Introduction and Literature Review [304209]
Chapter One
Introduction and Literature Review
1.1 Introduction
Geographical regions where the earthquakes were common and suspected the design of general construction should take the seismic action in consideration to resist seismic action and avoid collapse. The design of seismic action was depend on basis of return period of time and events of maximum intensity occur at the place. [1]
[anonimizat], also earthquakes give very good chance for engineers to improve the criteria of structural design and provide effective solutions for the seismic design problems. [anonimizat]. [2] Many comparative studies have shown that the isolated structures are significantly less affected than the fixed base structure. [3, 4, 5, 6, 7]
1.2 Description of the Study
This dissertation is structured on six chapters; Chapter 1, Introduction – [anonimizat] 2, Seismic Action and basic principles of the seismic action. Chapter 3, Basic knowledge about the bridge. In Chapter 4, [anonimizat] 5 represents the case study calculation and results. The paper ends with chapter 6, the conclusion of the study.
1.3 [anonimizat] a [anonimizat].
1.4 Literature Review
1.4.1 Base Isolation Systems
1.4.1.1 Development of Seismic Base Isolation Systems
1.4.1.2 Historical Background
Surprisingly, J.A. Calanterients, in 1909, who is a medical doctor proposed the first seismic base isolation design; He claimed that if a structure is built on a [anonimizat], [anonimizat]. The primitive isolation system that Mr. Calanterients proposed was earthquake resistant design; the basic idea is same with the seismic base isolation today. [8]
[anonimizat], balls, sliding rollers etc.; he performed his first seismic base isolation trials on sliding balls and rollers to improve the wind resistance of seismic isolation system. [9]
1.4.2 Basic principle of Seismic Base Isolation System in bridges
1.4.2.1 Base Isolation System (Overview)
The basic concept of isolation systems is fairly simple. [anonimizat], while also providing more flexibility to the structure to decrease or eliminate inelastic deformations.
Energy dissipation devices increase damping of the structure to lessen the seismic energy input. [10]
Bridges are highly important structures in a modern transportation system. There were 570,000 bridges in the United States by 1994, and 60% of them were constructed prior to 1970. [11] Any collapsed or heavily damaged bridges will possibly delay transportation of emergency services after or during a seismic event. [anonimizat] place of conventional design for bridges.
Several research projects have been conducted on the seismic isolation of structures. Significant amount of those researches conducted during investigation of seismic isolation were mainly focused on material science. Material properties were investigated and different types of isolation devices were invented.
One type of isolator device, which has been used on several projects, is lead-rubber bearings. It was first proposed by W. H. Robinson in 1975. [12] A study done by Robinson (1982) [13] showed the advantages and disadvantages of the use of lead-rubber bearings in the seismic isolation of structures.
As awareness of seismic isolation has increased, questions and research have also increased. Simo and Kelly (1984, 170) [14] studied stability of multilayer elastomeric bearings, which act as bridge bearings and as seating pads. It was found that the horizontal stiffness of the bearings decreases due to roll-off at the end plates with increased load and displacement. A finite element formulation was presented for elastic stability problems to be used for adequate end plate design for multilayer elastomeric bearings.
Since seismic isolation has been a growing earthquake-resistant design technique, isolation design methods as well as isolation devices are being developed. Kikuchi and Aiken (1997, 230) [15] proposed an analytical lagged effect model for elastomeric bearings in order to suspect the seismic response of base-isolated structures. Accuracy of the proposed analytical model was confirmed by comparing earthquake simulation tests of two isolated structures with dynamic analysis, using a total of four different types of elastomeric bearings. The advantage of this proposed model is its strong dependence on shear strains, being applicable for high shear strains. It was concluded that the analytical behavior well captured the experimental response.
Du, Han, and Zhan (2008) [10] studied nonlinear seismic response of continuous girder bridges isolated with lead-rubber bearings. Purpose of their study was proposing a solution method to account for changing bilinear force-deformation behavior of lead-rubber bearings due to bidirectional interaction of restoring forces under bidirectional horizontal seismic excitations. A two-span bridge model with a dimensional scale factor of 0.1 was used in shake table tests to validate the accuracy of the proposed method. It concluded that the differences in peak bearing force between experimental and analytical results are within 10 percent. This study should be considered for design of isolated bridges under bidirectional excitations regardless of selected isolation method, either partial or full isolation.
Typically, seismic isolation is applied to the whole bridge structure. However, there are some examples of partial isolation of bridges. Tsai (2008) [16] studied transverse earthquake response of commonly used bridge types in Taiwan, which are partially restrained in the transverse direction with abutment restrainers. From the analytical experiment, it was noted that the composite damping ratio of the transverse behavior may be decreased due to partial restrains. It is concluded that partial isolation is effective for the purpose of reducing shear forces developed at bridge piers, but increased seismic demands of the abutments in the transverse direction may result.
Another well examined seismic behavior of partial isolation was studied by Hu and Ryan (2011). [17] Typical highway Bridge structures used in the state of Utah, which consists of two spans, was examined and compared with conventional and fully isolated design options. A static design procedure was developed with a parametric study. Partial isolation in this study considers decoupling of the columns from the superstructure while the bridge is fixed at its abutments. Hu and Ryan (2011) [17] achieved reduced displacement demand and inertial force of bridge columns with nonlinear type isolators, which have elastic perfectly plastic force-displacement hysteresis. Increased force demands of the columns and abutments occur with linear type isolators, even though the displacement demand of the columns is reduced.
1.4.2.2 Types of Base Isolation System in Bridges
Until the ends of 70’s, development of laminated elastomer isolators was the practical method of earthquake resistant design in Japan. [8] Firstly, rubber was used under building basements to isolate the structures; then steel plates were vulcanized with rubber to increase the vertical stiffness of the isolators (Figure 1-1).
Figure 1-1: The steel reinforced elastomer isolator, SREI
Lead-plug rubber bearings are steel reinforced elastomeric isolators with one or more circular holes as shown in Figure 1-2. [8] The lead-plug is introduced in these holes to increase the damping of the isolator. The adding of damping may increase the contribution of secondary modes on the response of structure; this may lead to decreasing of efficiency of the isolation system.
Figure 1-2: The lead-plug rubber bearing
Because of steel plates inside the SREI samples used under buildings, it is very heavy; that makes the production and construction of elastomeric isolators’ difficult task, and causes significant increase in the cost of the isolation system, to solve this problem, Kelly et al. use lightweight fiber reinforcement inside the elastomeric isolators. [18]
Fiber-reinforced elastomeric isolators are a newly introduced technique in the field of seismic base isolation. [19] The elasticity of the fiber material is an important factor affecting the compression modulus of fiber reinforced elastomeric isolators. Later on Tsai, Kelly and Takhirov were conducted an experimental and analytical studies on compression behavior of fiber reinforced elastomeric isolators. [19, 20, 21, 22, 23]
Early sliding-based seismic isolation methods were deficient to re-center the building to its original location. Then Zayas et al. studied the friction pendulum system to handle re-centering deficiency of sliding rollers. [24] The system is formed of two concave Teflon spherical surfaces with an articulated slider in between these concave surfaces. The Friction Pendulum System is capable of re-centering the structure to its original position by the effect of horizontal component of the resting structure weight on the isolation system. The energy dissipation is ensured through the friction between the surface of the concave plates and the articulated slider that are presented in Figure 1-3.
Figure 1-3: The Friction Pendulum System, FP
The equation below indicated the period of the structure resting on friction pendulum system depends on the radius of the concave surface of friction pendulum system (FP), but did not indicate the mass of the structure. This characteristic of friction pendulum system (FP) also enabled seismic base isolation of lightweight structures.
The TASS system, the Resilient-Friction Base Isolator System, Sliding Disc Bearing and Helical Spring system are common other restoring systems considered to isolate the structures. [25, 26]
In TASS system, parallel rubber blocks were used to produce the re-centering capability, Figure 1-4.
