up wood beam. 1

[RARWOOD]

Is any one aware of a design procedure to determine the allowable bending stress of a Nailed Built-Up Beam laterally unsupported except at the ends.

The NDS has provisions for Nailed Built-Up Columns but I have not found any provisions for beams.

I would think the use of the same reduction factors per the column equations would be reasonable. Another engineer I work with thought that would be too conservative.

 

[boo1]

see:

http://jan.ucc.nau.edu/~dsl/egr437/class/woodbeams/adjustment/stability.html

 

[ARLORD]

I normally check the nailing or bolt pattern for the shear in the beam, for example k/ft. Then check the bult-up section using normal procedures with the full cross section, based on the unbraced length.

 

[Lion06]

If they are side by side, their is no real "composite" action, just load-sharing based on stiffness. If they are positiviely attached, just use the beam stability factor for the gross section of all the laminiations.

 

[RARWOOD]

Thank you for your thoughts. I have a case where a solid glulam was specified. The contractor instead subsituted a beam made up of three seperate LVL plys. The design is controlled by the unbraced length of the beam. For a solid section you use equation 3.3-6 in the 2005 NDS to calculate the beam stability factor. Equation 3.3-6 is similair to equation 3.7-1 that is used for columns.

If the built-up beam was a column instead of a beam then you could use equation 15.3-1 to calcualte the capacity of the built-up column. When you use equation 15.3-1 you reduce the capacity when bucklin in the weak direction controls.

When a column is built-up of seperate members the buckling resistance is increased. However testing has shown that slip between the laminations will reduce the column efficiencies by 20%-30% or more.

Applying this to my beam it seems that I should reduce the capacity below that given by equation 3.3-6. I should be conservative if I use the same reduction factors as those used in equation 15.3-1. I was just curious if any one is aware of any similair testing that has been down on built-up beams.

 

[boo1]

https://www.aitc-glulam.org/shopcart/Pdf/aitc_117-04-with _errata_incorporated.pdf

ANSI/AF&PA. 2001. ANSI/AF&PA NDS-2001.

 

[Lion06]

The additional reduction factor for columns is to account for the fact that the fasteners will likely not load all laminations equally in compression. You have a scenario where beams are being loaded (hopefully from the top) and not relying on the fasteners to transfer load, but only to ensure equal deflection and cause any buckling that would take place to happen to the composite section and not to an individual ply. This is akin to local buckling in steel. As long as you ensure that buckling of the
entire section will control and not buckling of an individual lamination, I think you are fine using the beam stability equation. Compression and flexure of side-by-side laminations have different behaviors.

Also, technically, you may have to reduce the strong axis buckling of a built-up column (and not the weak axis) if the l/d of the built-up strong axis is greater than the l/d of the built-up weak axis (which could be the individual member strong axis).