How did they arrive at each of the shear values listed in the UBC?

Plywood diaphragm research preceeded plywood shear wall research, so many of the formulas used to determine shear wall strength were adapted from the eariler research.  Here's an example from the American Plywood Association's Research Report #154, Structural Panel Shear Walls, revised in May, 1993 and available from APA - The Engineered Wood Association (206) 565-6600 (ask for publications):

(beginning of text)
APPENDIX B

CALCULATION OF DESIGN SHEARS

The building codes allow calculation of diaphragm and shear wall values using the principles of mechanics. Such a calculation involves several factors not shown in the building codes, such as influence of framing lumber width and panel thickness versus nail size. This appendix lists these factors and details the steps required to calculate design shears.

The currently accepted shear wall design values were based on applying a load factor to ultimate loads from tests of actual shear walls. The relationship between the design shears for different nail sizes is based on the relative lateral design values for the nails in the Uniform Building Code at the time the basic shear wall research was conducted. Lateral nail values and their relationships were changed in the 1964 Code; however, the original tabulated values based on tests were never adjusted to accommodate these changes or to make them correlate with the new individual nail design values. As a result, computation of shear wall design loads using currently accepted nail values and the design factors listed in this appendix will give conservative results.

Shear Wall Design Factors

Previous tests of fasteners, shear walls and diaphragms have established the following factors to be used in the calculation of design shears.

1. Plywood containing Species Group 2, 3 or 4 veneer.
   • Design shears are 90 percent of those for Structural I (all-Group 1) panels of the same thickness, for the same nail size and spacing; or
   • Design shears are 100 percent of those for Structural I (All-Group 1) panels one size thinner, for the same nail size and spacing, if minimum nail penetration into framing is maintained.
2. Design shears are reduced 11 percent when 2-in. nominal lumber is used.
3. Framing at adjoining panel edges shall be 3-in. nominal or wider, and nails shall be staggered where nails are spaced 2 or 2-1/2 in. o.c., or where 10d nails having penetration into framing of more than 1-5/8 in. are spaced 3 in. o.c. [Also note the new requirement for 3x's at panel edges in the 1997 UBC where design shears exceed 350 pounds per foot. - Ed.]
4. Nailing 4 in. o.c. at panel edges of blocked shear walls is used as the "basic" shear to derive the following values for unblocked shear walls.
   • For studs 16 in. o.c., use 50 percent.
   • For studs 24 in. o.c., use 33 percent.
5. 4. Design shears for 3/8-in. panels placed parallel to framing 24 in. o.c., must be reduced 17% to account for panel buckling (8.5% for 7/16-in. panels).

Nailing Examples

Example No. 1

8d common nails, 3".o.c., 3/8" APA Rated Sheathing, parallel to 2x Douglas Fir-Larch framing 24" o.c.

76 x 1.1 x 1.6 x 0.89 x 0.90 x 4 x 0.83 = 356 plf

• (0.83) - 17% reduction for framing 24 o.c.
• (4) - nails per foot
• (0.90 )- reduction for non-Structural I Rated Sheathing
• (0.89) - 11% reduction for 2x lumber
• (1.6) - 60% increase for wind and seismic
• (1.1) - 110% increase for nails used in diaphragm construction
• (76) - design lateral load for 8d common nail.

Use 355 plf (Recommended shear value in building code is 410 plf)

Example No. 2

10d common nails, 3" o.c., 15/32" APA Rated Sheathing, 3" Douglas fir-Larch framing 24" o.c.

90 x 1.1 x 1.6 x 0.90 x 4 = 570plf

Use 570 plf (Recommended shear value in building code is 600 plf)
(end of text)

Example No. 3

For comparison, let's redo the same examples using uncoated box nails as fasteners...

Nail Design Values for Single Shear Connections - Combined Table
(Both members of identical species)
Side Member Thickness (inches)	Nail Length (Inches)	Nail Diameter (Inches) D		Penny Weight	G=0.55 Southern Pine (lbs.)		G=0.50 Douglas-Fir Larch (lbs.)		G=0.42 Spruce-Pine-Fir (lbs.)
ts	L	Common	Box		Common	Box	Common	Box	Common	Box
0.5	2	0.113	0.099	6d	67	55	59	48	47	38
	2.5	0.131	0.113	8d	85	67	76	59	61	47
	3	0.148	0.128	10d	101	82	90	73	73	59
	3.25	0.148	0.128	12d	101	82	90	73	73	59
	3.5	0.162	0.135	16d	117	89	105	79	87	65
	4	0.192	0.148	20d	137	101	124	90	103	73
	4.5	0.207	0.148	30d	148	101	134	90	112	73
	5	0.225	0.162	40d	162	117	147	105	123	87
	5.5	0.244	-----	50d	166		151		127
	6	0.263	-----	60d	188		171		144

Example No. 1

8d box nails, 3".o.c., 3/8" APA Rated Sheathing, parallel to 2x Douglas-Fir Larch framing 24" o.c.

59 x 1.1 x 1.6 x 0.89 x 0.90 x 4 x 0.83 = 276 plf as compared with 355 plf using common nails.