The Resilient-Friction Base Isolator system was consisting of Teflon-coated steel plates with a rubber core to produce the re-centering of the isolation system.
Figure 1-4: The TASS System
Chapter Two
Seismic Action
2.1 Seismic Action
2.1.1 Introduction
Earthquake was take place duo to seismic waves which arise from sudden movements in the rupture zone (active fault) of the earth crust. These waves of different types and velocities were traveled in different directions before reaching the building locations and causing various motions to the ground. The ground subjected to moves rapidly backward and forward and in all directions, mainly in horizontal plan, but also may be in vertical direction. [27]
The effects of each earthquake on buildings are determined by the time histories of the three directions of ground motion parameters; ground acceleration (ag), velocity (vg), and displacement (dg), with their own specific frequency contents for each parameters. The study of an example of the linear horizontal ground motion chart shown that the dominant frequencies of acceleration (ag) are substantially higher than the velocity (vg) and much higher than the displacement (dg). [27]
The ground motion parameters (ag), (vg) and (dg); as well as, the other characteristic values at a location caused by an earthquake of a certain magnitude may vary strongly. The ground motion parameters depend on numerous factors, for example the distance, direction, depth and the mechanism of the fault zone in the earth crust (epicenter), in addition to, other factors particularly the local soil criteria such as layer thickness and shear wave velocity. In comparison with rock the softer soils are more subjected to significant local increase of the seismic waves. Regarding the response of structures to the ground motion, it depends mostly on the structure criteria like Eigen frequency, type of structure and ductility. [27]
2.1.2 Basic representation of the seismic action
2.1.2.1 General spectrum for the representation of Seismic Action
The earthquake motion in certain location of the surface of a structure was usually represented by the so-called “elastic response spectrum”. Such a structure is considered to be subject to a uniform displacement applied to the base support. It is thus implicitly assumed that all support points are subject to the same uniform excitation. If this assumption cannot be made realistically, a so-called spatial model of the seismic action should be used. [28]
The horizontal seismic action is described by two orthogonal components considered as independent and represented by the same elastic response spectrum. The vertical component of the seismic action should be represented by the response spectrum as defined for the horizontal seismic action. [28]
Application of the above described elastic response spectrum assumes a linear elastic behavior of structures were subjected to the seismic action. However most structures will rather show a non-linear behavior in such circumstances, as a considerable amount of energy will be dissipated due to the ductile behavior of the structural elements and connections. [28]
For the representation of the seismic action the seismic movement in certain location of the surface should generally be represented by the elastic response spectrum representing the acceleration of the ground. [29]
2.1.2.1.1 Ground condition
The seismic action is defined as being a geotechnical action, it is an action transmitted to the structure by the ground or the water of the land. As the seismic action consists of a movement of the ground under the structure, the characteristics of these grounds will be of great importance for its definition. [29]
2.1.2.1.1.1 Identification of ground types
Studies of characterization will have to be done in order to classify the geotechnical conditions in accordance with the types of ground, defined in table 3.1 of § 3.1.2 of the EN 1998-1, decreasing in terms of rigidity and resistance of the ground from A to S2. Each country will be able to define in its national annex the parameters S (Amplitude, translated by the peak ground acceleration) and TC (Content of frequency), which translate the influence in the seismic action of the local geotechnical conditions. [29]
Ground types A, B, C, and D described by the stratigraphic profiles and parameters given in Table 2-1 (4) may be used to account for the effect of local ground conditions on the seismic action. Also description may be done by taking in consideration the effect of deep geology on the Seismic action. [30]
Compared to the RSA, that only categorizes the ground in 3 types, EC8 enters into greater detail regarding this aspect, where 7 types of ground are defined. This greater detail not only allows the consideration of a larger number of types of ground foundation, but also defines in a more precise way the parameters of each type of ground that will influence the definition of the seismic action. [29]
Table 2-1: Ground types [29]
2.1.2.1.2 Seismic zones
In accordance with the EC8, [31] the seismic zoning will have to be made by the national authorities of each country, dividing the domestic territory in zones with constant seismic hazard. The seismic hazard of each zone will have to be described by an only parameter, which corresponds to the reference value of the peak of the acceleration in a rock type ground, agR, whose value will have to be determined by each national authority. This parameter is defined as being the maximum absolute acceleration obtained for the component of the seismic movement in one determined direction, being obtained, on the basis of attenuation relations, for a period of return of 475 years (or a probability of exceedance of 10% in 50 years). For Portugal the following seismic zoning is proposed for a near seism and of small magnitude (Type II seism in EN 1998-1) and for a distant seism and of moderate or raised magnitude (Type I seism in EN 1998-1), for a seismic hazard associated to a period of return of 475 years: [31]
Figure 2-1, Figure 2-2 and Figure 2-3: Proposal to the Portuguese seismic zoning for the two types of seism prescribed in EC8 (indicating the accelerations of project in rock, agR, for an importance factor of 1,0) and seismic zoning in RSA, proposed by the Task Group GT-8 of the Portuguese National Annex of the EN 1998-1.
Comparatively to the RSA, it is observed the continuation of the existence of two types of seism’s (distant and near) being that in the EC8 the zoning is different consonant the type of seism, situation that did not occur in the RSA, where the zoning was only one for the two types of seism. [29]
It is noticed that this fact represents a clear evolution facing the previous regulation, because it did not make sense to establish the same seismic zoning for two different seismic design situations. [29]
Another fact that is suggested by the analysis of the above displayed figures is the reduction of the area of the zone corresponding to the biggest hazard (red-color) from the RSA to the EC8. [29]
Relatively to the values of the reference acceleration, agR, it is verified a great disparity of values, being that, e.g., for a distant seism, it is observed an increase from 107 to 250 cm/s2 (RSA value to the EC8 value), in the zone of bigger hazard. For a near seism, the comparison of the values of RSA/EC8 presents sufficient similarities, being that for the zone of bigger hazard, it is prescribed the value of 177 cm/s2 in the RSA and the value of 150 cm/s2 in the EC8. It is noticed however that this consideration between ground accelerations should not be overrated, because there are some supplemental factors that have to be taken in account:
Effect of the consideration of the design envelope of two scenarios;
Different partial security factor of the seismic action in the EC8 (1,0) facing to the RSA (1,5);
Different configuration of the response spectrum in the two regulations (e.g, the different value of the behavior factor in the two norms and the new soil parameters).
