Εφαρμογή σε Στατικώς Αόριστη Γέφυρα Ο.Σ. - PDF

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Ε Θ Ν Ι Κ Ο Μ Ε Τ Σ Ο Β Ι Ο Π Ο Λ Υ Τ Ε Χ Ν Ε Ι Ο ΣΧΟΛΗ ΠΟΛΙΤΙΚΩΝ ΜΗΧΑΝΙΚΩΝ - ΤΟΜΕΑΣ ΓΕΩΤΕΧΝΙΚΗΣ Ηρώων Πολυτεχνείου 9, Πολυτεχνειούπολη Ζωγράφου Τηλ: , Fax: ,

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Ε Θ Ν Ι Κ Ο Μ Ε Τ Σ Ο Β Ι Ο Π Ο Λ Υ Τ Ε Χ Ν Ε Ι Ο ΣΧΟΛΗ ΠΟΛΙΤΙΚΩΝ ΜΗΧΑΝΙΚΩΝ - ΤΟΜΕΑΣ ΓΕΩΤΕΧΝΙΚΗΣ Ηρώων Πολυτεχνείου 9, Πολυτεχνειούπολη Ζωγράφου Τηλ: , Fax: , ΠΡΑΞΗ: «ΘΑΛΗΣ- ΕΜΠ: ΠΡΩΤΟΤΥΠΟΣ ΣΧΕΔΙΑΣΜΟΣ ΒΑΘΡΩΝ ΓΕΦΥΡΩΝ ΣΕ ΡΕΥΣΤΟΠΟΙΗΣΙΜΟ ΕΔΑΦΟΣ ΜΕ ΧΡΗΣΗ ΦΥΣΙΚΗΣ ΣΕΙΣΜΙΚΗΣ ΜΟΝΩΣΗΣ» MIS Επιστημονικός Υπέυθυνος: Καθ. Γ. ΜΠΟΥΚΟΒΑΛΑΣ ΔΡΑΣΗ 7 Εφαρμογή σε Στατικώς Αόριστη Γέφυρα Ο.Σ. ΠΑΡΑΔΟΤΕΑ: Πιλοτικός σχεδιασμός με χρήση «φυσικής» σεισμικής μόνωσης (Π7β) Ιούλιος 2015 NATIONAL TECHNICAL UNIVERSITY OF ATHENS SCHOOL OF CIVIL ENGINEERING GEOTECHNICAL DEPARTMENT 9 Iroon Polytechniou str., 15780, Zografou Campus, Zografou, Greece Tel: , Fax: , PROJECT: «THALIS-NTUA: INNOVATIVE DESIGN OF BRIDGE PIERS ON LIQUEFIABLE SOILS WITH THE USE OF NATURAL SEISMIC ISOLATION» MIS: Coordinator: PROF. G. BOUCKOVALAS WORK PACKAGE 7 Application to Statically Indeterminate R.C. Bridges DELIVERABLES Pilot Study of a Statically Indeterminate R.C. Bridge pier with the new methodology of natural seismic isolation (D7b) July 2015 Table of Contents 1. Introduction Brief description of the natural seismic isolation methodology Application to a statically indeterminate RC bridge Conventional bridge design Tolerable settlements and rotations for the statically indeterminate bridge system with shallow foundation Seismic scenarios Design of the shallow foundation Design of the improved crust Dynamic impedance functions for footings on liquefiable soil Bridge design Operational limit state No-Liquefaction case: Seismic scenario B (T ret=225y) Pier reinforcement Pier confinement Bearings and Expansion Joints Verification Bearings verification Expansion joints verification Liquefaction case: Seismic scenario A (T ret=1000y) Horizontal liquefaction-induced differential displacements Summary and Conclusions References INTRODUCTION 1. Introduction This Technical Report constitutes Deliverable #7 of the Research Project titled: THALIS-NTUA (MIS ) Innovative Design of Bridge Piers on Liquefiable Soils with the use of Natural Seismic Isolation carried out under the general coordination of Professor George Bouckovalas, NTUA (Principal Investigator). It presents the work and the corresponding results on tolerable ground deformations carried out for Work Package WP7, titled: Application to statically indeterminate RC bridges The Scope of Work Package WP7, has been described in the approved Research Proposal as follows: The aim of this WP is to explore the feasibility of the proposed new design methodology, and the resulting advantages over conventional design methods, in the case of a statically indeterminate RC bridge (with continuous box-girder type deck). The main work tasks required to achieve this aim are the following: (a) Initially, the allowable foundation movements (settlements and rotations) will have to be established for a statically indeterminate RC bridge system, in terms of the tolerable damage and serviceability level (e.g. driving discomfort, repairable damage, irreparable damage) and the anticipated seismic hazard level (e.g. seismic excitation with 90, 450 or 900 years return period). The allowable foundation movements will result from a joint evaluation of: an extensive survey of relevant codes and guidelines (e.g. Eurocode 2-Part 2, Eurocode 8-Part 2, Eurocode 7, MCEER & FHWA-chapter 11.4), examples of actual bridge performance during recent earthquakes, and parametric (theoretical and/or) experimental studies of various bridge components (e.g. piers, bearings) under static and cyclic-dynamic loading. (b) Next, an actual bridge will be selected, with