Two equal and opposite forces balance This is known. So if we apply corresponding compressive forces to the tensile forces, they will balance. This is the prestressing mechanism that static civil engineers use to achieve large bridge spans, so large that this would be impossible to achieve with simple linear reinforcement and the cooperation mechanism of concrete and steel, that of relevance that they use in construction. I will try to explain to you the reason.
As a span increases, the bending loads increase, so the tension and compression in the cross-section also increase. To receive the compression, we increase the concrete, that is, we increase the cross-section height. When I increase the dimensions of the cross-section, the loads also increase. To receive the loads we also increase the steel reinforcement. Steel has superior tensile strength, but to receive the tension it needs the help of concrete. That is, the concrete must have the ability to hold the steel inside it when it pulls from the right and left so that the steel does not slip through the concrete and their cooperation is broken. This pulling force applied at the interface of the two materials of steel and concrete is called shear.
The concrete not being able to withstand the shear caused by the pull of the steel breaks, their cooperation is lost and the bridge falls. As the span of the bridge increases, so we increase its mass and its loads, but without having the possibility to increase above a limit the strength of the concrete coating in terms of shearing. This is why we cannot construct large spans of 50 meters in bridges with the simple linear reinforcement that we construct in buildings. Concrete with the simple reinforcement method of this relevance has a problem because it cannot withstand the shear caused by the high tensile strength of the steel. Concrete, however, has superior strength in receiving the torsional force. So what do we do? We apply large compressive forces to the cross-section to neutralize the tensile forces and balance the forces and this means that along with the tensile forces we have also neutralized the shear forces at the interface of the concrete and the steel, since we have neutralized the tension that causes them.
In large earthquakes the seismic loads are three times the static loads. The shear failure of the concrete is given by the tripling of the stresses In order for this not to happen, the static civil engineers must apply prestressing to the walls and not only place reinforcement of the relevance With only two prestressing tendons on the slopes of the walls, they would replace 80% of the linear steel reinforcement, reduce the concrete cross-sections and increase the earthquake resistance of the structure. The other crazy thing that the statics do is that they try to stop the large moment of the overturning of the walls, which comes from the inertia of the vertical slabs, with the cross-sections of the slabs, without drawing external balance forces from the ground. If the prestressing tendon we just mentioned were anchored to the ground and not to the base, then all the forces of the overturning moments would be diverted into the ground and the cross-sections of the slabs would not break.
The shear base cuts the cross-sections of the walls near the base, and its force is equal to the magnitude of the acceleration, multiplied by the mass of the structure. This is also the power of inertia of construction. The cross-section of the wall increases its strength in relation to the shear base by 40% when we apply compression to the cross-section of 70% of the breaking point of the concrete. Basic and well-known engineering data which, for some unknown reason, do not apply to the statics of earthquake-resistant structures. Still compacting with the ground ensures a strong foundation.
Thank you dear Dr Ioannis Lymperis for sharing this interesting question !!!!
Dear Ioannis Lymperis ,
Thank you for your discussion... In it your explanation is very logical and good, I have no experience in architecture, but I have ideas about how to design buildings that can withstand earthquakes. We witnessed some of the events that took place during the recent catastrophic earthquakes in Turkey. You can use this information to decide whether you meant what you meant... It is worth travelling to the site and paying attention to the traces left by the earthquakes in the buildings. And if you have an idea that wasn't quite accurate then correct it.
Regards,
Laszlo
Dear László Attila Horváth. I noticed from the videos I saw that the walls were completely absent. Only columns with an absence of serious reinforcement. And that all the corner buildings fell. One floor fell on top of the other starting from the ground floor They didn't take any slope when falling. Buildings are demolished in this way from the intersecting base and soft floor
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