Load Sharing of Multi-Ply Wood Plates

[dengebre]

In order for multi-ply wood plates (oriented flat-wise) to behave as a single unit, they must be sufficiently fastened in order to transfer the stress across the shear plane (a shear flow calculation). However, sometimes the amount of fasteners required can be problematic. Is it acceptable to assume the plies share the load equally, and just divide the force by the number of plies? This assumption makes sense for top-loaded multi-ply wood beams (plies oriented edge-wise), but I am unclear of the load sharing mechanism across the contact surface for wood plies oriented flat-wise.

 

[EngDM]

It depends on where the load is applied. Are you talking about loading laterally in the strong direction, or loading gravity such that plies are flatwise perpendicular to load?

If the latter, I typically calculate the resistance of a single ply, and share this load across multiple plies. For this you use the section properties of a single member. If you want/need to get a stronger calculation for this, then yes you need to perform a shear flow calculation to ensure the plys engage together and using the combined member section properties.

 

[TRAK.Structural]

Load will be shared equally if they are not adequately fastened to one another. Deflection and stress calculations will be based on 2x the section properties of a single plate, NOT composite section properties.

 

[dengebre]

My question pertains to wood plates that are loaded in weak-axis bending, such as double top plates in a wood bearing wall. I have a case where the nailing is excessive in order for the plies to act compositely, but works with minimal nailing if I assume the plies share the load equally. Just trying to convince myself that that is a reasonable assumption.

 

[XR250]

Is it acceptable to assume the plies share the load equally, and just divide the force by the number of plies? This assumption makes sense for top-loaded multi-ply wood beams (plies oriented edge-wise), but I am unclear of the load sharing mechanism across the contact surface for wood plies oriented flat-wise.

Yes, that would be a conservative assumption (assuming for instance - a dbl. top plate loaded vertically) . Friction and fasteners would add to the strength and stiffness.

 

[phamENG]

Load will be shared equally if they are not adequately fastened to one another. Deflection and stress calculations will be based on 2x the section properties of a single plate, NOT composite section properties.

They would share equally.

If they were composite, the stresses would be much higher in the top and bottom plies, and negligible in the center ply (so, not equal). If not composite, each ply works on its own, and will effectively carry 1/n of the load (where n is the number of plies).