Skip to content Skip to sidebar Skip to footer

Shear Wall Size Wood Construction

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.


PreviousPage DocumentIndex TableOfContents

Source: http://www.mcvicker.com/vwall/page035.htm

Posted by: ayeshaswetlande0193595.blogspot.com

Post a Comment for "Shear Wall Size Wood Construction"