Definition of a Shear Wall
A shear wall contains vertical shear panels or slabs that resists shear forces in its own plane due to wind, earthquake forces, or explosions and thus stiffens the structure against deformation by such forces. [1]
The main structural system in light framed homes are its foundation and framing system. They must resist various loads which causes stress to the structure itself. All loads are a type of stress and can be reduced to two terms: tension and compression. [2]
Tension: forces that stretch and pull.
Compression: forces that press, push, or squeeze.
The origin of the word “shear” in old English is to ‘cut through with a weapon’. [3] So think of a shear wall as the most vulnerable area in a home that could be strained by any of those two forces explained above. However, in today’s language, a shear wall is a wall that is already reinforced to withstand various loads. These loads include:
Live Loads: Moving loads such as occupants, snow, and rainwater
Dead Loads: Static loads such as the weight of home itself including as the fixed structure such as cabinetry, appliances, and furniture.
Seismic Loads: Forces from impact loads, shock waves, and vibrations.
Wind Loads: Negative and positive pressure buildup within the house and its exterior environment.
What is a Shear wall made of?
A shear wall, as mentioned above contains panels or slabs. In conventional light framed homes, these panels are usually made of plywood according to standards by the Engineered Wood Association (www.apawood.org). Plywood (cross-laminated wood veneer) or oriented strand board (OSB) are common panels used as sheathing for the exterior walls, floors, and roofs of most light framed homes. [4]
Sheathing is like skin for a building's frame made of studs attached to the foundation. It's meant to protect against air and moisture, providing structure and strength.
A deeper look in the terminology is that a shear wall is a component of structural sheathing. Every shear wall contains sheathing but not all sheathing is a shear wall.
APA Sheathing Panels
Each panel of plywood or OSB has an APA Grade Stamp. Which is the performance standard from Engineered Wood Association. The APA acronym, which stands for American Plywood Association is the trade organization’s previously held name, and is shown as stamp on each sheet of panel.
Source: Engineered Wood Association
1. Panel grade
2. Span rating
3. Tongue and groove
4. Bond classification
5. Decimal thickness declaration
6. Mill number
7. Product standard
8. Performance Category
9. Siding face grade
10. Species group number
11. HUD recognition
15. Panel face orientation
installing apa wall sheathing for shear wall
Wall sheathing panels can be installed vertically or horizontally but certain considerations must be made. If the foundation is on a relatively flat surface, panels can be installed vertically where the strength axis is parallel to the studs. However, if the foundation is on a slope or contains a stepped foundation, it would be wise to orient the panels horizontally to prevent a ‘domino effect’. This horizontal orientation is applied with strength across studs.
A 1/8” spacing is recommended at all seams, edges and corners, unless the APA manufactured panel indicates otherwise. [4]
All edges of the panel shall be fastened onto a stud or blocking installed between the studs. Horizontal sheathing requires a lot more blocking since they are made 4 feet in height, whereas the vertical sheathing can ideally go from the sill plate all the way to the top plate if the height is 8 feet.
structural design IMplications
A shear wall is made to be incredibly stiff and resists the vertical and horizontal forces acting on its plane. Therefore the internal forces within the shear wall created complications if things fail.
There are four critical failure mechanisms that should be understood when designing structural supports on walls. [5]
Vertical shear
Horizontal shear
Flexural failure
Buckling
The four critical failure mechanisms in shear wall when the force acting on the shear panel is greater than its resistance.
Figures 1-4 are prime examples of lateral forces acting in-plane on the shear wall panel. Figures 1-3 are common during earthquakes; however, figure 4, is an example of axial loading during compression in-plane.
Buckling has been an issue since the beginning of housing construction. It was not until a polymath named Leonard Euler found a solution to the elastic buckling problem in 1759. The problem was finding a formula where high ratio of length to thickness columns, in this case a shear panel, would buckle while being overloaded on its axis but would recover to its original position once unloaded. [1]
Shear Wall solutions
The solution to shear wall panels from buckling are in its thickness, size and type of nails, and nail pattern. Finding the adequate thickness to prevent Eulid in-plane buckling is important as well as Eulid out-of-plane buckling when lateral torsion is acting on the panel.
Fortify openings, such as windows and doors, by using wall bracing and continued shear panels all around. This is known as the coupling effect. Neighboring walls can transition any lateral forces so that the energy transfer in the house due to any of loads mentioned above can safely move back down to the ground. Shear wall at adjacent corners are great to prevent torsional stress and gyrations.
Nail pattern: follow the local county for nail patterns as well thickness of the APA rated sheathing. I found it intuitive to increase the number of nails towards the ends of the house. For example if the house is on a slope. The frequency of nails on the panels should increase towards the lower end of the slope.
