Acknowledgements
I would like to express the deepest appreciation to my supervisor Doctor Kai Tai Wan for his support and valuable guidance for this project.
I would also like to thank my family and my girlfriend for their continuous encouragement support and attention.
Abstract
This dissertation reports the study of seismic analysis and analyze the method which been followed in order to avoid failures and damages from the structures combined both low cost and strong structures. For the last 17 years this methodology called Eurocodes and is common for all the countries which belong in European Union. Despite that method, one other factor which related to the stiffness of buildings is the choice of materials. In the past decade, carbon fiber reinforced polymer (CFRP) composite has been applied in several cases and the results were significantly encouraged about their contribution in seismic sustainability. This dissertation is going to examine an existing reinforced concrete building in Greece under an equivalent seismic activity and indicate a failure in structure through a structural analysis software called “FESPA” with consideration of Eurocode 8. After evaluation of the existing reinforced concrete building, a seismic retrofitting scheme by CFRP will be proposed and verified by FESPA.
Table of Contents
Acknowledgements 2
Abstract 3
Chapter 1 – Introduction 4
Chapter 2 – Literature Review 5
2.1 Seismic activity in Greece 5
2.2 Eurocodes 7
2.3 Seismic Retrofit with Fiber Reinforced Polymer 8
Chapter 3 Computer Model 10
3.1 Buildings Description 10
3.2 Brief Description of Fespa 11
3.3 Procedure of Seismic Pushover – Non linear analysis method 11
Chapter 4 Results Analysis 14
4.2 Global Behaviour 14
4.3 Discussion 14
Chapter 5 – Seismic Retrofitting Scheme 14
5.2 Configuration 14
5.3 Implementation in FESPA 14
5.4 Result Discussion 14
Chapter 6 – Conclusion 14
References 15
Chapter 1 – Introduction
Seismic analysis is a subset of structural analysis and is the calculation of the response of a building structure to earthquakes. It is part of the process of structural design, earthquake engineering or structural assessment and retrofit in regions where earthquakes are prevalent. In Europe the most seismically active region is Greece and also rank high (sixth position) on a global scale. Greece has suffered dramatically from earthquakes in the past several times. The problem is greater when you consider that earthquake is a phenomenon that can neither be predicted nor prevented. For this reason there is always a need to try minimizing the damages, and to achieve that, civil engineers created specific seismic rules. The major concern, of course of this need was to mainly protect human life and after construction.
For that reason civil engineers have to consider carefully the seismic factor in order to design a structure safe and try to limit the bending in a possible earthquake. The last 17 years this factor is resulting from the methodology of Eurocode 8. Their use is to convert the design to a common way of creating constructs in EU society. With Eurocode 8 the structures became more durable than before, combined with both lower cost and more stiffness.
Despite that method, one other factor which related to the stiffness of buildings is the choice of materials. In last few years fiber reinforced polymer (FRP) elements have been developed and tested in several cases and the results were significantly encouraged about their contribution in seismic sustainability. . The primary reason for this acceptance is the superior performance of FRP reinforcement in corrosive environments, its long term durability, high tensile strength-to-weight ratio, electromagnetic neutrality and resistance to chemical attacks. The use of FRP bars as concrete reinforcement is relatively new, with very few applications in practice, although externally applied FRP sheets, strips and bars for rehabilitation and seismic retrofit purposes are not uncommon. This is an outcome of the many advantages that FRP reinforcement produces such as high strength-to-weight ratio and favorable fatigue strength.
Chapter 2 – Literature Review
2.1 Seismic activity in Greece
Greece was always faced with the phenomenon of earthquake. As mentioned in the introduction is settled in the most seismically active region in Europe and has suffered dramatically from earthquakes in the past several times.
