Is Your Home Protected From Earthquake Disaster?

Structural Retrofits
When an earthquake strikes, your home’s structure is put to the test. The skeleton must absorb the earthquake’s energy and provide a stable path to transfer the forces back into the ground. For this to happen, your home’s structure must be tied together; that is, your home’s roof should be tightly attached to the walls, and your walls should be fastened to each other, braced and anchored to a strong foundation. Remember that an ideal time to inspect and retrofit your home’s structure is when you are making a significant change to your home such as adding on a room or remodeling. In either case, it is important that your work conforms to local building code requirements. Although the existing portion of your home may not need to be upgraded to current code requirements, now may be a good time to do so. Contact your local building code official to find out what is required for your project.

Foundation Systems
Earthquakes can create ground motion in any direction. During a quake, your home’s foundation moves with the earth, but the rest of your home reacts more slowly due to its inertia. This creates a tremendous amount of stress on the connections between the foundation and the remaining structure. If these connections are not strong enough, your home may slide or fall off its foundation. In fact, this is one of the most common and costly types of structural damage. Depending upon the foundation, however, this may be relatively easy to fix.

Slab-on-grade Foundations
Slab-on-grade foundations are just that: concrete slabs that rest on the ground. In an earthquake-prone area, a home’s wood frame structure should be connected to the slab with either anchor bolts or other steel connectors (including steel plates).

Crawl Space and Basement Foundations
A foundation with a crawl space or basement typically has enough room underneath the first floor so that you can inspect the foundation and the underside of the floor-framing members, or joists, as long as the space is unfinished. The main difference between a crawl space and a full basement is the amount of headroom available. The walls that rise from the foundation footings to the first floor are called foundation walls. They are typically made with masonry blocks or concrete. In some cases, a short wood stud wall, or cripple wall, is positioned above ground between the top of the concrete or masonry foundation wall and the first floor.

Steel plates or minimum 1/2-inch-diameter anchor bolts should connect the wood framing sill plate to the concrete or masonry wall. These connections should be spaced no more than six feet apart. All components should be undamaged and rust-free. Also, examine the overall condition of the foundation wall. Make sure you don’t see any severe cracks (wider than the edge of a dime) in the masonry or concrete. If a wood cripple wall is present, it should show noevidence of termites or rot. If the foundation wall needs repair, or you need to add additional anchorage, check with a professional engineer for a suitable retrofit method.

For cripple walls, exterior lap siding alone cannot adequately resist the earthquake’s lateral forces. You may need to add interior bracing, if it is not already there, to prevent the cripple wall from collapsing in an earthquake: Nail 3/8-inch minimum structural grade plywood or oriented strand board (OSB) sheathing to the inside of the wall. Ideally, the entire wall length should be covered. However, if you have limited access, place sheathing in each corner of your home. While the sheathing panels can be oriented in any direction, take care to ensure that each edge is supported by a stud or solid blocking.

Post and Pier Foundations
A home can also be supported by a post-and-pier foundation. Large beams run under the home’s floor joists and are held up by posts. Each post rests on a separate concrete footing or pier. Some post-and-pier foundations are hidden from view by a cripple wall that runs around the home’s outside perimeter. This type of foundation is very susceptible to collapse during an earthquake. To better resist seismic forces, all of the foundation’s components, including the beams, posts and piers, must be securely tied together. When inspecting the foundation for possible problems, carefully examine the way the components are anchored together. The connection between the beam and the post should be strong and without rust, rot or evidence of poor workmanship. One way to help your home better withstand an earthquake is to have the connection reinforced with steel plates or with plywood or OSB connectors. Pre-manufactured metal straps or fixtures are available at most hardware stores. This reinforcement is especially important if the joint in the beam falls on top of the post. The other critical joint in this foundation lies between the post and the pier. Examine this area carefully. The post should be securely fastened to the pier, and all components should be well constructed, free of rust, severe cracks (anything wider than the edge of a dime) and rot. Most new construction will have the post attached to the block with a built-in metal fixture. In older construction, the post may simply rest on the top of the block. Reinforce the connection by nailing heavy gauge straps onto at least two sides of the post and then bolting them into the concrete block. Keep in mind that strong connections between the various components may not be enough - the earthquake’s movement may still knock the home off its foundation. That is why extra connections and lateral bracing may be necessary. If your home has an exterior cripple wall, bracing and anchoring it further can provide the necessary protection. Refer to the previous discussion on cripple wall foundations for inspection and retrofit techniques. If your home does not have an exterior cripple wall, lateral bracing and strong connections between the posts are critical. Simple toe nailing is not sufficient. Since the seismic evaluation of post-and-pier foundations is complex, you should consult a professional engineer.

Floor Systems
An earthquake exposes the floor to substantial forces that can distort and damage the floor system, jeopardizing the strength of your home. The floor system typically consists of floor joists, floor sheathing and band joists, which are located along the floor’s perimeter. If you have access to the underside of your floor, make sure that your floor system is tied together and that the sub-floor is securely connected to the underlying floor joists. To reduce the possibility of rotation in an earthquake, each joist should be nailed to a band joist. Blocking or bridging can also be placed between joists to keep them from falling over. The forces absorbed by the band joist or blocking must, in turn, be transferred to the foundation. Secure this connection by using metal ties or framing anchors.

Finally, make sure you do not find any evidence of poor workmanship, rust or rot. It may be difficult for you to access these areas. Often, the best time to evaluate your floor system is when you are planning to remodel. If your inspection reveals any problems, consult a professional engineer for the best way to retrofit your floor system.

Wall Systems
During an earthquake, the walls in your home, especially the exterior walls, play an important role in preventing your home from collapsing. The walls along with the floors and roof create a box. As the ground shakes, the floors and roof sway back and forth, while the walls in between try to stop your home from moving too far. To do their job, your walls must be strong and securely tied to the roof, floor and foundation.

Wood-framed Walls
Traditionally, the exterior walls of wood frame houses are supported with wood studs attached to structural-grade plywood, oriented strand board (OSB) or diagonal wood sheathing. To protect the exterior walls from the elements, they are covered with lap siding, stucco, stone or brick veneer. In order for this system to resist damage from earthquake forces, it must be well designed with the appropriate hardware in place to ensure a strong connection between all of the elements. Also, consider the number, size and location of the windows and doors, including garage doors, in your home. Too many can weaken your walls and lead to possible collapse in an earthquake. Imagine a closed box with several openings: it will cave in much easier than one with no openings. Unlike wood siding, brick and stone veneers require special attention because of their weight. During an earthquake, this heavy veneer can fall off, causing injury and significant damage. It is very important that these walls be tied to the wood-frames behind them with simple metal ties secured in the mortar. Sound building practices usually provide sufficient ties for the first story - even when not designed specifically for earthquakes. Special attention, however, may be required for the size and spacing of the ties in the upper stories. Because it is difficult to access these areas, the best time to inspect is while you are remodeling or adding on to your home. If you have any concerns about your home’s exterior walls, openings or veneer, contact a professional engineer. Ask him or her to determine how well your walls can withstand an earthquake and to recommend necessary retrofit measures.

Unreinforced Masonry Walls
If your home’s walls are made entirely of brick, stone, clay tile, concrete block or adobe, they could be susceptible to earthquake damage. In newer masonry homes on the West Coast, these types of walls are often reinforced with steel bars grouted inside the walls. If the walls are reinforced and well anchored to the foundations, floors and roofs, they can usually withstand an earthquake. But masonry that is in poor condition, unreinforced or not securely tied to the rest of the structure, has the potential to collapse.

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