How do elastic and rigid structures react differently to an earthquake?
We all know that the structure rests on the ground and the weight of the structure is directed vertically, while the ground reacts with forces in opposite directions to create a balance of forces. All constructions are based on the ground, but from the ground and above each construction has its own design and its own shape. How does an elastic structure with columns react? How does a structure with reinforced concrete walls react to the earthquake? How does one react that is made entirely of reinforced concrete and is rigid? How does a frame structure from another asymmetric react to the earthquake? All these structures lose ground below their base when an earthquake occurs, yes or no? The truth is somewhere in the middle. A rigid high-rise construction made of reinforced concrete ... example .. an elevator shaft, during the rocking of the earthquake is will be overturned without breaking. When overturned, one side of the base will rise in the air and lose ground support. Without ground support the shaft loads remain in the air and create a torque, in the opposite direction, from that of the shaft. These opposite torque forces tend to cut the shaft in two, but it withstands because its vertical section is large and strong, and instead of cutting it reverses. This is because the strong vertical section withstood the weight of unsupported static loads. But if we have a large rigid house made of reinforced concrete with doors and windows, then the unsupported loads will be larger, the vertical sections above the doors and windows smaller and therefore weaker, and the shear failure at these points will be inevitable. In this construction we see that what demolishes the house is not the earthquake, but the same unsupported weight which is created by the overturning moment of the rigid house. If in these rigid houses we apply compressive intensities in their cross sections with simultaneous anchoring with the ground, from their highest points, by the mechanism of the patent they will never lose ground support and will not have brittle failures. Houses with columns and beams do not react in the same way. During the rocking of the earthquake in the elastic constructions with columns and beams, bending and tipping moment is created. But the cross sections of the beams are so small that they are unable to lift the structure in the air. So one side of the structure never rises from the ground to the air, because the cross section of the beam does not have the strength to lift it. So no unsupported loads are created. What is done is to bend the beams elastic and then inelastic until they break.
I agree with what you say. However, flexible structures have a limit of elasticity that does not present a problem and after this limit they are vulnerable because their inelastic displacement is not controlled. So when the ground acceleration is large, the duration is long, and the construction is coordinated with the earthquake then there is a problem. The rigid structure has no damping mechanism but when you join the rigid structure with the ground it withstands high acceleration with long seismic duration and coordination never occurs.
The answer to this question can vary depending on how you look at it. Personally, I think this question can be examined from four perspectives:
1) Cost-effective construction perspective;
Obviously, the elastic behavior of the structure during an earthquake is very desirable. Because the damage after the earthquake is very low and more importantly, the building will be able to be serviced with minor repairs.
But the point is that achieving such a structure will cost a lot and is by no means cost-effective. Have you ever wondered why when you enter a hospital, your eyes see a set of super columns? This is because the designer and builder expect perfectly elastic behavior from the structure.
Therefore, the modification factor that is presented in the building codes, relying on the ductility of the structure and as a result, the decrease in strength compared to the elastic design, will naturally lead to a reduction in construction costs. This ductility and entry into the inelastic area will definitely have consequences.
2) Earthquake-based design perspective;
The fact is that if we expect the desired behavior of a structure, we must design that structure separately for each earthquake. Only then is almost everything in order. While so far, the approach used in the designs is based on the efficiency of the structure in different earthquakes, which causes uncertainties.
3) Elastic frame with repairable (or replaceable) fuse perspective;
In the last two decades, appropriate approaches with the aim of keeping the frame elastic after the earthquake and dissipating the earthquake energy by directing damage to the fuses have opened a new window for the designer. Where these tools are delivered and repaired or replaced immediately after an earthquake and the building can be serviced in the shortest possible time. In addition, all the elements of the frame remain elastic.
This can be found in rocking and seesaw frames.
4) Sobhan-Origin transfer perspective;
I must say that this view has not been raised anywhere so far, and this is the first time I have said it. I may be doing something wrong, but I believe that hiding in knowledge is not in anyone's interest.
So far, in all the foundations of structural dynamics, the earth has been assumed as the origin of the coordinate system, and all the results have been presented on this basis. Suppose you are standing on the ground and watching the oscillations of a structure during an earthquake. This is exactly the way in which dynamic foundations have been laid.
Now suppose you are on the roof of a structure looking at a shaky building during an earthquake. In fact, the origin of the coordinate system has been transferred to the roof of the structure. In this situation, everything changes compared to before. If from the beginning instant of the earthquake (for example, an earthquake with a time duration of 40 seconds), we follow each member of the structure in Lagrangian, ie design the structure for the demands of each second of the earthquake (a total of forty design steps), all member resistances are designed very ideal. But this process requires the generation and development of a very special algorithm that measures each step according to the structural reactions and completes the chain loops.
Dear Sobhan Ghasemi Thank you for your interest and help.
Pre-tensioning is a method by which they are imposed compressive forces on reinforced concrete sections The result of prestressing is the reduction of tensile stresses in the cross section to a point where they do not exceed the cracking stress. Therefore the concrete does not crack! In this way it is possible to treat concrete as a material with elasticity.
The method I propose involves prestressing and anchoring at the same time to the ground under the foundation.
The method is more effective when placing on walls with a large width, on which we impose compressive stresses on all its sides + anchoring in the foundation ground, with the same mechanism. Constructions consisting of large walls are the prefabricated houses made of reinforced concrete, which are 50% cheaper than conventional houses, because they are industrialized. If you apply the design method I suggest in prefabricated houses then you will have cheap and seismically very strong construction.
What is the innovation of the method I propose.
The novelty of the invention can be expressed in two ways 1) Receives seismic loads over the construction and through the mechanism diverts them into the ground. That is, it removes seismic loads from the structure before they cause shear failures through deformation. 2) The mechanism receives a force from the ground and transfers it to the structure where the seismic loads are, in order to balance the seismic loads. This is not done by any other seismic design in the world.
If a structure is designed to be elastic, it will not be easily overturned, but in small earthquakes nothing will happen, in medium-sized earthquakes it will have some failures and in large earthquakes it can stand but it can collapse, and that depends on unbalanced factors.
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