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Lecture+20-small - 440:221 Intro to Engineering Mechanics:...

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Unformatted text preview: 440:221 Intro to Engineering Mechanics: Statics Instructor: Alberto Cuitino TA: Pam Carabetta and Jingru Zhang Lecture 20 20 Fall 2009 Board Meeting HWY07MH024 Slides from http://www.ntsb.gov/events/2008/Minneapolis-MN/presentations.htm 1 Office of Research and Engineering Bridge Description and Collapse Jim Wildey Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite element modeling analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Bridge Bridge Description and Collapse 1. General description of bridge – Structural elements elements – Bridge modifications 2. Sequence of collapse – Initiation location – Secondary damage and fractures 3. Factors excluded Board Meeting HWY07MH024 Bridge Bridge Description and Collapse Spans 9 - 14 Board Meeting HWY07MH024 Spans 6-8 Spans 1-5 U10 West North South approach Deck truss North approach General General Description of the Bridge Board Meeting HWY07MH024 Bridge a Bridge Description and Collapse General General Description of Bridge • Deck truss original weight: 18.3 million pounds million pounds • 1977 Modification – Deck thickness increased – Added over 3 million pounds over million pounds • 1998 Modification – Barriers / deicing system – Added 1.2 million pounds Board Meeting HWY07MH024 Sequence Sequence of Collapse Camera Deck truss Board Meeting HWY07MH024 Video Video Evidence North Pre-Collapse South Video Video Evidence Board Meeting HWY07MH024 Video Video Evidence Video Video Evidence North Pre-Collapse South Video Video Evidence South North L11W Collapse Video - Frame #1 Video Video Evidence South North L11W Collapse Video - Frame #2 Video Video Evidence South North L11W Collapse Video - Frame #3 Video Video Evidence South North L11W Collapse Video - Frame #4 Video Video Evidence North Collapse Video - Frame #5 South Video Video Evidence North Collapse Video - Frame #6 South Video Video Evidence North Collapse Video - Frame #7 South Video Video Evidence North Collapse Video - Frame #8 South Video Video Evidence North Collapse Video - Frame #9 South Video Video Evidence North Collapse Video - Frame #10 South Video Video Evidence North Collapse Video - Frame #11 South Video Video Evidence North Pre-collapse South Video Video Evidence • No initiation in center section or north section north section • Collapse initiated south of node L11 • Node L11W remained intact after collapse initiated • Deck lifted from stringers above node L11 and further to the south node L11 and further to the south Board Meeting HWY07MH024 Basis Basis of Collapse Sequence • Video evidence • Fracture and damage patterns and damage patterns • Finite element analysis Board Meeting HWY07MH024 Fracture Fracture and Deformation North U10W Upper chord U10E Lower chords Upper Board Meeting HWY07MH024 chord Sequence Sequence of Collapse Methodology • Initiating event occurs from forces acting along the members acting along the members – Buckling under compression Board Meeting HWY07MH024 Sequence Sequence of Collapse Methodology • Initiating event occurs from forces acting along the members acting along the members – Buckling under compression – Tension fracture without bending Board Meeting HWY07MH024 Sequence Sequence of Collapse Methodology • Deformation adjacent to fracture – Occurs during fracture process – Not from subsequent collision or ground impact Board Meeting HWY07MH024 Sequence Sequence of Collapse Methodology • Documentation – Fractures – Deformation patterns – Impact marks • Damage evaluation – Early in collapse process? – From subsequent ground or riverbed impact? Board Meeting HWY07MH024 Sequence Sequence of Collapse Methodology • Methodology used in other investigations – TWA Flight 800 – Rail from Baltimore railroad tunnel fire Board Meeting HWY07MH024 Sequence Sequence of Collapse Results • Main trusses fractured between nodes 9 and 10 • Fractures in node U10 gusset plates in node U10 gusset plates met initial event criteria Board Meeting HWY07MH024 Drawing Drawing of Fractures in Node U10E West Gusset Plate West Gusset Plate Initial compression failure Initial tension fracture Initial Initial Tension Fracture U10 West L9/U10W Up North Up Up North Folded gusset plate tips on compression diagonal at node U10 west Board Board Meeting HWY07MH024 2003 Photo Board Meeting HWY07MH024 Compression diagonal Collapse Collapse Initiation • Initial failure in U10 gusset plates – Compression failure above failure above compression diagonal – Tension fracture fract • Collapse was unavoidable once gusset plates failed at node U10 Board Meeting HWY07MH024 U10 West Board Meeting HWY07MH024 South U10 U10 Compression Diagonal L9/L10 Board Meeting HWY07MH024 L9 Secondary Secondary Damage - Bent and Fractured Member T C Bending Fracture Secondary Damage Sequence Sequence of Collapse 1. Gusset plates at nodes U10 failed, compression diagonals translated