- Tips to prepare family for earthquakes
- Terrified about That Earthquake? Here’s what to do about it | Grist
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Figure shows several types of retrofit solutions for old buildings. The walls may be strengthened by infill walls, by bracing, by post-tensioning, by external buttresses beautifully displayed by medieval Gothic cathedrals in western Europe , by adding an exterior or interior frame, or by base isolation.
The building needs to behave as a unit during shaking, because the earthquake is likely to produce failure along weak joins. There are several lateral force-resisting systems for withstanding the earthquake-induced forces, including moment resistant frames, shear walls, and braced frames, for example. In addition, the lateral-resisting system may be a combination these systems. These lateral-resisting systems can be constructed out of reinforced concrete, structural steel, reinforced masonry, or even timber.
At the floor levels, the lateral resisting forces are transferred through a diaphragm. The term diaphragm is used for a horizontal element of the building, such as a floor or a roof, that transfers horizontal forces between vertical elements such as walls or columns Figure a. The diaphragm can be considered as an I-beam, with the diaphragm itself the web of the beam and its edges the flanges of the beam Figure b.
In most buildings, holes are cut in the diaphragm for elevator shafts or skylights Figure c. These holes interrupt the continuity and thereby reduce the strength and stiffness of the diaphragm Figure d. Lateral forces from diaphragms are transmitted to and from the ground through shear walls or moment resisting frames. The forces are shear forces, those tending to distort the shape of the wall, or bending forces for slender structures like a skyscraper Figure Construction may include walls that have higher shear strength or diagonal steel bracing, or both.
Moment-resistant frames are more flexible than shear-wall structures; they are less likely to undergo major structural damage but more likely to have damage to interior walls, partitions, and ceilings Figure Several steel-frame buildings failed in the Northridge Earthquake, but the failures were in large part due to poor welds at the joints—a failure in design, construction, and inspection. The normal approach to providing seismic resistance is to attach the structure firmly to the ground. All ground movements are transferred to the structure, which is designed to survive the inertial forces of the ground motion.
This is the reason why your house is bolted to its foundation and your cripple wall is reinforced. In large buildings, these inertial forces can exceed the strength of any structure that has been reinforced within reasonable economic limits.
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The engineer designs the building to be highly ductile, so that it will deform extensively and absorb these inertial forces without collapsing. Moment-resistant steel-frame structures are good for this purpose, as are special concrete structures with a large amount of steel reinforcing. In deforming, they can cause extensive damage to ceilings, partitions, and building contents Figure such as filing cabinets and computers.
Equipment, including utilities, will stop operating. High-rise buildings will sway and might cause occupants to become motion-sick and panicky. In addition, staircases may fail, hindering evacuation of the building after an earthquake. The problem with attaching the building firmly to the ground is that the earthquake waves are absorbed by the building and its contents, often destructively.
Is there a way to dissipate the energy in the foundation before it reaches the main floors of the building? In base isolation, the engineer takes the opposite approach: the objective is to keep the ground motion from being transferred into the building. This is the same objective as in automobile design—to keep the passengers from feeling all the bumps in the road.
To accomplish this, the automobile is designed with air-inflated tires, springs, and shock absorbers to keep its passengers comfortable. One way to do this is to put the building on roller bearings so that as the ground moves horizontally, the building remains stationary Figure A problem with this solution is that roller bearings would still transmit force into the building through friction.
In addition, once the building began to roll, its inertia would tend to keep it moving. We need a structure that allows horizontal movement with respect to the ground, but restrains, or dampens, this movement so that as the ground vibrates rapidly, the building vibrates much more slowly with slower velocities and accelerations. The solution is to separate the requirement for load bearing vertical loads from that for movement horizontal loads. One way to do this involves a lead-rubber bearing Figure This bearing consists of alternating laminations of rubber and steel, which allow for up to six inches of horizontal movement without fracturing but are strong enough to support the building.
A cylindrical lead plug is placed in the center of this bearing to dampen the oscillations in the ground produced by an earthquake, just like the shock absorbers in a car. The energy of the earthquake waves is absorbed by the lead plug rather than by the building itself. Lead recovers nearly all of its mechanical properties after each deformation from an earthquake. This is analogous to the solid-state ductile deformation of lower crustal rocks without producing earthquakes.
Tips to prepare family for earthquakes
The lead-rubber bearings allow the ground under a building to move rapidly, but the building itself moves much more slowly, thereby reducing accelerations and maximum shear forces applied to the building. The building is allowed to move about six inches horizontally. A six-inch slot around the building is built for this purpose and covered by a replaceable metal grating.
The damage to architectural and mechanical components of the building, and the ensuing costly repairs, are greatly reduced and, in some instances, almost eliminated. There are some new systems, which go beyond the lead-bearing base isolator. Examples are the single and triple pendulum bearing systems. The single pendulum system maintains constant friction, lateral stiffness, and dynamic period for all levels of earthquake motion and displacements.
