ADVANCE EARTHQUAKE RESISTANT DESIGN OF STRUCTURES.pdf

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ADVANCE EARTHQUAKE RESISTANT DESIGN OF STRUCTURES

BY

B.UMAMAHESHWARA

G.SOHAIL AHAMMED

III BTECH, CIVIL

III BTECH, CIVIL

Email : [email protected]

[email protected]

Ph.No: 8125133426

Mobile: 9703483437

DEPARTMENT OF CIVIL ENGINEERING G.PULLAREDDY ENGINEERING COLLEGE

Abstract: Existence of non-engineered buildings, no doubt, has been one of the major cause of collapse of buildings in most of the affected regions. Indian Standard Codes for Earthquake Resistant Design & construction prescribes, besides other things, use of properly designed structures will reduce the effect of earthquakes. It is important to follow the provisions of the codes in a sincere way in construction of buildings for mitigating the effect of such hazards in future. Earthquake resistant buildings are required to have the ability to sway back and forth during an earthquake and to withstand earthquake effects with some damage, but without collapse. For this the structure, particularly their main elements, need to be built with ductility in them. To achieve the required performance it is also important to have right selection of building designing techniques.

Advanced Earthquake Resistant Design Techniques Introduction: The conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity which are great enough to withstand a given level of earthquake–generated force. This is generally accomplished through the selection of an appropriate structural configuration and the careful detailing of structural members, such as beams and columns, and the connections between them.

Keywords: Base isolation, earthquake generated forces, spherical sliding isolation systems, energy dissipation device, energy dissipation device, layouts. Fig 1

INTRODUCTION: Earthquake: An earthquake is a sudden, rapid shaking of the Earth caused by the breaking and shifting of rock beneath the Earth’s surface. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth’s surface move slowly over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free causing the ground to shake. Most earthquakes occur at the boundaries where the plates meet; however, some earthquakes occur in the middle of plates.

In contrast, we can say that the basic approach underlying more advanced techniques for earthquake resistance is not to strengthen the building, but to reduce the earthquake–generated forces acting upon it. Among the most important advanced techniques of earthquake resistant design and construction are base isolation and energy dissipation devices. Base Isolation It is easiest to see this principle at work by referring directly to the most widely used of these advanced techniques, which is known as base isolation. A base isolated structure is supported by a series of bearing pads which are placed between the building and the building's foundation.(See Figure 1) A variety of different types of base isolation bearing pads have now been developed. For our example, we'll discuss lead–rubber bearings. These are among the frequently–used types of base isolation bearings. (See Figure 2) A lead–rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the bearing is a solid lead "plug." On

top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.

particular point of its earthquake response.

Earthquake Generated Forces:

fig 3

Fig 2 To get a basic idea of how base isolation works, first examine Figure 3. This shows an earthquake acting on both a base isolated building and a conventional, fixed–base, building. As a result of an earthquake, the ground beneath each building begins to move. In Figure 3, it is shown moving to the left. Each building responds with movement which tends toward the right. We say that the building undergoes displacement towards the right. The building's displacement in the direction opposite the ground motion is actually due to inertia. The inertial forces acting on a building are the most important of all those generated during an earthquake.

In addition to displacing toward the right, the un– isolated building is also shown to be changing its shape– from a rectangle to a parallelogram. We say that the building is deforming. The primary cause of earthquake damage to buildings is the deformation which the building undergoes as a result of the inertial forces acting upon it. The different types of damage which buildings can suffer are quite varied and depend upon a large number of complicated factors. But to take one simple example, one can easily imagine what happens to two pieces of wood joined at a right angle by a few nails, when the very heavy building containing them suddenly starts to move very quickly — the nails pull out and the connection fails. Response of Base Isolated Building

It is important to know that the inertial forces which the building undergoes are proportional to the building's acceleration during ground motion. It is also important to realize that buildings don't actually shift in only one direction. Because of the complex nature of earthquake ground motion, the building actually tends to vibrate back and forth in varying directions. So, Figure 3 is really a kind of "snapshot" of the building at only one

By contrast, even though it too is displacing, the base–isolated building retains its original, rectangular shape. It is the lead–rubber bearings supporting the building that are deformed. The base–isolated building itself escapes the deformation and damage— which implies that the inertial forces acting on the base–isolated building have been reduced. Experiments and observations of base–isolated buildings in earthquakes have been shown to reduce building accelerations to as little as 1/4 of the acceleration of comparable fixed–base buildings, which each building undergoes as a percentage of gravity. As we noted above, inertial forces increase, and decrease, proportionally as acceleration increases or decreases.