2.1.2.1.3 DESIGN CONCEPTS
To assure that a structure remains elastic during the design seismic action, typically associated to a probability of exceedance of 10% in 50 years, it is necessary that the same is designed for lateral forces in the order of magnitude of 50% or more than its weight. Despite it being technically possible to design a structure to answer in elastic regimen to the seismic action, it is unnecessary to do it so, because being a seism a dynamic action, it represents for the structure a certain total of energy input and a requirement of tolerance to a certain level of displacements and deformations, but not a requirement of resistance to specific forces. Besides, an elastic design to the seismic action would result in prohibitive economic costs and numerous practical difficulties. In this way, it is generally accepted by the various seismic norms to take advantage of the non linear behavior of the structure for the seismic action, provided that the magnitude of the inelastic deformations does not place in danger the integrity of the diverse structural elements and of the structure as a whole. Traditionally, to take in consideration the non-linear behavior of the structure in the analysis, a linear elastic analysis is executed, that provides the design forces in each element through the action of a specified set of lateral forces in the structure. These design forces are obtained from a design response spectrum of accelerations, which generally is obtained through the division of the elastic response spectrum with a behavior factor, q. In this way, each structural element may designed to resist to the forces obtained from analysis, having equally to be detailed (especially in the “critical regions”) to be capable to develop the inelastic deformations associated to the respective value of the behavior factor. Thus, the value of the behavior factor to be adopted will depend on the type of structure, of the permissible degree of ductility of the same and, finally, of the nature of the materials that constitute it. The global behavior of a reinforced concrete element, subject to repeated and alternated loads, will be a function of the behavior of the materials that constitute it, reflecting the available ductility one of the basic aspects for the description of the global seismic behavior of the structure. [29]
2.1.2.1.4 Capacity design
A good performance of a structure during an intense seism consists in the development of a mechanism that dissipates efficiently, in a hysteretic way, the absorbed energy. The zones of the structure where this energy dissipation will be given (critical zones), that is, the zones where the inelastic deformations will be situated, will have therefore, at the conception level, to be detailed to be able to accommodate the ductility adopted in the design processes and models of analysis. Currently, the normative philosophy consists in the adoption of rules that look to define conception norms and designs that guarantee an appropriate behavior of the structures during a seism, taking advantage of the effective resistant capacity of each structural element. This purpose is considered an explicit way in diverse regulations, because in these there are adopted rules of design that are based on the attribution of a relative resistance for evaluation of the effective resistant capacity of the various elements, for each type of effort, or either to adopt a capacity design, as alternative to a direct design, which considers only one distribution of the elastic efforts for the structure, diminished by the behavior factor. The capacity design allows the designer to define the form of how the structure will behave during an intense seism, independently of the characteristics of this one. With this procedure, the designer imposes the zones where the plastic hinges will form, as well as their respective order of formation. So, the designer imposes the values of resistance and ductility on the diverse structural elements through different disposals consonants of zone to consider, guaranteeing, on one hand, the existence of an excess of resistance in the zones where he does not intend that plastic hinges form, and, on the other hand, that in the zones where he intends that these occur, that the calculated effort reaches the value of the resistant effort. It should be noticed that in this way the plastic hinges will have to be characterized by an adequate ductility and capacity of energy dissipation, preventing fragile rupture or loses of resistance during the formation of these plastic hinges, resulting of the deformations in these zones imposed by the seismic action. [29]
2.1.2.1.5 Ductility
The concept of ductility can be defined as the possibility of a structure to dissipate, by an hysteretic process, the energy that the dynamic action transmits to it, imposing this same capacity of waste that the constituent elements of the structure have the possibility to deform themselves beyond its elastic limits, accommodating, without great reductions of resistance and rigidity, successive cycles of alternated loads of great amplitude. Therefore, the global behavior of the structure, are influenced by the available ductility. [29]
For this effect, the EN 1998-1 defines a zone for each primary seismic element called "critical region" in which the biggest efforts proceeding from the most unfavorable combination will occur and where probably the plastic hinges will be formed. It is expected that it is in these so called "critical regions" where the biggest energy dissipation will occur (zones where the inelastic deformations will be situated), having therefore, at the conception level, these to be detailed to be able to accommodate the ductility adopted in the design processes and models of analysis. Out off the critical regions, the design and detailing will have to observe the disposals related in the EN 1992-1-1. However, the Eurocode 8 gives the freedom to the designer for, having in mind the concepts of ductility above described, to design the reinforced concrete building with a bigger resistance and minor ductility, or vice versa, defining for this effect the following classes of ductility, which will influence the rules of design and the value of the behavior factor to adopt: [29]
1) DCL (Ductility Class Low) – It corresponds to the philosophy of design for resistance instead of ductility (without practically any requirements being added to those of the EN 1992-1-1). Thus, practically all the structure will reply elastically, being that the resistance to the horizontal forces of a seism will have to be assured by the proper resistance of the structural elements and not for its ductility. A behavior factor, q, could be admitted, with the maximum value of 1, 5 (with this value, in a common design practice it is assured an over-resistance).
2) DCM (Ductility Class Medium) – This Class corresponds to the design strategy for energy dissipation and ductility. This class will give origin to structures designed according to the principles of seismic resistance of the EC8, that is, with good ductility and the capacity to suffer hysteretic cycles, without the occurrence of a fragile rupture.
3) DCH (Ductility Class High) – Corresponding equally to the philosophy of design for energy dissipation and ductility, this class is characterized by high requirements of ductility and of high plasticity levels, leading therefore that the design requirements and detailing to be more complex and expensive that those of DCM. These reasons will cause that, in the current design practice, this class of ductility will not be used frequently.
2.1.2.2 Seismic design spectrum for linear analysis
2.1.2.2.1 Use of the design spectrum
The philosophy for developing design specifications of bridges depends on various criteria defining their performance based on their importance category in case of seismic events. Force based design and displacement based design are determined in a way that the required performance of the bridge is met. In recent years, it has become more crucial to understand the behavior of the structures under flexible performance criteria. These criteria were established to consideration for the design and retrofit of bridges. Under this philosophy, capacity design is used for the seismic design of capacity protected members assuming unity for the importance factor and modification factor. [32]
2.1.2.2.2 Design and analysis requirements in codes
Several codes have addressed the issue of structural base isolation. Parameters of analytical design of such elements and the structure itself have been mostly discussed in codes like:
American Association of State Highway and Transportation Officials-AASHTO LRFD Bridge Design Specifications (AASHTO, 2007 and 2012). [33]
AASHTO guide specifications for seismic isolation design (2010). [34]
National Building Code of Canada (NBCC, 2015). [35]
Canadian Highway Bridge Design, CSA S6 (2006, 2014). [36]
Eurocode 8 – Design of Structures for Earthquake Resistance. [31]
Bridges (BSI, 2005). [37]
2.1.2.2.3 Seismic parameters
The key parameter in the seismic design is the elastic seismic response coefficient, which describe the seismic hazard at a site. Codes have proposed similar equations to calculate this coefficient. The ductility however is considered differently in various codes. In the following, the provisions for the calculation of seismic response coefficient for isolated and non-isolated bridges are discussed briefly for different codes. [32]
2.1.2.2.3.1 Eurocode 8
Shape of elastic response spectrum in Eurocode 8 (2005) is taken as being the same for the two levels of no-collapse requirement (ultimate limit state design) and for the damage limitation requirement. The Importance factor in this code is used to reflect the impact of a bridge failure on the society. This factor modifies the design hazard level (Fardis et al., 2005). [38]
For non-isolated structures, the elastic response spectrum Se (T) was defined by four expressions at four ranges of period of a linear single degree of freedom system. The soil factor in design ground acceleration (ag) on rock or other rock-like ground types was determined at a probability of exceedance of 10% in 50 years and the damping correction factor are three parameters that define the coefficient.
The first descending branch of the elastic response spectrum was inversely proportionate to the period (1/T), and the second branch up to T=4s is inversely proportional to the second order of the period (1/T2). [39]
Deep geology features describe the geological settings criteria of the site by the scale of kilometers. [39] For shallow or local soil conditions with site geotechnical description by the scale of ten meters [40] (Bulajić et al., 2013) with the purpose of probabilistic hazard assessment, the earthquakes that contribute most to the seismic hazard go with a different type of spectrum.
2.1.2.2.3.2 Canadian Highway Bridge Design, CSA S6 (2006) and AASHTO (2007 and earlier)
According to the “Recommended Provisions for the Development of Seismic Regulations for Buildings” prepared by the National Earthquake Hazards Reduction Program (NEHRP, 1998), the design spectra are defined by the Peak Ground Acceleration (PGA) and the Peak Ground Velocity (PGV). Later, AASHTO brought a modification to the spectra by taking the PGA only and similarly the CSA S6 (2006) adopted the same approach (CSA S6 commentary, 2006). The zonal acceleration ratio in CSA S6 (2006) is based on the PHA from NBCC (1995) representing a seismic event with a 10% probability of exceedance in 50 years.
2.1.2.2.3.3 Canadian Highway Bridge Design, CSA S6 (2014)
It is the basis to find out the seismic hazard in 2006 edition of CSA S6 and its previous editions was the 10% in 50-year probability of exceedance, (return period of 475 years).
Seismic hazard levels in the 2014 version of CSA S6 are at 2%, 5% and 10% probability of exceedance in 50 years for the performance assessment of bridges. For all three return periods, the spectral response accelerations are presented with 5% damping up to 10s.