to the west, and remaining portions of nodes U10 were pulled down and through diagonals U10 Compression Diagonal Diagonal Sequence Sequence of Collapse 2. Dropping of center span created tension loading in main truss lower chords and secondary structural members. This loading pulled south portion of deck truss to the north, off pier 5, causing loss of support for south approach causing loss of support for south approach spans Sequence Sequence of Collapse 3. Other main truss members fractured in bending fractured in bending Sequence Sequence of Collapse 4. Lower chord L7/L8 east L7/L8 east fractured from its nodes allowing nodes, allowing south portion of truss to topple to truss to topple to the east South South Portion Toppled to the East East Center section Sequence Sequence of Collapse 5. Gusset plates at node U10 failed, and additional secondary failures in this portion of the truss completely separated the center section of the center span from remainder of deck truss Sequence Sequence of Collapse 6. Deck truss collapsed above pier 8, causing loss of support for north approach spans, and this collapse spread southward toward pier 7 Pier 8 North Factors Factors That Did Not Contribute • Corrosion damage • Fracture of a floor truss fl • Pre-existing cracking • Bearings and piers Board Meeting HWY07MH024 Corrosion Corrosion Damage • Some corrosion present present – Secondary members members – Lower chord gusset plates gusset plates • Inspection records documented corrosion Board Meeting HWY07MH024 Corrosion Corrosion Did Not Contribute L11 gusset plates failure eliminated as factor as shown by: by: L11W – Fracture and deformation patterns deformation patterns – Video recording – Finite element element analysis Board Meeting HWY07MH024 Collapse Video - Frame #3 Fracture Fracture of a Floor Truss Fracture Floor truss 10 upper chord Board Meeting HWY07MH024 West Fracture Fracture of a Floor Truss Floor Truss 10 Truss 10 Cut West Board Meeting HWY07MH024 Floor truss 10 – secondary damage damage Fracture Preexisting Preexisting Cracking • Primarily in approach spans • Main truss welds truss welds – Near node U14 – Near node U7 node U7 • No fatigue cracking in fractured pieces • U10 gusset plates – Ductile overstress tension, shear, and bending fractures Board Meeting HWY07MH024 Bearings Bearings and Piers • Rollers were moving • Wear in center of contact plates • No horizontal shifting of piers horizontal shifting of piers Pier Pier Movement Pier 7 Overall Overall Summary • Collapse initiated with failure of gusset plates at nodes U10 • Complete collapse was unavoidable once these gusset plates failed • Other possible initiation locations and factors were ruled out • All findings were supported by – Fracture and deformation patterns – Video evidence – Finite element analysis Board Meeting HWY07MH024 Office of Research and Engineering Construction Activity & Traffic Conditions Traffic Conditions on I-35W Bridge Robert Accetta Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite Element Analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Roadway Roadway Construction Work 2 closed outside northbound traffic lanes Board Meeting HWY07MH024 2 closed inside southbound traffic lanes Lane Lane Designations Northbound Completed - CLOSED Northbound - OPEN Northbound - OPEN Southbound Milled - CLOSED Southbound Completed - OPEN Completed Southbound Completed - OPEN Board Meeting HWY07MH024 Staging Staging of Construction Material 07/06/07 07/23/07 08/01/07 U10 Node Board Meeting HWY07MH024 Construction Construction Work – Day of Accident Piles of aggregate Construction equipment and vehicles U10 west node Board Meeting HWY07MH024 Construction Construction Materials & Vehicles Construction materials materials 383,000 lbs Construction equipment 195,500 lbs Center span combined Center span combined Construction load 578,500 lbs Board Meeting HWY07MH024 Office of Research and Engineering Gusset Plate Inadequacy Carl R. Schultheisz Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite element analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Overview Overview • Bridge design basics – Allowable Stress Design methodology St – Design forces • I-35W design – Review of design documents of design documents – Evaluation of gusset plates – Clearly inadequate capacity of gusset plates at U10 and other nodes Board Meeting HWY07MH024 Bridge Bridge Design Basics • Allowable Stress Design methodology • Stress = Force/Area • Truss member forces counteract the weight of structure and traffic weight of structure and traffic Board Meeting HWY07MH024 Bridge Bridge Design – Allowable Stress • AASHO allowable stress no more th 55 than 55 percent of the yield stress th • Yield stress defines limit of usable load-carrying capacity of a material • The allowable stress level ensures allowable stress level ensures a substantial margin of safety Board Meeting HWY07MH024 Bridge Bridge Design - Forces • Design force: calculated in accordance with AASHO specifications with AASHO specifications • Design force calculated from dead load plus live load plus impact plus live load plus impact • Live load = AASHO-specified lane loads • AASHO-specified lane load – Represents unusual, heavy traffic load – Provides additional margin of safety Board Meeting HWY07MH024 U10 - Member Forces Transferred by the Gusset Plate Upper chord 924,000 lbs compression Tension diagonal 1,975,000 lbs lbs Board Meeting HWY07MH024 Upper chord 2,147,000 lbs tension Compression diagonal 2,288,000 lbs Vertical 540,000 lbs lbs tension U10 - Design for Shear Upper chord 924,000 lbs compression A Tension diagonal Horizontal component 1,363,000 lbs lbs Board Meeting HWY07MH024 A Upper chord 2,147,000 lbs tension Compression diagonal Horizontal component 1,361,000 lbs U10 - Design for Shear Upper chord Upper chord 924,000 lbs 924,000 lbs compression compression Tension Tension diagonal diagonal Horizontal Horizontal component component 1,363,000 lbsbs lbs 1,363,000 lbs l Board Meeting HWY07MH024 SHEAR Upper chord Upper chord 2,147,000 lbs 2,147,000 lbs tension tension Compression Compression diagonal diagonal Horizontal Horizontal component component 1,361,000 lbs 1,361,000 lbs U10 - Design for Shear Total = 3,071,000 lbs Upper chord 924,000 lbs compression Tension diagonal horizontal component 1,363,000 lbs lbs Board Meeting HWY07MH024 SHEAR Total = 2,724,000 lbs Upper chord 2,147,000 lbs tension Compression diagonal horizontal component 1,361,000 lbs Bridge Bridge Design – U10 Calculations Shear stress = stress Shear force Area 3,071,000 pounds pounds = 100 square inches = 30,710 pounds per 30,710 pounds per square inch Board Board Meeting HWY07MH024 Bridge Bridge Design – U10 Calculations • Demand = Shear stress from member design stress from member design forces = 30,071 pounds per square inch • Capacity = AASHO specified allowable stress = 15,000 pounds per square inch pounds per square inch • Demand-to-Capacity ratio = 2.05 Board Meeting HWY07MH024 Bridge Bridge Design – U10 Calculations • The Demand should be less than the Capacity Capacity • The Demand-to-Capacity ratio should be less than be less than 1 • U10 Demand-to-Capacity ratio = 2.05 • To make the Demand-to-Capacity ratio equal to 1, the U10 gusset plate thickness would have to increase from ½ inch to slightly more than 1 inch Board Meeting HWY07MH024 Gusset Plate D/C ratio Results - Gusset Plate Shear Results Analysis 2.5 U4 U4 2.0 U10 L11 1.5 1.0 0.5 0.0 L1 U2 L3 Board Meeting HWY07MH024 L5 U4 U2 L1 U4 L3 U6 L7 L9 U10 L11 U12 L13 U14 U8 U6 L5 U8 L7 U12 U10 L9 L11 U14 L13 Gusset Gusset Plate Analysis Summary • Analysis showed that the U10 gusset plates had clearly gusset plates had clearly inadequate capacity • Other gusset plates had similar inadequate capacity • No records were found for design of main truss gusset plates main truss gusset plates • No other design deficiencies found Board Meeting HWY07MH024 Office of Research and Engineering Finite Element Analysis Carl R. Schultheisz Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite Element Analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Overview Overview • Finite Element Analysis approach • Models and inputs and inputs • Loads in the U10 gusset plates at th ti the time of the accident th • Stress levels in U10 gusset plates over the life of the bridge • Failure initiation mechanism initiation mechanism • Factors that did not contribute to collapse collapse Board Meeting HWY07MH024 Finite Finite Element Analysis Computer model of interconnected of interconnected simple elements Board Meeting HWY07MH024 Computer-calculated deformation and and stress in the model Global Global Model of Bridge • Truss members shown as lines that join at points • No gusset plates Board Meeting HWY07MH024 FHWA FHWA Global Model • Calculate forces in members under various applied loading conditions various applied loading conditions • Transfer the applied loads and any deformations of the truss to the detailed models of U10 and L11 • Showed bridge design (other than gusset plates) was consistent with gusset plates) was consistent with AASHO specifications Board Meeting HWY07MH024 Detailed Detailed Models U10 West L11 West Board Meeting HWY07MH024 Results Results from Computer Model • Contribution of loads over the history of • • • • the bridge the bridge Stresses in the gusset plates at U10W Failure initiation mechanism Bowing of gusset plates at U10W Comparison of U10W and U10E Board Meeting HWY07MH024 Increasing Increasing Loads on U10W Gusset Plate Total load at collapse Construction Materials and Vehicles Traffic Critical Load on U10W Gusset Plates 1998 Modified Barriers Modified Barriers Board Board Meeting HWY07MH024 1977 Added Deck (Less Milled-off Lanes) Dead Load of Original Bridge Design Increasing Increasing Loads on U10W Gusset Plate Expected capacity of gusset plates for proper AASHO for proper AASHO design Total load at collapse Missing reserve capacity for proper design Construction Materials and Vehicles Traffic Critical Load on U10W Gusset Plates 1998 Modified