BY: Hadi Golabi, BSc, MSc Senior Design Engineer

Transcription

1 THE EFFECT OF EARTHQUAKE RELATED TREMORS ON BUILDINGS IN MALAYSIA BY: Ir Ng Pek Har, BSc, MIEM, PEng Director Hadi Golabi, BSc, MSc Senior Design Engineer WEB STRUCTURES Singapore 146 Robinson Road #05-01 Singapore Tel : (65) Fax : (65) webstruc@webstruc.net Malaysia # 1503 Plaza Jalan Ampang Kuala Lumpur M a l a y s i a Tel : (603) Fax : (603) webkl@webstruc.net Date : May 2005 Ref : WEB/INFORMATIONDOC.04

2 THE EFFECT OF EARTHQUAKE RELATED TREMORS ON BUILDINGS IN MALAYSIA TABLE OF CONTENTS 1 : Introduction 2 : What causes Earthquakes? 3 : Mapping of Areas More Prone To Earthquakes 4 : Measurement of Earthquakes 5 : An Understanding of The Richter Scale 6 : An Understanding of The Response of Buildings to Earthquakes 7 : Earthquake Resistant Structures 8 : Web Structures Approach Regarding Earthquake Considerations for Design of Buildings in Malaysia 9 : Approximate Guide on Additional Structural Costs for Each Option References Glossary of Common Seismic Terms UBC 97 Seismic Zone Classification for Selected Cities in South East Asia NOTE: This is a Web structures Information document which has been compiled for the sole use and benefit of selected clients and associates of Web Structures. This Information document is PRIVATE & CONFIDENTIAL. While every attempt has been made to ensure the accuracy of this compilation, Web Structures recommends that those interested in more in-depth understanding of the concepts touched upon in this document should refer to relevant standards and expert literature.

3 THE EFFECT OF EARTHQUAKE RELATED TREMORS ON BUILDINGS IN MALAYSIA 1. INTRODUCTION It is the aim of this report to provide the reader with a basic understanding of the effects of tremors in buildings, experienced in Malaysia, arising from the earthquakes off the coast of Sumatra, after the tsunami-causing earthquake on 26 December The reader should gain a clearer idea about the risk factor to the structure of tall buildings and the basic principles governing the behaviour of the structure in response to the tremors. Malaysia does not lie in any presently demarcated seismic zone. Hence, there is at present, no code or regulation requiring buildings to be designed for earthquakes in Malaysia. However, aftermath of the tsunami and the recurrent tremors, the statutory bodies have announced that they will conduct a study on this and review the building regulations if necessary. The Uniform Building Code (UBC97) classifies Malaysia/Kuala Lumpur as seismic zone 1, in a scale of six seismic zones of 0, 1, 2A, 2B, 3 and 4. The seismic forces recommended in UBC97 for zone 1 fall, by-and-large, within the minimum Notional Horizontal Forces stipulated in the British standards prevailing in Malaysia. This applies to UBC97 static forces on buildings with reinforced concrete walls/moment frames of fundamental period of more than 4 seconds, on stiff soil profile with undrained shear strength between 50 and 100kN/m 2. This report also outlines Web Structure s position and approach for the design of current and future tall buildings in Malaysia, which will be adopted by Web Structures until such time as the statutory bodies in Malaysia come up with more specific requirements for general design. 2. WHAT CAUSES EARTHQUAKES The earth s crust is not one continuous shell, but comprises of many plates abutting each other. These plates are called tectonic plates. (Fig.1) Fig. 1: Tectonic plates of earth 1