continuous deck system, long spans 2 INTRODUCTION between piers (in excess of 40m) and extensive liquefiable soil layers underneath one or more of the bridge piers. Note that, following an initial survey, we have already identified a number of such bridges constructed as part of the Egnatia Motorway in Northern Greece, such as the large bridge on Nestos River, with approximately 500m length, and a number of shorter bridges along the motorway connection with the City of Serres. The piers of the selected bridge will be (re-)designed using the conventional foundation approach, i.e. pile groups with ground improvement between and around the piles. (c) Finally, the static and seismic design of this bridge will be repeated with the new methodology of natural seismic isolation (i.e. shallow foundation and partial improvement of the top part only of the liquefiable soil), in connection with the allowable foundation movements which were established in work task (a) above. The comparative advantages and limitations of the new design methodology, relative to the conventional one, will be consequently evaluated on the basis of structural performance, as well as cost, criteria. The work described herein corresponds to Work task (b) above. It has been carried out by the following members of the Aristotle University of Thessaloniki (Department of Civil Engineering) Research Team: Andreas Kappos, Professor Anastasios Sextos, Associate Professor 3 BRIEF DESCRIPTION OF THE NATURAL SEISMIC ISOLATION METHODOLOGY 2. Brief description of the natural seismic isolation methodology Motorway bridge piers are often founded on fluvial and alluvial soil deposits consisting of loose, saturated sands and silty sands as they frequently cross bodies of water such as rivers, streams and lakes. These deposits are generally weak and/or soft enough and conventional design approaches require the construction of deep foundations to avoid excessive settlements or damages due to phenomena such as erosion and scour. Furthermore, in seismically active environments, loose, saturated soil deposits are susceptible to soil liquefaction. In common usage, liquefaction refers to the loss of shear strength in saturated, cohesionless soils due to the build-up of pore water pressures during dynamic loading. The common practice for designing bridges built on liquefaction-susceptible soils is to construct deep (pile) foundation systems along with extensive improvement of the surrounding soil (Figure 2.1, left). Based on the current state of knowledge, deep foundations appear to be the only adequately safe, albeit conservative, solution, resulting to a significant increase in the project cost, compared to cases where shallow foundations could be used [FHWA (1982), FHWA (1987), Sargand and Masada (2006)]. Conventional seismic isolation methods aim to mitigate structural damage by isolating the structure from earthquake ground motions through energy absorption and modification of the structural properties (using for instance, lead rubber, steel neoprene/rubber and fiber-reinforced, elastomeric bearings, combined sliding or elastomeric bearings with fluid dampers, as well as passive and active mass damping systems). The idea proposed herein suggests a fluidizable foundation isolation system intentionally designed to directly reduce the induced seismic ground motions transmitted to the structure (Figure 2.1, right). The underlying physical concept is that shear waves can hardly propagate through a fluidized medium; hence, a liquefied soil 4 BRIEF DESCRIPTION OF THE NATURAL SEISMIC ISOLATION METHODOLOGY layer may act as a seismic isolation barrier to the upward propagating seismic waves. To maintain the bearing capacity of the shallow foundations of the bridge, a nonliquefiable surface crust needs also to be assured, in the form either of a nonliquefiable (e.g. clay) layer or an improved