The horizontal spectral response acceleration values in the National Building Code of Canada (2015) are given on the website of the Geological Survey of Canada (www.earthquakescanada.ca). The uniform hazard spectral acceleration values at periods 0.2s, 0.5s, 1.0s, 2.0s, 5.0s and 10s are given for each seismic hazard level which is modifiable for each site. The values between the designated periods can be obtained by interpolation that may cause some conservatism, especially in longer periods.
2.1.2.2.3.4 French standard PS92 [41]
In the French standard PS92, the design ground acceleration is referred to as the nominal acceleration aN. Values for aN are specified not only as a function of the seismic zone (0, Ia, Ib, II & III), but also as a function of the so called hazard class:
Class A: for the buildings of minor importance for public safety, e.g. Agricultural buildings.
Class B: for the “ordinary” buildings, of which, collapse during earthquakes establishes a normal risks for the inhabitants
Class C: for the buildings required a seismic resistance as an importance in view of the civil and economic sequel associated with damage, e.g. public buildings.
Class D: for the buildings with integrity during earthquakes was of vital importance for civil survival and protection, e.g. hospitals, fire stations, power plants.
Chapter Three
Basic Knowledge about Bridges
3.1 Structural Types of Bridges
3.1.1 Bridges Types
Bridges can be categorized in several different ways. The following characteristics are used in bridge type identification: [42]
Function:
Transportation of vehicles
Railroad transportation
Use of pedestrians
For the material handling
Span length:
Short: less than 100-200 feet.
Intermediate: from 100-600 feet.
Long: greater than 400-600 feet.
Span type:
Simple span (beam, girder or truss)
Figure 3-1: Simple span (beam, girder or truss)
Rigid frame
Figure 3-2: Rigid frame
Cantilever (beam, girder or truss)
Figure 2-3: Cantilever (beam, girder or truss)
Continuous (beam, girder or truss)
Figure 3-4: Continuous (beam, girder or truss)
Arch (girder or truss)
Figure 3-5: Arch (girder or truss)
Suspension
Figure 3-6: Suspension bridge
Cable stayed
Figure 3-7: Cable stayed bridge
Movable
Swing: (girder or truss)
Figure 3-8: Movable Swing: (girder or truss)
Lift: (girder or truss)
Figure 3-9: Movable Lift: (girder or truss)
Bascule: (girder or truss)
Figure 3-10: Movable Bascule: (girder or truss)
Floating
Non-bridges:
Culverts
Tunnels
Structure materials
Timber
Steel
Concrete:
Reinforced
Pre-stressd
Post-tesioned
Composite: (decks and girders)
Others: less frequently used in modern construction.
Masonry
Figure 3-11: Masonry Bridge
Iron
Aluminum
Cross section
Deck
Figure 3-12: Cross section Deck
Half through
Figure 3-13: Cross section (Half through)
Through
Figure 3-14: Cross section (Through)
Degree of redundancy
Determinate
Indeterminate
Floor system
Conventional deck
Orthotropic deck
Figure 3-15: Floor system (Conventional and Orthotropic deck)
Bridges Component
Figure 3-16: Bridges Component
Superstructure
The superstructure is defined as, the entire part of the bridge structure which primarily receives and supports the load and, in turn, transfers the resulting reactions to the bridge substructure. The superstructure consists of beam, girder, and truss or cable construction. The superstructure components include: [42]
Figure 3-17: Superstructure of bridge
The floor beams
The girders
The stringers
The diaphragms:
Intermediate
End
Continuity
The deck
The roadway
The sidewalk / overhang
The parapet and railings
The expansion dam
The truss members
The chords (top and bottom)
The vertical and web members
The lateral bracing
The portal
The end post
The struts and wind bracing
The cable system
The hangers: fixed and expansion type
Substructure
The substructure is the foundation portion of the bridge that supports the superstructure and transfers the load to the earth. The substructure includes: [42]
Abutments
The breast wall
The wing walls
The bridge seat
The back wall
The footing or pile cap
Figure 3-18: The footing or pile cap
The piers
The stem wall
The column or pier shaft
The web wall
Figure 3-19: The column or pier shaft
The pier cap
The footing or pile cap
Figure 3-20: solid and open Pier
The pile bent
The piles (steel, concrete, or timber)
The bent cap
Figure 3-21: The Pile Bent and Steel Bent
The caisson
The piling
The dolphins and fenders: pier protection
Bearings
Bearing transmit and distribute the superstructure load to the substructure and permit the superstructure to undergo necessary movements without development harmful overstress. The two general types of bearings are fixed and expansion. The principle difference is that fixed bearing allow rotation but no translation; while the expansion bearing permit both rotation and translation. Without the ability of bearing to rotate, otherwise determinate structures would become statically indeterminate; bending moment would be induced in piers and footings.
Expansion bearings are designed primarily to allow longitudinal movement (translation) resulting from thermal growth and contraction. Inhibiting this movement can result in buildup of stresses reaching enormous values. [42]
3.2 Basic Behavior of Bridges
3.2.1 Introduction
The long span suspension bridges design of, particularly the suspended bridges; the primary concern was the wind action. The realization of the potential aeroelastic nature of the wind phenomena, which causing the failure of the Tacoma Narrows suspension bridge in 1940 in Washington, was established whole over the world.
The aerodynamic effects of wind on bridges were primarily vortex shedding, galloping, torsional-divergence, flutter and buffeting. [43]
3.2.2 Criteria for the bridge design
The criteria for the long spanned suspension bridges design were concentrated on the static and dynamic responses of a bridge exposed to wind load.
The design of long span suspension bridges was mainly controlled by the aeroelastic instability. The aerodynamic design concerned with calculation of critical velocity for the onset of flutter. It is essentially important issue to be considered that the wind velocity does not exceed the suspected critical velocity to avoid failure due to flutter.
Arrol and Chatterjee (1981) [44] reported that frequencies other than the basic ones should be considered in the bridge design. They declare that the bridge designers should keep in mind that the position of maximum stress should not be always at mid-span, or a support, moreover the stress value will depend on the mode shape of bridge. In a simply supported span, the second mode of maximum stress is at the quarter points and will have a value four times than that of the fundamental mode maximum stress, which occur at the mid span.
There are static and dynamic concerns, which should be considered for a safer designing of bridges; as discussed by Simiu and Scanlan (1986) [45] and Larsen (1992). [46] Which are described below in Figure 3.22.
Figure 3.22: Relative bending moment diagrams due to 1st and 2nd modes of vibration. (Picture from Arrol and Chatterjee, 1981)
3.2.2.1 Static behavior
The considerations in static behavior are the reverse, excessive lateral deflection, divergence, and lateral buckling (Selvam et. al., 1998). [46] The static phenomena, in general, are not critical issue for the design of bridges design. The issues that correlated to static behavior could be checked by the aerodynamic force components like the drag force, lift force and the pitching moment. The static issues are concerned with the plot of the coefficients of drag force, lift force and pitching moment against the angle of incidence of wind.
3.2.2.2 Dynamic behavior
According to the Newton’s second law, the movement of mass is described by the differential equation
Where:
F(t) is the time dependent load which act on the mass, k is the stiffness coefficient and c is the coefficient of damping. This equation can be rewritten in the form
Where .
Here ω is the natural circular frequency and 2mω is the critical damping coefficient. Three cases arise based on ζ being less than, equal to, or greater than unity resulting in under-damped, critically damped and over-damped response respectively.
Selvam found in 1998 that the dynamic behavior includes the responses obtained due to vortex shedding excitation, self-excited oscillations and the buffeting by the wind turbulence. [46] Also Sachs in 1978 states that suspension bridges could oscillate in two natural modes, vertical and torsional modes. [48] In the vertical mode, all the joints shift the same distance in vertical direction, while in the torsional mode every cross-section was twisted about a longitudinal axis parallel to the horizontal direction.
The importance of dynamic behavior was unlike the static behavior because it was critical and considered in the bridge design.
