Earthquake Engineering Module 2 PDF

DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dh...

DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] EARTHQUAKE RESISTIVE CONSTRUCTION Earthquake engineering is the study of the behavior of buildings and structures subject to seismic loading. It can be defined as the branch of engineering devoted to mitigating earthquake hazards. In this broad sense, earthquake engineering covers the investigation and solution of the problems created by damaging earthquakes, and consequently the work involved in the practical application of these solutions, i.e. in planning, designing, constructing and managing earthquake-resistant structures and facilities. General Goals in Seismic-Resistant Design and Construction The philosophy of earthquake design for structures other than essential facilities has been well established and proposed as follows: a. To prevent non-structural damage in frequent minor ground shaking b. To prevent structural damage and minimize non-structural damage in occasional moderate ground shaking c. To avoid collapse or serious damage in rare major ground shaking EARTHQUAKE RESISTANT TECHNIQUES The technologies in earthquake engineering to protect buildings from damaging earthquake effects are vibration control technologies and, in particular, base isolation. I. SEISMIC VIBRATION CONTROL Seismic vibration control is a set of technical means aimed to mitigate seismic impacts in building and non- building structures. All seismic vibration control devices may be classified as passive, active or hybrid where:  passive control devices have no feedback capability between them, structural elements and the ground;  active control devices incorporate real-time recording instrumentation on the ground integrated with earthquake input processing equipment and actuators within the structure;  hybrid control devices have combined features of active and passive control systems. When ground seismic waves reach up and start to penetrate a base of a building, their energy flow density, due to reflections, reduces dramatically: usually, up to 90%. However, the remaining portions of the incident waves during a major earthquake still bear a huge devastating potential. After the seismic waves enter a superstructure, there are a number of ways to control them in order to soothe their damaging effect and improve the building's seismic performance, for instance:  to dissipate the wave energy inside a superstructure with properly engineered dampers;  to disperse the wave energy between a wider range of frequencies;  toabsorb the resonant portions of the whole wave frequencies band with the help of so called mass dampers. Devices of the last kind, abbreviated correspondingly as TMD for the tuned (passive), as AMD for the active, and as HMD for the hybrid mass dampers, have been studied and installed in high-rise buildings, predominantly in Japan, for a quarter of a century. Seismic Base Isolation Base isolation, also known as seismic base isolation or base isolation system, is one of the most popular means of protecting a structure against earthquake forces. It is a collection of structural elements which should substantially decouple a superstructure from its substructure resting on a shaking ground thus protecting a building or non-building structure's integrity. It is a passive structural vibration control technology. 14 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] A base isolated structure is supported by a series of bearing pads, which are placed between the buildings and building foundation. The concept of base isolation is explained through an example building resting on frictionless rollers. When the ground shakes, the rollers freely roll, but the building above does not move. Thus, no force is transferred to the building due to the shaking of the ground; simply, the building does not experience the earthquake. Now, if the same building is rested on the flexible pads that offer resistance against lateral movements (fig 1b), then some effect of the ground shaking will be transferred to the building above. If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground, namely a fixed base building (fig 1c). The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure. As a result, 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. The first evidence of earthquake protection by using the principle of base isolation was discovered in Pasargadae, a city in ancient Persia, now Iran: it goes back to 6th century BCE. Mausoleum of Cyrus, the oldest base-isolated structure in the world 15 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] SEISMIC VIBRATION CONTROL TECHNOLOGIES a. Dry-stone walls control Dry-stone walls of Machu Picchu Temple of the Sun, Peru People of Inca civilization were masters of the polished 'dry-stone walls', called ashlars, where blocks of stone were cut to fit together tightly without any mortar. The stones of the dry-stone walls built by the Incas could move slightly and resettle without the walls collapsing, a passive structural control technique employing both the principle of energy dissipation and that of suppressing resonant amplifications. b. Lead rubber bearing LRB being tested at the UCSD Caltrans-SRMD facility Lead Rubber Bearing or LRB is a type of base isolation employing a heavy damping. It was invented by Bill Robinson, a New Zealander. Heavy damping mechanism incorporated in vibration control technologies and, particularly, in base isolation devices, is often considered a valuable source of suppressing vibrations thus enhancing a building's seismic performance. The bearing is made of rubber with a lead core. It was a uniaxial test in which the bearing was also under a full structure load. 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. c. Spherical sliding base isolation 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. 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. The forces 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 bearings curved surface, this property can be used to design bearings that also lengthen the buildings period of vibration. 