Shulou(Shulou.com)11/24 Report--

Seeing the recent seismic activity, people may think that the earth may be a little active and shaking badly. There were intermittent earthquakes in Chile between 2010 and 2011, starting with an 8.8 magnitude earthquake off the coast near Concepcion in February 2010. Subsequently, a magnitude 9.0 earthquake struck Japan in March 2011, triggering a tsunami, killing an estimated 29000 people and damaging nuclear reactors. Then, in August 2011, a 5.8-magnitude earthquake struck near the mineral center of Virginia, spooking residents along the Atlantic coast and damaging the Washington Monument.

Although these events seem to hint at the ominous future of multiple earthquakes, earthquakes have always been common, and so is human determination to survive them. Over the centuries, engineers have become more and more aware of one thing: earthquakes don't kill people, but buildings do. Of course, this is too simple, because tsunamis can also kill many people, but not all earthquakes cause tsunamis. However, earthquakes do cause sudden lateral acceleration of buildings, bridges and other structures, resulting in building damage. All this raises the logical question of whether buildings can be kept upright and intact in the event of catastrophic earthquakes, such as the Chile earthquake in February 2010 and the Japan earthquake in March 2011.

Many engineers and architects now believe that a perfect earthquake-resistant building can be built that can withstand the scariest earthquakes and remain fresh after the tremors have stopped. However, the cost of this kind of building will be astonishingly high. In contrast, architectural experts strive to pursue the less ambitious goal of designing an earthquake-resistant building that can prevent complete collapse, protect lives and have the right budget.

In recent years, the development of earthquake-resistant buildings has developed by leaps and bounds, but this is not a brand-new topic. In fact, although some ancient buildings are located in seismically active areas, they still stand today. The most famous of these is Santa Sophia Cathedral, a domed church (now a museum) built in 537 AD in Istanbul, Turkey. About 20 years after it was built, an earthquake occurred in the area and a huge dome collapsed. The engineers assessed the building and decided to rebuild the dome, but on a smaller scale. They also reinforced the whole church from the outside.

Today, although the technology for building earthquake-resistant buildings is a little different, the basic principles are the same. Before delving into the details of building earthquake-resistant structures, let's review some basic knowledge, that is, what forces are generated during an earthquake and how these forces affect the building structure.

1. The influence of earthquake on buildings

two。 Earthquake-resistant Building Design: rescue by the United States Geological Survey

3. Earthquake-resistant architectural design: self-protection

4. Anti-seismic foundation and materials

5. The future of earthquake-resistant buildings

The impact of earthquakes on buildings the complete content of earthquakes is introduced in the book "how earthquakes occur", but it is helpful to review the basics here. An earthquake occurs when the rocks in the local crust slide against each other. This movement occurs most often in faults, which are faults in rock bodies that can extend for miles or even hundreds of miles. When rocks in the local crust suddenly slide or move, they release huge amounts of energy and then propagate through the earth's crust in the form of seismic waves. On the earth's surface, these waves can cause the ground to shake, sometimes violently.

Geologists divide seismic waves into two categories: solid waves and surface waves. Solid waves include longitudinal waves (P waves) and shear waves (S waves), which travel in the interior of the earth. Longitudinal waves are similar to sound waves, which means that they compress and expand matter as they pass through. Shear waves are similar to water waves, which means that they move matter up and down. Longitudinal waves can travel in solids and liquids, while shear waves can only travel in solids.

After an earthquake, longitudinal waves always reach the surface first, while shear waves arrive later. Then there are the slower surface waves, which geologists call Love wave and Rayleigh wave. Both waves move the ground horizontally, but only Rayleigh waves move the ground vertically. Surface waves form long wave trains that travel long distances, causing most of the vibration and most of the damage in an earthquake.

