ASSESSMENT AND REHABILITATION OF EXISTING STRUCTURES ICE LEARNED EVENT SULTAN QABOOS GRAND MOSQUE LECTURE HALL (MUSCAT SULTANATE OF OMAN)

Transcription

1 ASSESSMENT AND REHABILITATION OF EXISTING STRUCTURES ICE LEARNED EVENT SULTAN QABOOS GRAND MOSQUE LECTURE HALL (MUSCAT SULTANATE OF OMAN) Speaker: (BSc, MSc, PE, CEng. MICE)

2 Speaker s Background Bachelor of Science in Civil Enegineering (Bsc CEng) Master of Science in Structural Engineering (Msc SEng) Professional Engineering Degree (Italian Board of Engineers) 7+ Years of Professional Experience in Structural Analysis and Design of Bridges 2 Years in Italy (Rome) 5+ Years in The Sultanate of Oman (Muscat) Particular Interests: Seismic Retrofitting, Wind-Induced Vibrations and Rehabilitation Member of the following Institutions: Institution of Civil Engineers (ICE), Oman Society of Engineers (OSE) Italian Board of Professional Engineers

3 Table of Content Part 1 Definition of some Key Words Around the world: Lessons to be Learned Guide-lines for Assessment and Rehabilitation Programmes Common Deficiencies in Existing Structures Typical Structural Assessment Tests Part 2 Study case: Analysis of a Historical Bridge Site Activities: Topographic, Thermal, Static and Dynamic Tests Desk Activities: Post-Processing of Collected Data Setting-Up of a Reliable Mathematical Model Possible Structural Solutions and Main Conclusions

4 PART 1

5 Definition of Key Words Assessment: The act of judging or deciding on the actual conditions of an existing structure in order to plan future rehabilitation activities Rehabilitation: To restore to a condition of good health under the main applied loads Retrofitting: Modifications of an existing structure to make it more resistant under the applied loads (earthquake, fire, traffic load, etc.) Degradation: Process of damaging or ruining something (material, mechanical) Failure: Loss of load-carrying capacity of a component or member within a structure or of the structure itself The above definitions are very well understood by every engineer, at least theoretically. But WHY do we still have to face collapses and casualties???

6 Earthquake in Italy Central Area ( ): Catastrophic Consequences Peak Ground Acceleration: 0.45 g Ground Motion: 300 mm/s Depth of Epicenter: 4.2 km Number of Casualties: 300 people Peak Ground Acceleartion: 0.18 g (from Oman Highway Design Manual, OHDS 2010) MUSANDAM Event of

7 Before After It has been estimated that the cost of new constructions and repairs (after the earthquake) is much greater than the cost of the rehabilitation and retrofitting measures that would have been spent before the seismic event, without counting the number of casualties involved.

8 Designing to Withstand Natural Forces: understand the territory in which we operate USA: bridges are classified as structurally deficient and need to be rehabilitated and retrofitted Corrosion of PT Cables Extensive Cracking Increase of Traffic Loads and Frequences Tacoma Narrows Bridge Wind-induced Collapse (1940) Wind-induced effects not included in the original design Aerodynamic Forces induced by wind actions Pedestrian Bridge (China) Intensity of the Design Pedestrian Load? Were Dynamic Effects accounted for? Second Order Effect? Eulerian Buckling? Construction Details? Brand new structures can be potentially classified as STRUCTURALLY DEFICIENT Design & Construction Mistakes

9 Video 01: ROAD EMBANKMENT

10 Video 02: PEDESTRIAN BRIDGE

11 Video 03: TACOMA NARROWS BRIDGE

12 An operating highway bridge facing severe damages Most Stressed and Affected Zone of the Bridge Deck Vehicles are still running on it Longitudinal Tension Bars completely exposed Absence of Concrete Cover Stirrups broken Degradation of large portions of the concrete beam

13 General Guidelines for Assessment and Rehabilitation Programmes VISUAL INSPECTION (Designers and Client: Kick-Off Site Inspection) DOUMENTS COLLECTION (As-Built Drawings, Tech. Reports, Material Test Certificates) BRAINSTORMING (Understand Causes of Destress, Make Reasonable Assumptions) MATHEMATICAL MODEL (Prepare a numerical model as close as possible to the real structure) STRUCTURAL RESPONSE (Compare the Math Model with the Real Structure to validate the former one) FINAL DECISIONS AND PRACTICAL ACTIONS

