Earthquake Resistant Structures

Earthquake Resistant Structures

Earthquake ResistantStructures Designing Earthquake Resistant Structures is indispensable. Every year, earthquakes take the lives of thousands of people, and destroy property worth billions. It is imperative that structures are designed to resist earthquake forces, in order to reduce the loss of life. Structural design plays an important role. Here, we will discuss different tips and techniques used in designing Earthquake Resistant structures."Earthquake don't kill people, buildings do."

HORIZONTAL EARTHQUAKE EFFECTS A typical RC building is made of horizontal members (beams and slabs) and vertical members (columns and walls), and supported by foundations that rest on ground. The system comprising of RC frame. The RC frame participates in resting the earthquake forces. Earthquake shaking generates inertia forces in the building, which are proportional to the building mass. Since most of the building mass is present at floor levels, earthquake induced inertia forces primarily develop at the floor levels. These forces travel downwards - through slabs and beams to columns and walls, and then to foundations from where they are dispersed to ground. As inertia forces accumulate downwards from the top of the building, the columns and walls at lower storey experience higher earthquake- induced forces and are therefore designed to be stronger than those in storey above.

HORIZONTAL EARTHQUAKE EFFECTS

Under gravity loads, tension in the beams is at the bottom surface of the beam in the central location and is at the top surface at the ends. The level of bending moment due to earthquake loading depends on severity of shaking and can exceed that due to gravity loading. Thus, under strong earthquake shaking, the beam ends can develop tension on either of the top and bottom faces. Since concrete cannot carry this tension, steel bars are required on both faces of beams to resist reversals of bending moment.

STRENGTH HIERARCHY For a building to remain safe during earthquake shaking, columns should be stronger than beams, and foundations should be stronger than columns.

If columns are made weaker, they suffer severe local damage, at the top and bottom of a particular storey.

PLANNING - A TOOL OF ARCHITECTUREARCHITECTURAL FEATURES The behavior of building during earthquakes depends critically on its overall shape, size and geometry. Hence, at planning stage itself, architects and structural engineers must work together to ensure that the unfavorable features are avoided and a good building configuration is chosen. If both shape and structural system work together to make the structure a marvel.

"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 cannot harm its ultimate performance too much".

SIZE OF BUILDINGS In tall buildings with large weight-to-base size ratio the horizontal movement of the floors during ground shaking is large. In short but very long buildings, the damaging effects during earthquake shaking are many. And, in buildings with large plan area, the horizontal seismic forces can be excessive to be carried by columns and walls.

HORIZONTALLAYOUT OF BUILDINGSBuildings with simple geometry in plan perform well during strong earthquakes. Buildings with re-entrant corners, like U, V, H and + shaped in plan sustain significant damage. The bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts by using a separation joint at the junction.

VERTICAL LAYOUT OF BUILDINGS

Earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path, any deviation or discontinuity in this load transfer path results in poor performance of building. Buildings with vertical setbacks cause a sudden jump in earthquake forces at the level of discontinuity. Buildings that have fewer columns or walls in a particular storey or with unusually tall storey tend to damage or collapse which is initiated in that storey. Buildings on sloppy ground have unequal height columns along the slope, which causes twisting and damage in shorter columns that hang or float on beams have discontinuity in load transfer. Buildings in which RC walls do not go all the way to the ground but stop at upper levels get severely damaged.

ADJACENCY OFBUILDINGS

When two buildings are close to each other, they may pound on each other during strong shaking. When building heights do not match the roof of the shorter building may pound at the mid- height of the column of the taller one; this can be very dangerous.

Construction Techniques for Earthquake Resistance

EARTHQUAKE RESISTANCE DESIGN APPROACHConventional Approach Design depends upon providing thebuilding with strength, stiffness and inelastic deformationcapacity which are great enough to withstand a given levelof earthquake-generated force. This can be accomplished by selection of an appropriatestructural configuration and careful detailing of structuralmembers, such as beams and columns, and the connectionsbetween them.

Basic Approach Design depends upon underlying moreadvanced techniques for earthquake resistance is not to strengthen the building, but to reduce the earthquakegenerated forces acting upon it. This can be accomplished by de-coupling thestructure from seismic ground motion it is possible toreduce the earthquake induced forces in it by three ways.

