The main objective of having a house earthquake retrofitted is to make sure that they are protected structural damage that’s often accompanied by earthquakes. The retrofitting simply makes it more difficult for a house to get uprooted from the foundation. Both existing homes as well as new ones need to be retrofitted so that they can withstand the effects of earthquakes.
While insurance can help you get prepared for an earthquake as you will be able to finance rebuilding, it won’t replace the house and certainly not the contents inside. Retrofitting houses for earthquakes on the other hand minimizes the damage and as such, all you may have to after an earthquake are minor repairs. Discussed below are several things you can expect from an earthquake retrofitting.
Expansion foundation bolting
Also known as mechanical foundation anchor bolting, expansion foundation bolting refers to anchoring the foundation’s mudsill. This is an affordable part of the retrofitting process, but to work well the concrete strength has to be good. This is because if the concrete cracks, then the bolts will not hold. They are usually installed in houses with new foundations or where the concrete is in good shape.
Recently, it was discovered that washers installed in houses prior to the 1990s did not provide enough holding strength as they weren’t long enough. This necessitated the upgrade of clamping washers usually installed with retrofit anchor bolts. To allow efficient clumping of the mudsill to the foundation, hot-dipped galvanized plate washers measuring 3” by 3” by 0.25” have to be used.
This basically refers to improving the connection between the concrete foundation and the frame of the building using bolts. Usually, this involves installing bolts that run through the mudsill into the underlying concrete. Mudsill refers to the piece of wood that lies on the foundation. For optimal effect, care should be taken when planning, placement as well as installation.
Cripple wall bracing
Many houses come with a short wall made of a wood frame that runs from the foundation to the bottom of the ground floor. This wall is known as cripple wall. The collapse of this wall is one of the main causes of damages during earthquakes. This makes it important to brace the wall against seismic movements.
To protect them from earthquakes, cripple walls are either stiffened or braced. A bracing effect is achieved by attaching the wall framing with high quality plywood. Stiffening, on the other hand, is achieved when plywood is attached along the walls of the house. The plywood that runs along the sides, braces the cripple wall in a front to back direction, while those that run along the front and back brace the cripple wall in a side to side direction.
Good detailing of connections from the ground, foundation, sill, wall and the floor is important. This makes it possible for load transfer to occur. Load transfer basically refers to transfer of seismic effects from one component of the building to the next.
Epoxy-set foundation washers
These work best for older homes whose concrete foundations are not as strong as those of new buildings. They also work in situations where seismic movements cause bolts to uproot upward from the concrete. Their biggest advantage is that they longer and therefore sit deeper in concrete, offering added protection against earthquakes.
Home without cripple walls
While cripple walls are common, not all houses have them. This makes them less susceptible to earthquake damage, but a considerable level of risk still exists. Toenails found between the foundation and the frame are not strong enough to handle earthquakes and slight movement can cause a lot of damage. Fortunately, there are a number of connection systems that one can use to protect their houses against earthquakes. The options will depend on the age of the home, access condition as well as the size of the frame.
Angle iron struts and foundation bolting
This custom upgrade provides a more efficient, economic, functional way of connecting the foundation with floor joists. The reason we are describing them as custom is because they are not ready made and as such cannot be directly purchased from a store. They have to order and custom made by a qualified steel fabricator. They are especially preferred for homes that sit on hillsides.
Foundation holdown brackets
Shear walls, sometimes require additional holdown brackets in order to effectively anchor them. This ensures that they are resistant to rolling and lifting, which often occur during earthquakes. Holdown brackets are installed at the ends of shear walls rather than their locations during house earthquake bolting in Los Angeles.
Soft story conditions
These are common in houses that are attached to garages. A large opening between the garage and the main house, usually the garage door, has little or no strength against earthquakes. Thus, the name soft story, referring to a story house built over a soft wall. Such walls are strengthened by bracing plywood on a wall that’s in line and next to the opening.