The high seismic activity of the country is due to the fact that it is located at the boundary of the Africa-Eurasia convergence. Within this framework, the Anatolian plate rotates counterclockwise (fig. 1). From the west, the Adria microplate rotates counterclockwise. As a consequence, the Aegean microplate moves fast towards SW. The external Aegean area is subject to a general compressional stress field and the inner Aegean area experiences a general extensional stress field. Greece often hosts large magnitude earthquakes, whilst a moderate or small magnitude earthquake is felt every 2-3 days on average (fig. 2). Although the majority of these earthquakes are shallow, a few cases have been recorded as «devastating» for the human environment or for life loss (e.g., the 1881 Chios, 1953 Cefallonia, 1999 Athens earthquakes). No historical information is provided for extensive migration of populations and obliteration of civilizations in Greece due to earthquakes. This, however, can be due to the fact that the earthquakes which usually cause damage in Greece are relatively small. Major shallow earthquakes (M > 8, return period of about 1000 years), which can cause such extensive destructions occur rarely. (Vicki Kouskouna and Kostas Makropoulos, 2004)
Figure 1. Active tectonics in the Aegean and surrounding area
Figure 2. 20th century earthquake epicenters with M>= 3 distribution in the boarder area of Greece (Makropoulos ,2001) Red and blue colour ranges represent shallow and intermediate depth earthquakes, respectively.
2.2 Eurocodes
Eurocodes is a multiannual program created by the European Committee of Standardization (CEN) has as a main purpose to produce a series with all the appropriate regulations and calculations of the static adequacy of the structures that will be use by all the members of the European Union.The Eurocodes consist of 10 major European standards, including all construction methods (concrete, steel, timber) and are subdivided into parts depending on the construction behavior of the earthquake or fire.
In this dissertation the Eurocode Methodologies which are going to be used in order to simulate the existing building are the Eurocode 1, Eurocode 2, Eurocode 3 and Eurocode 8. Eurocode 1 is responsible for all the actions which should take into account on the structure. It includes characteristic values for various types of loads and densities for all materials which are likely to be used in construction. Eurocode 2 related to design of concrete structures. More specifically describes technical rules for the design of concrete, reinforced and prestressed concrete structures, by using all the limit state design philosophy. On the other hand Eurocode 3 related to design of steel structures.
The most important methodology which should be considered very carefully is Eurocode 8. Eurocode 8 concerns the design of structures in seismic zone and was approves on April by the European Committee for Standardization (CEN). In case of an earthquake Eurocode 8 (EN 1998 or EC) ensures first of all that humans are not endangered and every possible damage will be avoided. The achievement of these goals is partially possible and is only measurable in probabilistic terms because of the nature of the seismic events which is random and due to the fact that the available resources are not enough to counter the effects of the earthquakes. The amount of the protection that is provided to each building depends on the allocation of the resources and is different in each country considering the seismic risk of every place. Some special structures, like nuclear power plants, are not included in the scope of EN 1998. EN 1998 contains only those provisions that must be observed for the design of structures in seismic regions.
2.3 Seismic Retrofit with Fiber Reinforced Polymer
Eurocode 8 is not the only factor which depends on minimizing the damages or the demolition of the buildings. One other important factor which helps to prevent those losses is the selection of materials.
The reinforcement of structure with composite materials is a new technique which spreads rapidly the last few years and applied more and more. The basic materials that used are carbon, glass or aramid fibers which combined with special epoxy resins and form a high strength composite material. This material adheres to properly prepared surfaces of the structural element, making it a permanent lining for repair and reinforcement. The most common applications of the composite materials are the attachment of strips to tensile soles and the wrapping of columns with fabrics. In addition to the above, composite materials are applied to repairs as sheets or grids. Particularly at the corners of the columns, the fabrics may install in one or two directions and form as whole or in layers.
The purpose of using composite materials is to improve the mechanical behaviour of the structural elements. More specifically they help to improve the strength, the deformation failure (and thus the ductility) and the energy absorption capacity. By using composite materials engineers can also increase the relevancy in critical areas of splice arrays, as well as reduce the possibility of local reinforcement buckling. Other cases of composite materials preference are under severely corrosive conditions as well as reinforcement of existing structures in areas of high seismicity as they provide significant advantages over conventional reinforcement.