14 Typical Deficiencies in Existing Buildings and Retrofitting Measures Retrofitting of a Column to Beams Connection Bending Retrofitting using Fiber Composites Shear Bending Section A-A (Bending) Section C-C (Shear) Retroftting Measures at Support (Shear) and Mid-Span (Flexure) using Fiber Reinforced Composites

15 Common Tests Executed on Existing Structures Scanner: check rebar position inside concrete Cover Meter: identify concrete cover Cores: extract concrete cores for Carbonation and Compressive Tests Crack Meter: measure crack width Core after Compressive Crushing Test Core Extracted from the Column Crack Pattern over the investigated beam Smidth Hammer Test

16 MOVE ON AND MOVE ON AND Let s exhamine a real bridge...

17 PART 2 STUDY CASE: STRUCTURAL HEALTH CHARACTERIZATION OF AN OLD RIVETED IRON BRIDGE 1 of 15

18 Location of the Investigated Bridge Year of Construction: 1930 Vertical Strut Top Chord Bottom Chord 2.00 m 3.50 m

19 Actual Condition of the Superstructure Bottom View Top View 5.50 m Concrete Deck Main Girders Wind Bracings Truss System Secondary Beams Detail of Top Chord Severe Damage due to Corrosion Detail of Girder Connection

20 Absence of Wing Walls Actual Condition of the Sub-Structure Abutment: Side View Deck-to-Abutment Connection: Bottom View Settlements of Approaching Road Wind Bracings Masonry Front Wall Vertical Strut Damaged: Hole into the steel member

21 Scope of Work and Main Constraints Scope of Work: Assessment of the Structural Capacity through field tests Prepare a Reliable Mathematical Model of the Structure (F.E.M.) Estimate the Residual Life and present Retrofitting Measures Main Constraints: Short Time-Frame to perform experimental field tests Limited Financial Resourses Absence of Technical Documents and Geometry Bridge currently opened to local traffic Extensive Corrosion (reduced stiffness and material strength)

22 Experimental Campaign and Performed Field Tests Determine the Actual Geometric Layout before load application (Carry out the Initial Deformed Shape of the Bridge) [TOPOGRAPHIC TOTAL STATION, GPS, 3D LASER SCANNER, DIRECT MEASUREMENT] Long-Term Thermal Load Test (12 hours) (Effects induced by a Temperature Gradient) [TOPOGRAPHIC TOTAL STATION] Static Load Tests (Application of Distributed and Concentrated Traffic Loads) Dynamic Load Tests (Application of Moving Traffic Loads at different speeds, with and without obstacles along the roadway)

23 Geometric Layout and Thermal Test Actual Deformed Shape of the Bridge under its own weight: Deformed Shape from Survey using Total Station and GPS Elevation (m.a.s.l.) Total Station on approaching embankment Span Length (m) Arrangement of Total Station and Benchmarks to detect thermal elongations

24 Acquisition through 3D Laser Scanner

25 Description of Static Load Tests PHASE 1 Uniformly Distributed Load Eccentrically Placed to Maximize Bending and Torsional Effects PHASE 2 Removal of n.2 cars at extreme ends of the bridge PHASE 3 Placement of a concentrated load at midspan to increse sagging effects

26 Main Results recorded during the Static Load Tests Vertical Deflections vs Time due to statically applied Loads: Vertical Displacement Test Phases DOWNLOADING PHASE Maximum Vertical Deformation fully recovered : ELASTIC BEHAVIOUR Span Length (m) Test Duration: minutes Sampling Frequency: 10 Hz Spatial Resolution: 0.50 m

27 Description of Dynamic Load Tests Ordinary Vehicle (PTOT =10kN) travelling at different speeds (20 50 km/hr) with and without obstacles along the roadway Loaded Pick-up Truck (PTOT =25kN) travelling at different speeds (20 50 km/hr) with and without obstacles along the roadway ADDITIONAL DYNAMIC FIELD TEST: A GROUP OF PEOPLE WAS ALLOWED TO RANDOMLY JUMP ON THE BRIDGE DECK IN ORDER TO INVESTIGATE THE STRUCTURAL BEHAVIOUR UNDER PEDESTRIAN LOADING

28 Main Results recorded during the Dynamic Load Tests Frequency Domain First Frequency of Vibration in the Vertical Plane FIRST FREQUENCY OF VIBRATION: 3.40 Hz FIRST PERIOD OF VIBRATION: ~ s Test Duration: 3 11 minutes Sampling Frequency: 100 Hz Spatial Resolution: 0.50 m CRITICAL DAMPING RATIO FROM ANALYSIS ξ = 0.60% Time Domain

