What is the philosophy of seismic design?

Should we abandon the design of earthquake-resistant buildings? or should we design it; So that there is no damage during strong earthquakes? It is clear that the first approach can lead to great disaster, and the second approach is very expensive. Thus, the philosophy of seismic design must be somewhere between these two situations. [1]

Content of the article:

seismic design philosophy

It can be summarized as follows:

  1. With slight vibration, the main elements of the building should not be damaged; which support vertical and horizontal forces, as for load-bearing parts of the building; You may suffer irreparable damage.
  2. With moderate vibration, key elements can be repaired and other parts of the building can be damaged to such an extent that they may need to be replaced after an earthquake.
  3. With strong vibrations, key elements can be severely damaged (nor repaired), but the building should not collapse.
Seismic performance objectives: to receive little repairable damage with light vibration and to prevent collapse with strong vibration

After a slight vibration, the building will operate at full power within a short time and repair costs will be low. After a moderate vibration, the building will function after the repair and strengthening of the main damaged elements has been completed. After a strong earthquake, the building may be broken and unusable again, but it must remain in place so that people can be evacuated and the properties restored.

The consequences of damage must be considered in the seismic design philosophy. for example; Important buildings, such as hospitals and fire stations, play an important role in post-earthquake activities and should function as soon as they occur. These structures should suffer very little damage and be designed to provide a higher level of earthquake protection. The collapse of dams during earthquakes can cause flooding, which in itself can be a secondary disaster. Therefore, dams (and similarly, nuclear power plants) should be designed for significant levels of seismic motion. [1]

Seismic damage is inevitable

Designing buildings to withstand earthquakes involves controlling for acceptable levels of damage and cost. Different types of injuries occur They appear mainly through cracks; Especially in concrete and stone buildings – during earthquakes. Some of these cracks are acceptable (in terms of size and location), while others are not. for example; In a building with a concrete frame reinforced with stone walls between the columns, cracks between the columns and stone walls are acceptable, but diagonal cracks passing through the columns are not acceptable.

Diagonal cracks in columns jeopardize the ability to withstand vertical loads

Therefore, earthquake resistant design is about ensuring that seismic damage is of an acceptable type as well as occurring in the right places and in the right quantities. This approach is very similar to usage “Electric fuses” in home; To fully protect wires and electrical appliances in a home, you sacrifice a few small pieces of an electrical circuit, called fuses. These fuses are easily replaced after an increase in voltage. Also, to save the building from collapsing, you will allow some predetermined parts to be subjected to an acceptable type and level of damage. [1]

compliance

Let us take two rods of the same length and the same cross section, one of them of material “Ductile compliant” And the other of the material “fragile-fragile”. Now, tighten them until they break! You will notice that a ductile rod extends by a large amount before it breaks, while a brittle rod suddenly breaks when its maximum resistance is reached with a relatively small elongation. For materials used in construction; Steel is malleable, and building stone and concrete are brittle materials. [2]

Engineers define the special properties that allow a material to bend back and forth with a large amount,“Ductility”. Acceptable forms of damage and the desired behavior of the building during an earthquake must be determined. It also needs to be designed and built “Earthquake resistant buildings”, especially its key elements, to be consistent. These buildings have the ability to oscillate back and forth during an earthquake and withstand its effects with some damage but not collapse. Compliance is one of the most important factors affecting the performance of the building. Thus, earthquake-resistant design seeks to pre-identify the sites where damage occurs and then provide good design details at these locations to ensure appropriate building behavior. [1]

Design of earthquake resistant buildings

The forces of seismic inertia created at floor levels are transmitted through various rollers and columns to the ground. Proper construction components must be compatible. can affect “Failure-Failure” The column stabilizes the entire building, and beam failure causes a localized effect. Therefore, it is better to design the shells to be forged rings that are weaker than the columns (this can be achieved by choosing the right size of elements and providing them with the right amount of reinforcing steel). This method of designing reinforced concrete buildings is called; Strong and weak column method permissible. [2]

Strong pillar and weak beam

sources

[1] What is the philosophy of seismic design for buildings?

[2] How to make ductile buildings for good seismic performance?

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