3.2.3 Aerodynamic Instability
Aeroelasticity is the subject which deals with the study of interaction between aerodynamic forces and structural motions. When a structure is exposed to wind flow, it will either vibrate or suddenly deflected in the airflow. The motion of the structure leads to change in the pattern of air flow around the structure. When pattern of wind around the structure was modified by aerodynamic forces that will escalate rather than reducing the vibration, thus the giving rise to subsequent deflections of oscillatory and/or divergent character aeroelastic instability will occur (Simiu and Scanlan, 1986). [45] The aeroelastic processes that are dealt with in wind engineering are vortex shedding, torsional divergence, galloping, flutter and buffeting.
3.2.4 Vortex Shedding
Simiu and Scanlan (1986) [45] found that when a structure was subjected to wind flow, the separation of air flow will occur around the structure. Separation of air flow will produce forces on the structure, which are the pressure force on the facing side and the suction force on the opposite side. The pressure and suction forces result in the formation of tornadoes in the wake region leading to deflections on the structure. The shedding of vortex balances the change in momentum of fluid along the structure surface (Larsen and Walther, 1997). [49]The frequency of the vortices shedding dictates the response of the structure. When the vortex was induced and the natural frequencies overlap; that lead to what is called lock-in. During the lock-in state, the structural member will oscillates with increasing amplitude but seldom exceeding half of the across wind dimension of the structure (Simiu and Scanlan, 1986). [44] The lock-in condition is explained in Figure 3.23.
Figure 3.23: Qualitative trend of vortex shedding frequency with wind velocity during lock-in (Simiu and Scanlan, 1986).
3.2.5 Galloping
Simiu and Scanlan (1986) [44] proved that galloping is an instability state which is typical of slender structures. This is a relatively low-frequency oscillatory process of elongated, direct structures operated upon by a wind stream. The frequency at which the object reacts is much lower than the frequency of vortex shedding. There are two types of galloping: Wake and Across-wind galloping.
3.2.5.1 Wake galloping
It was produced when two cylinders one facing the wind, and the another one opposite the air flow direction, within that wake separated by distance away from each other. In wake galloping the downstream cylinder is subjected to galloping oscillations induced by the turbulent wake of the upstream cylinder; so that the upstream cylinder tends to rotate clockwise and the downstream cylinder rotate in anti-clockwise direction that will lead to torsional oscillations. Figure 3-24 explain the process.
Figure 3-24: Wake galloping Picture from Simiu and Scanlan (1986)
3.2.5.2 Across wind galloping
The across-wind galloping is an instability state of bridge that is initiated by a turbulent wind blowing in transfer direction across the deck. It causes a crosswise vibration in the bridge deck (Liu, 1991). [49]As the cross section vibrates crosswise in a stable wind velocity U, so it changing the angle of attack (α). Because of the change in α, an enhancement or reduction on the lift force of the cylinder will occur. In case of increase of α an increase in the lift force in the opposite direction of motion; that is mean the situation is stable, but on the vice versa, when an increase of α causes a decrease in lift force, subsequently the situation is unstable and galloping occurs. Figure 3-25 explain the process.
A classical example of this phenomenon is observed in ice covered power transmission lines. Galloping is reduced in these lines by decreasing the distance between spacing of the supports and increasing the tension of the lines.
Figure 3-25: Across wind galloping: Wind and motion components, with resultant lift and drag, on a bluff cross section. (Picture from Simiu and Scanlan, 1986)
Where: U= wind velocity, Ur= relative wind velocity with respect to moving body, ẏ= velocity across-wind, B= dimension of the section, L= lift force, D= drag force.
3.2.6 Torsional divergence
The torsional divergence is an example of a static response of structures. Torsional divergence was at first related to aircraft wings because of their susceptibility to twisting off at excessive speeds of air flow (Simiu and Scanlan, 1986). [44] Liu, 1991, [49] found that when the wind flow occurred a drag, lift, and moment are created on the structure. The created moment cause a twist on the structure, and that lead to increase of the angle of incidence α. Later on, the increase in α will resulted in higher torsional moment with the increase of the wind velocity. If the structure hasn’t sufficient torsional stiffness in order to stand against this increasing moment, the structure will becomes unstable and will be twisted till failure. Simiu and Scanlan, 1986, [49]found that the torsional divergence depends on the structural flexibility and the manner in which the aerodynamic moments develop with twist; and didn’t depend on the ultimate strength of structure. Figure 3-26 explain the process.
Figure 3.26 a. Torsional divergence of an airfoil, b. Torsional divergence of Bridge deck (Picture from Liu, 1991)
3.2.7 Flutter
The flutter is a very serious issue in the bridges design. Now, it is well realized that failure of the Tacoma’s narrows bridge was due to the flutter. The term flutter has been used widely to describe different types of wind induced behaviors of bridges. The flutter was defined as a state of negative aerodynamic damping in which the deflection of the structure grows to high levels till failure was started. It is also known as classical flutter. [44]
3.2.8 Critical wind speeds for Flutter
When the critical wind speed expected to cause flutter is exceeded, the structure will become unstable and developed excessive deflections. Thus it is an important factor to be measured in the design of bridges. [43]
3.2.9 Buffeting
Buffeting is defined as the unsteady loading of a structure by velocity fluctuations in the incoming flow and not self-induced (Simiu and Scanlan, 1986). [44] The buffeting vibration was produced by the turbulence of air flow. There are two types of buffeting. The first is generated by turbulence in the airflow, and the second type is caused by disturbances generated by an upwind beside structure or obstacle. The first type of buffeting caused a noticed vertical and torsional motions of the bridge even at low speeds. The motion produced by buffeting causing a gradual transition to large amplitude torsional oscillations, which may lead to the failure of the bridge.
3.3 Seismic Damages of Bridges
Study effects of seismic damages on bridges to provide a strategy for undertaking bridge inspections following a significant earthquake and to give guidance on where to look for, and how to evaluate damage to atypical highway bridges.
Emergency works to open the bridges, or to clear roads or waterways below may sometimes commence very shortly after the earthquake. Even if total demolition is going to be the final fate of the bridge it is desirable to investigate, record and photograph the damage before the evidence is a destroyed. It is preferable that this work be done by an experienced bridge designer, who has experience in forensic investigation of seismic damage and who is not involved in the emergency work. [50]
3.3.1 Objectives of Seismic Damage Assessment of Bridges [51]
Tire overall objectives of seismic damage assessment are to
Minimise loss of life;
Minimise the economic loss to the region.
For the reading networks, the hierarchy of objectives will be to ensure the safety of:
Bridges known to be vulnerable, with potential for loss of life
Primary routes for the passage of emergency vehicles concerned with the saving of life and property
Primary routes for the passage of vehicles concerned with the distribution of essential supplies and restoration of essential services
Secondary route for the passage of emergency vehicles
All routes for general use.
3.3.2 STRATEGY AND INSPECTION LEVELS
To ensure that a route or network is safe for the public, bridges should be inspected following an earthquake of sufficient intensity to cause concern about the possibility of damage. All bridges within an area subjected to MM VIII intensity shaking or greater should be inspected. Two levels of inspection are appropriate:
A Preliminary Safely Check, conducted immediately following the earthquake to check for safely for immediate use and for obvious damage.
A Detailed Structural Check, which may or may not be required, and which would be conducted at some later time.
In most areas of the country, seismic screening of the State Highway bridges will have already identified those bridges likely to be most vulnerable to damage with potential to cause loss of life.
3.3.3 PRELIMINARY SAFETY CHECK
The fallowing order of priority should be considered for the Preliminary Safely Check of the bridges:
Inspect first those brides known to be most vulnerable with potential for loss of life (e.g. as identified by the seismic screenings), giving priority to those carrying the highest traffic volumes
Inspect all other bridges along the primary routes required for the passage of emergency vehicles concerned with the saving of life and property
Inspect all other bridges all other primary routes required for the distribution of essential supplies and restoration of essential cervices
Inspect all remaining bridges.