ground zone. Given the particular characteristics of the proposed methodology, it is evident that the current seismic code framework is not adequate for designing bridges with shallow foundations on liquefaction-susceptible soils. Hence, the design process needs to be tailored to the salient features of the novel soil isolation concept, while at the same time complying with the legislative requirements of seismic code provisions. The procedure given in detail in Deliverable 7 [WP07] is applied herein for the case of a statically indeterminate RC bridge. This methodology involves initial design of the superstructure and a shallow foundation according to modern seismic codes (the Eurocodes are used in the present study) with due tailoring to account for the effect of liquefaction on the design seismic loads and displacements. Figure 2.1. Bridge design on liquefaction susceptible soils (a) common practice (left) and (b) according to Bouckovalas et al. (2014a) (right). Σχήμα 2.1. Σχεδιασμός γεφυρών σε ρευστοποιήσιμα εδάφη (α) συμβατική προσέγγιση (αριστερά) και (β) σύμφωνα με τη μεθοδολογία των Bouckovalas et al. (2014a) (δεξιά). 5 BRIEF DESCRIPTION OF THE NATURAL SEISMIC ISOLATION METHODOLOGY Along these lines, the aim of the proposed research is to present a novel methodology for seismic design of low cost bridge foundations on liquefiable soils underlain an intact crust. Figure 2.2 presents a flow chart of the proposed methodology. The key milestones with brief reference to the associated Deliverables and Work Packages of the current project are listed below: (a) Conventional design of the selected bridge founded on liquefaction-susceptible soil. Based on Eurocode provisions, appropriately designed pile groups are used along with extensive improvement of the surrounding soil [Deliverable 7a, WP07]. (b) Analytical estimation of the tolerable settlements and rotations of the studied bridge. Tolerable settlements and rotations are defined based on performance criteria associated with the acceptable damage level at the bridge [Deliverable 7b, WP07]. (c) Estimation of the seismic ground motion (PGA, PGV and design spectra) considering the non-linear response of the liquefied soil layers [Deliverable 4, WP04]. (d) Analytical expressions for the frequency-dependent parameters of the soil springs and dashpots which will have to be attached at the base of the superstructure in order to simulate the interaction of the foundation with the pre-liquefied and the post-liquefied subsoil [Deliverable 5, WP05]. (e) Bridge design considering static, seismic and liquefaction-induced horizontal differential displacements between the abutments and the adjacent piers [Deliverable 7c (presented herein), WP07]. The application of the novel methodology for the case of a statically indeterminate RC bridge is presented in the following. 6 BRIEF DESCRIPTION OF THE NATURAL SEISMIC ISOLATION METHODOLOGY Figure 2.2. Flow chart of the proposed methodology for bridge design on liquefaction-susceptible soils. Σχήμα 2.2. Διάγραμμα ροής της προτεινόμενης μεθοδολογίας σχεδιασμού γεφυρών σε ρευστοποιήσιμα εδάφη. 7 3. Application to a statically indeterminate RC bridge For the purposes of this research project, a typical three-span bridge of Egnatia Motorway having a total length of 99.0m is analysed. The two outer spans have a length of 27.0m each, while the middle span is 45.0m long. The slope of the structure along the bridge longitudinal axis is constant and equal to 7% ascending towards the west abutment. The deck consists of a 10m wide, prestressed concrete box girder section and the two piers are designed with a solid circular reinforced concrete section of diameter equal to 2.0m (Figure 3.1). Both piers are monolithically connected to the deck. The heights of the left and the right pier are 7.95m and 9.35m, respectively. A full description of the studied bridge is available in Deliverable 7a [WP07]. Figure 3.1: Transverse section at the location of the bridge pier. Σχήμα 3.1: Εγκάρσια τομή στη θέση του βάθρου της γέφυρας. 