Increase natural period of structures by Base Isolation. Increase damping of system by Energy DissipationDevices. By using Active Control Devices.EARTHQUAKE DESIGN PHIOSOPHY

Severity of ground shaking at a given location during an earthquake can be minor, moderate and strong. Thus relatively speaking, minor shaking occurs frequently; moderate shaking occasionally and strong shaking rarely. For instance, on average annually about 800 earthquakes of magnitude 5.0-5.9 occur in the world while about 18 for magnitude range 7.0-7.9. So we should design and construct a building to resist that rare earthquake shaking that may come only once in 500 years or even once in 2000 years, even though the life of the building may be 50 or 100 years?

Engineers do not attempt to make earthquake proof buildings that will not get damaged even during the rare but strong earthquake; such buildings will be too robust and also too expensive. Instead the engineering intention is to make buildings earthquake-resistant; such buildings resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake. Thus, safety of people and contents is assured in earthquake-resistant buildings, and thereby a disaster is avoided. This is a major objective of seismic design codes throughout the world.

DESIGN PHILOSOPHY

Under minor but frequent shaking, the main membersof the buildings that carry vertical and horizontalforces should not be damaged; however buildingsparts that do not carry load may sustain repairabledamage.

b) Under moderate but occasional shaking, the mainmembers may sustain repairable damage, while theother parts that do not carry load may sustainrepairable damage.

c) Under strong but rare shaking, the main membersmay sustain severe damage, but the building shouldnot collapse.

Earthquake resistant design is therefore concerned about ensuring that the damages in buildings during earthquakes are of acceptable variety, and also that they occur at the right places and in right amounts. This approach of earthquake resistant design is much like the use of electrical fuses in houses: to protect the entire electrical wiring and appliances in the house, you sacrifice some small parts of electrical circuit, called fuses; these fuses are easily replaced after the electrical over-current. Likewise to save the building from collapsing you need to allow some pre-determined parts to undergo the acceptable type and level of damage.

Earthquake resistant buildings, particularly their main elements, need to be built with ductility in them. Such buildings have the ability to sway back-and-forth during an earthquake, and to withstand the earthquake effects with some damage, but without collapse.

EARTHQUAKE RESISTANT DESIGN CONCEPTIf 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.

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 then 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.

QUALITY CONTROL IN CONSTRUCTION

The capacity design concept in earthquake resistant design of buildings will fail if the strengths of the brittle links fall below their minimum assured values. The strength of brittle construction materials, like masonry and concrete is highly sensitive to the quality of construction materials. Workmanship, supervision and construction methods. Similarly, special care is needed in construction to ensure that the elements meant to be ductile are indeed provided with features that give adequate ductility. Thus, strict adherence to prescribed standards, of construction materials and processes is essential in assuring an earthquake resistant building. Regular testing of materials to laboratories, periodic training of workmen at professional training houses, and on-site evaluation of the technical work are elements of good quality control.

BASICS OF EARTHQUAKE RESISTANCE 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)

Traditional Earthquake Mitigation Techniques

Base Isolation Technique Due to the flexibility in the structure, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed, to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building. Many of the base isolators look like large rubber pads, although there are other types that are based on sliding of one part of the building relative to other. Also, base isolation is not suitable for all buildings. Mostly low to medium rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation.

Lead-rubber bearings are the frequently-used types of base isolation bearings. A lead rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the 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.

Energy Dissipation Devices for Earthquake Resistance Another approach for controlling seismic damage in buildings and improving their seismic performance is by installing Seismic Dampers in place of structural elements, such as diagonal braces. These dampers act like the hydraulic shock absorbers in cars - much of the sudden jerks are absorbed in the hydraulic fluids and only little is transmitted above to the chassis of the car. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building.

Commonly used types of seismic dampers include: Viscous Dampers (energy is absorbed by silicone-based fluid passing between piston cylinder arrangement)

Friction Dampers (energy is absorbed by surfaces with friction between them rubbing against each other)

Yielding Dampers (energy is absorbed by metallic components that yield)

Viscoelastic dampers (energy is absorbed by utilizing the controlled shearing of solids)

Thus by equipping a building with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building.

Working Principle The construction of a fluid damper is shown in (fig). It consists of a stainless steel piston with bronze orifice head. It is filled with silicone oil. The piston head utilizes specially shaped passages which alter the flow of the damper fluid and thus alter the resistance characteristics of the damper. Fluid dampers may be designed to behave as a pure energy dissipater or a spring or as a combination of the two.

A fluid viscous damper resembles the common shock absorber such as those found in automobiles. The piston transmits energy entering the system to the fluid in the damper, causing it to move within the damper. The movement of the fluid within the damper fluid absorbs this kinetic energy by converting it into heat. In automobiles, this means that a shock received at the wheel is damped before it reaches the passengers compartment. In buildings this can mean that the building columns protected by dampers will undergo considerably less horizontal movement and damage during an earthquake.