4 Though most of us are largely unaware of it, these plates undergo many small movements against each other. The plates can slide horizontally against each other or pull away from each other to form new crust or come towards each other causing one plate to dive beneath the other. (Fig.2) Fig. 2: Different type of plate s movements Most of the time, these movements are quite smooth and are not generally perceptable. But when these movements involve large plates, the sudden movement causes huge energy to be released in the form of waves. The waves travel inside the earth and along the ground. These waves are felt by us as shakes and tremors. We call this event, an earthquake. The intersecting edges of the tectonic plates are called faults. There are also smaller faults within each plate but strong earthquakes happen along the bigger and more major faults and the resulting movement lasts for a longer time (Fig 3) Kilometers Seconds Magnitude Magnitude Bigger Faults Make Bigger Earthquakes Bigger Earthquakes Last Longer Time Fig. 3: Relationship between earthquake size, length of faults and duration South East Asia is made up of the Indian plate, the Australian plate, the Eurasian plate & the Philippine plate (Fig 4). There are a number of different faults between these plates but most of the earthquakes in this region is caused by the interaction of the Indian & Australian plates with the Eurasian plate, which actually forms the Sunda Trench to the south of Indonesia (Fig 5) The nearer one is to this fault line, the more seismic activity one experiences. Hence, the southern parts of Malaysia, for example Kuala Lumpur, which lie closer to this fault are more prone to be affected. All the tremors felt recently in Malaysia are due to the earthquakes along the Sunda Trench. 2

5 Fig. 4: Names of tectonic plates Sunda trench Fig. 5: Global distribution of earthquakes 3

6 3. MAPPING OF AREAS MORE PRONE TO EARTHQUAKES The hazards caused by earthquakes take the form of ground shaking, landslide, liquefaction,surface faults,tsunamis and tectonic deformations. Which type of hazard occurs depends on the geographical location and the tectonic conditions such as sliding, pulling away or coming together. With respect to building design, the main hazard that affects the structure is the effect of ground shaking. An earthquake risk map or seismic hazard map gives the correlation between geographical location and the amount of probable danger experienced in a particular location, if a likely earthquake occurs on a particular fault. Earthquake risk depends on many factors such as distance of the place from the nearest faults, ground conditions, amount of tectonic activity along the faults, etc. It is pertinent to note that a place with a high earthquake risk, meaning greater chances of seismic activity occurring there, need not necessarily mean that the hazards experienced there are more severe. Figure 6 shows part of the Global Seismic Hazard map from the Global Seismic Hazard Assessment Program (GSHAP) These earthquake risk maps are used together with each country s seismic design code to design buildings against earthquake loads. Except for the UBC97 classification, there are no seismic design codes for Malaysia as it does not presently lie in a demarcated earthquake-likely zone. But after the tsunami, organizations like MACRES (Malaysian Centre for Remote Sensing) are carrying out research to prepare the required risk maps and seismic design codes. Peak Ground Acceleration Fig. 6: Global Seismic Hazard Map for South East Asia & Australia by GSHAP It is helpful to understand that GSHAP s Global Seismic Hazard Map is measured by the magnitude of peak ground acceleration. This is because when the ground accelerates from its initial stationary state due to the earthquake waves, the buildings resting on the ground will also be subjected to the same acceleration. A body that accelerates is subjected to a force that is directly proportional to the size of that acceleration and the mass of the body. This force causes earthquake loads on the building. 4

7 By cross referring to the Earthquake Risk Map of Indonesia & Thailand shown in Figure 7 and The Seismic Hazard Map of 28 March 2005 in Northern Sumatra by USGS (Fig 8), we can reasonably anticipate that should Kuala Lumpur be assigned an earthquake risk zone, it could be an extension of the current Indonesian earthquake risk zones. This is based on proximity and the fact that the tremors in Kuala Lumpur originate from the Sunda Trench earthquake. Kuala Lumpur Fig.7: Earthquake Risk Map of Indonesia & Thailand Seismic Hazard is expressed as peak ground acceleration (PGA) on firm rock, in meters/sec 2, expected to be exceeded in a 50-yr period with a probability of 10 percent Fig. 8: Seismic hazard map of 28March northern Sumatra earthquake by USGS 5

