CBTC - Shear Wall Basics

This presentation is directed toward the practical use of shear walls.  It is not intended for engineers.  I will avoid formulas and equations unless absolutely necessary.  Instead, we'll see how shear walls came to be used and the materials we currently use to make them.  We'll look at the basic elements of a shear wall and how they function together to resist shear forces.  And we'll highlight common mistakes that should be avoided.

Shear Wall Types

There are a variety of products currently in use as shear-resisting materials.  A collection from the current Uniform Building Code is included in your presentation handouts and repeated below for your reference.  Learning to recognize these and other types of shear walls in the field is important.  Correct identification can save your from costly errors.  Here's a short list:

Shear Wall Materials - A Collection
Plywood and Siding Panel Grade	Min. Nom. Panel Thickness (inches)	Min. Nail Penetration in Framing (inches)	Panels Applied Directly to Framing					Panels Applied over 1/2-inch or 5/8-inch Gypsum Sheathing
			Nail Size (Common or Galv. Box)	Nail Spacing at Panel Edges (inches)				Nail Size (Common or Galv. Box)	Nail Spacing at Panel Edges (inches)
				6	4	3	2		6	4	3	2
Structural I	5/16	1 1/4	6d	200	300	390	510	8d	200	300	390	510
	3/8	1 1/2	8d	230	360	460	610	10d	280	430	550	730
	7/16			255	395	505	670
	15/32			280	430	550	730
	15/32	1 5/8	10d	340	510	665	870	n/a	n/a	n/a	n/a	n/a
C-D, C-C Shtg. plywood panel siding and other grades covered in UBC Std. 23-2 or 23-3	5/16	1 1/4	6d	180	270	350	450	8d	180	270	350	450
	3/8			200	300	390	510		200	300	390	510
	3/8	1 1/2	8d	220	320	410	530	10d	260	380	490	640
	7/16			240	350	450	585
	15/32			260	380	490	640
	15/32	1 5/8	10d	310	460	600	770	n/a	n/a	n/a	n/a	n/a
	19/32			340	510	665	870
Plywood panel siding in grades covered in UBC Std. 23-2	5/16	1 1/4	6d	140	210	275	360	8d	140	210	275	360
	3/8	1 1/2	8d	160	240	310	410	10d	160	240	310	410
Particleboard Panel Grade	Min. Nom. Panel Thickness (inches)	Min. Nail Penetration in Framing (inches)	Panels Applied Direct to Framing					The 1997 Uniform Building Code (UBC) no longer provides values for particleboard panels applied over 1/2" gypsum sheathing. Please reference the 1994 UBC for such values.
			Nail Size (Common or Galv. Box)	Nail Spacing at Panel Edges (inches)
				6	4	3	2
M-S and M-2	3/8	1 1/2	6d	120	180	230	300
	3/8	1 1/2	8d	130	190	240	315
	1/2			140	210	270	350
	1/2	1 5/8	10d	185	275	360	460
	5/8			200	305	395	520
Fiberboard Size and Application	Nail Size		Shear Value with 3-inch Nail Spacing Around Perimeter, 6-inch at Intermediate Points
1/2" x 4' x 8'	No. 11 gage galvanized roofing nail, 1 1/2" long, 7/16" head		125
25/32" x 4' x 8'	No. 11 gage galvanized roofing nail, 1 3/4" long, 7/16" head		175
Type of Material	Thickness of Material	Wall Construction	Max. Nail Spacing (inches)	Shear Value		Minimum Nail Sizes
Expanded metal or woven wire lath and portland cement plaster	7/8"	Unblocked	6	180		No. 11 gage, 1 1/2" long, 7/16" head or No. 16 gage staple, 7/8" legs
Gypsum Lath	3/8" lath and 1/2" plaster	Unblocked	5	100		No. 13 gage, 1 1/8" long, 19/64" head, plasterboard blued nail
Gypsum Sheathing Board	1/2" x 2' x 8'	Unblocked	4	75		No. 11 gage, 1 3/4" long, 7/16" head, diamond-point, galvanized
	1/2" x 4'	Blocked	4	175
	1/2" x 4'	Unblocked	7	100
Gypsum Wallboard or Veneer Base	1/2"	Unblocked	7	100		5d cooler (0.086" diam., 1 5/8" long, 15/64" head) or wallboard (0.086" diam., 1 5/8" long, 9/32" head)
			4	125
		Blocked	7	125
			4	150
	5/8"	Unblocked	7	115		6d cooler (0.092" diam., 1 7/8" long, 1/4" head) or wallboard (0.0915" diam., 1 7/8" long, 19/64" head)
			4	145
		Blocked	7	145
			4	175
		Blocked Two Ply	Base:9 Face:7	250		Base ply - 6d cooler or wallboard (see above) ; Face ply - 8d cooler (0.113" diam., 2 3/8" long, 9/32" head) or wallboard (0.113" diam., 2 3/8" long, 3/8" head)
Allowable shears on materials in italics are subject to a 50% reduction in seismic zones 3 and 4.

