Each material is more resistant to a certain force. Concrete and above all prestressed concrete withstands compressive stress. In tensile bending torsion and shear it has a problem. Reinforcing steel has super tensile strength. And we make these two materials work together so that the concrete receives the compression and the steel receives the tension. Great combination. Is it so or not? No, its not like that. Ideally, the steel and concrete would have exhausted their compressive and tensile strengths before they failed. But this does not happen. Both concrete and steel fail before they exhaust their strength. This is because during the bending of the body of the load-bearing elements, in addition to the compressive and tensile forces, another force, the shear force, appears on the interface where the concrete and steel are in contact. The concrete covering the steel having no resistance to the shear force breaks along the steel and their cooperation stops. Thus before the steel and concrete exhaust their tensile and compressive strengths, shearing cancels their strengths. This problem grows even more when the critical area of failure occurs at the ends of the load-bearing elements, because apart from the mentioned problem we also have the potential difference in adhesion. Another problem is that the cover concrete does not withstand bending and breaks leaving the steel reinforcement exposed so the bond is cancelled. The ideal would be if we could eliminate the bending of the beam and the shearing that occurs at the concrete-steel interface when the steel begins to stretch. Then only concrete and steel would exhaust 100% of their ultimate compressive and tensile strengths before failing. There is a solution? Yes there is a solution but it is rarely used. It's called, prestressing. Prestressing uses the steel to compress the concrete with the help of hydraulic pullers, and compaction systems at their ends. The compression in the concrete makes it capable of receiving the developing tensile forces. It reduces the bending of the trunk, thus also the deformation of the load-bearing element. It increases the effective cross-section because the compressive force is distributed throughout the cross-section, effectively eliminating the inert concrete cover. The main one is that prestressing has strong ductility and is considered elastic since it restores the structure (compression ratio) to its original position by tilting the developing cracks after a strong inelastic displacement of the structure. Now why they don't do this to the walls that are the cause of the distortion in the whole structure I don't know. If prestressing is applied at the ends of the longitudinal rigid walls and is combined with compaction in the foundation soil then the overturning moment and the bending moment and the shear failure of the cover concrete will be stopped and the response of the cross section will increase with respect to the other intersecting that of basis. Consider that a Φ/50mm cross-section steel lifts a two-story building into the air and we place 8000 kg of steel on the two-story building and have an earthquake problem due to shear failure.
Dear Aparna Sathya Murthy The magnitude of the acceleration reaching below the base of the structure, the duration of the earthquake, the coordination of soil and structure, the height of the structure end the type of building are the five main factors that increase the forces for shear failure to occur. The reduction of the diameter of the reinforcing steel bars and the quality of the concrete, reduce shear failure. Increasing horizontal reinforcement reduces cracking
Interesting topic ........ Thanks dear Dr Ioannis Lymperis for sharing !!!
Dear Doctor
"Wind shear can occur at many different levels of the atmosphere, however it is most dangerous at the low levels, as a sudden loss of airspeed and altitude can occur. Plenty of altitude is normally needed to recover from the stall produced by the abrupt change in wind speed and direction, which is why pilots need to be aware of the hazards and mitigation of low-level wind shear. So how does the phenomenon occur? There are a couple of ways, with the most dangerous coming from thunderstorm updrafts and downdrafts. The lifecycle of a thunderstorm starts with just an updraft as warm moist air at the surface rises in an unstable atmosphere to heights as high as 50 to 60 thousand feet in the air. As the updraft matures, precipitation forms and falls out as rain. This is when the downdraft portion of the thunderstorm occurs. Rain-cooled air sinks along with the downward drag from falling precipitation. The harder the rain falls, the stronger the downdraft can become. Also, stronger atmospheric winds aloft are sometimes able to be pulled down to the surface by the already established downdraft, adding to the intensity of the downward moving winds.
Another way that low-level wind shear can occur is when a “nocturnal” or nighttime inversion sets up as the lowest levels of the atmosphere “decouple” from the upper levels and stronger winds. A nocturnal inversion is basically a layer of warm air that develops in the lowest few thousand feet of the atmosphere as the planetary boundary layer becomes decoupled or “disconnected” from the stronger winds just above it. The layer of warm air marks the interface between the calmer and much cooler layer of air near the surface and the stronger winds and more well-mixed atmosphere above the inversion. This interface is another area where wind shear can occur. Many times, the change in temperature with height between the inversion and the atmosphere above it can create the pressure gradient necessary for a low level wind maximum between 25 (weaker) and 60 (stronger) knots. So, imagine taking off on a calm evening and climbing through 2 thousand feet. Suddenly, you encounter serious turbulence as a headwind or tailwind of 40 knots rocks your aircraft and changes your airspeed dramatically. This effect can quickly cause a pilot to lose control of the aircraft, especially if he or she is caught off guard or have never experienced the phenomenon before.
As mentioned above, many changes to how we detect and react to low-level wind shear have changed since the landmark crash of Flight 191. With the advent of the National Weather Service Doppler Radar, low-level velocity data has aided in the detection of lowlevel wind shear. Many large airports now have TDWRs (Terminal Doppler Weather Radar) installed near the airport to help detect changes in wind speed and direction. Also, at the time of the Flight 191 crash, the Federal Aviation Administration was in the process of testing and implementing LLWAS (Low Level Windshear Alert System). Although the new system had shown promise in detecting low-level wind shear in other incidents in recent years, it was not able to detect wind shear before the Dallas/Fort Worth crash in 1985. As a result, the nation demanded something be done. In 1986, the FAA announced the National Integrated Windshear Plan. This included better training for pilots in how to detect and handle windshear situations and eventually led to the development of the TDWRs at airports today."
Airborne Trailblazer, Chapter 5: “The Best We Can Do”, Taming the Microburst Windshear, Wallace, Lane E. 1994, 198p. (http://oea.larc.nasa.gov/trailblazer/SP-4216/chapter5/ch5.html)
I would add that, if the tendons of the prestressing are combined with their ''footing'' in the foundation soil using anchors to do this, then we deflect the earthquake stresses out of the structure.
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