Again, the statics does not change. The second floor shearwall plate
transfers the shear from the diaphragm above. The sheathing, extended to the
edge of framing below the plate) transfers the diaphragm shear from that
level - therefore, the plate of the shearwall need only transfer the wall
demand through the sheathing below and into the blocking between floors.
Remember that the diaphragm shear is uniformly distributed at the diaphragm
edge. The nailing of the diaphragm is responsible for the transfer of shear
to the wall below.
However, the loads are cumulative so you need to account for all loads at
the bottom of the shearwall and make sure that the sum of these loads is
properly transferred down to the level below.
There has always been a question in my mind as to whether or not it is
necessary to assure that the capacity of the wall above is accounted for
within the confines of the shearwall. In my opinion, if the plate is
continuous, the capacity of the wall need only be distributed through the
continuous plate with a minimum nailing as stipulated by code. If a plate
splice is provided which maintains the continuity of the plate, it would
seem reasonable that the demand from the wall can be distributed through a
long plate and therefore advance the spacing of the nails (or bolts in the
foundation) in such a manner as the total shear in that line is accounted
for or no less than the minimum specified in the code (i.e., 4 or 6 feet on
center for anchor bolts and 4 or 6" on center with nails and spikes).
The more common approach is to consider the total cumulative value of the
shear at the second floor diaphragm (Roof + 2nd Floor diaphragm) and to
transfer the shear through the side of the plate and the face of blocking
below by mechanical connector (i.e., Simpson A35F's or equal).
The critical area (if we consider the nailing of the shearwall plate and the
diaphragm nailing above to be sufficient) is the connection of the blocking
to the double wall plate below. Again, this is where mechanical connectors
make a lot of sense for a very small price.
The most important thing to understand is that the shear is cumulative from
roof to foundation and needs to be accounted for within the connection of
the bottom of the walls and the top of the walls in the level below.
Finally, in a long about way I guess I showed you that you can distribute
the shear further along a continuous plate as long as you account for any
shear loss due to a discontinuity or plate splice.
Dennis Wish PE
From: Jake Watson [mailto:jwatson(--nospam--at)inconnect.com]
Sent: Sunday, January 30, 2000 8:05 AM
Subject: Re: 3x_ versus (2)-2x_
> Jeff Creagan wrote:
> A common request by contractors is can they substitute (2)-2x_ for the
> 3x_ studs we specify heavily nailed shear walls and bearing studs in
> the lower levels of multi-story buildings (here in Seattle, 5 story
> wood-framed structures are permitted) so long as the studs are spiked
> together adequately to ensure shear transfer at panel edges.
Another question, if I design the A.B.'s for double the force (or half
stress) I can use 2x's below 600 plf. What do you do with the wall
above on a two story residence? There is no anchor bolts, so is 2x ok
up to 600 plf?
Jake Watson, E.I.T.
Salt Lake City, UT