Shear walls can overturn just like when you were a kid dong cartwheels. This video explains why it happens and what to do about it.
In this video you will discover why hold down installations can be extremely dangerous and how to keep from getting sick.
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.
This is an example of a shear wall trying to overturn as shown by the half circle arrows. The weight of the house prevents the shear wall from overturning so it does not need hold downs.
When are Hold Downs Necessary?
If the earthquake force is strong enough the cripple wall is tall and will therefore tend to tip over. Usually the weight of the house will prevent the overturning but if not hold downs are necessary.
This diagram explains the simple science behind overturning and when hold downs are required to resist it.
- 3,600 pounds of earthquake force are spread along the top of a 12′ long shear wall.
- Each linear foot must resist 300 pounds (3600#/12 linear feet) = 300# per linear foot.
- This 300 pounds per linear also applies to the two sides of the shear wall and try to overturn it.
- Upward force on the left side it is an overturning force, on the right side it is a downward or compression force.
- When the earthquake reverses direction, the compression arrow will become an overturning arrow and the overturnig arrow will become a compression arrow .
- Hold Downs resist the upward force that alternates between the two sides of the shear wall.
Why short and wide shear walls don’t need Hold Downs.
Here is an example of a shear wall overturning and the kind of damage that can occur. 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..
Overturning of shear walls damages the plywood to mudsill connection.
The shear walls on either side of this garage door opening were tall and narrow which cause them to overturn.
The building on the left used to be two stories. This collapse was caused by overturning of tall, narrow shear walls on either side of the garage that could not resist the overturning forces. Even if this house were bolted, it would not have made any difference unless it had hold downs to resist this force.
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.
This is an interesting video that shows the reaction of a house on a shake table before and after shear walls were built. Hold downs were not used. It takes a trained eye to know when and when not to use them.