The advantages of composite materials are quite important. Initially, they exhibit very high tensile strength and good linearity.
Figure Stress-Strain relationships for composite materials in tensile strength
Their weight is small, not corroded and available in very long lengths. In addition, the aid technique is characterized by ease, flexibility and fast application. On the other hand the main disadvantage which needs to be considering, is the cost which is high but decreases as the demand increases. Furthermore they produce low resistance to high temperatures, and their durability is doubtful. Finally it is worth mentioning that the composite materials have attracted the interest of many researchers and attempts are made to predict even the exact inelastic behavior and the corresponding strength and ductility of the reinforced structural elements. (Konstantinos Gousis, 2010)
Chapter 3 Computer Model
3.1 Buildings Description
The building which is going to be examined is located in the northwestern part of Greece, in the city of Kastoria. It is an individual structure that belongs to a school complex where it offers the opportunity to students to practice in case of bad weather conditions (snow, rain).
The structure is made of reinforced concrete. The form of the roof is rectangular with dimensions of 10.00m long and 5.00m wide. Total surface area is 50.00 m2. The 2 horizontal four-square reinforced concrete slabs distribute their weight and loads (snowloads, wind etc) to the ground through seven horizontal beams and 6 identical vertical columns (30×30 cm).
The building contains a basketball court made by wooden floor and a metal-movable standing structure which based under the 2 square concrete slabs (capacity 50 people).
For more detailed information a about the dimensions of the structure you can observe the following figures made both in AutoCAD software and by hand.
Figure 3 Floor plan of the structure
Figure 4 Cross-section handmade sketch of the structure
3.2 Brief Description of Fespa
Fespa is a 3D software program for the analysis and design of structure comprised of linear and surface elements, of different materials and arbitrary cross-sections, subjected to various loading. In other words it can “run” non-linear analysis (PushOver) with the needed data. After the simulation, Fespa provides documentation and series of drawings for each floor and details about beams and columns.
3.3 Procedure of Seismic Pushover – Non linear analysis method
The first step in assessing the carrying capacity of a building is to simulate it in the available computer program. In other words compute the static system that takes the main actions that act and stress the building. The static system can only consist of the main elements or in combination with the secondary ones. Depending on the available data and their validity, three levels of data are defined: high, satisfactory, and tolerable.
Then, depending on the existing plans of static analysis, it is able to define the cross sections of the structural elements with the corresponding reinforcement and the quality of the materials (concrete, reinforcement, structural steel etc.)
The next step is to determine the vertical loads which acted in the structure. These loads will be included as uniformly distributed per meter in the beams, coming from the influence surfaces of the slabs, due to lack of simulation of the latter.
In addition, forces and moments are entered into the positions that act by creating the corresponding nodes. A table with the standard assumptions (materials, loads, safety factors etc.) is going to be shown below. In case of take into account the soil-foundation interaction, it is able to use elastic springs (horizontal and vertical motion, torsion) that are characterized by similar stiffnessses with the corresponding soil compressibility index.
Finally additional parameters to be taken into account in the simulation of the vector are the torsional phenomena, the morphology of the structure, whethere characterized as normal or not, the effects of the 2nd order (static and dynamic) and taking into account relative assumptions about stiffnesses and resitances of the structural elements. (Mpoursianis Charis, 2014)
The appropriate data needed for the simulation are given in table 1.
Table 1.