SECOND TYPE OF ENERGY DISSIPATION DEVICES

The innovative methods for control of seismic vibrations such as frictional and other types of damping devices are important integral part of seismic isolation systems as they severe as a barrier against the penetration of seismic energy into the structure. In this concept, the dampers suppress the response of the isolated building relative to its base.

The novel friction damper device consists of three steel plates rotating against each other in opposite directions. The steel plates are separated by two shims of friction pad material producing friction with steel plates.

When an external force excites a frame structure the girder starts to displace horizontally due to this force. The damper will follow the motion and the central plate because of the tensile forces in the bracing elements. When the applied forces are reversed, the plates will rotate in opposite way. The damper dissipates energy by means of friction between the sliding surfaces.

The latest Friction-ViscoElastic Damper Device (F-VEDD) combines the advantages of pure frictional and viscoelastic mechanisms of energy dissipation. This new product consists of friction pads and viscoelastic polymer pads separated by steel plates as shown below.A prestressed bolt in combination with disk springs and hardened washers is used for maintaining the required clamping force on the interfaces as in original FDD concept.

Active Control Devices for Earthquake Resistance

After development of passive devices such as base isolation and TMD. The next logical step is to control the action of these devices in an optimal manner by an external energy source the resulting system is known as active control device system. Active control has been very widely used in aerospace structures. In recent years significant progress has been made on the analytical side of active control for civil engineering structures. Also a few models explains as shown that there is great promise in the technology and that one may expect to see in the foreseeable future several dynamic "Dynamic Intelligent Buildings" the term itself seems to have been joined by the Kajima Corporation in Japan. In one of their pamphlet the concept of Active control had been explained in every simple manner and it is worth quoting here.

People standing in swaying train or bus try to maintain balance by unintentionally bracing their legs or by relaying on the mussels of their spine and stomach. By providing a similar function to a building it can dampen immensely the vibrations when confronted with an earthquake. This is the concept of Dynamic Intelligent Building (DIB).

The philosophy of the past conventional a seismic structure is to respond passively to an earthquake. In contrast in the DIB which we propose the building itself functions actively against earthquakes and attempts to control the vibrations. The sensor distributed inside and outside of the building transmits information to the computer installed in the building which can make analyses and judgment, and as if the buildings possess intelligence pertaining to the earthquake amends its own structural characteristics minutes by minute.

Active Control System The basic configuration system consists of three basic elements:

1) Sensors to measure external excitation and/or structural response.

2) Computer hardware and software to compute control forces on the basis of observed excitation and/or structural response.

3) Actuators to provide the necessary control forces.

Thus in active system has to necessarily have an external energy input to drive the actuators. On the other hand passive systems do not required external energy and their efficiency depends on tunings of system to expected excitation and structural behavior. As a result, the passive systems are effective only for the modes of the vibrations for which these are tuned. Thus the advantage of an active system lies in its much wider range of applicability since the control forces are worked out on the basis of actual excitation and structural behavior. In the active system when only external excitation is measured system is said to be in open-looped. However when the structural response is used as input, the system is in closed loop control. In certain instances the excitation and response both are used and it is termed as open-closed loop control.

Control Force Devices

Many ways have been proposed to apply control forces to a structure. Some of these have been tested in laboratory on scaled down models. Some of the ideas have been put forward for applications of active forces are briefly described in the following: 1) Active tuned Mass Dampers (TMD): These are in passive mode have been used in a umber of structures as mentioned earlier. Hence active TMD is a natural extension. In this system 1% of the total building mass is directly excited by an actuator with no spring and dash pot. The system has been termed as Active Mass Driver (AMD). The experiments indicated that the building vibrations are reduced about 25% by the use of AMD.

2) Tendon Control: Various analytical studies have been done using tendons for active control. At low excitations, even with the active control system off, the tendon will act in passive modes by resisting deformations in the structures though resulting tension in the tendon. At higher excitations one may switch over to Active mode where an actuator applies the required tension in tendons.

3) Other Methods:

The liquid sloshing during earthquakes has assumed significance importance in view of over flow of petroleum products from storage tank in post earthquakes. One of the important consideration with sloshing is that is associated with a very low damping. The wave height was controlled through force applied to the side wall by a hydraulic actuator. The active control successfully reduced wave heights to the level of 6% of those without control, for harmonic excitations at sloshing frequency. For earthquake type excitation the wave heights were reduced to 19% level.

CONCLUSION

Conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity. But the new techniques like Energy Dissipation and Active Control Devices are a lot more efficient and better.

Civil Engineering DeptPage 6