EARTHQUAKE RESISTANT STRUCTURES- CONCEPT AND TECHNOLOGY

EARTHQUAKE RESISTANT STRUCTURES- CONCEPT AND TECHNOLOGY

An Earthquake is a natural phenomenon associated with violent shaking of the ground.

How does an earthquake occur?

The inside of our earth consists of many layers (crust, mantle, inner and outer cores). Formed by complex processes over countless years, they continue to be active. Once in a while, the disturbances below the earth get transmitted to the surface, causing earthquakes.

 The waves generated in the soil during an earthquake travel long distances in many directions in a very short time, shaking the ground. The buildings that cannot resist this ground shaking can collapse, causing disaster and loss of human life.

Earthquake hazard in India:

Based on historical occurrences, regions in India are classified into low, moderate, severe and very severe earthquake-prone zones. The zones are denoted as II, III, IV and V respectively. More than half of the country’s population lives in moderate to very severe regions, where high-magnitude earthquakes can occur.

The extent of damage to a building during an earthquake depends not only on the magnitude of the earthquake, but also on the soil, building configuration, quality of design and construction. In developed countries, because of better awareness and regulation of design and construction practices, the buildings survive earthquakes and damage and loss of life is less. India should also achieve this standard.

Concepts used in Earthquake resistant structures

If two bars of same length and same cross-sectional area – one made of ductile material and another of a brittle material. And a pull is applied on both bars until they break, then we notice that the ductile bar elongates by a large amount before it breaks, while the brittle bar breaks suddenly on reaching its maximum strength at a relative small elongation. Amongst the materials used in building construction, steel is ductile, while masonry and concrete are brittle. Comparison of Brittle and Ductile Building materials The correct building components need to be made ductile. The failure of columns can affect the stability of building, but failure of a beam causes localized effect. Therefore, it is better to make beams to be ductile weak links than columns. This method of designing RC buildings is called the strong-column weak-beam design method. Special design provisions from IS: 13920-1993 for RC structures ensures that adequate ductility is provided in the members where damage is expected.

Popular Earthquake Resistant Techniques Conventional seismic design attempts to make buildings that do not collapse under strong earthquake shaking, but may sustain damage to non-structural elements (like glass facades) and to some structural members in the building. This may render the building non-functional after the earthquake, which may be problematic in some structures, like hospitals, which need to remain functional in the aftermath of earthquake. Special techniques are required to design buildings such that they remain practically undamaged even in a severe earthquake. Buildings with such improved seismic performance usually cost more than the normal buildings do. Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers. The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building or at least greatly reduced. Seismic dampers are special devices introduced in the buildings to absorb the energy provided by the ground motion to the building (much like the way shock absorbers in motor vehicles absorb due to undulations of the road).

Advanced Earthquake Resistant Design Techniques

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.

                                  (fig. 1)

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.

Earthquake Generated Forces

                                 (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.

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 particular point of its earthquake response.

                                   (fig. 3)

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

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.)

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.

                                  (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.

Energy Dissipation Devices

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
• Visco-elastic 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 damper that has been widely utilized and has proven to be very effective in a wide range of applications.

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 technology in Designing earthquake-resistant buildings

As many as 100,000 earthquakes of + 3 magnitude hit the earth every year. According to estimates, some 15 million people have died with losses approximating hundreds of billions of dollars in recorded history.
Mercifully, the devastating effects of quakes can be minimised considerably through mitigation and preparedness. Damages and spiralling costs could be avoided in future with the development of new techniques such as the one being worked on by Hyderabad boy Chandra Mouli Vemury who is doing his Ph.D. at Newcastle University, England.

Earthquakes are impossible to predict and safety precautions must be taken to make buildings safer. If Chandra’s work pays off it could make future earthquakes far less costly both in the number of lives lost and the financial impact.
Chandra's doctoral programme is titled Seismic Analysis of Shape Memory Alloy Based Dampers, aimed at making buildings more earthquake resistant, potentially making seismic events much less devastating in future.
He is working with Nitinol, an alloy of nickel and titanium which has “many unusual properties including shape memory, which means it can be deformed but then go back to its original shape.”
Nitinol belongs to a family of smart materials called Shape Memory Alloys (SMA). The unique properties of these materials significantly help reduce the damage associated with severe earthquakes.
Chandra says he is, “Looking at the possibility of using the alloy to make energy absorbing dampers which will absorb energy with little or no damage to the building and therefore help to reduce the repair bill after a very severe earthquake, as well as by reducing casualties.”
 This could potentially save lives and money. It is an expensive material, but in terms of the benefits it can bring it is very cost effective.”
The system works with a series of dampers at the bottom of buildings which absorb and dissipate the energy of the shaking earth and stop it pulling the building apart.
The work is still at an early stage and is being fully tested in our labs, but we are confident there is something here which could make a big difference to the safety of cities in earthquake zones. Chandra’s research can be applied to local construction here and shared internationally.”

