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SEISMIC DESIGN PHILOSOPHY FOR BUILDINGS

The Issue of Earthquakes

During an earthquake, the severity of ground shaking at a given area might be minimal, moderate, or intense. Minor shaking happens a lot, moderate shaking happens a lot, and intense shaking happens a lot. For example, roughly 800 earthquakes of magnitude 5.0-5.9 occur annually on average over the planet, but only about 18 earthquakes of magnitude 7.0-7.9 occur annually. So, even if the building’s life span is only 50 or 100 years, should we plan and construct it to withstand the unusual seismic shaking that may occur only once every 500 years or perhaps once every 2000 years at the chosen project site?

Because providing greater earthquake safety in buildings costs money, a tension arises: Should we abandon building design for earthquake effects? Should we construct structures to be “earthquake resistant,” so that no damage occurs during the powerful but infrequent earthquake shaking? Clearly, the former technique can result in a big calamity, while the latter approach is too expensive. As a result, the design philosophy should fall somewhere in the middle of these two poles.

Earthquake-Resistant Buildings

Engineers do not attempt to design earthquake-proof structures that would not be damaged even during a rare but powerful earthquake; such structures would be too large and expensive. Instead, the technical goal is to design buildings earthquake-resistant; these structures will withstand the effects of ground shaking, even if they are severely damaged, and will not collapse during a big earthquake. As a result, earthquake-resistant structures ensure the safety of people and possessions, preventing a calamity. Seismic design codes all around the world include this as a main goal.

Earthquake Design Philosophy

The following is a summary of the earthquake design philosophy (Figure 1):

(a) The primary members of the building that carry vertical and horizontal forces should not be damaged by modest but frequent shaking; however, building parts that do not carry load may incur repairable damage.

(b) The primary members of the building may sustain repairable damage with modest but infrequent shaking, but other parts of the building may be damaged to the point that they may need to be replaced after the earthquake; and

(c) The primary members of the structure may experience significant (even irreversible) damage during strong but rare shaking, but the structure should not collapse.

Figure 1: Performance objectives for different earthquake shaking intensities – targeting minimal repairable damage under moderate shaking and high repairable damage under major shaking.

Preventing collapse in the event of a major tremor.

As a result, after moderate tremors, the building will be fully functioning in a short period, with little repair expenditures. After the damaged main members are repaired and strengthened, the building will be operational after moderate shaking. However, following a large earthquake, the structure may become unfit for further use, but it will remain standing so that people may be evacuated and property can be salvaged.

The repercussions of damage must be considered while developing a design philosophy. Important structures, such as hospitals and fire stations, for example, play a key part in post-seismic activities and must stay operational immediately following the earthquake. These constructions must withstand minimal damage and should be built to withstand earthquakes to a greater extent. Dam failure during earthquakes can result in flooding in downstream areas, which can be a secondary calamity. As a result, dams (as well as nuclear power facilities) should be designed to withstand much more earthquake motion.

Building damage is unavoidable.

Controlling the damage to acceptable levels at a reasonable cost is an important part of earthquake-resistant structure design. Engineers designing earthquake-resistant buildings know that some damage is unavoidable, contrary to popular belief that any fracture in a building after an earthquake signifies the building is hazardous for habitation. During earthquakes, different sorts of damage (often visible through fractures; especially in concrete and masonry constructions) occur in buildings. Some of these cracks are okay (both in terms of size and position), but others are not. For example, cracks between vertical columns and masonry filler walls are permissible in a reinforced concrete frame building with masonry filler walls between columns, but diagonal cracks running through the columns are not.

Qualified technical personnel are familiar with the causes and degree of damage in earthquake-resistant structures in general.

Buildings’ vertical weight carrying capacity is jeopardised by diagonal cracks in columns, which is an unacceptable risk.

As a result, earthquake-resistant design is concerned with ensuring that building damages during earthquakes are of an acceptable type, as well as that they occur in the correct places and in the right amounts. This approach to earthquake-resistant design is similar to the use of electrical fuses in homes: to protect the entire electrical wiring and appliances in the house, some small parts of the electrical circuit, called fuses, are sacrificed; these fuses are easily replaced after the electrical over-current.

Similarly, in order to prevent the building from collapsing, you must allow some pre-determined parts to sustain damage of an acceptable type and severity.

Ductility is an acceptable form of damage.

As a result, the goal today is to determine what types of damage are acceptable and what types of building behaviour are desired during earthquakes. To do so, we must first comprehend how various materials react. Consider the usage of solid-headed steel pins to hold sheets of paper together and white chalk to write on blackboards. Yes, a chalk is easily broken!! A steel pin, on the other hand, can be bent back and forth. Engineers call ductility the quality that allows steel pins to bend back and forth in huge amounts; chalk is brittle.

Earthquake-resistant structures, especially their primary parts, must be constructed with ductility in mind.

Such structures can wobble back and forth during an earthquake and endure the effects of an earthquake with considerable damage but not collapse (Figure 3). One of the most critical aspects determining building performance is ductility. As a result, earthquake-resistant design aims to predict where damage will occur and then provide good detailing at these spots to ensure the building’s ductile behaviour.

(a) Earthquake performance of buildings: ductile and brittle extremes

(b) A reinforced concrete column fails due to brittleness

Figure 3: Brittle and ductile structures; seismic design tries to avoid the latter.

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