Study Material(s)

SYNOPSIS OF PROJECT [B.TECH]

Implementation of GEO-SYNTHETIC POLYMER{Waste Plastic Except PVC} In a FLEXIBLE PAVEMENT to from MODIFIED BITUMENIOUS MIX Compactible as ALL WEATHER ROAD in INDIA.

MAIN STREAM SPECIALIZATION

INTRODUCTION ABOUT EARTHQUAKE RESISTANT BUILDING

Earthquake-Resistant Buildings

DYNAMIC ACTIONS ON BUILDINGS – WIND versus EARTHQUAKE Dynamic actions are caused on buildings by both wind and earthquakes. But, design for wind forces and for earthquake effects are distinctly different. The intuitive philosophy of structural design uses force as the basis, which is consistent in wind design, wherein the building is subjected to a pressure on its exposed surface area; this is force-type loading. However, in earthquake design, the building is subjected to random motion of the ground at its base (Figure 1.1), which induces inertia forces in the building that in turn cause stresses; this is displacement-type loading. Another way of expressing this difference is through the load-deformation curve of the building – the demand on the building is force (i.e., vertical axis) in force-type loading imposed by wind pressure, and displacement (i.e., horizontal axis) in displacement-type loading imposed by earthquake shaking.

Wind force on the building has a non-zero mean component superposed with a relatively small oscillating component. Thus, under wind forces, the building may experience small fluctuations in the stress field, but reversal of stresses occurs only when the direction of wind reverses, which happens only over a large duration of time. On the other hand, the motion of the ground during the earthquake is cyclic about the neutral position of the structure. Thus, the stresses in the building due to seismic actions undergo many complete reversals and that too over the small duration of earthquake.

The design for only a fraction of the elastic level of seismic forces is possible, only if the building can stably withstand large displacement demand through structural damage without collapse and undue loss of strength. This property is called ductility It is relatively simple to design structures to possess certain lateral strength and initial stiffness by appropriately proportioning the size and material of the members. But, achieving sufficient ductility is more involved and requires extensive laboratory tests on full-scale specimen to identify preferable methods of detailing.

In summary, the loading imposed by earthquake shaking under the building is of displacement-type and that by wind and all other hazards is of force-type. Earthquake shaking requires buildings to be capable of resisting certain relative displacement within it due to the imposed displacement at its base, while wind and other hazards require buildings to resist certain level of force applied on it . While it is possible to estimate with precision the maximum force that can be imposed on a building, the maximum displacement imposed under the building is not as precisely known. For the same maximum displacement to be sustained by a building , wind design requires only elastic behaviour in the entire range of displacement, but in earthquake design there are two options, namely design the building to remain elastic or to undergo inelastic behaviour. The latter option is adopted in normal buildings, and the former in special buildings, like critical buildings of nuclear power plants.

What are the Four Virtues?

All buildings are vertical cantilevers projecting out from the earth’s surface. Hence, when the earth shakes, these cantilevers experience whiplash effects, especially when the shaking is violent. Hence, special care is required to protect them from this jerky movement. Buildings intended to be earthquake-resistant have competing demands. Firstly, buildings become expensive, if designed not to sustain any damage during strong earthquake shaking. Secondly, they should be strong enough to not sustain any damage during weak earthquake shaking. Thirdly, they should be stiff enough to not swing too much, even during weak earthquakes. And, fourthly, they should not collapse during the expected strong earthquake shaking to be sustained by them even with significant structural damage. These competing demands are accommodated in buildings intended to be earthquake- resistant by incorporating four desirable characteristics in them. These characteristics, called the four virtues of earthquake-resistant buildings, are:

1. Good seismic configuration, with no choices of architectural form of the building that is detrimental to good earthquake performance and that does not introduce newer complexities in the building behaviour than what the earthquake is already imposing;

2. At least a minimum lateral stiffness in each of its plan directions (uniformly distributed in both plan directions of the building), so that there is no discomfort to occupants of the building and no damage to contents of the building;

3. At least a minimum lateral strength in each of its plan directions (uniformly distributed in both plan directions of the building), to resist low intensity ground shaking with no damage, and not too strong to keep the cost of construction in check, along with a minimum vertical strength to be able to continue to support the gravity load and thereby prevent collapse under strong earthquake shaking; and

4. Good overall ductility in it to accommodate the imposed lateral deformation between the base and the roof of the building, along with the desired mechanism of behaviour at ultimate stage.