Acceleration is decreased because the base isolation system lengthens a building's period of vibration, the time it takes for the building to rock back and forth and then back again. And in general, structures with longer periods of vibration tend to reduce acceleration, while those with shorter periods tend to increase or amplify acceleration. Finally, since they are highly elastic, the rubber isolation bearings don't suffer any damage. But what about that lead plug in the middle of our example bearing? It experiences the same deformation as the rubber. However, it also generates heat as it does so. In other words, the lead plug reduces, or dissipates, the energy of motion—i.e., kinetic energy—by converting that energy into heat. And by reducing the energy entering the building, it helps to slow and eventually stop the building's vibrations sooner than would otherwise be the case —in other words, it damps the building's vibrations. (Damping is the fundamental property of all vibrating bodies which tends to absorb the body's energy of motion, and thus reduce the amplitude of vibrations until the body's motion eventually ceases.)

(fig. 4) During an earthquake, the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically (See Figure 4.) The force needed to move the building upwards limits the horizontal or lateral forces which would otherwise cause building deformations. Also, by adjusting the radius of the bearing's curved surface, this property can be used to design bearings that also lengthen the building's period of vibration. For more information read this article titled protective systems for Buildings: Applications of spherical sliding isolation systems as it describes one particular type of spherical sliding isolation system, and its successful use in making some structures more earthquake resistant. Energy Dissipation Devices:

Snapshot from shake table video of testing base isolated (right) and regular (left) building model

Spherical Sliding Isolation Systems: As we said earlier, lead–rubber bearings are just one of a number of different types of base isolation bearings which have now been developed. Spherical Sliding Isolation Systems are another type of base isolation. The building is supported by bearing pads that have a curved surface and low friction.

The second of the major new techniques for improving the earthquake resistance of buildings also relies upon damping and energy dissipation, but it greatly extends the damping and energy dissipation provided by lead–rubber bearings. As we've said, a certain amount of vibration energy is transferred to the building by earthquake ground motion. Buildings themselves do possess an inherent ability to dissipate, or damp, this energy. However, the capacity of buildings to dissipate energy before they begin to suffer deformation and damage is quite limited.

The building will dissipate energy either by undergoing large scale movement or sustaining increased internal strains in elements such as the building's columns and beams. Both of these eventually result in varying degrees of damage. So, by equipping a building with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building, and thus decrease building damage. Accordingly, a wide range of energy dissipation devices have been developed and are now being installed in real buildings. Energy dissipation devices are also often called damping devices. The large number of damping devices that have been developed can be grouped into three broad categories: 

Friction Dampers– these utilize frictional forces to dissipate energy  Metallic Dampers– utilize the deformation of metal elements within the damper  Viscoelastic Dampers– utilize the controlled shearing of solids  Viscous Dampers– utilized the forced movement (orificing) of fluids within the damper Fluid Viscous Dampers: Once again, to try to illustrate some of the general principles of damping devices, we'll look more closely at one particular type of damping device, the Fluid Viscous Damper, which is one variety of viscous dampers that has been widely utilized and has proven to be very effective in a wide range of applications. The article, titled application of fluid viscous dampers to earthquake resistant design, describes the basic characteristics of fluid viscous dampers, the process of developing and testing them, and the installation of fluid viscous dampers in an actual building to make it more earthquake resistant.

Damping Devices and Bracing Systems:

(fig. 5) Damping devices are usually installed as part of bracing systems. Figure 5 shows one type of damper– brace arrangement, with one end attached to a column and one end attached to a floor beam. Primarily, this arrangement provides the column with additional support. Most earthquake ground motion is in a horizontal direction; so, it is a building's columns which normally undergo the most displacement relative to the motion of the ground. Figure 5 also shows the damping device installed as part of the bracing system and gives some idea of its action.