If the earthquake only moves the ground vertically, the damage to the building may be minimal, because all buildings are designed to withstand vertical forces, that is, forces related to gravity. However, the rolling waves of an earthquake, especially the Love waves, exert great horizontal forces on stationary buildings. These forces cause lateral acceleration, which scientists measure as G force (G-forces). For example, an earthquake of magnitude 6.7 can produce an acceleration of 1 G and a peak velocity of 40 inches (102 centimeters) per second. This sudden lateral movement (almost like a sharp push) exerts great stress on the structural components of the building (including beams, columns, walls and floors) and the connectors that connect them together. If these stresses are large enough, the building will collapse or suffer serious damage.

Another key factor is the foundation of the house or skyscraper. Buildings built on bedrock are usually stable because the ground is solid. Buildings located on soft or filled soil tend to collapse completely. In this case, there is the greatest risk of liquefaction, that is, when loose, flooded soil temporarily behaves like a liquid, it will cause the ground to sink or slide, and the building will sink or slide with it.

Obviously, the engineer must choose the building address carefully. Next, we will introduce how engineers plan and design earthquake-resistant buildings.

When the local seismic wave reaches the surface of the earth, it will cause the ground and any object on the ground to vibrate at a certain frequency. During an earthquake, buildings tend to vibrate around a specific frequency (that is, its natural frequency or fundamental frequency). When the vibration frequency of the ground is the same as the natural frequency of the building, they are called resonance. This is a very bad situation. Resonance will magnify the impact of the earthquake and cause more damage to buildings. In September 1985, an earthquake in Mexico City produced seismic waves with the same frequency as the natural frequency of a 20-story building. As a result, 20-story buildings were more damaged in the earthquake in Mexico than other types of buildings. In some cases, the damaged 20-story building is next to undamaged buildings of different heights.

Earthquake-resistant building design: the earthquake-resistant building design of the United States Geological Survey engineers must first assess seismic activity at the construction site before major construction projects begin. In the United States, they can use a resource to help assess the national seismic risk map produced by the United States Geological Survey (USGS). These maps show the probability of ground shaking exceeding a certain value over the next 50 years. In order to calculate the value of a particular site, geologists use historical earthquake data and then speculate about ground tremors at all possible distances from the site based on the magnitude of possible future earthquakes. The result of the calculation is a color contour map showing which parts of the country have the highest seismic risk. As you might expect, the entire coast of California has a high incidence of earthquakes. Other earthquake hotspots in the United States include Alaska, Hawaii, South Carolina, and areas including southeastern Missouri, southern Illinois, western Kentucky, Tennessee and northeastern Arkansas.

Examples of hazard maps that engineers may use source: the USGS Building Code, such as the International Building Code used in most parts of the United States, is based on the USGS seismic hazard map. In high-risk areas, engineers and architects must adhere to stricter standards when designing buildings, bridges and highways to ensure that these structures are resistant to earthquake shaking. At the same time, in low-risk areas, engineers can avoid designing too many buildings that are less likely to move violently in an earthquake.

Once the engineer has determined the earthquake risk of a site, he must come up with an appropriate architectural design. Generally speaking, they will try to avoid irregular or asymmetric designs. These designs include L-shaped or T-shaped buildings or hierarchical structures. Although this design increases visual interest, buildings are also more vulnerable to torsion or vertical distortion. As a result, seismic engineers prefer to keep the building symmetrical so that the force is evenly distributed throughout the structure. They also restrict decorations, such as cornice, vertical or horizontal cantilever protruding or lintel stones, because earthquakes can easily cause these building components to shift and fall to the ground.

Symmetry alone cannot save buildings. Next, we will further discuss what can.

Earthquake-resistant architectural design: self-protection even symmetrical buildings must be able to withstand huge lateral forces. Engineers want to counteract the horizontal and vertical forces in the building's horizontal and vertical structures. The horizontal partition is a key component of the horizontal structure. They are used on the floors and roofs of buildings. Engineers usually place each horizontal partition on its own platform and reinforce it horizontally so that it can share the transverse force with the vertical structural members. On the roof, because there are not often strong platforms, engineers reinforce the horizontal partition with trusses, which are diagonal structural members inserted into the rectangular area of the frame.