Behaviour of buildings during earthquakes depend critically on these four virtues. Even if any one of these is not ensured, the performance of the building is expected to be poor.

(a) Who Controls the Four Virtues?

Henry Degenkolb, a noted earthquake engineer of USA, aptly summarized the immense importance of seismic configuration in his words: “If we have a poor configuration to start with, all the engineer can do is to provide a band-aid - improve a basically poor solution as best as he can. Conversely, if we start-off with a good configuration and reasonable framing system, even a poor engineer can’t harm its ultimate performance too much.” Likewise, Nathan M. Newmark and Emilo Rosenbleuth, eminent Professors of Earthquake Engineering in USA and Mexico, respectively, batted for the concepts of earthquake-resistant design in their foreword to their book: “If a civil engineer is to acquire fruitful experience in a brief span of time, expose him to the concepts of earthquake engineering, no matter if he is later not to work in earthquake country.”

In many countries, like India, in the design of a new building, the architect is the team leader, and the engineer a team member. And, in the design of retrofit of an existing building, the engineer is the team leader, and the architect a team member. What is actually needed is that both the architect and the engineer work together to create the best design with good interaction at all stages of the process of the design of the building. Here, the architect brings in perspectives related to form, functionality, aesthetics and contents, while the engineer brings the perspectives of safety and desired earthquake performance during an expected earthquake. There is a two way influence of the said parameters handled both by the architect and by the engineer; their work has to be in unison.

(b) How to Achieve the Four Virtues?

The four virtues are achieved by inputs provided at all stages of the development of the building, namely in its planning, design, construction and maintenance. Each building to be built is only one of the kind ever, and no research and testing is performed on that building, unlike factory- made products like aircrafts, ships and cars. The owner of the building trusts the professionals (i.e., architect and engineer) to have done due diligence to design and construct the building. Thus, professional experience is essential to be able to conduct a safe design of the building, because it affects the safety of persons and property.

Traditionally, in countries that have advanced earthquake safety initiatives, governments have played critical role through the enforcement of techno-legal regime, wherein the municipal authorities arrange to examine, if all requisite technical inputs have been met with to ensure safety in the building, before allowing the building to be built, the construction to be continued at different stages, or the users to occupy the building. These stages are: (1) conceptual design stage, (2) design development stage through peer review of the structural design, (3) construction stage through quality control and quality assurance procedures put in place. Senior professionals (both architects and engineers) are required to head the team of professionals to design a building; these senior professionals should have past experience of having designed buildings to resist strong earthquakes under the tutelage of erstwhile senior professionals.

EARTHQUAKE DEMAND VERSUS EARTHQUAKE CAPACITY

Unlike all other loading effects, e.g., wind loads, wave loads (excluding tsunami loads), blast loads, snow loads, imposed (live) loads and dead loads, earthquake shaking is the most severe, because it imposes displacement under the building, which is time varying. This, in turn, demands lateral deformation in the building between its base and upper elevations. Higher is the seismic zone, larger is the severity of this imposed relative deformation (Figure 1.12). Therefore, the main challenge is to meet the double demand – the building should be able to withstand this imposed deformation with damage under small intensity shaking, and with no collapse under high intensity shaking. The building needs to possess large inelastic deformation capacity and needs to have the strength in all its members to sustain the forces and moments induced in them.

The method of design of buildings should therefore take into account the deformation demand on the building, and the deformation capacity of the building. The former depends on the seismo-tectonic setting of the location of the building, but the later is within the control of the design professionals (i.e., architects and engineers). The concern is that both of these quantities have uncertainties. On one hand, even though some understanding is available on the maximum possible ground dispalcement at a location, earth scientists are not able to clearly provide the upper bound for these numbers. Each new damaging earthquake has always provided surprises. And, on the other hand, analytical tools are not available to estimate precisely the overall nonlinear behaviour of an as-built structure, and its ultimate deformation capacity.