16 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] d. Tuned mass damper Tuned mass damper in Taipei 101, the world's third tallest skyscraper Typically, the tuned mass dampers are huge concrete blocks mounted in skyscrapers or other structures and moved in opposition to the resonance frequency oscillations of the structures by means of some sort of spring mechanism. e. Friction pendulum bearing FPB shake-table testing Friction Pendulum Bearing (FPB) is another name of Friction Pendulum System (FPS). It is based on three pillars:  articulated friction slider;  spherical concave sliding surface;  Enclosing cylinder for lateral displacement restraint. f. Building elevation control Building elevation control is a valuable source of vibration control of seismic loading. Pyramid-shaped skyscrapers continue to attract the attention of architects and engineers because such structures promise a better stability against earthquakes and winds. The elevation configuration can prevent buildings' resonant amplifications because a properly configured building disperses the shear wave energy between a wide range of frequencies. Transamerica Pyramid Bldg. A tapered profile of a building is not a compulsory feature of this method of structural control. A similar resonance preventing effect can be also obtained by a proper tapering of other characteristics of a building structure, namely, its mass and stiffness. 17 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] g. Simple roller bearing Simple roller bearing is a base isolation device which is intended for protection of various building and non- building structures against potentially damaging lateral impacts of strong earthquakes. This metallic bearing support may be adapted, with certain precautions, as a seismic isolator to skyscrapers and buildings on soft ground. Recently, it has been employed under the name of Metallic Roller Bearing for a housing complex (17 stories) in Tokyo, Japan. h. Springs-with-damper base isolator Springs-with-damper base isolator installed under a three-story town- house, Santa Monica, California is shown on the photo taken prior to the 1994 Northridge earthquake exposure. It is a base isolation device conceptually similar to Lead Rubber Bearing. One of two three-story town-houses like this, which was well instrumented for recording of both vertical and horizontal accelerations on its floors and the ground, has survived a severe shaking during the Northridge earthquake and left valuable recorded information for further study. Springs-with-damper i. Hysteretic damper Hysteretic damper is intended to provide better and more reliable seismic performance than that of a conventional structure at the expense of the seismic input energy dissipation There are four major groups of hysteretic dampers used for the purpose, namely:  Fluid viscous dampers (FVDs) – energy is absorbed by silicone-based fluid passing between piston cylinder arrangement.  Metallic yielding dampers (MYDs) – energy is absorbed by metallic components that yield  Viscoelastic dampers (VEDs) – energy is absorbed by utilizing the controlled shearing of solids.  Friction dampers (FDs) – energy is absorbed by surfaces with friction between them rubbing against each other. Thus by equipping a building with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building. How Dampers Work 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 18 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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. Fluid Viscous Dampers Active Control Devices for Earthquake Resistance After development of passive devices such as base isolation and TMD. The next logical steps 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). 19 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] Active mass driver system 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. The basic configuration of an active control system is schematically shown in figure. The 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. II. SEISMIC RETROFITTING is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. Prior to the introduction of modern seismic codes in the late 1960s for developed countries (US, Japan etc.) and late 1970s for many other parts of the world (Turkey, China etc.), many structures were designed without adequate detailing and reinforcement for seismic protection. The retrofit techniques outlined here are also applicable for other natural hazards such as tropical cyclones, tornadoes, and severe winds from thunderstorms. Even as current practice of seismic retrofitting is predominantly concerned with structural improvements to reduce the seismic hazard of using the structures, it is similarly essential to reduce the hazards and losses from non-structural elements. It is also important to keep in mind that there is no such thing as an earthquake-proof structure, although seismic performance can be greatly enhanced through proper initial design or subsequent modifications. 20 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] Strategies  Increasing the global capacity (strengthening). This is typically done by the addition of cross braces or new structural walls.  Reduction of the seismic demand by means of supplementary damping and/or use of base isolation systems.  Increasing the local capacity of structural elements. This strategy recognizes the inherent capacity within the existing structures, and therefore adopts a more cost-effective approach to selectively upgrade local capacity (deformation/ductility, strength or stiffness) of individual structural components.  Selective weakening retrofit. This is a counter intuitive strategy to change the inelastic mechanism of the structure, whilst recognizing the inherent capacity of the structure.  Allowing sliding connections such as passageway bridges to accommodate additional movement between seismically independent structures. Performance objectives  Public safety only. The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passersby, and that the structure can be safely exited. Under severe seismic conditions the structure may be a total economic write-off, requiring tear-down and replacement.  Structure survivability. The goal is that the structure, while remaining safe for exit, may require extensive repair (but not replacement) before it is generally useful or considered safe for occupation. This is typically the lowest level of retrofit applied to bridges.  