29 Mathematical Model of the Bridge using F.E.M. Discrete Bridge Model Extruded Bridge Model UNDERSTANDING THE BOUNDARY CONDITIONS BETWEEN THE BRIDGE AND THE SUPPORTING SUB-STRUCTURES WAS ESSENTIAL TO PICK-UP THE RIGHT STRUCTURAL BEHAVIOUR

30 Details of the Mathematical Model Increased number of vertical struts to increase Shear Resistance Detailed view of the Top Chord at Support and Mid-Span Typical Cross- Section of the Secondary Beam COMPOSITE IRON BEAM

31 Details of the Mathematical Model Extruded View of the Model highlighting the connection between the secondary beams, the vertical struts and the wind bracings Bottom overall view of the mathematical model with all the structural members

32 Main Results obtained from the Mathematical Model Deformed Shape under Dead Loads Overall Bridge Cross-Section at Mid-span THE F.E. MODEL WAS BUILT USING DATA RECORDED ON SITE DURING THE EXPERIMENTAL TESTS AN OPTIMIZATION PROCESS HAVE BEEN IMPLEMENTED TO COME OUT WITH A RELIABLE MATHEMATICAL MODEL First Global Deformation Mode Second Global Deformation Mode

33 Validation of the Mathematical Model through Field Test Results Overall Structural Mass of the Bridge: M = 201 tons Cross-Sectional Stiffness at Mid-span: Ixx = 0.42 m4 SUMMARY TABLE SHOWING THE MAIN OBTAINED STRUCTURAL PARAMETERS MONITORED PARAMETER FIELD INVESTIGATIONS FINITE ELEMENT MODEL FREQ. OF VIBRATION (Hz) PERIOD OF VIBRATION (s) VERTICAL DEFLECTION UNDER OWN WEIGHT (mm) MAXIMUM LONGITUDINAL THERMAL ELONGATION (mm) ~ ~ F.E. MODEL DISCRETIZATION, UNCERTAINTIES IN GEOMETRY AND BOUNDARY CONDITIONS CAN BE SOURCES OF ERRORS IF NOT SUPPORTED BY ACCURATE EXPERIMENTAL FIELD TESTS THE ACCURACY OF THE F.E. MODEL STRONGLY DEPENDS ON THE QUALITY OF THE DATA RECORDED ON SITE DURING EXPERIMENTAL TESTS, PERFORMED HERE BY MEANS OF STATIC AND DYNAMIC LOADING CONDITIONS, BASED ON AMBIENT AND FORCED VIBRATIONS.

34 Documents officially submitted to the Client Typical deck cross-section at support Miscellaneous Structural Details Overall Longitudinal Section of the Bridge / River Bed Eng. Clemente Caruso

35 Critical Aspects in the Structural Response of the Bridge Some of the elements of the Bottom Chord fail under the Fatigue Load Case. In particular, the element of the bottom chord, placed close to the support section, doesn t reach the target of 2 million cycles. Detail of the Bottom Chord at Support Section Axial Force Envelope Diagram for Fatigue Analysis

36 Final Remarks and Conclusions RECORD HUGE AMOUNT OF DATA IN ONLY TWO DAYS OF FIELD WORK REAL-TIME UNDERSTANDING OF THE BRIDGE BEHAVIOUR SETTING-UP OF A RELIABLE MATHEMATICAL MODEL OF THE BRIDGE ADVICE TO THE CLIENT ON ITS ACTUAL STRUCTURAL CONDITIONS: MAXIMUM AXIAL LOAD PERMITTED REMAINING SERVICE LIFE SUGGEST RETROFITTING MEASURES

37 Main Takeaways Meaning of Assessment and Rehabilitation Guide-lines for conducting a rehabilitation program Solutions to practical problems through a proposed study case Importance of a Reliable Mathematical Model Discussion of main results Possible solutions to extend service life and improve structural capacity Bibliography SETRA: Assessment of Vibrational Behaviour of Footbridges Under Pedestrian Loading Bridge Rehabilitation Wojciech Radomski (Imperial College Press) Eurocode no.3: Design of Steel Structures Guide to Design Criteria for Bolted and Riveted Joints (American Institute of Steel Construction, Chicago) Recommended Specialist Books ICE Manual of Bridge Engineering (ICE) Bridge Engineering: A global Perspective (Troyano) Why do buildings collapse? (Mario Salvatori)

38 THANK YOU FOR YOUR ATTENTION QUESTIONS?