Each bridge should be examined quickly but with sufficient care to identify problems that could lead to collapse and compromise public safety.
Fortunately serious damage can often be detected at read level. Nevertheless the underside of the deck and the substructure should also be briefly examined.
At the bridge, the lines of the handrails, kerbs and centreline markings should be checked for horizontal and vertical discontinuities as these will be quick indicators of problems below. Differential settlement between piers and abutments, of any one support relative to the others, may indicate serious damage to substructure members. Also, it will alter the stress distribution in continuous superstructures which could lead to overstressing and damage at some sections.
Other indicators of problems that can be seen at deck level are:
Evidence of excessive movement of expansion joints during the earthquake;
Expansion joints closed up;
Knock-off devices at abutment backwalls displaced backwards and / or upwards by impact;
Spalling of kerbs and decks either side of expansion joints;
Buckling of handrails or traffic barriers.
If damage is found, several courses of action can be taken. The bridge may be:
Left open to the public unrestricted, but noted for a Detailed Structural Check at a later date;
Left open to the public but with restricted speeds and/or axle loads;
Left open only for emergency vehicles;
Closed until temporary repairs are completed or until shoring has been installed;
Closed indefinitely.
In deciding on what course of action to take, the engineer should take account of the risk of the bridge collapsing against the consequences of placing restrictions on it. That in turn will depend on the importance of the route and the alternatives available. The likely effect of aftershocks should also be considered.
It should be remembered that a bridge can sustain a great deal of superficial damage including loss of cover concrete without the vertical load-carrying capacity being affected too greatly.
Lastly, any damage discovered should recorded, photographed, and confirmed as recent and likely to have been caused by the earthquake.
3.3.4 DETAILED STRUCTURAL CHECK
3.3.4.1 Approach Embankments
High approach embankments on soft ground are notorious for settling or slumping in earthquakes. If settlement is associated with failure of underlying soils, especially liquefaction, then soil flow through the abutment is likely to have occurred.
If lateral displacement has occurred it can be detected by such evidence as:
Heave at the toe of the embankment;
Longitudinal cracking of the approach road surface;
Movement of the abutment;
Sand volcanoes and / or ground cracking on the flat ground.
Soil flow through the abutments will increase the lateral load on piles and it may have caused damage to them that can only be seen by excavation. If the rotational and lateral movements of the abutment can be quantified it may be possible to carry out a back analysis which gives an indication of the risk of pile damage. Abutments on raked pile groups cannot sustain much movement without damage.
Flow through the abutments can occur even where there are no approach embankments but where the natural ground is weak. There are many recorded cases of river banks moving closer together in earthquakes (usually but not necessarily associated with liquefaction).
3.3.4.2 Abutments
Apart from the problems with moving ground discussed above, abutments are vulnerable in other ways. Particularly with more recently designed bridges, the superstructures are often designed to act independently of abutments in an earthquake and large relative movements between the abutment and the superstructure can take place. If the intensity was sufficient to cause yielding in the substructure this relative movement could be greater than that provided for normal service. In such cases damage is likely to include:
Hammering and concrete spalling;
Failure of expansion joints or at least of their seals;
Knock-off devices displaced backward and/or upwards;
Linkage hardware distressed;
Shear keys damaged.
Such damage is considered to be acceptable, repairable and not necessarily cause for closing the bridge completely.
In the ideal situation, bearings should still cope with post-elastic movements but they should be checked carefully for damage.
Where the abutment is independent of longitudinal superstructure movements but provides lateral restraint, the mechanism for providing that restraint should be checked, i.e. check for damage to shear keys, linkage devices, mechanical dampers, etc.
3.3.4.3 Piers
Cracking of piers to a greater or lesser extent should be noted, and considered on a case-by-case basis for sealing, repair, encasing or replacing.
The piers are the visible part of the substructure and ideally are the locations chosen by designers for the development of plastic hinges in the design intensity earthquakes.
For modern bridges at least, spalling of cover concrete at the top or bottom of piers may indicate that the bridge has been subject to a design intensity earthquake and has yielded according to prediction.
If the reinforcing cage is largely intact and the core concrete properly confined, the vertical load capacity is likely to be adequate. Repairs will be required but it may be possible to carry them out with the bridge in service.
Piers should be checked for verticality. If they have moved out of plumb significantly during the earthquake the reason should be determined. If it is caused by yielding of the pier and displacement of superstructure then it may be straighten them before repair to yield zones are attempted. However, if piers are out of plumb and the superstructure is not displaced, there is a strong inference of foundation displacement.
3.3.4.4 Foundations
The foundations are the hidden part of the substructure and ideally, in modern (post-1972) bridges, have been designed with sufficient overstrength to ensure they remain elastic up to and after the bridge structure has started to form a collapse mechanism.
However, that design approach will not have been applied in older bridges, and even in modern bridges that ideal is not always achieved and piles can yield.
Tell tale signs are:
Large relative movements between piles and the soils;
Relative displacement between pile caps;
Piers out of plumb.
From each of these signs a deflection of the top of the pile can be measured and a back analysis using a range of soil parameters can be used to indicate the likelihood of pile yielding. The final check is to dig them out and examine them but usually that is not easy to do.
Vertical displacement of piles does not always mean that pile yielding has taken place, particularly if there are no raked piles in the group. Furthermore, temporary loss of bearing capacity (e.g. from liquefaction) does not necessarily mean that capacity is lost permanently.
Vertical displacement can have serious consequences for the superstructure andparticularly so for indeterminate (continuous) spans. The effect should be assessed.
For simply supported spans, the rotational capacity of bearings may have been exceeded because of the settlement.
For short deep continuous spans, distress could have been caused by small settlements that are only detected by taking levels. This should be considered as part of a thorough investigation.
3.3.4.5 Linkage Devices and Shear Keys
Linkage devices and shear keys are installed in bridges to limit relative movements between adjacent spans, and between spans and theirsupports. If they have been worked very hard by the earthquake they may have been damaged and require replacement.
To check the linkages properly it may be necessary to remove some for close inspection.
3.3.4.6 Expansion Joints
Expansion joints that have operated beyond their design range may be damaged and require repair or replacement. Many joints have very little capacity for lateral displacement and damage can be caused by very small movement.
3.3.4.7 Holding Down Bolts
Holding down bolts that have been subject to large transverse shears are likely to have yielded. That does not necessarily mean that replacement is required. Recent design practice is to provide no holding down bolts at all in most cases. Each case should be assessed on its merit.
3.3.4.8 Bearings
Bearings are vulnerable in earthquakes, particularly if they are also required to carry transverse shear.
(a) Elastomeric Bearings:
Bearings that have been deflected beyond their design shear may be ruptured internally, but usually damage manifests itself at the surface and can be seen.
Deep lead-rubber bearings are used for base isolation and can be subject to large lateral deflections. They should be carefully checked after an earthquake. If there is any suspicion of damage it may be necessary to remove one and check its capacity for continued performance.
Bearings not positively restrained in position may "walk". There is a recorded case of a lead-rubber bearing escaping from its keeper ring during an earthquake.
(b) Sliding Bearings:
If the design capacity for sliding has been exceeded, damage is likely.
(c) Pot Stay Bearings:
If pot stay bearing has failed in shear it will be obvious.
3.3.5 INVESTIGATION REPORT
The results of the investigation should be recorded in a report that includes:
An assessment of the ground acceleration at the site.
An assessment of how the loads were transmitted to and from the ground and their magnitude (i.e. trace the load paths).
A record, including photographs, of damage and permanent deformations.
One of the potential difficulties of investigating earthquake damage is confirming that all the damage observed occurred during the earthquake. If investigation is sufficiently soon after the event it is easier to differentiate between new and old damage.
Chapter Four
Seismic Isolation System
4.1 Basic Principles of Seismic Isolation for Bridges
Two important concepts have been considered in earthquake resistance design of the structures.