8 4. MEASUREMENTOF THE EARTHQUAKES When an earthquake occurs, the released energy travels as waves. Each earthquake produces several types of waves that apply forces on buildings in different ways and directions as shown in Fig 9. The P wave is a longitudinal wave that causes regions of compression and expansion like sound wave while the S wave is a transverse wave that causes the medium to vibrate up and down like a boat bobbing on a water wave. The P Waves have higher speed than other waves and arrive first. S- Waves have lowers speed but higher vibration. Other wave type also exist as shown in Figure 9. P Wave Travel Simulation Fig. 9: Earthquake waves S Wave Travel Simulation Scientists record characteristics of these waves on seismographs, which give information on acceleration, velocity and displacements caused by the earthquakes at different locations. Once this information from at least 3 different locations are available, the scientists can calculate the focus of the earthquake. The point on the surface of the earth, directly above this focus point is called the epicenter and the distance of the focus point from the surface of the earth gives the depth of the earthquake (Figs 10 & 11) 6

9 Fig. 10: Mechanism and terms definition of earthquake Fig. 11: Seismographs and defining the focus point of earthquake 7

10 5. AN UNDERSTANDING OF THE RICHTER SCALE There are several intensity scales used to measure earthquakes of which the Richter scale is one of the more popularly known. Figure 12 illustrates that the Richter number depends on the maximum ground displacement recorded and the distance of the location of the recording seismometer from the epicenter of the earthquake. Hence, an earthquake reported as magnitude 5 means that at a distance 220km away from the epicenter, the maximum measured ground displacement is approximately 23mm. The same magnitude of 5 for the said earthquake would be recorded on another seismometer located at a distance of 360km where amplitude of 5mm would be recorded. The salient points are:- a) The reduction in amplitude decreases rapidly as we go further away from the epicenter as the Richter scale is a logarithmic scale. b) The real force exerted by earthquakes on buildings is reduced as the earthquakes travel from the epicenter through the different layers of the earth. Table 1 shows some approximate relationship between earthquake intensities and the effects on buildings. The effects are related to ACCELERATION as this is the factor that creates forces on a particular building. c) Each earthquake has a single magnitude value, but its effect will vary place to place. THE RICHTER SCALE To determine Richter Mangnitude at varying distance from epicenter, connect on the chart: A) Pick the Maximum amplitude recorded by a standard seismometer B) Pick the distance of seismometer from the epicenter of the earthquake (or difference in arrival times of P and S waves) 360 C) Connect these two points with a straight line D) Read the magnitude on the center scale Fig. 12: The evaluation of Richter scale based on seismogram of an earthquake 8

11 Approximate Acceleration Effect - Felt slightly g Felt indoors g Some crackimg g Some movement. Alarm g Some damage. Chimneys fall g g g Panels deformed. Some buildings collapse Considerable damage. Frames out of plumb. Masonry buildings collapse Most frame structures seriously damaged. Landslides g Few structures survive Table 1: Table for approximate description of earthquake intensities 6. AN UNDERSTANDING OF THE RESPONSE OF BUILDINGS TO EARTHQUAKES Though both wind forces and earthquake loads apply horizontal forces on buildings, there is a major difference between them. While wind loads damage a building by externally applied pressures, earthquake damage is caused by internally generated inertial forces induced by vibration of the building s mass. The building s mass, size and shape/configuration are factors that determine the magnitude of these forces and how well the building can stand up to these forces. The following points are noteworthy: a) Inertial forces are measured by the product of mass and acceleration (Newton s F = m x a) Acceleration is the rate of change of velocity and depends on the nature of the earthquake while mass depends on the nature of the building. Increase in mass cause increase in the forces. Hence, lightweight construction is preferred in seismic design as a heavier building is subjected to more force than a light building with the same height and gross floor area. So one of the ways to make buildings cost-effective in regions with higher earthquake risk is to use more lightweight materials, especially for non-structural members like finishes and partitions. b) Compared to wind loads, the earthquake loads are applied on buildings over a much shorter period of time in opposite directions (typically the longest period of an earthquake vibration is only seconds). The earthquake load increases from zero to its maximum value in just a few seconds and then decreases to zero and increases in an opposite direction. Hence, earthquake loads apply a shock-type effect on buildings. c) When the height of the building increases, the effect of earthquake loads on it becomes more severe, with the additional risk of the building undergoing resonance, which causes the movements to be greatly amplified. This can be destructive. d) The configuration of lateral force resisting elements in buildings is very important. The continuity of vertical members and the distribution of these members on plan is the building s most effective response to counter earthquake loads. This is because the configuration controls the vibration period, the damping characteristics of the building and therefore changes the reaction of the building to earthquake loads. 9