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The triple pendulum system includes three pendulum mechanisms that are sequentially activated as the earthquake motions become more intense. Although base isolation adds to the cost of construction, some cost savings are possible within the building itself because so much of the earthquake force is absorbed at the base of the building rather than transmitted into the structure. The Pioneer Courthouse in Portland, constructed in , is the oldest surviving federal building in the Pacific Northwest, and it has been designated a National Historic Landmark.
It houses the Ninth District Court of Appeals. The challenge of a seismic retrofit of this unreinforced-masonry building was to strengthen the building without totally disrupting its character, including its sandstone-block walls.
The solution was base isolation, installed below the existing foundations of the building, which minimized construction in the historic sections of the structure. The retrofit was completed in Research is underway in Japan, New Zealand, and the United States to design other methods of base isolation and other ways to dissipate seismic energy in a building. After the Loma Prieta Earthquake, the California State Legislature passed Senate Bill , requiring the state architect to select one new and two existing buildings to demonstrate new engineering technologies, including base isolation.
The new international airport terminal in Istanbul, Turkey, is the largest base-isolation building on Earth. Each large building presents its own set of design problems in surviving earthquake forces, which means that architects must consider earthquake shaking in designing a large structure in a seismically hazardous region such as the Pacific Northwest.
I consider the problem of a soft ground floors and the issue of the tuning fork. In a building with a soft ground floor , the ground floor is weaker than the higher floors.
Terrified about That Earthquake? Here’s what to do about it | Grist
The ground floor is taken up by a parking garage or contains large amounts of open space occupied by a department store or hotel ballroom. Instead of load-bearing walls, these spaces are supported by columns. Building codes commonly limit the height of soft stories to two normal stories, or thirty feet. But the result is that the ground floor is less stiff has less strength than the overlying floors. Without a soft ground floor, the earthquake forces are distributed more equally throughout the entire building. With a soft ground floor, there is a tremendous concentration of forces on the ground floor and at the connection between the ground floor and the second floor.
This can cause collapse or partial collapse of the higher floors, as happened at the Los Angeles County Olive View Medical Center during the Sylmar Earthquake Figures and The upper floors were relatively undamaged, but the lowermost floor and basement absorbed much of the force.
The acceleration came from the right, and the building was forced toward the right, almost knocking down the stairwell. The problem can be alleviated by adding more columns, stiffening the existing structure. A second problem is illustrated by Figure , in which apartments were constructed over a garage in the basement, which acted as a soft story.
This brings us to the tuning fork problem. A large pipe organ has pipes of different lengths so that the organ can play different notes. The deep bass notes are played on long pipes, and the high notes are played on short pipes. A xylophone works the same way: the high notes are played on short keys, and the low notes are played on long keys.
These instruments are designed to take advantage of the vibrational frequency of the pipes or keys to make music. A tuning fork works in the same way. Strike the tuning fork and place it tines-up on a hard surface. You will hear a specific note, related to the length of the tuning fork, which generates sound waves of a specific frequency—the vibrational frequency of the tuning fork.
I recall a TV commercial in which a wine glass is shattered when a Wagnerian soprano sings a certain high note. Buildings work in the same way. A tall building vibrates at a lower frequency than a short building, just like a tuning fork.
The problem comes when the earthquake wave transmitted through the ground vibrates at the same frequency as the building. The building resonates with the earthquake waves, and the amplitude of the waves is intensified. All other things being the same, a building with the same vibrational frequency as the earthquake waves will suffer more damage than other buildings of different height. In the Mexico City Earthquake of , surface waves with a period of about two seconds were amplified by the soft clay underlying most of the city, which also extended the period of strong shaking. Buildings between ten and fourteen stories suffered the greatest damage, because they had a natural vibrational period of one to two seconds Figure When waves of that characteristic frequency pushed the foundations of those buildings sideways, the natural resonance caused an accentuation of the sideways shaking and resulted in great structural damage.
In contrast, a thirty-seven-story building built in the s, with a vibrational period of 3. Freeways and bridges are lifelines, and their failure can disrupt the economy and kill people on or beneath them during an earthquake Figure The television images of people sandwiched in their cars in the collapse of the double-decker Interstate Cypress Viaduct in Oakland, California, the collapsed span of the Oakland-San Francisco Bay Bridge, and the pancaked freeway interchanges in Los Angeles after the Sylmar and Northridge earthquakes were dramatic reminders of the vulnerability to earthquakes of highways and railroads.
Structural engineers in the Bridge Division of the Oregon Department of Transportation visited the collapsed freeway overpasses after the Northridge Earthquake, and their recommendations led to the first thorough appraisal of the earthquake potential of Oregon faults. However, most of the overpasses on Interstate 5 have not yet been repaired. As pointed out above, the Oregon Legislature failed to pass a transportation bill that would have begun retrofitting seismically-dangerous bridges that, if they failed, would cut off the Oregon coast from the Willamette Valley in a subduction-zone earthquake.
The resilience survey pointed out the problem, but the legislature did nothing about it.