New Earthquake –Resistant Design pulls Buildings upright after violent quake:

Keeping Buildings Upright During Quakes A new structural system dissipates energy to replaceable fuses and pulls buildings back upright after violent earthquakes. Xiang Ma, Stanford University When a quake strikes, the new system dissipates energy through steel frames in the building's core and exterior. These frames are free to rock up and down within fittings fixed at their bases. Steel tendons made from twisted steel cables run the length of each frame, keeping the frames from moving so much that the building could shear. When the quake stops, these tensile tendons pull the frames back down into the "shoes" at their bases, returning the building to its plumb, upright position. So where does all that energy go? At the base of each frame is a flexible steel "fuse" that takes the brunt of the force, keeping the frame and constituent tendons from shouldering the entire load. The fuses are easily replaceable when they blow -- just like an electrical fuse -- so after a quake, the building can be refitted with fresh fuses for its next bout with Earth's occasional tectonic fits. Many elements of the system have been tested before, this is the first time they've been melded into a complete system and successfully put through the motions. For testing, the team constructed a three-quarters-size model of a standard three-story office building, with a footprint 120 by 180 feet, and a mass comparable to a full-size building. Then they shook the hell out of it. Even at a magnitude 1.75 times that of the 1994 Northridge earthquake -- itself a 6.7 on the Richter scale -- the only damage recorded in the frame was in the replaceable fuses. Effect of shape of a building during earth quake: According to building codes earthquake-resistant structures are meant to withstand the largest earthquake of a certain probability that is likely to occur at their location. This means the loss of life should be minimized by preventing collapse of the buildings for rare earthquakes while the loss of functionality should be limited for more frequent ones

Destruction due to undesirable design: Earthquakes cause massive vibrations in the Earth’s crust. This can cause a number of problems in the ground, which in turn becomes a hazard to all life and property. The effect depends on the geology of soil and topography of the land.

(5) Analytical tools for reliable prediction of structural response. (6)It is fairly well accepted that earthquakes will continue to occur and cause disasters if we are not prepared. Assessing earthquake risk and improving engineering strategies to mitigate damages are the only options before us. Geologists, seismologists and engineers are continuing their efforts to meet the requirements of improved zoning maps, reliable databases of earthquake processes and their effects; better understanding of site characteristics and development of EQRDs. As for the engineer, the ultimate goal will remain the same: to design the perfect, but costeffective structure, that behaves in a predictable and acceptable manner. The ongoing research and development activities in the area of EQRD of structures offer significant promise in realizing that goal in the coming years.

1964 Niigata earthquake The most destructive of all earthquake hazards is caused by seismic waves reaching the ground surface at places where human-built structures, such as buildings and bridges, are located. When seismic waves reach the surface of the earth at such places, they give rise to what is known as strong ground motion. Strong ground motions cause’s buildings and other structures to move and shake in a variety of complex ways. Many buildings cannot withstand this movement and suffer damages of various kinds and degrees.

Conclusions: In the coming years, the field of EQRD of structures is most likely to witness the following significant developments: (1) A complete probabilistic analysis and design approach that rationally accounts for uncertainties present in the structural system will gradually replace deterministic approaches, especially in the characterization of the loading environment. (2) Performance-based design processes will take centre stage, making conventional descriptive codes obsolete. (3) The acceptable risk criterion for design purposes will be prescribed in terms of performance objectives and hazard levels. (4) The development of new structural systems and Devices will continue for base-isolation, passive Energy dissipation and active control systems, along With the proliferation of non-traditional civil engineering materials and techniques.

References: 1. Hamburger, R. O. and Holmes, W. T., Vision statement: EERI/ FEMA Performance-based seismic engineering project, Background document for EERI/FEMA action plan, Earthquake Engineering Research Institute, Oakland, CA, 1998. 2. Der Kiureghian, A., Advances in Earthquake Engineering, University of California, Berkeley, CA, 1994. 3. Priestley, M. J. N., Proc. 12WCEE, Auckland, New Zealand, Paper No. 2831 on CD-ROM, Feb. 2000. 4. Freeman, S. A., Proc. 6US NCEE, Seattle, 1998. 5. Moehle, J. P., Earthq. Spectra, 1992, 3, 403–428. 6. Elsesser, Eric, Proc. 50th Annual meeting of Earthquake Engineering Research Institute, San Francisco, CA, Feb. 1998. 7. Power, M. S., Chiou, S. and Mayes, R. L., in Research Progress and Accomplishments, MCEER, NY, July, 1999.