Built-Up Sawn Lumber Column

[medeek]

I'm trying to create a new calculator for timber and sawn lumber columns. However, I decided to get fancy and add in an eccentricity check for both the x and y axis.

My question is: Does it make sense to use a flat use factor (C_fu) in calculating the allowable bending of the minor axis (F_by) when you more than one 2x4 or 2x6 mechanically fastened together (nailed). One stud or post makes sense to me but as soon as you start laminating the plies together the shear/slip of the nails would seem to negate the flat use factor in bending similar to the reduction that is taken in C_p with the K_f factor. Note that C_L is taken as unity since the aspect ratio with bending in this direction prevents buckling.

 

[KootK]

I think that the flat use factor still applies when you're bending the column about an axis parallel with the wide dimension of the individual laminations. While I'm not sure of this, I speculate that the flat use factor is a structural reliability thing. When a random knot shows up in a member bent strong axis it is more detrimental to flexural capcity than if the same knot shows up in a member bent about its weak axis. I've been out of of the wood game for almost a decade so you'll want to confirm this theory with another source. Woodman will probably jump in any moment now if we're lucky.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

I think I've applied the interaction equation correctly (15.4-3). Anyone is more than welcome to look at this excel spreadsheet and if you notice anything amiss please advise.

I've temporarily posted it here:

http://design.medeek.com/resources/column/

 

[medeek]

In the NDS 2012 supplement on page 30 it states that the Cfu should be applied when the loading is flatwise and not load applied to the narrow face. In the situation with 3 or 4 2x4 studs nailed together and bending about an axis parallel to the wide dimension of the laminations, the nailed beam-column as a whole is actually bending about its strong axis and hence I think the Cfu factor should not apply. However, the bending is about the minor axis of the 2x4 studs if you consider the laminations separately, so in this sense it should apply.

I guess my question is with this sort of situation do you consider the built up beam as a whole or the laminations separately with respect to applying the flat use factor.

 

[msquared48]

Unless this a stand alone column or has sheathing on only one side of the wall, this a moot point due to the lateral support of the sheathing in the weak axis.

Mike McCann, PE, SE (WA)

 

[medeek]

If the flat use factor is applied when the moment is causing bending about the minor axis for a (3)-2x4 stud built up column (considering the built up column as a whole) then it would affect the outcome since the sheathing is bracing in other direction.

 

[medeek]

It really comes down to how to apply the flat use factor in the case of a built up column where the width of the built up 2x4 or 2x6 exceeds the depth of the lumber. For example three 2x4's would be 4.5 inches which is greater than the 3.5 inches of the 2x4 depth.

 

[medeek]

For a 2x6 column it would require 4 laminations (6 inches) before the strong axis would change 90 degrees. If you compare this situation to how you treat glulam beams you will see what I'm getting at. When glulam beams are oriented so that they are loaded in bending about the y-y axis and d_y is less than 12" a C_fu is applied. In my mind a built up column is essentially equivalent to a glulam beam but with mechanical fastening between laminations instead of glue.

With that being said the real question becomes what is the appropriate C_fu to use for this situation?