Shear Wall Anchoring 1

[Deutero]

RE: NDS

I've often wondered why there is not a value for shear perpendicular to wood.

The tension of the overturning forces of a shear wall, that would be resisted at the sill anchor certainly will cause stress at the sill plate cross section. What kind of force is applied to the sill plate?

The bolts' washer applying compression perpendicular to the wood does'nt make sense since the washer area would be less than one square inch, thus yielding a very small resisting value.

If this is a negligible consideration, I would like to know why?

 

[jike]

The holdown anchor carries the tension load directly into the foundation. Therefore the sill plate never sees the tension load.

 

[Deutero]

jike,

I should have been more descriptive. I am not using a "holdown anchor" as in Simpson or other. I am referring to the "old-fasioned" shear wall with spikes or threaded fasteners at 32" oc through the sill plate.

Also, I am not anticipating the sill plate to experience tension. I realize the sill anchor will be in tension. However, the uplift of the shear walls sill plate will cause the anchor to be tension. Therefore, exactly what stresses are occuring at the sill plate cross section?

 

[DaveAtkins]

I have conservatively used the compression perpendicular to grain to determine the uplift capacity of a sill plate anchor bolt, and the value is fairly low (315# for 1/2" diameter anchor bolt with 1" diameter washer). This method ensures that no sill plate deformation will occur.

I have also used "punching shear" through the sill plate, which yields a MUCH higher value. For this method, I use Fv' = Fv*CD*CH times the failure area (shear perimeter times thickness of sill plate). While this method prevents complete failure, I think it would allow deformation in the sill plate.

DaveAtkins

 

[UcfSE]

The NDS gives the weaker of the shear perpindicular to the grain and parallel to grain. Horizontal and vertical shear stresses at a given point are equal so it makes sense to use only the weaker design value.

 

[Deutero]

DaveAtkins,

Where did you find a value for Fv since we are dealing with shear perpendicular?

What is CH?

Would there be considerable plate deformation if the shear panels are fastened to the sill plate at 6" oc?

Thank you!

 

[frogit22]

oversize washers are often used (in modern buildings) in this application. It improves the load capacity considerably.

 

[DaveAtkins]

UcfSE,

I think we're talking about bolt pullout, not bolt shear (but I could be wrong).

Deutero,

Fv is listed in the NDS Supplement for each species of wood, and CH is the Shear Stress Adjustment Factor.

DaveAtkins

 

[Deutero]

DaveAtkins,

NDS does not list a value for Fv perpendicular. According to UcfSE, the weaker of the two shear values is listed. Does this mean I can apply the parallel value to a perpendicular cross section?

Where in NDS is "CH" listed. I cannot find it.

Thank you!

 

[IceNine]

CH is found in the NDS Supplement as an adjustment factor for Tables 4A-D

 

[Deutero]

I found it!

However, that is in the 1991 Edition. "CH" is not in the 2001 or reportedly in the latest 2005 edition. Apparently it is discontinued and no longer used since the shear parallel values have nearly doubled.

 

[RARWOOD]

Deutero, you are correct the Ch factor was eliminated when the allowable shear values were increased in the 2001 & 2005 NDS.

There are a few problems in using the sill plate bolts to resist the plate uplift. 1. The force in the shear wall chord has to be transfered into the sill plate. 2. Shrinkage of the sill plate may leave a small gap between the top of the sill plate and the nut.

If you assume that the washer is held tight to the sill plate, then as the plate attempts to lift up, the washer is bearing perpendicular to the grain. The bearing stress must be less than or equal to the allowable compression perpendicular to the grain.

Under certain conditions you can apply the bearing area factor to increase the allowable compression perpendicular to the grain. See section 3.10.4 Bearing Area Factor, Cb in the 2005 NDS.