cross grain bending in 3x top plate

[struct_eeyore]

I have an odd condition on a building I'm repairing that requires us to raise the exterior walls by 3".
Ideally, I'm looking to do this by adding (2) additional plates on top of an existing p.t. plate (over a conc beam - see attached snip)
However, because this is an exterior wall, I'm questioning if under lateral load this top plate will experience cross grain bending sufficient enough to cause problems (we're looking at about 1000 in-lbs)
I cant find anything in literature giving a limit stress.
Any and all comments appreciated.

 

[jayrod12]

Can you sketch out how you envision lateral load causing cross grain bending in that scenario?

What is the source of the lateral load?

What supports the concrete beam? Is it possible that the beam just goes along for the ride with any lateral movement of the diaphragm?

 

[Eng16080]

I second the comment above. I’m not really seeing how this is in cross-grain. I may well be missing something though.

Generally speaking, it’s best to avoid cross-grain bending (or really cross-grain tension), if possible. I want to say that I read somewhere that a stress of 50 to 100 psi might be ok. Can’t remember the details though so don’t take my word for it.

Is the lateral load that you’re concerned about perpendicular to the plane of the wall?

 

[ChorasDen]

Agreed with the other comments, perhaps the shear walls are designed as perforated, requiring uplift to be included in the design, and concern is uplift resolved into bending around the wall plate and anchor bolts? If that's the case, I would suggest installing wider washers to prevent the cross grain bending.

I want to say that I read somewhere that a stress of 50 to 100 psi might be ok.

This number sounds about an order of magnitude too high to me, but it's been quite a while since I've looked, I believe the Forest Products Lab has a design for wood handbook that might include some information, but keep in mind it's highly variable and the values are low, so the industry tends to treat the capacity as '0'.

 

[phamENG]

I believe the Truss Plate Institute allows up to 100psi for certain conditions when designing truss plates, but I tend to avoid it whenever possible.

 

[ChorasDen]

I do not recall a number of tension perp provide, but I recall something like a 3kip allowable load below the neutral axis, so perhaps you back calculated a value from TPI?

There is a long history of tension perp within the wood industry, but long story short, it's not a value that is controlled for, in that it is not tested when determining member properties, and because it's not tested for, and with it's low value, it has a high possibility to drift from sample to sample, as well as over time.

 

[struct_eeyore]

To clarify, the bending in the plate is a result of it acting as a continuation of the wall below, which is bending due to an external wind load.
If we imagine that the connection is slightly loose, and ignoring contribution of the uplift strap, then you get the image below. (arrow sizes are not proportional, and shears not shown). I would probably also ignore the reaction at the top left in a proper hurricane, since the selfweight of the structure is less than a blanket uplift.

 

[phamENG]

Best bet there is a plate washer.

 

[struct_eeyore]

@Pham - duly noted.
However, the threaded rods are only at 24" o.c., which also leaves section of the 3ply unrestrained in between.
I'm thinking to add another strap on the opp side to create full moment restraint - question is, would this be overkill?

 

[phamENG]

If the concern is cross grain tension caused by bending from the couple formed by the anchor tension and compression against the concrete interface, the material between anchors isn't a concern.

Look at uplift in a sill plate. We only put plate washers at the anchors, because there's nothing to make it bend that way anywhere else.

 

[struct_eeyore]

@Pham,

Not quite. If we take the the 3ply plate as an extension of the wall below, which is is a beam out of plane - then the cross grain moment is the moment at the end of an otherwise simply supported wall-beam - which is then roughly resolved as a couple between the edges of the wood and the anchor rod.

 

[dold]

would this be overkill?

Yeah. I'm with pham. Just use a plate washer and be done. I think you might be a little lost in the weeds on this one.

 

[phamENG]

which is then roughly resolved as a couple between the edges of the wood and the anchor rod.

Exactly. Between the anchor rod and the plate. So that's where you need to prevent cross grain bending. At the anchor rod.

 

[struct_eeyore]

I'm going to beat this one a little more. I've revised the wall from being a pinned-pinned to fixed at base to maybe clarify. There will be unrestrained sections of the top plate between anchor rods. The cross grain moment in the plate is the same as the moment toward the end of the cantilever. Granted it is not large, but it is there nonetheless.