12 7. EARTHQUAKE RESISTANT STRUCTURES To design a building against earthquakes is to ensure that the structure is capable of resisting the loads and movements caused by earthquakes, preventing collapse and irreparable damage. Different types of buildings are categorized under different levels of importance, where seismic design codes are concerned. Hospitals, power plants, fire stations, communication centers, for example, are considered high priority buildings and the requirements are therefore more stringent. Residential and office buildings are considered medium priority which means that the basic structural frame of the building must not collapse but damages like cracks in partitions and non-structural members are expected and considered acceptable. Besides considerations about the resistance of the building against lateral loads, the codes of practice also limit the lateral sway and acceleration. The lateral sway limit controls the verticality of structural elements while the acceleration limit provides comfort for the occupants. It is good to note that usually, a building s acceleration due to wind loads which lasts longer than earthquake loads, is more likely the governing factor for acceleration limits. So usually a building that has been checked for acceleration due to wind loads satisfies the acceleration limits controlling earthquake loads. The CIRIA Report 102 limits the sideways drift of each storey to be H/200, where H is the storey height. In summary, the most important considerations in the design of earthquake resistant buildings that are considered in the seismic design codes of various countries are: a) The seismic risk depending on geographical location. b) The classification of the building s importance. c) The site s soil conditions d) The structural characteristics of the building in terms of resistance against lateral loads. 8. WEB STRUCTURES S APPROACH REGARDING EARTHQUAKE CONSIDERATIONS FOR THE DESIGN OF BUILDINGS IN MALAYSIA In view of the current situation, whereby buildings in Malaysia have suffered repeated tremors due to seismic activities in the Sunda Trench and in the interim period while the Malaysian authorities are carrying out studies to determine changes, if necessary, to the building codes, Web Structures responds to the concern of clients and end users of buildings designed in Malaysia by offering the following options: Option 1) Design and Detail the Structural Frame in Accordance with the Current Standards Prevailing in Malaysia Notwithstanding the Effects of Recent Tremors In this option the building will comply with the prevailing Structural Codes Of Practice for Malaysia and will be designed for the stipulated horizontal loads due to Wind and Notional Horizontal forces only. The wind loads stipulated are generally not for winds that occur weekly or monthly, but for maximum wind forces that by probability, could occur say, once in 30 years, Therefore, by-and-large, most existing buildings in Malaysia which have not been designed for seismic loads, are still structurally sounds after tremors experienced recently, though the swaying was perceptible to occupants. Of course we need to bear in mind that the actual effect as mentioned depends on the specifics of a particular building, such as height, stiffness, etc. 10