8 3.1 Conventional bridge design Based on the geotechnical study [Deliverable 4, WP04] the soil profile at the site of interest is located within the bed of Strymonas River in Greece and consists mainly of river deposits. More precisely, loose, liquefiable silty sands and soft clays are met while the ground water table is located on the ground surface, a fact that is further enhancing the liquefaction susceptibility. Due to the high uncertainty associated with the liquefaction phenomenon, verification against liquefaction was performed for two possible seismic scenarios corresponding to a liquefaction case and a non-liquefaction case. Namely: Seismic scenario A (liquefaction case): Mw=7.0, PGAb=0.32g (T ret = 1000yr) Seismic scenario B (non-liquefaction case): Mw=6.7, PGAb=0.22g (T ret = 225yr) Results from both scenarios indicated high risk of extended liquefaction within a depth of 0.0 to 20.0m. Therefore, the subsoil is improved by installing gravel piles through vibro-replacement. To avoid the liquefaction in the selected geotechnical site, a minimum replacement rate as=19.6% is needed, corresponding to a quadratic gravel pile grid of diameter D=0.80m with axial distance S=1.60m and length L=24m. As previously mentioned, the common practice for designing bridges built on liquefaction-susceptible soils is to construct deep (pile) foundation systems. In this case, a 3 3 pile group of 15.0m long piles (D=1.0m) was adopted connected with a m pile cap. Linear elastic static and response spectra analyses were then performed for the static and seismic loads acting on the bridge. A full description of the loads is available in Deliverable 7a [WP07]. The finite element model of the studied bridge is illustrated in Figure 3.2. It was found that 176Ø25 and 2Ø16/7.5 are required for the longitudinal and the transverse pier reinforcement, respectively. Finally, two sets of elastomeric bearings with dimensions 500x600mm and thickness of rubber equal to 110mm were used at each deck end and an expansion joint AGFLEXJ 140 (±70) or similar was chosen based on displacement calculation or an expansion joint AGFLEXJ 200 (±100) based on gap calculation. A full description of the bridge design can be found in Deliverable 7a [WP07]. 9 Figure 3.2: Finite element model of the bridge. Σχήμα 3.2: Μοντέλο πεπερασμένων στοιχείων της υπό μελέτη γέφυρας. 3.2 Tolerable settlements and rotations for the statically indeterminate bridge system with shallow foundation According to the proposed methodology tolerable settlements and rotations have to be defined for the studied bridge. Tolerable settlements and rotations were derived using nonlinear static analysis. More precisely, a predefined pattern (Δ) of displacements (settlements, ρ) and rotations (around the x (θx) and y (θy) bridge axes) are applied at the base of the piers until the bridge collapses. This pattern was defined assuming that settlements and rotations triggered from the liquefaction phenomenon will act as permanent loads in the structure after the earthquake. The following combinations were applied: Δ = ρ ± θy(ρ) ± 0.3θx(ρ) [3.1] Δ = ρ ± θx(ρ) ± 0.3θy(ρ) Rotations θy and θx were defined as a function of the imposed settlement ρ according to the empirical equation: θx = θy = 0.05 ρ [3.2] where rotations θy and θx are expressed in [deg] while settlement ρ is expressed in [cm]. 