Materials • Concrete Grade C20/25
• Steel Reinforcement S420
• Structural Steel S235
• Structural Timber C24/11E
Loads (Dead-Permanent) • Weight of concrete 25 kN/m3
• Weight of steel: 78 kN/m3
• Masonry brickwork: 3.6 kN/m2
• Slabs Cover: 1.2 kN/m3
• Roof: 2 kN/m2
Imposed Loads • Roof: 0.5 kN/m2
• Gymnastic Rooms: 5 kN/m2
• Railway Platform/Standing: 5 kN/m2
• Physical Activities: 5 kN/m2
Safety Factors • Dead Loads: γg =1.35
• Imposed Loads: γQ = 1.5
• Concrete: γc = 1.5
• Concrete: γc = 1.5
• Compressive strength factor: acc= 0.85
• Steel Reinforcement: γs =1.15
• Steel: γΜ0 =1, γΜ1 = 1. γΜ2 = 1.25, γΟν =1.25
• Timber: γΜ = 1.5
• ΕCO combination (6.10 α) + (6.10 b) ξ=0.85
Ground Parameters • National Annex – GR (Hellas)
• Seismic Zone: Z1 agR 0.16 , aVgR=0.149
• Ground Type: B , S=1.20
• Frequency Spectrum: TB=0,15, TC=0.5, TD=2.5
• Damping Ratio: ξ= 5.00%
• Topography Ratio: ST= 1,00
Nonlinear Analysis – PushOver • Load Distribution – Uniform
• Load Combination Coefficient Ratio: 30%
• Eccentricity: Only in the transverse direction
Behavior factor • Seismic behaviour factor (horizontal): qx=1.5 qz =1.5
• Seismic behaviour factor (vertical): qv=1.5
• Main behaviour factor value: qoX=1.5 qoZ=1.5
• Wall coefficient: Kw_x=1,00 Kw_z=1,00
• Allowable stress: σallow = 200,00 kN/m2
• Friction angle at the base: δ= 40.00(o)
• Safety factors (sliding): static γRh=1.1,
Seismic γRh= 1.00
• Safety factors (carrying capacity): static γRv=1.4,
Seismic γRv= 1.00
Chapter 4 Results Analysis
4.2 Global Behaviour
4.3 Discussion
Chapter 5 – Seismic Retrofitting Scheme
5.2 Configuration
5.3 Implementation in FESPA
5.4 Result Discussion
Chapter 6 – Conclusion
References
• Cimilli Erkmen, Serra (2009) “Analysis and design of earthquake resistant FRP reinforced concrete buildings” Online available at: http://www.ruor.uottawa.ca/handle/10393/29760
• Theodoros M. Tsapanos (1991) “Earthquake Monitoring and Seismic Hazard Mitigation in Balkan Countries” Online available at: https://link.springer.com/chapter/10.1007/978-1-4020-6815-7_17
• Eurocodes building the future (2017) “EN 1998: Design of structures for earthquake resistance” Online available at: http://eurocodes.jrc.ec.europa.eu/showpage.php?id=138
• BBC (2017) “ Risks and implications of Information and communications technology” Online available at: http://www.bbc.co.uk/education/guides/zkyg87h/revision/3
• Vicki Kouskouna and Kostas Makropoulos (2004) “Historical earthquake investigations in Greece”. Online available at: http://www.geophysics.geol.uoa.gr/papers/makro/makro205.pdf
• CSi (2005) “Introductory Tutorial Parts I and II”. Online available at:
http://docs.csiamerica.com/manuals/etabs/Introductory%20Tutorial.pdf
• Konstantinos Gousis (2010) “Reinforcement of columns with composite materials”. Online available at: file:///C:/Users/Admin/Pictures/6.%20%CE%93%CE%9F%CE%A5%CE%A3%CE%97%CE%A3%20%CE%9A..pdf
• Mpoursianis Charis (2014) “Non-linear analysis (Pushover) of an existing building” Online available at: http://www.episkevesold.civil.upatras.gr/Files/Content/58/%CE%9C%CE%A0%CE%9F%CE%A5%CE%A1%CE%A3%CE%99%CE%91%CE%9D%CE%97%CE%A3.pdf