The vertical structure system of a building is composed of columns, beams and braces, and its function is to transfer seismic forces to the ground. Engineers have several options when building vertical structures. They usually use braced frames to build walls, which rely on trusses to resist lateral movement. Cross bracing is a commonly used method for the construction of wall trusses, which is composed of two diagonal members in X shape. Engineers can use shear walls instead of bracing frames or as a supplement, shear walls are vertical walls that strengthen the structural frame of a building to help resist rocking forces. Engineers usually install shear walls on walls that have no openings, such as elevator shafts or walls around stairwells.

However, shear walls do limit the flexibility of architectural design. In order to overcome this shortcoming, some designers choose the torque resistance frame. In these structures, columns and beams can be bent, but the joints or connectors between them are rigid. Therefore, the whole frame will move under the action of transverse force, but compared with the shear wall structure, there are fewer obstacles inside the building. This gives designers more flexibility in placing building components, such as exterior walls, partitions, and ceilings, as well as building contents, such as furniture and bulk equipment.

Of course, the structural members of the building are based on the foundation. The following will describe how engineers can improve the building foundation to make it more earthquake resistant in the event of a strong earthquake.

Case study: the Trans-American Pyramid in San Francisco, towering 853 feet (260 meters) high, has been a symbol of San Francisco since 1972. Its beauty alone is awe-inspiring and amazing, but it is no less impressive in terms of design and engineering. The strength of the pyramid comes from its unique truss system, which is characterized by the use of X-shaped braces on more than one floor. The truss system can bear both vertical load and horizontal load, especially the torsion force caused by earthquake. In addition to the external frame, the internal frame extends to the 45th floor. Therefore, the structure of the building is very strong and the anti-seismic effect is excellent in the earthquake. During the 7.1 magnitude Loma Prieta earthquake in the Santa Cruz Mountains in 1989, the top of the pyramid shook more than 12 inches (30 centimeters) left and right, but did not cause any damage.

Earthquake resistant foundation and materials if the foundation of the building is on soft or filled soil, the whole building may collapse in the earthquake no matter what advanced engineering technology is adopted. However, if the soil under the building is solid, engineers can greatly improve the response of the building foundation system to seismic waves. For example, earthquakes often collapse buildings from their foundations. One solution is to tie the foundation to the building so that the whole structure moves as a whole.

Another solution, known as base isolation, is to float the building on a system of bearings, springs, or padded cylinders. Engineers use a variety of support pad designs, but they usually choose lead rubber bearings, which contain a solid lead core that is alternately wrapped in rubber and steel. Lead makes the bearing hard and firm in the vertical direction, while rubber and steel belt make the bearing elastic in the horizontal direction. The supports are fixed on the building and the foundation by steel plates. When an earthquake occurs, the foundation moves, but the structure above does not move. As a result, the horizontal acceleration of the building will be reduced, and the deformation and damage will be greatly reduced.

Even if the base isolation system is installed, the building will still receive a certain amount of vibration energy in the earthquake. The building itself can dissipate or resist these energies to a certain extent, but its dissipation or resistance is directly related to the ductility of building materials. Ductility refers to the ability of a material to undergo large plastic deformation. Brick and concrete buildings are less malleable, so they absorb little energy, and they are particularly vulnerable to mild earthquakes. In contrast, reinforced concrete buildings perform much better because embedded steel bars increase the ductility of the material. Buildings made of structural steel (steel members have a variety of preformed shapes, such as beams, angles and plates) have the highest ductility, allowing the building to bend substantially without breaking.

Ideally, engineers do not have to rely entirely on the inherent energy dissipation capacity of the structure. In more and more earthquake-resistant buildings, designers are installing damping systems. For example, the active mass damping system relies on heavy mass blocks installed on top of the building and connected viscous dampers, which act like shock absorbers. When the building starts to wobble, the mass moves in the opposite direction, thus reducing the amplitude of mechanical vibration. Smaller damping devices can also be used in the support system of the building.