Structure functionality. Primary structure undamaged and the structure is undiminished in utility for its primary application. A high level of retrofit, this ensures that any required repairs are only "cosmetic" - for example, minor cracks in plaster, drywall and stucco. This is the minimum acceptable level of retrofit for hospitals.  Structure unaffected. This level of retrofit is preferred for historic structures of high cultural significance. Techniques Common seismic retrofitting techniques fall into several categories: One of many "earthquake bolts" found throughout period houses in the city of Charleston subsequent to the Charleston earthquake of 1886. They could be tightened and loosened to support the house without having to otherwise demolish the house due to instability. The bolts were directly loosely connected to the supporting frame of the house. a. External post-tensioning The use of external post-tensioning for new structural systems has been developed in the past decade. Under the PRESS (Precast Seismic Structural Systems) a large-scale U.S./Japan joint research program, unbounded post-tensioning high strength steel tendons have been used to achieve a moment-resisting system that has self-centering capacity. An extension of the same idea for seismic retrofitting has been experimentally tested for seismic retrofit of California bridges under a Caltrans research project and for seismic retrofit of non-ductile reinforced concrete frames. Pre-stressing can increase the capacity of structural elements such as beam, column and beam-column joints. It should be noted that external pre-stressing has been used for structural upgrade for gravity/live loading since 1970s. 21 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] b. Supplementary dampers Supplementary dampers absorb the energy of motion and convert it to heat, thus "damping" resonant effects in structures that are rigidly attached to the ground. In addition to adding energy dissipation capacity to the structure, supplementary damping can reduce the displacement and acceleration demand within the structures. In some cases, the threat of damage does not come from the initial shock itself, but rather from the periodic resonant motion of the structure that repeated ground motion induces. In the practical sense, supplementary dampers act similarly to Shock absorbers used in automotive suspensions. c. Slosh tank A slosh tank is a large tank of fluid placed on an upper floor. During a seismic event, the fluid in this tank will slosh back and forth, but is directed by baffles - partitions that prevent the tank itself becoming resonant; through its mass the water may change or counter the resonant period of the building. Additional kinetic energy can be converted to heat by the baffles and is dissipated through the water - any temperature rise will be insignificant. d. Active control system Very tall buildings ("skyscrapers"), when built using modern lightweight materials, might sway uncomfortably (but not dangerously) in certain wind conditions. A solution to this problem is to include at some upper story a large mass, constrained, but free to move within a limited range, and moving on some sort of bearing system such as an air cushion or hydraulic film. Hydraulic pistons, powered by electric pumps and accumulators, are actively driven to counter the wind forces and natural resonances. These may also, if properly designed, be effective in controlling excessive motion - with or without applied power - in an earthquake. In general, though, modern steel frame high rise buildings are not as subject to dangerous motion as are medium rise (eight to ten story) buildings, as the resonant period of a tall and massive building is longer than the approximately one second shocks applied by an earthquake. e. Ad hoc-addition of structural support/reinforcement The most common form of seismic retrofit to lower buildings is adding strength to the existing structure to resist seismic forces. The strengthening may be limited to connections between existing building elements or it may involve adding primary resisting elements such as walls or frames, particularly in the lower stories. e.1. Connections between buildings and their expansion additions Frequently, building additions will not be strongly connected to the existing structure, but simply placed adjacent to it, with only minor continuity in flooring, siding, and roofing. As a result, the addition may have a different resonant period than the original structure, and they may easily detach from one another. The relative motion will then cause the two parts to collide, causing severe structural damage. Proper construction will tie the two building components rigidly together so that they behave as a single mass or employ dampers to expend the energy from relative motion, with appropriate allowance for this motion. e.2.Exterior reinforcement of building e.3.Exterior concrete columns 22 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 DHVSU Main Campus, Villa de Bacolor, Pampanga URL: http://dhvsu.edu.ph E-Mail Address: [email protected] Historic buildings, made of unreinforced masonry, may have culturally important interior detailing or murals that should not be disturbed. In this case, the solution may be to add a number of steel, reinforced concrete, or post stressed concrete columns to the exterior. Careful attention must be paid to the connections with other members such as footings, top plates, and roof trusses. e.4. Infill shear trusses Infill shear trusses — University of California dormitory, Berkeley, California Shown here is an exterior shear reinforcement of a conventional reinforced concrete dormitory building. In this case, there was sufficient vertical strength in the building columns and sufficient shear strength in the lower stories that only limited shear reinforcement was required to make it earthquake resistant for this location near the Hayward fault. e.5. Massive exterior structure In other circumstances, far greater reinforcement is required. In the structure shown at right — a parking garage over shops — the placement, detailing, and painting of the reinforcement becomes itself an architectural embellishment. External bracing of an existing reinforced concrete parking garage (Berkeley) IV. References: 1. Encyc. Brit. – Encyclopedia Britannica 2. World Almanac 2015, 2020 3. Phivolcs - Dost 4. National Structural Code of the Philippines 2015 23