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PLYWOOD REDUNDANCY SHEAR WALL -NOT BAD

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REDUNDANCY FACTORS FOR PLYWD SHEAR WALLS

I agree with the code defined rho factor and its application for wood 
design. Added construction costs seem small. The change mostly will not 
increase wall size. One can still design small walls with increased
nailing. 
There may be a way to reduce shear demands. There may be ways to 
reduce the amount of engineering.

It is the intent of the code, to identify a system with low redundancy and
to 
increase the design forces. The intent has been accomplished. 

Redundancy of plywood shear walls should be one of our goals. Now we 
have a code argument to defend our attempts for redundancy. 

Other walls exist as unintended shear walls (gypsum-sheathed walls) and 
may act as a redundancy.  Assuming unintended shear walls help to 
achieve redundancy, is short of properly using gypsum walls as shear 
walls with the uplift and nailing designs. Maybe they are not as redundant 
as the old assumption.

********
GYPSUM VS PLYWD SHEAR WALLS 

Using the gyp-board walls for shear will increase the seismic-force at
least 
22% and may reduce the redundancy factor to 1.0. 

The following case results in a redundancy factor of 1.22 when ignoring 
the gypsum. In this case, either way the base shear is about the same and 
redundancy is addresses per code. 

This is a single story rectangular roof. Weight=20psf, area=35x75=2625 
sqft, Shear wall schedule: 15/32? 10d@ 4 double ply.  One 4 ft wall at 
each exterior wall. Either approach results in the same base shear the 
gypsum board system has rho=1 with an R=5.5, and the plywood system 
has a rho=1.22 with R=4.5. 

With same capacity, the double-sided gypsum sheathing needs to be about 
12 feet long. The 12 feet of construction has much more redundancy than 
the 4 feet of plywood. It is very rational that the small plywood wall
needs 
to be designed with a higher force level to match the redundancy. With out 
the redundancy factor for the short plywood wall the gypsum sheathing is 
the better design.

For the integrity of gypsum shear walls, I have found a testing report that

suggests that with all design factors, gypsum performs as well as plywood 
and better than OSB. See web site:  
http://www.engr.sjsu.edu/dmerrick/shearwalls/

*********
CONVENTIONAL CONSTRUCTION

One complaint I have, is that there is more engineering effort in using the

new code. The additional engineering seems to have been important to 
better the weakness of previous codes. There is added wording for 
conventional construction where we can now engineer parts of a building 
that does not comply with the conventional construction provisions. This 
could reduce some of the work, if the building official allows it.

*********
DYNAMIC PERIOD

Another complaint I have is that the wood frame seems to perform better 
than that recognized by the code. The building is more flexible than what 
the static analysis assumes to derived the base shear. With the code, I
have 
found a constant reduction factor for all plywood shear wall systems, for 
each soil and foundation condition. The constant is derived from two 
simplifying and conservative assumptions. 

1. All plywood designs have the same deflection of at least 0.4 inches. 
2. Assume an infinite story stiffness at all but one level. Only one of 
multiple stories will break free from the unintended-shear-walls to 
deflect to its capacity of 0.4 inches.  This is a dynamic single spring 
and mass model. Conservative, because if more floors break free then 
the period increases and the shear reduces even more. The higher 
stiffness will not deliver a higher shear to a plywood wall. As the wall 
receives the load, it will deflect and that increases the dynamic period, 
reducing the shear to the concluded shear or less because of the 
conservative assumptions.

In solving the static code equation, using the natural period, the Mass and

stiffness terms drop out and one is left with a constant for any plywood 
building on a specific site. For the better soils, the reduction is only 
limited by the code?s allowed minimums. This can be set up as a generic 
proof for office files. This is code, the law and an incredible reduction
of 
plywood shear demands. 

One argument against analyzing the dynamic properties of plywood shear 
walls is that it is incredible and never done. I find a plywood dynamic 
property analysis is used when the frame is over concrete or masonry 
floors. The base of the plywood is allow to be considered at the top of the

rigid system. This is allowed only if relative deflections and periods are 
analyzed (dynamic properties analyzed) and proven to be substantially 
different. This is using the static force procedure, method B using
equation 
30-10.

***********
REDUNDANCY NEEDS TO BE CONSIDERED.

I agree with the code redundancy factors for all buildings. The affects on 
plywood systems are not that bad. There may be a way to reduce the 
demand. There may be a way to reduce the amount of engineering.

David B. Merrick, SE
MERRICKGROUP(--nospam--at)COMPUSERVE.COM