Even though a large number of tests have been carried out on the laboratory shaking table, any seismic engineering design concept is still a prototype before experiencing an actual earthquake. Only by experiencing earthquakes can the scientific community evaluate its performance and use what it has learned to promote innovation. In the next section, we will discuss some of these innovations, as well as the future development of earthquake engineering.

Case study: Taipei 101 was the tallest skyscraper in the world until the Burj Dubai was built in 2010. However, the 1667-foot (508m) giant tower is still a miracle of architectural innovation, one of its most impressive features is a 730t (662m) active mass damper located between the 88th and 92nd floors at the top of the building. The giant sphere is located in a bracket of eight steel cables and is connected to eight viscous dampers. If the building starts to shake, the function of the damper is to counteract the shaking movement, thereby reducing the vibration and the pressure that may make residents feel uncomfortable and put on the structure of the building.

The role of earthquake-resistant buildings in the future is to protect life. This means that even if it is eventually demolished, a building that does not collapse and allows residents to escape is useful. However, the building will be deformed in the earthquake, how will it be restored to its original state? For some researchers, such as Greg Deierlein of Stanford University and Jerome Hajjar of Northeastern University, this is what the future of earthquake engineering needs to consider.

Darling and Hajar collaborated to create a building technology called cradle, which consists of three basic components: a steel frame, a steel cable and a steel fuse. Its working principle is as follows: when an earthquake occurs, the steel frame will shake violently up and down. All the energy is transferred down to a fitting with multiple toothed fuses. The fuse teeth are clenched together, and even until the fuse fails, the frame itself remains intact. Once the shaking stops, the cable in the frame pulls the building back to its upright position. Then the worker inspects the fuse and replaces the damaged fuse. In this way, buildings can be quickly restored to their original state after the earthquake.

Another innovative technology, known as the "seismic invisibility cloak", allows buildings to remain invisible when surface waves generated by an earthquake reach them. To achieve this goal, engineers will bury a series of more than 100 concentric plastic rings under the foundation of the building. When the local shock wave encounters these plastic rings, they enter the plastic ring and then enter the plastic bottleneck. The waves basically glide under the foundation of the building, and then flow out of the ring on the other side and restore the original velocity and amplitude, which is equivalent to letting the surface waves generated by the earthquake cross the building.

Interestingly, earthquake engineering is mainly about reviewing the past, not looking forward to the future. This is because retrofitting old buildings with improved designs and materials is as important as building new ones from scratch. Engineers have found that adding a base isolation system to buildings is both feasible and economically attractive. According to the National earthquake Hazard reduction Program, more than 200 buildings in the United States, including many municipal, fire and emergency buildings, are equipped with seismic isolation systems. After the 1989 Loma Prieta earthquake, engineers renovated a number of buildings, including city halls in San Francisco, Oakland and Los Angeles. The aseismic structures of these buildings are bound to face the test of a serious earthquake. The only problem is that we don't know when it will happen and how earthquake-resistant it will be.

Case study: San Francisco City Hall engineers in California need to consider not only the height of the building, but also the civilian or cultural significance of the building when evaluating the available anti-seismic technology. For example, the construction of earthquake-resistant facilities in hospitals may be more noteworthy than warehouses. After the 1989 Loma Prieta earthquake, which killed 3500 people and damaged 100000 buildings in San Francisco, Oakland and Santa Cruz, engineers and city planners retrofitted some important buildings, such as San Francisco City Hall. Engineers cut the two-block building from the foundation and made it float on 530 basic isolators. If the earthquake waves hit, the building will sway up to 26 inches (66 centimeters) horizontally, so it will not shake and fall apart.

Author: William Harris

Translation: sweeping monk

Revision: * 0

Original link: How Earthquake-resistant Buildings Work

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: William Harris

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