The Japanese Are Using Levitation Technology To Make Earthquake-Proof Buildings

The innovative Japanese are trying out a new way to make earthquake-resistant buildings. Air Dashin Systems installs artificial foundations that detect an earthquake and within a second lift a building 3 cm off the ground until the earth stops rumbling. The levitating foundations have been implemented in more than 88 buildings and supposedly costs one-third of other damage prevention systems with the added benefit of very little maintenance.

Hydraform technology

The Hydraform building system replaces the conventional brick and mortar by using Hydraform blocks. The other components of the conventional building system remain largely unchanged. Hydraform system is a largely dry stacked-Interlocking masonry system that enables speedier construction of high quality, aesthetic and affordable building. The Blocks have an extremely appealing face-brick finish and provide a pre-pointed straight masonry. The walls may be left exposed, plastered or finished with cement paint. These blocks are made using Hydraform Block making machine with an option to make at the site of construction itself. The interlocking construction is suitable for both Load bearing structures as well as framed structures – single or multistorey.

In conventional construction, brick could be good but is mortar used is normally weak thus making the over all masonry weak. This is taken care by Hydraform blocks which virtually use NO MORTAR making masonry uniform and strong.

The user also has option to use mortar slurry within the blocks. These are solid blocks for all kind of wall applications. Blocks can be made with fly ash based combinations or soil based combinations to meet relevant technical requirements.

 Dry interlocked masonry provides flexibility in application and technically superior for making earthquake resistant structures. For Earthquake resistant construction, blocks with Vertical & Horizontal cavities for reinforcements are also available. These blocks have been successfully used in making earthquake resistant houses / community centres in Gujarat in line with GSDMA guidelines and HUDCO.            EARTHQUAKE RESISTANT CONSTRUCTION                                                  Hydraform interlocking block masonry can be effectively used in Earthquake resistant constructions incorporating necessary structural design and reinforcements – horizontal / vertical. HUDCO in consultations Dr. Arya, Seismic advisor to Gujarat State Disaster Management Authority arrived at suitable designs to use these blocks in Earthquake resistant houses and buildings in Gujarat. HUDCO & other NGOs implemented these projects Hydraform machine made compressed earth interlocking block masonry. Vertical & Horizontal reinforcements can be effectively done for Earthquake construction. We continue to improve Earthquake construction techniques using Hydraform Building Systems with its continuous research process to improve the construction standards to meet Earthquake construction requirements.        Hydraform has done Experiment on Earthquake Loading of a full scale dry stacked masonry structure, the elaborated report for which is available on http://www.hydraform.com/BuildingSystems/Earthquake.asp    

                

     Fig1 Loading the soil                                                                

  Fig2.Mixing, soil, cement and water

         

             

                                                                                                           

           Fig3. Loading the Hydraform machine                                                      

Fig4.Carrying the block

    

Fig5.Stacking and curing the blocks                                         

Fig6. Laying the blocks

INTERLOCKING BLOCK SPECIFICATION: 

Type	HF 220 / 200 Conduit	HF150 / 115
Use	External walls / Fencing / Load bearing constructions	Interior / Partition Walls / Framed Constructions
Width	220 mm (9")	150 / 115mm (6/4.5")
Height	115 mm	115 mm
Length	120 mm to 240 mm	120 mm to 240 mm
Weight	9-11  Kg approx (Full block)	4.5-5.5  – 7 kg approx

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             By size, one HF220 is equivalent to three to three & half size of  conventional fired bricks. Other sizes are also available

Raw Material for making blocks:                                               

The blocks can be made in Hydraform machine using Fly ash – Lime  – Gypsum – Cement- Sand, Fal-G or other fly Ash based tested combination OR  Soil-Cement (compressed Stabilized Earth Blocks) OR Stone Dust - Cement.

Black cotton soils are not suitable. The block strength is determined by Soil / Fly Ash type,   quantity of Cement used and other materials used and the extent of curing after   manufacture of block.