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RE: Wood Shear Wall Top Plate Shear Transfer

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I’m not sure if I am getting the question correct or not but thought I might help to simplify it somewhat with respect to the distribution of shear through a diaphragm to a shear wall below.

1.       At the building edge, the length of the collector is determined based on the capacity of the nailing (size and spacing) that equals the capacity of the shear wall(s) below. If the wall is narrow and the double top plate is spliced with a typical 48-inch overlap splice, then the length of the double 2x top plate will determine the size and spacing of the nails. In short if the shear wall is something like 2400 pounds in an 8’-0” long x 8’-0” tall shear wall then the structural panel connection to the wall needs to resist 300 plf of force. If the double top plate is 10-feet long, then an 8d nail at 6” o.c. or 240 plf should be sufficient to transfer the 2400 pounds of diaphragm force to the wall. If the joist is parallel to the wall, then the diaphragm transfers the shear to the joist and the joist must make the connection to the double top plate of the wall. If the joist is parallel to the wall, then the blocking can be connected to the double top plate with an A35, L50 or similar lateral clip. I generally use one clip per block as a minimum requirement to make the shear transfer to the wall. The clip, if good for 500 pounds (these values are only approximations for learning) and the blocks are spaced at 16” o.c. then you need to verify if you have sufficient clips from blocking to double top plate to make up the 2400 pounds that were originally required.

2.       If the clips or nailing is either not sufficient or there are multiple walls, then you need a collector that can pick up the necessary capacity at the discontinuity if placed end to end. Some engineers use continuous straps above the roof diaphragm while others will nail the necessary additional length of strapping needed to the bottom of the blocking and then to the top of the double top plate if the joists or trusses are perpendicular to the wall.

 

If you are dealing with an interior shear wall, you can pick up shear from both sides of the diaphragm tributary to the wall, but you still need to develop the shear through the diaphragm and down into the shear wall using some method (mechanical connector, drag truss, joist parallel, shear clips such as the L50’s or new replacements for the old A35-f etc.)

 

Once done, you still have to address the tension and compression on the wall and carry the shear down through the wall into the diaphragm below where the reaction in this line of shear is added to the shear distributed through the diaphragm on the level below. In most cases if the walls do not stack then there is a vertical plan irregularity that must be dealt with and this triggers in the need of boosting the diaphragm shear (horizontal shear) using an overstrength factor and the redundancy factor in the wall to compensate for the irregularity that is allowed for the Seismic Design Category you are located in (or wind design requirements whichever govern). In any case, you need not forget that you may have an overabundance of moment acting at the base of the upper shear wall that needs to be designed into the double joist or beam that the wall bears upon (or on a foundation and/or foundation in a slab on grade).

 

I like to put it in terms of molasses running down the load path from roof to foundation and follow the flow through the nails, diaphragm, drags and collectors and on down to an adequate foundation.  This analogy also helps a layperson like the client or an inexperienced contractor understand the principle of a load path. The one point I would like to make is that where I use trusses, I will always specify a drag truss concentric with the wall below. I think it is worth the price of the additional truss or two to keep the load path directly over the wall and try whenever possible to keep my walls stacked. This last part is, for most practical applications, nearly impossible when designing a custom home but generally the rule of thumb in commercial/industrial structures of light-framing where as much shear is placed at the exterior of the building to allow for future expansion and maximization of leasable space. Residences do not generally look that far into the future to see what the next owner will want to do with the design.

I hope this helps clarify the answer you are seeking.

 

Regards,

Dennis

 

From: Gordon Goodell [mailto:GordonGoodell(--nospam--at)harmonydesigninc.com]
Sent: Tuesday, August 12, 2008 7:42 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wood Shear Wall Top Plate Shear Transfer

 

Michel,

 

The diaphragm load is distributed along the entire length of the dbl top plate assuming, as David points out, adequate top plate splicing.  For an interior shearwall, with load being taken out of the diaphragm to each side, the allowable load of the diaphragm along that line is effectively doubled.  Looked at another way, the nail or connector  spacing to the top plate is halved.  The load is taken out of the top plate and into the shearwalls with shearwall edge nailing, which is determined based only on length of shearwalls on that line.

 

regards,

Gordon Goodell