13 Option 2) In Addition to Option 1 Above, Design the Structural Frame to Withstand the Static Earthquake Loads as Defined in the Uniform Building Code (UBC97) Zone 1 In this option, the structural frame will be designed to the current codes prevailing in Malaysia; However an additional horizontal static load case for Malaysia which is that given in the Uniform Building Code (UBC97) is applied to the structural frame. Option 3) In Addition to Option 1 Above, Design the Structural Frame to Withstand the Static Earthquake Loads as Defined in one of The Zones Defined in the Indonesian Seismic Code Since the source of the tremors in Kuala Lumpur is earthquake in Sunda Trench (off the coast of the Indonesia island of Sumatra), Web Structures will apply the static loads specified in the Indonesian Seismic Code for buildings located in the demarcated seismic zones of Indonesia, to the buildings designed for Kuala Lumpur. With reference again to Fig 8, these seismic zones are labeled zone 2 (most severe / closest to Sunda Trench) to zone 6 (no effect/present zone for Kuala Lumpur). Depending upon the level of comfort desired, our client will have the option of selecting the seismic zone to be designed for. Then, Web Structures will conduct the computer modeling to assess the response of the building subjected to the earthquake loads specified for that particular zone. Based on this analysis, we will be able to DESIGN the building for the seismic condition. Option 4) In Addition to Option 3 Above, Detail All Structural Components to Strictly Adhere to the Most Stringent Detailing Requirements of the Indonesian Seismic Code Though not required by present Malaysian building codes, Web Structures can also then, DESIGN AND DETAIL the structure in full compliance with the Indonesian Seismic Code, in addition to the current detailing required by British Standards for Malaysia. This detailing includes more stringent control of minimum reinforcement sizes and maximum spacing of bars. 9. APPROXIMATE GUIDE ON ADDITIONAL STRUCTURAL COSTS FOR EACH OPTION For our clients who want to offer their end-users an additional measure of comfort and more likelihood of the building complying more closely to any probable amendments to the Malaysian Structural Code to account for seismic hazards, we recommend any one of options 2, 3 or 4 above. As a guide, the following approximate cost increases may apply for each option: Option 1: Basic cost of structural frame Option 2: 5% more than Option 1 Option 3: 5% to 10% more than Option 1 per seismic zone Option 4: 10% more than Option 3 11

14 REFERENCES: 1. Uniform Building Code (UBC-91), International Conference of Building Officials, Code of Planning And Practice for Building to Withstand Earthquake (Tata Cara Perencanaan Ketahanan Gempa Untuk Bangunan Gedung). Badan Standardisasi Nasional BSN ( SNI ) 3. CIRIA Report 102, Design of Shear Wall Buildings, Construction Industry Research and Information Association. 4. The Nature of Ground Motion and its Effect on Building, Christopher Arnold, NISEE ( National Informational Service for Earthquake Engineering), University of California, Berkeley. 5. Poster of the Sumatra-Andaman Islands Earthquake of 26 December 2004 Magnitude 9.0, USGS (United States Geological Survey). 6. Poster of the Northern Sumatra Earthquake of 28 March 2005 Magnitude 8.7, USGS (United States Geological Survey). 7. The GSHAP Global Seismic Hazard Assessment Program, D. Gardini (ETH Zurich, Switzerland), G.Grunthal (GFZ Potsdam, Germany), K Shedlock (USGS, Golden, CO, USA) and P. Zhang (CSB, Beijing, China), Global Seismic Hazard Assessment Program, United Nations, International Handbook of Earthquake Engineering, Mario Paz, 1994 Chapman & Hall Inc. GLOSSARY OF COMMON SEISMIC TERMS: Earthquake Earthquake is a term used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth. Epicenter The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins. Crust The crust is the outermost major layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost km of crust is brittle enough to produce earthquakes. Earthquake Hazard Earthquake hazard is anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunamis, and seiches. Focus/Hypocenter The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth. Also commonly termed as the focus. Ground Motion Ground motion is the movement of the earth's surface from earthquakes or explosions. Ground motion is produced by waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its surface. 12

15 Intensity The intensity is a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the earth's surface and on humans and their structures. Several scales exist, but the ones most commonly used in the United States are the Modified Mercalli scale and the Rossi-Forel scale. Love wave A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling. Seismometer A seismograph, or seismometer, is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium. This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground. Richter scale The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter scale, magnitude is expressed in whole numbers and decimal fractions. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. Tectonic Plates The tectonic plates are the large, thin, relatively rigid plates that move relative to one another on the outer surface of the Earth. 13

16 UBC 97 SEISMIC ZONE CLASSIFICATION FOR SELECTED CITIES IN SOUTH EAST ASIA Note: Zone classification is noted thus 4 for each city. 14

17 / / / / / / #1503, PLAZA 138, 138, JALAN AMPANG, KUALA LUMPUR MALAYSIA T F webkl@webstruc.net MALAYSIA WEB STRUCTURES (M) SDN BHD / SINGAPORE WEB STRUCTURES PTE LTD, WEB STRUCTURES (SINGAPORE) / INDONESIA P.T. BALI WEB STRUKTUR / ENGLAND WEB STRUCTURES