The first concept is to increase the capability of the structures to resist the earthquake load effects (which are mainly horizontal forces) or to boost the dynamic stiffness such as like the seismic energy dissipation capacity by adding damping systems (both devices and/or structural fuses). [52]
The second concept considered the seismic isolation systems to decrease the input load effects on the structures. Apparently, both concepts can be integrated to get an optimal design for the seismic flexible structures. [52]
From the structural response point of view of the community control, the seismic protective systems are usually classified as passive, active and semi-active systems. The passive control system formed of many different categories like energy dissipation systems, toned mass systems and the vibration isolation systems. [53]
The seismic isolation and energy dissipation systems offered effective techniques to improve the resist seismic forces of the structures. These techniques decrease the seismic forces by altering the stiffness and/or damping of the structures, while the conventional seismic design is required to provide additional strength and ductility to resist seismic forces. [54]
4.1.1 CLASIFICATION OF SEISMIC ISOLATION SYSTEMS
The seismic isolation systems of structures were usually used as method to separate the foundation of the structure from its superstructure. Two types of classification of the devices can be considered according to their location in the building and its operation principles.
According to the classification of the seismic isolation systems by their location in the building, isolators can be considered as external and internal types.
The external types are located outside the building and they are often fixed to the foundations.
The internal types are depending on the energy dissipation mechanisms.
Also the response control systems are classified according to their principles of action as active control systems, passive control systems and hybrid systems. [54, 55]
4.1.1.1 Active Control Systems
The mechanism of action of the active control systems was depending on providing of continuous energy from outside. The system will manage the acceleration, displacement or velocity of the structure. The active control systems were usually consisting of electronic components like computers, actuators and starters. The system changes its stiffness or the amount of action according to the severity of the ground motion. There are three major application forms for the active control systems:
Active Mass Damper: controlled by computer systems which affected by lateral forces affected by the actuator control force, the acceleration, displacement and velocity of the structure.
Active Variable Stiffness: In this system, there is no need for forming of actuator control force; it’s developed for utilization in case of strong ground motion.
Active Passive Composite Tuned Mass Dumper: it considered as hybrid structural control systems, which were developed recently, it depend on employment of both active and passive systems. [54, 55]
4.1.1.2 Passive Control Systems
The action of passive control systems did not use any external energy source. These systems could be able to control displacement for a certain limit. Passive control systems are consisted of dampers and isolators. Passive control systems have many types. [54, 55]
Irreversible displacement systems consist of balls or rolls (Figure 4-1).
Figure 4-1: Irreversible displacement system
Sliding systems: They consist of plenty of rolls placed vertical to each other or steel balls between the steel plates.
Plastic systems: depend on the plasticity of lead which facilitates energy absorption for seismic isolation (Figure 4-2).
Figure 4-2: Plastic system
Elastic systems: Rubber or neoprene dampers were usually used as examples for these systems. The isolation was achieved by insertion of dampers between the foundation and the columns.
Viscous systems: These systems consist of polymer or viscous liquid which fitted between two cross sections.
Kinematical systems: These systems consist of balls, rolls, elliptic balls and small columns with elliptic ends. The geometric shapes of these devices permit its return its original positions. Figure 4-3.
Figure 4-3: Kinematic-Elastic systems
Friction sliding systems: The principle of action depends on dissipating energy by the friction forces. These systems are used both as base isolation systems between the foundation and the columns, and as energy dissipating mechanisms in the superstructure. Figure 4-4(a).
Figure 4-4: New Zealand bearing system (a) Sectional details (b) Schematic diagram (c) Force deformation behavior
Another type of kinematical-elastic systems is the Hercules Systems (Figure 4-4(b). Hercules system was suitable isolator for the isolations of small and medium scale foundation of structures.
4.2 Types of Seismic Isolators
The strength of the structure in usual seismic design is measured to stand against earthquake forces. The base isolation technology act to separate the structure from the seismic ground motions, by separation of the structure from its foundation. The base isolation systems have the criteria of flexibility and energy absorbing capacity.
Currently the base isolation techniques are principally classified into three types:
Passive base isolation devices.
Compound base isolation with semi-active devices.
Compound base isolation with passive energy devices.
4.2.1 Types of Passive base Isolation Devices
Mud layer below the structure [56, 57]
Flexible first storey [57]
Roller bearings in foundations
Rubber layer as foundation support [57]
Laminated rubber bearing system [57]
New Zealand bearing system [58]
Resilient – friction base isolation system
Electric de-France system
Sliding resilient- friction system
High damping rubber bearing
Pure friction system. [59]
Friction pendulum system
Spring type systems [59]
Sleeved pile isolation system [59]
Rocking systems [59]
Base isolation using Geo- Synthetic materials [60]
BS cushion [61]
4.2.2 COMPOUND ISOLATION SYSTEM WITH SEMI-ACTIVE DEVICES [62]
Compound isolation system adopted the passive isolation systems with the semi-active or active controlling systems. The analyses showed that considerable diminution of the structure accelerations (up to 50%) can be achieved with the compound system. [62]
4.2.3 COMPOUND BASE ISOLATION WITH PASSIVE ENERGY DEVICES
These types of devices developed whole over the world; it can be fitted at the foundation of the structure or in superstructure at suitable places. They can be mixed with passive base isolation devices.
4.3 Advantages/Disadvantages of Seismic Isolation
The base isolation structures, besides being more efficient, safe, functional and economical, it may achieve new design prospects in seismic regions. [63]
4.3.1 Benefits and savings of seismic isolation system [64]
Base isolation reduces number of component of the structure with less ductile details.
Crawl spaces or basements which created can be of multiple benefits such as generating additional income from a car park and flexibility for the future planning and development.
Protection of the contents found inside the building from damage.
Protection of the integrity of the internal structures of the structures.
Building being safer for habitants.
Life activities could be Continue after seismic events.
4.3.2 Maintenance of seismic isolation system [64]
Unlike the belief, seismic isolation devices need no maintenance, during the life of the structure.
After any seismic event isolation systems should be inspected to ensure that its components are still working effectively.
There is no need to replace the isolation devices after an earthquake unless it was damaged; some devices recommend for removal for testing purposes.
Repairing costs after seismic events will be lower, because the structure is protected from major damage.
The effectiveness of seismic isolation in the protection of structures is not only of the external structure but also the contents and in maximizing the potential for immediate business continuation.
4.3.3 Historic and heritage structures [64]
Traditional earthquake strengthening methods can detract from the aesthetics of historic and heritage buildings
Many heritage buildings are appropriate for base isolation
Base isolation can be retrofitted to achieve earthquake protection without compromising the aesthetic integrity of the building
4.3.4 Economic and social benefits [64]
If functions and offered services of the buildings and places of work were continued and secure the employee; then they will be able to survived and share later on in the reconstruction and recovery of their community after the earthquake.
4.3.5 Advantages of Base isolation systems [65, 66]
Reduced the seismic protection requirements of structure, so that reducing the cost of building.
Reduce the displacements of constructions during an earthquake.
Improves safety measures of buildings.
Reduced the damages occurred due to an earthquake. That will facilitate the performance of structure after seismic event; and maintains functions after an earthquake or speed up the recovery of function.
Improve the functions of structure under seismic effect.
Protection of property.
Prevent furniture and fixtures from overturning and falling
Ensure an escape route
Ease anxiety of habitants.
Chapter Five
Calculation
5.1 Structure Analyzed
A 4-aperture concrete bridge was analyzed. The bridge consists of two collections, 3 lamellar piles, prestressed concrete beams and reinforced concrete slab. The geometric characteristics of the bridge are shown in Figure 5-1 and Figure 5-2
Figure 5-1: Longitudinal section
Figure 5-2: Cross section
5.2 Purpose of the Study Implemented
In this paper, the behavior of a structure subjected to seismic action, using accelerograms, was studied in two situations:
The first situation in which classical neoprene support appliances were used.