10 Settlement, ρ (m) APPLICATION TO A STATICALLY INDETERMINATE RC BRIDGE Tolerable settlements were then defined based on performance criteria associated with the acceptable damage level at the bridge. The performance criteria adopted in this case are presented in Figure 3.3. Specifically, for settlements (ρ) smaller than 0.08m, no damage is expected in the bridge as it responds in the elastic range. For settlements in the range 0.08 ρ 0.15m, minor damage is expected, while for settlements in the range 0.15 ρ 0.20m, moderate damage is expected. Finally, the bridge collapses for imposed settlements greater than 0.20m. In this case, a value of 0.15m was adopted for the settlements at the base of piers corresponding to minor damage. By further assuming a safety factor of 1.15, the tolerable settlement was set equal to 0.15/1.15=0.13m. A more detailed description of the adopted procedure is available in Deliverable 7b [WP07] Elastic Minor Moderate Collapse Rotational ductility, μ θ Right Pier (Base) Right Pier (top) Left Pier (Base) Figure 3.3. Applied settlements at the base of piers as a function of rotational ductility. Σχήμα 3.3. Επιβαλλόμενες καθιζήσεις στη βάση των στύλων συναρτήσει της πλαστιμότητας στροφών. 3.3 Seismic scenarios The next step of the proposed methodology requires the generation of design spectra for liquefiable soils for the site of interest. Based on the geotechnical study [Deliverable 4, WP04], response spectra were generated for the two seismic scenarios A (1000yr) and B (225yr). For each scenario, a suite of seven (7) earthquake motions, recorded on bedrock outcrop and having the target magnitude, was selected and properly scaled, for the average response spectrum to match as closely as possible the 11 Sa (m/s2) APPLICATION TO A STATICALLY INDETERMINATE RC BRIDGE design spectra of Eurocode 8 for soil type A, for peak ground acceleration at the bedrock outcrop PGAb = 0.32g (Scenario A) and PGAb = 0.22g (Scenario B), respectively. Subsequently, one-dimensional, nonlinear site response and liquefaction analyses were performed and the peak intensity measures (PGA, PGV) and the mean 5% damped elastic spectra were derived at the free ground surface. These spectra were then matched with those prescribed in Eurocode 8. The analytical procedure is described in detail in Deliverable 4 [WP04]. Figure 3.4 presents the elastic response spectra derived for the two examined seismic scenarios for the site of interest. The significant reduction of the ground surface acceleration observed for the liquefaction case (Tret=1000y) is attributed to the presence of the liquefiable soil layer. The shear waves can hardly propagate through the fluidized medium, hence, the liquefied soil layer acts as a seismic isolation barrier to the upward propagating seismic waves. Furthermore, the improved crust, located under the footing of the pier, seems to have little impact on the response of the soil surface which is dominated by the liquefiable layer. The resulting response spectra characteristics are then matched to the Eurocode 8 elastic spectra (Tret=225y with type D and Tret=1000y with type C). The spectral parameters are summarized in Table 3.1. It is further noted that the response spectrum of the vertical component was derived based on Eurocode 8 provisions Tret=225y Tret=1000y T(s) Figure 3.4. Elastic response spectra for the two examined seismic scenarios (T ret=225y and T ret=1000y). Σχήμα 3.4. Ελαστικά φάσματα απόκρισης για τα δύο εξεταζόμενα σεισμικά σενάρια (T ret=225y και T ret=1000y). 12 Table 3.1. Elastic spectra characteristics for the two examined seismic scenarios. Πι νακας 3.1. Χαρακτηριστικά ελαστικών φασμάτων απόκρισης. Spectrum characteristics T ret =225 years T ret =1000 years EC8 elastic soil Type D Type C Case No liquefaction Liquefaction S T B (sec) T C (sec) T D (sec) 2 2 a g A g η Design of the shallow foundation The innovative idea studied herein is that the existence of a surface crust of nonliquefiable soil (e.g. clay, dense sand and gravel, or partially saturated-dry soil) with sufficient thickness and shear strength may mitigate the consequences of liquefaction in the subsoil, to such an extent that t
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