What is an earthquake resistance building
Many regions of the world are regularly shaken by earthquakes. In order for houses, bridges and other structures to withstand these loads, engineers have developed a variety of methods - tailor-made for each individual structure.
Trace of the quake
When tectonic stresses in rock formations deep underground exceed their strength, abrupt fractures and shifts occur - an earthquake occurs. The strongest earthquakes can cause fractures several square kilometers in size, traces of which can sometimes even be seen on the surface of the earth. The energy released inside the earth is partly propagated in the form of seismic waves that penetrate to the earth's surface and set it in motion.
Terms such as magnitude and intensity characterize the strength of earthquakes, but a clear distinction must be made between them. The magnitude is a logarithmic measure of the energy released during an earthquake, while the intensity is a measure of the consequences of an earthquake in a specific location. This includes damage to buildings, but also the impact on people - such as feelings of fear or imbalance - and on nature. Magnitudes are often given according to the Richter scale; there are several, mostly twelve-point scales for intensity. In the strongest German earthquake zones, which are near Aachen and in the southern part of Baden-Württemberg, a possible intensity of up to 7.5 according to the European Macroseismic Scale is assumed, which corresponds to noticeable damage to buildings.
Acceleration of the ground
In general, the local intensity of an earthquake depends on its magnitude, the distance to the epicenter and, last but not least, on the subsurface conditions; While the soil acceleration on the earth's surface has a long-wave time course with relatively low peaks in loose sediments, short-term, high-frequency time courses with relatively high peaks occur on the rock bedrock (see graphic: Acceleration of the soil
If a structure is to be built in an earthquake-prone region, engineers first determine how much it could be impacted by an earthquake at its future location. In doing so, they fall back on the so-called response spectrum. In simplified terms, this diagram shows how single-mass oscillators with different periods of oscillation behave when exposed to an earthquake as a base point excitation. Physically, one can imagine such single-mass oscillators as masses on slender supports.
Single mass oscillator
An oscillator with a high natural frequency and a short period of oscillation represents, for example, structures with a strong resistance to horizontally acting forces, so-called “stiff” buildings. From the response spectrum it can then be seen that they have relatively high accelerations and thus inertia forces. “Soft” systems, on the other hand, experience little acceleration at the cost of greater deformations relative to the subsoil.
Before engineers can base any response spectrum on a given range of responses, they need to define the strength of the assumed ground motion. In this way, they ultimately determine how earthquake-prone the planned location is. For normal office and residential buildings, it has become common practice around the world to estimate the ground movements occurring at the construction site on average every 475 years; In the case of structures that are particularly important for the general public, such as hospitals or those with high damage consequences such as dams or nuclear power plants, much stronger earthquakes can be assumed, with average return periods of 1000, 2500 or 10,000 years.
This information can then be used to calculate how much cutting forces and deformations stress an existing or planned structure. On the basis of these results, the engineers then select a suitable design. Their load-bearing capacity must at least be sufficient to rule out failure as a result of the assumed earthquake impact. The protection of human life is thus guaranteed. In addition, a building can also be secured against weaker, more frequent ground movements (mean return period: 95 years for normal buildings). Costly repairs and downtime can be avoided.
Guidelines for Architects
When designing earthquake-proof structures, architects must observe a number of rules that can be found in the relevant standards, among other things. Compact structures with symmetrical floor plans are generally cheaper than those with a structured floor plan. Even resistance to horizontally acting inertia forces along the height of the building, i.e. no “soft” and “stiff” storeys in the same building, is an advantage. In addition, buildings with large masses on the higher floors or systems susceptible to torsional vibrations should be avoided.
In the case of structures of particular importance and / or high damage consequences, the supporting structure is dimensioned in such a way that it remains in an elastic state even under the strongest earthquakes to be assumed: Engineers therefore choose a very stiff system with high natural frequencies and very low deformations. In the case of conventional high-rise buildings, on the other hand, a certain degree of plastic deformability, which is also referred to as “ductility”, can be used explicitly, which leads to lower costs. This in no way means an increased susceptibility to failure, provided that the rules anchored in the standard are observed.
Ductility, stiffness and strength
The sensible combination of the three parameters ductility, rigidity and strength of a structure guarantees a high level of seismic safety. Sufficient ductility ensures that local overstressing does not lead to global failure, but is defused through redistribution; Adequate rigidity against horizontal displacement is not only important for stability itself, but also to avoid inventory damage and loss of use. After all, even after several load changes, the strength of the support links must not decrease so much that there is a risk of failure.
Effective protection against earthquake damage can be achieved through "seismic isolation": In the event of a strong earthquake, the structure is decoupled from the moving subsoil by means of suitable bearing structures. In this case, the structure remains essentially at rest or experiences only slight accelerations and thus inertia forces, while the ground moves back and forth due to the earthquake.
A structure without such insulation is caused to vibrate by seismic waves over the foundation. In this case, safety can be increased by other measures - special mechanisms convert the work carried out by the earthquake, for example, in a targeted manner into other forms of energy (e.g. using damping elements) or reduce building vibrations by distributing the energy over a broader frequency range. A distinction is usually made between passive and active mechanisms, but there are also hybrid systems that combine elements of both approaches.
Avoid building vibrations
Passive systems do not have any feedback between the vibrating components controlled by a control loop and therefore do not require any external energy supply, which ensures their function even if the power supply is interrupted. These include, for example, systems that convert part of the seismic energy through local plastic deformations on specially designed components. In active systems, the vibration amplitudes are minimized by generating a counterforce that is determined by a control loop and counteracting it.
Vibration dampers (tuned mass dampers) are also of practical importance; these are systems connected to the main structure, which are excited to vibrate in the event of an earthquake and thus extract energy from it. They can also function as passive, active or hybrid systems and are present in many structures, for example in the 500-meter-high Taipei 101 skyscraper, in the 60-story John Hancock Building in Boston or in the 300-meter-high Centrepoint Tower in Sydney .
Overall, it should be noted that the erection of earthquake-proof structures or the corresponding reinforcement of existing structures is always possible, whereby the effort for the measures to be taken in each individual case should be in a reasonable relationship to the importance of the structure and the existing earthquake hazard.
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