Shear wall panels are not limited to residential homes but large commercial buildings as well.
code and standard provisions
Provisions and design tables are made from the International Building Code (IBC) along with the Special Design Provisions for Wind and Seismic (SDPWS).
IBC: Section 2305 General Requirements for Lateral-Force-Resisting Systems
2305.1 General. Structures using wood-frame shear walls or wood-frame diaphragms to resist wind, seismic or other lateral forces shall be designed and constructed in accordance with AWC SDPWS and the applicable provisions of Sections 2305, 2306 and 2307.
Continuous Load Path
A continuous load path from the roof to the foundation is required per SDPWS Section 4.1.1
Source: Simpson Strong-Tie
Once a continuous load path has been determined for the shear wall and each partition has been fastened with the right type fastener, look to the SDWPS table on shear capacities.
Below is the table for nominal capacity of shear wall panels for both seismic and wind loads.
To understand the graph below, let me explain a couple variables in the table:
V = induced unit shear, plf (pounds per linear foot)
G = apparent diaphragm shear stiffness from nail slip and panel shear deformation, kips/in (1000 pounds-force per inch)
Source: SDPWS
For better resistance to lateral forces, use a thicker panel with more nails, placed closer together, and longer nails in earthquake and windy regions. Follow IBC and SDPWS rules, unless local regulations differ.
calculations for shear walls
Deformations and elasticity should be determined depending on length and height of your shear wall. “If a shear wall deflects an inch, its connections will also need to be able to travel the same distance without breaking the system.” [7]
Let’s do an example of a force-transfer shear wall with a large window opening in order to find what type of plywood and nails to use given the dimensions below:
If we want to be within the requirements of a 3,500 lb lateral wind load, we will need to find the compression and tension forces acting on this wall with the given dimensions.
T = C = 3,500 x 8’ (height)/15’ (span) = 1,867 lb for both Compression and Tension.
We’ll use the Diekmann Technique Analysis by treating each sheathing area to act like its own beam, carrying shear and flexural forces at either end. [7]
The Diekmann Technique treats the entire wall as one unit and will provide the limits to the nominal shear capacities for Table 4.3A.
In the schematic above, you can see the mechanics involves in each zone. For the following schematic below we will list out these zones by letter.
Let’s start with zones B and G. They are each 4’x4’ and will need to resist a total of 3,500 lbs. Our goal is to figure out the force in pounds per linear foot in each zone.
3,500 lb / (4’ + 4’) = 438 plf for both zone B and G for horizontal forces.
Zone D and E will need to resist a total of 1,867 lbs.
1,867 lb / (1.5’ + 2.5) = 467 plf for zone D and E for vertical forces.
We assume that the tension and compression cancel out at the mid-point of Zone E (i.e. the window sill) and bottom of Zone D (bottom of header). In other words those two horizontal lines that intersect the midpoint of Lines 2 and 3 = 0 lb in either compression and tension.
The difference between those midpoints and line 7 is 3.5’.
The T & C located on Line 2 and 7 and Line 3 and 7 = 467 plf x 3.5’ = 1,635 lb
Zones F & H on the horizontal: v = (1,635 lb/4’) = 409 pff = 438 pff - 29 plf
Zones A & C on the horizontal: v = 29 plf by symmetry
Finally, check the Maximum Induced Unit Shear from ASD. [8] In this case, 467 plf from zones D and E in the Diekmann analysis to that of the nominal value by multiplying by 2.0.
467 plf x 2.0 = 934 plf (nominal).
Look back at Table 4.3A to the Wind column and go down the nearest number by rounding up. You will find 980 plf is the capacity for 7/16” plywood with 8d galvanized nails on 4” nail spacing.
Footnotes:
Henry J. Cowan, “Science & Building: Structural and Environmental Design in the Nineteenth and Twentieth Centuries,” A Wiley-Interscience publication. (1978): 350
Julia McMorrough, “The Architecture,” Rockport Publishers, Inc. (2008): 72
"New Oxford American Dictionary (Second Edition)".
Charlie Wing, “The Visual Handbook of Building and Remodeling: A Comprehensive Guide to Choosing the Right Material and Systems of Every Part of Your Home,” The Taunton Press, Inc.: 196-205
“Shear Wall,” Wikipedia, last modified on April 22nd 2024, https://en.wikipedia.org/wiki/Shear_wall
See footnote 1: 18
Shearwall Design with 2015 SDPWS from Simpson Strong Tie Learning Center
“ASD Diaphragm Shear Forces,” Simpson Strong Tie, https://seblog.strongtie.com/2014/12/wood-shear-wall-design-example/