Home Mitigation
Mitigation is the process of modifying a structure to reduce its associated seismic risk. To select an effective mitigation strategy, the homeowner should understand the excitation caused by an earthquake, and the way in which the structure reacts to this stimulus. By simulating earthquakes on 'Shaking Tables,' structural engineers are able to study and isolate the important design characteristics of earthquake resistant structures. For example, the table at the University of California at Berkeley is capable of shaking a 45 ton structure at accelerations of up to 15 meters per second squared at frequencies of up to twenty cycles per second (Hertz).
To withstand an earthquake, a structure must first withstand the static load induced by the event, then respond dynamically by not absorbing a lot of energy in the earthquake's band of excitational frequencies. Older homes, which satisfied early building codes, are the most suceptible to earthquake damage, and will see the greatest benefit from mitigation efforts.
- Mudsills and Anchor Bolts
In the United States, the most common residential construction design is that of the Wood Frame Structure. In such a structure, wooden base plates, called mudsills, typically two by six lumber, are attached to a masonry foundation by anchor bolts tying the structure to the earth. In older construction, the anchors may not have been installed. - First Subfloor Attachment
Next, headers, seated on the mudsill and nailed to it, support the floor joist hangers. The subfloor, commonly 1-1/4 inch plywood, is nailed to the joists. The subfloor forms a shear panel, resisting horizontal shear forces through it's torsional rigidity. Joists are prevented from rolling by properly nailed hangers. - Studwalls
Studwalls are normally constructed 'on the flat,' then raised into position. They consist of a sole plate, studs, and a pair of top plates which form tap joints in the corners. Doors and windows form weak areas because they cannot be protected by shear wall or trusses. Headers and cripple studs which are sheathed with plywood can strengthen window openings. Doorways are a more difficult problem. Sole plates are typically nailed to the subfloor at the headers. - Shearwall
Shear walls are formed in framed construction by nailing plywood to the studwall. Properly installed, they resist the torsional moments caused by ground motion induced by seismic activity. Common alternatives to plywood sheathing include Oriented Strand Board (OSB), or engineered wood paneling. As shear sheathing, half inch plywood is typical. Our fourth checkpoint is plywood or steel shear wall installed on building faces. - Roof structure
The roof consists of ceiling joists, rafters, and various types of reinforcing, such as posts, collar beams, stress-skin panels, and trusses. The details depend upon the roof design, which in turn is a function both of esthetics and of load requirements. Snow country and hurricaine zones militate for carefully designed roof structures. - Shear Stresses
Earthquakes can generate severe torsional stresses on the structure, causing walls to fail from the generated strains. Mitigation strategies for framed structures strengthen the building by adding steel joint protections, and by making the structure more rigid by adding shear wall. - Steel Reinforcing
A36 mild steel has a tensile strength of 55,000 pounds per square inch. That means that, under tension, you might hang ten Jeep Grand Cherokees, weighing 4000 pounds each, from a steel rod one inch square. For purposes of comparison, utility pine has a tensile strength of 175 pounds per square inch, a factor of 314 times less strong under tension. The weight of a single 200 pound man would cause it to fail.This means that the additon of relatively little steel reinforcing to a structure can greatly increase it strength. Retrofitting steel reinforcing, such as Simpson Strongties, can increase the structural strength of an older structure at a relatively minor expense. - Normal Mode Oscillations and Shear Walls
Every extended structure possesses a set of resonant frequencies. If shaken at one of these frequencies, the structure is set into independent motion, slowly vibrating back and forth. The structure stores vibrational energy like a pendulum, with the amplitude of the oscillation becoming progressively greater as more and more energy is stored in the motion of the structure. As the amplitude of the vibration increases, the elastic limits of the structure can be exceeded resulting in structural failure. These driven oscillations may be reduced in amplitude by making the structure more rigid, so that the resonant frequencies are higher the frequencies caused by the earthquake.
Summarizing
Two general techniques mitigate earthquake damage to frame structures: steel tie points anchoring the main structural subassemblies, and adequate shear walls to stiffen the structure with respect to vibrational excitation. In older construction some of the substructures are often left without any ties. In newer construction, conventional nailing is sometimes used without steel ties reinforcing the connections. In older construction, plywood shear panels may not have been used. Significant improvements in seismic safety can be achieved by retrofitting these systems.