Materials of Note

Stucco - Even today, the stucco (cement plaster) wall is listed as a shear-resisting system.  If properly installed over building paper and with embedded wire mesh, the allowable shear on these walls is 180 pounds per foot.  Stucco has a dual purpose: it functions as a decorative finish (depending on texture) and it resists lateral loads.  It can be a confusing system for remodelers since stucco can be used alone or it can be used only as a finish over wood sheathing or structural paneling.  I will put off the topic of mixed materials until later in this presentation.

Lath and Plaster - In the building code you will find reference to a 3/8" lath nailed with plasterboard blued nails.  In this system, a thinner, perforated gypboard is nailed to the walls then the plaster finish is trowelled over the board to create the finish.  An older type of construction that used horizontal wood lath with a top coat of plaster should not be considered a shear-resisting system.

Plywood - Before the acceptance of performance-rated panels, this was the sheet product of choice for constructing wood shear walls.  The panel thickness, panel grade, nail type and nail spacing could be combined in different ways to achieve a wall with the right design strength.  With the advent of performance-rated products (like Oriented Strand Board) another variable was added; the strength of the panel.  Manufacturers have the ability to adjust the proportion, size and type of wood fibers and the amount of bonding material to suit particular job needs.

Gypboard and Gypsum Sheathing - Both types of materials are listed in the building code as shear-resisting materials, but their use is discouraged in earthquake regions of the country.  Gypboard and similar products lack the flexibility of other products.  Yes, shear forces are resisted, but the result of repetitive cycles of alternate direction loading is gypboard with larger and larger nail holes.

Straight or Lapped Siding - Provided each board-stud intersection is nailed with two nails, this system can resist lateral loads.  Of all of the systems listed it is the least efficient at its job because of the spacing of the fasteners.

Diagonally Sheathed Walls - In this system, boards are run at an angle to connect the top and bottom plates of a stud wall.  Values for this system are not currently available, but the 1961 Uniform Building Code gave a maximum value of 300 pounds per foot.

Foam Core Panels - This is a manufactured product in which solid structural foam takes the place of wood or metal studs between interior and exterior sheathing.  In almost every way a foam core panel may be treated as you would a wood shear wall as far as shear-resistance is concerned.

Brick, Clay Masonry, Concrete Masonry and Concrete - All these materials can be used to create shear walls if assembled in the proper way.  They are generally used where forces exceed those permitted for wood walls, where the construction type does not permit combustible materials and/or where the height of the structure exceeds the limit set for wood (3 stories).  The basic principles discussed later on apply to walls constructed with these materials, but an in-depth look at rules specific to these materials is beyond the scope of this presentation.

Steel - Where force levels exceed those permitted in all other materials or where dimensional constraints limit the width of shear walls, steel plate shear walls are used.  Taken to the extreme, one can regard a wide flange column as the smallest form of a shear wall...with the web resisting shear forces and the flanges stabilizing and in turn being stabilized by the web.