Timber Purlin - Biaxial Bending?

[waytsh]

Hi All,

Quick sanity check question on a timber purlin I am reviewing in an old barn. The purlin is not rotated with the roof as is typical but is vertically oriented relative to the ground. The top has pockets cut into it with the slope of the roof so the rafters can run over top of it.



In this situation can I consider the purlin to be loaded solely on its strong axis? The Timber Construction Manual seems to indicate that biaxial bending only needs to be considered when the purlin is rotated with the slope of the roof.





Thanks in advance

 

[jhnblgr]

In theory if idealized as a simple beam rafter wouldn’t the assumption be the strong axis is being loaded? Neglecting the lateral reactions per industry standard for pitches less than or equal to 12/12. But the section taken due to the notch would reduce the bending capacity of the purlin? If you have bi-axial loading how is that being resisted to prevent thrust outwards?

 

[HS_PA_EIT]

I don't think so. Considering the free body diagrams, it would be like a mass on an inclined plane. The reaction is normal to the plane of the interface, which results in loading about the strong and weak axis.
That's my take.

 

[waytsh]

I don't necessarily disagree, but that does not seem to be the way the Timber Manual is considering load application to a purlin. Maybe I am missing something.

 

[jhnblgr]

Post you FBD and design assumptions.
Is the rafter designed as a beam or a wall (typically only slopes exceeding 12/12.

Is there a ridge beam?

 

[JAE]

I assume this is a roof with DL/SL downward but wind perpendicular to the rafters to the purlin is definitely in biaxial bending in some load combinations

 

[HS_PA_EIT]

I don't necessarily disagree, but that does not seem to be the way the Timber Manual is considering load application to a purlin. Maybe I am missing something.

I see what you're saying, but when I read it, it sounds like they are just giving the case of the rotated purlin as one example of "many" possible cases where the member is loaded obliquely. But I'm just looking at the section you provided.

 

[waytsh]

Is there a ridge beam?

No ridge beam but the building a asymmetrical so there is a purlin on the opposite slope only about 5' down from the ridge. The slope that this purlin is on has two rows of purlins of which this is one. All purlins are oriented in this manner.

I assume this is a roof with DL/SL downward but wind perpendicular to the rafters to the purlin is definitely in biaxial bending in some load combinations

Correct, and the slope is 8:12. So would you be inclined to treat the gravity loading on the strong axis and the wind loading as oblique loading?

By the way, thank you all for taking the time to respond. I appreciate all the input.

 

[JAE]

I would treat the gravity load as strong axis bending (that's the direction of the load after all) and the wind as a pair of oblique loads.

 

[gte447f]

To be technically correct, you should analyze for biaxial bending. It is exactly as @HS_PA_EIT says: even though it is a vertical gravity load, the sloped interface, due to the notch cut out of the purlin, will transfer the force as a normal force perpendicular to the interface and a friction force parallel to the interface. These forces will result in biaxial bending in the purlin. This is not at odds with the excerpt you posted from the Timber Construction Manual. It is actually basically the same scenario, due to the sloped interface, even though the actual orientation of the purlins is different.

In practice, biaxial bending is often often ignored in these scenarios.

 

[waytsh]

To be technically correct, you should analyze for biaxial bending

I guess I better bite the bullet and design this reinforcing for biaxial bending. If any of you all have a good suggestion for how to do this I am all ears. My thought is stack a wood beam of the same width below the purlin to allow room for taller side plates. I will design the side plates for the vertical bending and the new wooden beam underneath the purlin for the lateral load. The side plates would be bolted to the existing purlin and the supplemental section. The purlin has cracked both top and bottom at mid-span so at this point it is basically worthless but the contractor does not want to have to remove it from the rafters and make a new one.