What are Hold Downs?
Shear walls must resist lateral forces that would make the house slide off its foundation. At the same time the earthquake will try to flip the shear wall over. This is known as overturning. Overturning is resisted with special hardware called Hold Downs. This video and text explain the basics behind overturning of shear walls and how hold downs prevent this from happening. Watching the video first will make the text easier to understand.
Overturning forces primarily act upon tall shear walls, much as a tall chest of drawers flips over if pushed across the floor from the top. A short squat dresser will not have this tendency, which is why overturning of shear walls is not a major concern in most cripple wall retrofits. Generally it is a waste of money to install overturning hardware, known as hold downs, on short shear walls.
Hold Downs are often Unnecessary
Many engineers and contractors are not aware of this and unfortunately recommend this expensive hardware when it is not necessary. The 2013 California Building Code does not even require their use so long as an 8 foot tall is at least 4 feet wide. You will see this on either side of too many garage doors, even when they support living areas above. This hardware is made by Simpson StrongTie and is abbreviated as HDU2, 4,5,8, etc. When looking at retrofit plans, you will often see this abbreviation.
Tall narrow shear walls can twist and deform and if extremely narrow, they will function like a post and simply tip over.
Let’s explain it using the illustration below.
If the top of the shear wall must resist 3,600 pounds of force distributed over 12 feet, then the top of the shear wall must resist 300 pounds per linear foot (3600/300). This 300 pounds per linear foot will then apply to the top, bottom, and sides of the shear wall. The illustration shows how this force is distributed. When the earthquake reverses direction, the direction of the arrows will also reverse. As shown in the example one side tries to life up the shear wall with 2400 pounds of tension while the other side pushes down in compression. Hold Downs resist the upward force that alternates between the two sides of the shear wall.
What if there are windows?
The perforated shear wall design method using IBC Section 2305.3.8.2 recognizes the strength and stiffness contributed by plywood above and below the wall openings. The value of the plywood above and below the windows does not count in terms of the shear wall’s capacity.
Why short and wide shear walls don’t need Hold Downs.
Using the same principle, if you have a 12-foot-long shear wall 2 feet tall that must resist 3600 pounds at the top (300 pounds per linear foot), the overturning will only be 600 pounds on each end (2 x 300 pounds per linear foot) and the weight of the house is sufficient to resist this force.
Here is an example of a shear wall overturning. This drawing is exaggerated in order to illustrate what happens. Most of the damage occurs where the plywood lifts up and away from the mudsill..
The building on the left used to be two stories. This collapse was caused by overturning of tall, narrow shear walls that could not resist the earthquake forces generated by the heavy living area above a garage. Even if this house were bolted, it would not have made any difference unless it had a seismic retrofit that resisted overturning forces.
Hold Downs resist overturning
The hold-down hardware shown at the ends of the shear wall in the figure above are designed to resist overturning forces. One hold down is connected to the vertical framing at each end.
As the shear wall tries to overturn, the left end of the shear wall pulls up on the hold down, which in turn pulls up on the hold down bolt, which in turn pulls up on the foundation. When the earthquake changes directions, the exact same thing happens but in the other direction.
Sometimes the overturning forces are so great that an un-reinforced concrete foundation breaks. However, the segment of foundation that the shear wall is built upon should remain in one piece and maintain the integrity of the plywood nailing.
Getting new Concrete under the Hold Downs
The best solution to prevent overturning in shear walls with un-reinforced concrete foundations is to provide additional weight to anchor the hold-downs. The existing un-reinforced concrete foundation shown here is only 8 inches deep and 12 inches wide. This is clearly insufficient to resist the potential overturning forces.
Here is the same view after the concrete has been poured and the plywood installed.
Cross Grain Bending
When the nails lift up on the mudsill, the mudsill is put into “cross-grain bending” and splits. Along with hold downs, one of the minor functions of plate washers is to reduce cross grain bending.