Construction Business & Technology Conference

Shear Walls - A History

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.

UNDERWRITING DIVISION
FEDERAL HOUSING ADMINISTRATION
Washington, D.C.

October 5, 1949

A STANDARD FOR TESTING SHEATHING MATERIALS
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.

TEST PROCEDURE

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.

(page 2)

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.

At load of 1200 lbs., average total deflection less than 0.2 in. Residual deflection less than 0.1 in.
At load of 2400 lbs. average total deflection less than 0.6 in. Residual deflection less than 0.3 in.

Wet Tests - Load increments 400 lbs.
Maximum load 4000 lbs. or 0.50 kips/ft.

At load of 1200 lbs. average total deflection less than 0.28 in. Residual deflection less than 0.14 in.
At load of 2400 lbs. average total deflection less than 0.8 in. Residual deflection less than 0.4 in.

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

Assistant Commissioner,

Underwriting
(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)

Specimen
Number

Recommended
Design Shear
(plf)

Deflection at
Recommended
Design Shear
(in.)

Ultimate
Load
(lbs.)

Ultimate Load
Divided by
Recommended
Design Shear
(Load Factor)

1 200 0.165 6800 4.3

1A 200 0.165 7600 4.8

2 200 0.152 7500 4.7

3 200 0.175 6800 4.3

4 400 0.145 12800 4 0

5 900 0.271 20000 2.8

6 200 0.140 5200 3.3

7 200 0.187 6000 3.8

8 200 0.185 5330 3.3

9 180 0.160 5800 4.0

10 180 0.146 6000 4.2

11 180 0.159 6000 4.2

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

15 140 0.159 4400 3.9

16 160 0.206 5200 4.1

17 160 0.16 6000 4.7

18 160 0.123 4700 3.7

19 140 0.105 5590 5.0

20 160 0.110 5200 4.1

21 240 0.216 9340 4.9

22 210 0.250 7170 4.3

23 210 0.15 8610 5.1

24 160 0.119 6400 5.0

25 200 0.088 10800 6.8

26 no plywood ----- 3200 ---

27 210 0.115 7700 4.6

28 210 0.190 8375 5.0

29 210 0.129 7650 4.6

30 430 0.160 14900 4.3

31 160 0.127 4850 3.8

32 210 0.150 6200 3.7

33 --- ----- 3870 ---

34 180 0.170 6610 4.6

35 180 0.175 6535 4.5

36 160 0.133 5095 4 0

37 770 0.335 20000 3.3

38 460 0.270 15050 4.1

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
holding power of the fasteners. The curve to the graphed lines indicate this.

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.

TABLE 8 - SUMMARY OF RECOMMENDATIONS FOR TEST PANELS
(From APA Laboratory Report #105. See that publication for footnotes)
Specimen Number	Recommended Design Shear (plf)	Deflection at Recommended Design Shear (in.)	Ultimate Load (lbs.)	Ultimate Load Divided by Recommended Design Shear (Load Factor)
1	200	0.165	6800	4.3
1A	200	0.165	7600	4.8
2	200	0.152	7500	4.7
3	200	0.175	6800	4.3
4	400	0.145	12800	4 0
5	900	0.271	20000	2.8
6	200	0.140	5200	3.3
7	200	0.187	6000	3.8
8	200	0.185	5330	3.3
9	180	0.160	5800	4.0
10	180	0.146	6000	4.2
11	180	0.159	6000	4.2
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
15	140	0.159	4400	3.9
16	160	0.206	5200	4.1
17	160	0.16	6000	4.7
18	160	0.123	4700	3.7
19	140	0.105	5590	5.0
20	160	0.110	5200	4.1
21	240	0.216	9340	4.9
22	210	0.250	7170	4.3
23	210	0.15	8610	5.1
24	160	0.119	6400	5.0
25	200	0.088	10800	6.8
26	no plywood	-----	3200	---
27	210	0.115	7700	4.6
28	210	0.190	8375	5.0
29	210	0.129	7650	4.6
30	430	0.160	14900	4.3
31	160	0.127	4850	3.8
32	210	0.150	6200	3.7
33	---	-----	3870	---
34	180	0.170	6610	4.6
35	180	0.175	6535	4.5
36	160	0.133	5095	4 0
37	770	0.335	20000	3.3
38	460	0.270	15050	4.1

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 holding power of the fasteners. The curve to the graphed lines indicate this.