The second situation where neoprene isolators were used.
5.3 Analytical Structure
To obtain the seismic response of the structure, a 3D model with finite elements was made according to the figure 5-3 below.
Figure 5-3: Structured discretization
5.3.1 Loads we take into account.
Load patterns are defined in "Load Patterns". After charging is defined, they are hooked up in Load Cases load cases to run specific analyzes. Load cases can be multiplied by different load factors and can form a Load Combinations load combination.
Only permanent loads and seismic loading were taken into account.
5.3.2 Results
Formulas for Calculation:
Determine the mass of the deck
[ton]
Where:
M(deck)=mass of the deck
g = ground Acceleration
Assume the period of the isolation system.
Tisolation = 2.5 (S)
Where:
Tisolation= period of isolation system
The angular frequency
Determine Stiffness of the isolation
Where:
Kisolation = Stiffness of the isolation
Take into account the Damping coefficient
ζ= 10%
Where:
ζ= Damping coefficient
The rigidity of the compression of isolation
ζ
Where:
The supports used are: 12 cm on the Abutment and 14 cm Pier classical.
Table 5-1: Period of Isolation
For the load case (time history) we used in sap2000 the Acceleration as shown in the figures below.
Figure 5-4: Acceleration INCERC-VRANCEA1977
Figure 5-5: Acceleration INCERC-VRANCEA1990
Figure 5-6: Acceleration Drumul Sarii-VRANCEA1990
By using SeismoSignal software we get the Amplitude of power spectrum for each Acceleration as shown in the figures below.
Figure 5-7: Acceleration power spectrum INCERC – Vrancea 1977
Figure 5-8: Acceleration power spectrum INCERC – Vrancea 1990
Figure 5-9: Acceleration power spectrum Drumul Sării – Vrancea 1990
By using sap2000 software we get the time period for Structure with classic supports (without Isolation)
Table 5-2: Mode of time period
Figure 5-10: Structure with classic supports Modal 1 T1=0.72
Figure 5-11: Structure with classic supports Modal 2 T1=0.72
Figure 5-12: Structure with classic supports Modal 3 T1=0.56
Figure 5-13: Structure with classic supports Modal 4 T1=0.36
Figure 5-14: Structure with classic supports Modal 5 T1=0.3
The time period for Structure with base Isolation supports (with Isolation 3 second)
Figure 5-15: Structure supports with Isolation 3 second Modal 1 T1=2.87
Figure 5-16: Structure supports with Isolation 3 second Modal 2 T1=2.83
Figure 5-17: Structure supports with Isolation 3 second Modal 3 T1=2.76
Figure 5-18: Structure supports with Isolation 3 second Modal 4 T1=0.35
Figure 5-19: Structure supports with Isolation 3 second Modal 5 T1=0.31
Table 5-3: Structure with and without Isolation system
After modify the modal in sap2000 and put the Isolation system with (2,2.5and,3second)We can see the difference in bending moment and displacement as shown in the figures below
Figure 5-20: The difference between Classic supports and supports with Isolation system (2second) in bending moment (X-Direction) pier1 (Drumul Sarii)
Figure 5-21: The difference between Classic supports and supports with Isolation system (2second) in bending moment (X-Direction) pier3 (Drumul Sarii)
Figure 5-22: The difference between Classic supports and supports with Isolation system (2second) in bending moment (Y-Direction) pier1 (Drumul Sarii)
Figure 5-23: The difference between Classic supports and supports with Isolation system (2second) in bending moment (Y-Direction) pier3 (Drumul Sarii)
Figure 5-24: The difference between Classic supports and supports with Isolation system (2second) in bending moment (X-Direction) pier1 (INCERC – Vrancea 1977)
Figure 5-25: The difference between Classic supports and supports with Isolation system (2second) in bending moment (X-Direction) pier3 (INCERC – Vrancea 1977)
Figure 5-26: The difference between Classic supports and supports with Isolation system (2second) in bending moment (Y-Direction) pier1 (INCERC – Vrancea 1977)
Figure 5-27: The difference between Classic supports and supports with Isolation system (2second) in bending moment (Y-Direction) pier3 (INCERC – Vrancea 1977)
Figure 5-28: The difference between Classic supports and supports with Isolation system (2second) in bending moment (X-Direction) pier1 (INCERC – Vrancea 1990)
Figure 5-29: The difference between Classic supports and supports with Isolation system (2second) in bending moment (X-Direction) pier3 (INCERC – Vrancea 1990)
Figure 5-30: The difference between Classic supports and supports with Isolation system (2second) in bending moment (Y-Direction) pier1 (INCERC – Vrancea 1990)
Figure 5-31: The difference between Classic supports and supports with Isolation system (2second) in bending moment (Y-Direction) pier3 (INCERC – Vrancea 1990)
Figure 5-32: The difference between Classic supports and supports with Isolation system (2second) in Displacement (X-Direction) pier1 (Drumul Sarii)
Figure 5-33: The difference between Classic supports and supports with Isolation system (2second) in Displacement (X-Direction) pier3 (Drumul Sarii)
Figure 5-34: The difference between Classic supports and supports with Isolation system (2second) in Displacement (X-Direction) pier1 (INCERC – Vrancea 1977)
Figure 5-35: The difference between Classic supports and supports with Isolation system (2second) in Displacement (X-Direction) pier3 (INCERC – Vrancea 1977)
Figure 5-36: The difference between Classic supports and supports with Isolation system (2second) in Displacement (X-Direction) pier1 (INCERC – Vrancea 1990)
Figure 5-37: The difference between Classic supports and supports with Isolation system (2second) in Displacement (X-Direction) pier3 (INCERC – Vrancea 1990)
Comparison between Classic supports and supports with Isolation (2, 2.5 and 3 second) in bending moment (X and Y Direction) and in displacement (X Direction) in pier (1) and pier (3)
Table 5-4: Bending moment (Y-Direction) in pier (1) and pier (3)
(Drumul Sarii-Vrancea-1990)
Table 5-5: Bending moment (Y-Direction) in pier (1) and pier (3)
(INCERC-Vrancea-1990)
Table 5-6: Bending moment (X-Direction) in pier (1) and pier (3)
(Drumul Sarii-Vrancea-1990)
Table 5-7: Bending moment (X-Direction) in pier (1) and pier (3)
(INCERC-Vrancea-1977)
Table 5-8: Bending moment (X-Direction) in pier (1) and pier (3)
(INCERC-Vrancea-1990)
Table 5-9: Displacement (X-Direction) in pier (1) and pier (3)
(Drumul Sarii-Vrancea-1990)
Table 5-10: Displacement (X-Direction) in pier (1) and pier (3)
(INCERC-Vrancea-1977)
Table 5-11: Displacement (X-Direction) in pier (1) and pier (3)
(INCERC-Vrancea-1)
Chapter Six
Conclusion
Comparing the structure before and after using the seismic isolation technique with 2, 2.5 and 3 seconds; the obtained reduction coefficient ranged 23% – 81%, this difference in reduction coefficient was due to strength of seismic action, vibration period content of the accelerograms and the vibration period of the isolation system.
When we shift the vibration period from 0.7 close to 2, 2.5 and 3 seconds in the case of the accelerogram from Drumul Sarii (Vrancea source, 1990) the reduction coefficient is around 70%, because the structure is far from the pick point on the seismic action as shown in Figure 5-9 and Figure 5-8; also for the accelerogram recorded at INCERC (Vrancea source, 1990).
In the case of the accelerogram recorded at INCERC (Vrancea source, 1977) we get reduction coefficient about 23% because the seismic action is more powerful due to the far distance of structure from the pick point on seismic action as shown in Figure 5-7.
Generally the seismic isolation achieve the purpose for its use and instillation; and could save the structure and preserve people safety which is an important target for the structure designing engineers.
We can implement the seismic isolation technique to ensure the construction of the earthquake resistant structures; moreover, we can recommend confidently the use of seismic isolation system in seismic area.
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