Example No. 2

10d box nails, 3" o.c., 15/32" APA Rated Sheathing, 3" Douglas fir-Larch framing 24" o.c.

73 x 1.1 x 1.6 x 0.90 x 4 = 462 plf as compared with 570 plf using common nails.

Plywood Strength

For a point of reference, what is the strength of the sheathing material versus the calculated shear based on nailing?  You'll need three bits of information:

1. The effective thickness in shear for the particular panel you are using.  This is available in Volume 3 of the 1997 Uniform Building Code, pages 3-420 and 3-421.  The column is labeled "Effective Thickness For Shear".  Pay attention to the treatment of the panels in manufacturing (sanded, unsanded, and touch-snaded) as this affects their strength characteristics.
2. The Species Group of the panel you are using.  This can be obtained from the manufacturer or supplier.  It is not to be confused with the Exposure Classification (Exterior, Exposure 1, or Exposure 2) which relates to the type of adhesives used in panel assembly. Engineers tend to call for Structural I panels by reflex because we know their component plies are all Group 1 species.  It eliminates possible confusion in the field.  If you want to learn more about panel grades, species and veneers, consult the APA Product Guide, Grades and Specifications, available from APA - The Engineered Wood Association.
3. The allowable shear in the plane of the sheathing plies. This table is from page 2-295 of the 1997 Uniform Building Code.

Allowable Unit Stresses for Construction and Industrial Softwood Plywood
(In pounds per square inch - normal loading)
(To be used with section properties in Plywood-design Specifications - See UBC Standard 23-2)
Stress	Species Group of Face Ply	Exterior A-A, A-C, C-C		Exterior A-B, B-B, B-C, C-C (PLUGGED)		All Other Grades of Interior Including C-D Sheathing
				Structural C-D (Use Group 1 Stresses)
				Structural II C-D (Use Group 3 Stresses)
		Structural I A-C, C-C (Use Group 1 Stresses)		C-D Sheathing (Exterior Glue)
				All Interior Grades with Exterior Glue
		Wet	Dry	Wet	Dry	Dry
1. Extreme fiber stress in bending (Fb) Tension in plane of plies (Ft) Face grain parallel or perpendicular to span (at 45 degrees to face grain use (Ft)/6)	1	1,430	2,000	1,190	1,650	1,650
	2,3	980	1,400	820	1,200	1,200
	4	940	1,330	780	1,110	1,110
2. Compression in plane of plies (Fc) Parallel or perpendicular to face grain (at 45 degrees to face grain use (Fc)/3)	1	970	1,640	900	1,540	1,540
	2	730	1,200	680	1,100	1,100
	3	610	1,060	580	990	990
	4	610	1,000	580	950	950
3. Shear in plane perpendicular to plies (Fv) Parallel or perpendicular to face grain (at 45 degrees to face grain use 2*Fv)	1	155	190	155	190	160
	2,3	120	140	120	140	120
	4	110	130	110	130	115
4. Shear, rolling, in the plane of plies Parallel or perpendicular to face grain (at 45 degrees to face grain use (4/3)*Fs)	Marine and Structural I	63	75	63	75
	Structural II	49	56	49	56
	All Others	44	53	44	53	48
5. Bearing (on face) Perpendicular to plane of plies	1	210	340	210	340	340
	2,3	135	210	135	210	210
	4	105	160	105	160	160
6. Modulus of elasticity In bending in plane of plies Face grain parallel or perpendicular to span	1	1,500,000	1,800,000	1,500,000	1,800,000	1,800,000
	2	1,300,000	1,500,000	1,300,000	1,500,000	1,500,000
	3	1,100,000	1,200,000	1,100,000	1,200,000	1,200,000
	4	900,000	1,000,000	900,000	1,000,000	1,000,000

Given the descriptions, we know were are not dealing with Structural I panels. We also cannot be certain we are working with only Group 1 species...so we'll assume Group 2 stresses (and verify with our supplier). No notation is made about how these panels were manufactured, so we'll have to look at all possible combinations.

Allowable Shear = (Effective Thickness in.) * (Shear In Plane, Fv) * (12 inches/foot) * (1.33 seismic increase)

3/8" APA Rated Sheathing
3/8" Unsanded Panel, Shear (lbs./ft.) = (0.278) * (140) * (12 inches/foot) * 1.33 = 622 plf versus 355 plf
3/8" Sanded Panel, Shear (lbs./ft.) = (0.288) * (140) * (12 inches/foot) * 1.33 = 645 plf versus 355 plf

15/32" APA Rated Sheathing
15/32" Unsanded Panel, Shear (lbs./ft.) = (0.298) * (140) * (12 inches/foot) * 1.33 = 665 plf versus 570 plf
15/32" Sanded Panel, Shear (lbs./ft.) = (0.421) * (140) * (12 inches/foot) * 1.33 = 943 plf versus 570 plf

Of note is the disparity between the answers for 15/32" panels. One provides a 16 percent margin above the nailing.  The other provides a 65 percent margin. Chose the panel that best suits your needs while providing the maximum overcapacity at the same cost.