Asymmetric bending in wood roof purlins
Asymmetric bending in wood roof purlins
(OP)
What is the best way to approach asymmetric bending forces in wood? For instance, if you have a pitched-roof purlin with the narrow face against the sheathing, such that dimension "d" is not vertical, how do you calculate Fbmax for snow load?
Would you break the vertical load into components perpendicular to the sawn member dimensions, calculate two Fb's, then add them together for a combined Fbmax at the most extreme fiber, which is at the vertex of the two member faces?
Would you break the vertical load into components perpendicular to the sawn member dimensions, calculate two Fb's, then add them together for a combined Fbmax at the most extreme fiber, which is at the vertex of the two member faces?






RE: Asymmetric bending in wood roof purlins
Take a look at these two free .pdf downloads:
"Structural Design Loads for One- and Two-Family Dwellings", Paragraph 7.
http://www.huduser.org/publications/destech/strdes...
"Residential Structural Design Guide, 2000 Edition", Paragraph 3.7
http://www.huduser.org/publications/destech/reside...
You may find both of these documents (and others at this website) useful for many situations.
RE: Asymmetric bending in wood roof purlins
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RE: Asymmetric bending in wood roof purlins
RE: Asymmetric bending in wood roof purlins
RE: Asymmetric bending in wood roof purlins
RE: Asymmetric bending in wood roof purlins
The shathing will only act as a diaphragm IF it is capable of doing so. If it cannot act as a diaphragm them one of two things can be considered.
Case one - sheathing is in compression due to parallel load component. A support is needed at eaves and is usually provided by the wall for vertical component and ceiling ties for horizontal. In this system the purlins are designed for major axis bending (as described by doejohn).
Case two - If there is no support at eaves AND no diaphragm action in the sheathing, the purlins are subject to biaxial bending. The exact approach to this will be specific to your design code. My normal method is to split the load into major and minor axis components. The stress at the outermost fibre is the sum of the two axis stresses. Don't forget to check deflection (using pythagoras theorem) as this is frequently the most onerous condition.
RE: Asymmetric bending in wood roof purlins
If pitch is small then you can probably ignore minor axis bending. Conservatively ignore the diaphragm action and design for biaxial bending. The Australian code has provision for this so I would suppose your local code does too.
If spans are long you may need bridging between the purlins.
RE: Asymmetric bending in wood roof purlins