Shear Walls - A
In the late 50's and early 60's, plywood walls were proposed
simply as alternates to diagonally braced wall sections. Their acceptance
for this purpose was based on their ability to meet certain load/deflection
criteria as set forth by the Federal Housing Administration. The following
is a copy of the relevant FHA Circular for your reference.
(beginning of text)
A STANDARD FOR TESTING SHEATHING MATERIALS FOR RESISTANCE TO RACKING
FHA Technical Circular No.12
The following section is a reprint in full of Federal Housing Administration Technical Circular No. 12, entitled, A STANDARD FOR TESTING SHEATHING MATERIALS FOR RESISTANCE TO RACKING. It is included here for reference only, giving the criteria used by FHA in determining acceptability of sheathing materials for use without corner bracing. No acceptance or approval by FHA is implied.
FEDERAL HOUSING ADMINISTRATION
October 5, 1949
A STANDARD FOR TESTING SHEATHING
FOR RESISTANCE TO RACKING
Technical Circular No. 12
The following test procedure is provided in view of the present demand for broadening use of wallboard sheathing, pending establishment by the industry of a Commercial Standard, an ASTM Standard or other nationally recognized standard for strength and rigidity properties.
Structural acceptability of wood frame wall construction without diagonal corner bracing will be subject to demonstration by an independent laboratory test report that the proposed construction provides resistance to racking stress at least equal to that provided by walls with horizontal wood board sheathing and let-in corner bracing complying with our Minimum Construction Requirements.
Test panels 8' x 8', double top plate, double end studs and single sole plate with wallboard sheathing shall be framed conventionally except for omission of diagonal braces, and sheathing shall be attached as recommended by the manufacturer. All materials shall be seasoned to not more than 19% moisture content. If desired by the manufacturer or if necessary to meet our minimum strength and rigidity requirements we will consider framing with studs less than 16 in. o.c. and attachments other than nailing.
All panels shall be tested to failure as described in Building Materials and Structures Report BMS-2 of the National Bureau of Standards.* Test results shall be based upon three separate specimens. Specimens shall be furnished for both dry and wet series, a minimum of six specimen panels being required.
For wet series, panels shall be mounted or suspended in vertical position in such a manner as to prevent continuous immersion of the bottom edge and exposed to water spray applied at and near the top of the panel simultaneously on both sides for periods of 6 hours. Three spray periods shall be applied with drying periods between of 18 hours at room temperature of approximately 700 F. Load test shall be commenced within two hours after completion of third period of spraying.
*BMS-2 has been superceded by ASTM E72.
The following is based upon current information regarding the strength of standard wood sheathed panels and until new test data have been developed the following loads and deflections shall be considered the criteria for comparison:
Dry Tests - Load increments 400 lbs.
Maximum load 5200 lbs. or 0.65 kips/ft.
Wet Tests - Load increments 400 lbs.
Maximum load 4000 lbs. or 0.50 kips/ft.
A basis for omission of corner braces with a particular wall board sheathing is provided when the test results indicate satisfactory physical characteristics. Manufacturers of sheathing materials which show such favorable test results will be requested by these Headquarters to certify to FHA field insuring offices in which they intend to operate that a wall sheathed with their particular board) when installed in accordance with the manufacturers instructions, is at least equal in racking strength to a horizontally wood board sheathed and corner braced wall.
s/Curt C. Mack
t/Curt C. Mack
(end of text)
So, to summarize, a plywood shear wall should only deflect a set amount under a given load and its permanent deformation should be limited.
In 1966, the American Plywood Association published Plywood Shear Walls (Laboratory Report 105) which provided test results for 39 different test combinations of sheathing, nailing and framing. You can reference that publication for the actual testing procedure but the results are summarized in the table below.
|TABLE 8 - SUMMARY OF RECOMMENDATIONS FOR TEST PANELS|
|(From APA Laboratory Report #105. See that publication for footnotes)|
|12||180 dry||0.206 wet||5370 wet||3.7|
|13||180 dry||0.191 wet||5440 wet||3.8|
|14||180 dry||0.187 wet||5145 wet||3.6|
As you can see, the test panels not only met the deflection requirements of FHA, they were capable of resisting a significant amount of load before failure. The additional FHA restriction involving limitation of permanent deformation is met by restricting the allowable shear in walls to a percentage of the ultimate value (Ultimate Load / Load Factor). This can best be seen by viewing by the following:
|Plywood Diaphragm Test Results|
|If you remember the discussion of E, the Modulus of
Elasticity, this graph should be familiar.
A lateral force-resisting system is subjected to increasing loads (Y axis) and its deflection is
plotted (X axis). Three test are plotted. The first test reached 800 pounds. Note that the
diaphragm almost returned to its same shape. In test two, the same diaphragm was loaded
to 1500 pounds...and when the load was removed the diaphragm retained a permanent
change in shape. In test three, the diaphragm was loaded until it failed.
Unlike the single material graph, this system depends
on the strength of the material and the
Based on this and other testing data and by incorporating knowledge gained from plywood diaphragm testing, the 1967 Uniform Building Code for the first time provided allowable load tables for plywood shear walls. If you are interested in how they arrived at the published values, go to the previous page and read all about it.