Barriers Board Meeting HWY07MH024 1977 Added Deck (Less Milled (Less Milled-off Lanes) Lanes) Dead Load of Original Bridge Design Crossing Crossing the Threshold of Failure Precise moment of failure Critical load (not scaled) Failure level 1977 modification 1998 modification 2007 modification Original design Upper bound of load Instantaneous load Lower bound of load Time (not scaled) Board Meeting HWY07MH024 Instantaneous Instantaneous Loads are Variable Upper bound of load Instantaneous load Lower bound of load Some of the many sources of variability at the time of the accident •Traffic •Amount •Position •Motion Board Meeting HWY07MH024 •Weather •Wind •Temperature •Construction Activity Activity •Amount •Position •Motion Dead Dead Load of Original 1967 Bridge Orange and red shading: exceeds yield stress Stress Yield stress Allowable 0 Tension diagonal Board Meeting HWY07MH024 Compression diagonal After After 1977 and 1998 Modifications Orange and red shading: exceeds yield stress Stress Yield stress Allowable 0 Tension diagonal Board Meeting HWY07MH024 Compression diagonal Loads Loads at Time of Accident Orange and red shading: exceeds yield stress Stress Yield stress Allowable 0 Tension diagonal Board Meeting HWY07MH024 Compression diagonal Accident Accident Loads on 1-Inch-Thick Gusset Plates Stress Yield stress Allowable 0 Tension diagonal Board Meeting HWY07MH024 Compression diagonal Loads Loads at Time of Accident Orange and red shading: exceeds yield stress Stress Yield stress Allowable 0 Tension diagonal Board Meeting HWY07MH024 Compression diagonal Simulation Simulation – Failure Initiation Oblique view Tension diagonal Board Meeting HWY07MH024 End view Compression diagonal Board Board Meeting HWY07MH024 Bowed Bowed U10 Gusset Plates 2003 Photo Board Meeting HWY07MH024 Computer Model Bowed Bowed Gusset Plates • Reduced load necessary to trigger instability instability • Upper end of the compression diagonal shifted to the outside of the bridge shifted to the outside of the bridge, consistent with physical observations • A symptom of the inadequate capacity of symptom of the inadequate capacity of the U10 gusset plates • Distortion such as bowing should be such as bowing should be identified and evaluated Board Meeting HWY07MH024 Summary Summary • Collapse began at node U10W when highly stressed gusset plates were unable to prevent unstable lateral shift of upper end of highly loaded compression diagonal • Gusset plates meeting AASHO specifications plates meeting AASHO specifications would have safely supported loads on bridge at time of accident • Bowed gusset plates reduced load necessary to trigger instability and resulted in lateral shift to outside of bridge consistent with physical to outside of bridge, consistent with physical observations Board Board Meeting HWY07MH024 Summary Summary – Factors That Did Not Contribute • Corrosion of gusset plates at the L11 nodes did not contribute to the collapse did not contribute to the collapse • Thermal expansion under uniform temperature change or differential temperature change did change or differential temperature change did not play a significant role in collapse Board Meeting HWY07MH024 Office of Research and Engineering Design Error Joe Epperson Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite element analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Potential Potential Sources of Error • Fabrication – Material mix-up mix – Transcription error • No shop drawing shop drawing changes • Tests and measurements and measurements found correct components installed • Design firm was source of error Board Meeting HWY07MH024 Source Source of Design Error • Design error possibilities – Transcription/drafting error error – Material change error – Calculation error error – Omission of calculations • U10 gusset plates same as in final fi design plans • Thickness and material did not change Thi did from earliest design Board Meeting HWY07MH024 Source Source of Design Error Design – Transcription/drafting error error – Material change error – Calculation error – Omission of calculations Board Meeting HWY07MH024 Office of Research and Engineering Bridge Load Rating Dan Walsh Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite element analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Office of Research and Engineering Bridge Inspections Dan Walsh Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite element analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Snooper truck inspection of of I-35W bridge bridge Board Meeting HWY07MH024 Main Truss Members Poor Weld Details Board Meeting HWY07MH024 Floor Truss Members Pack Rust Board Meeting HWY07MH024 Stringer Ends Surface Rust Corrosion Board Meeting HWY07MH024 Expansion Hinge in Span 2 No Longer Functioning Board Meeting HWY07MH024 Findings Findings • No findings were considered a threat to load bearing capacity of threat to load bearing capacity of structure • No findings were determined to have played a role in collapse Board Meeting HWY07MH024 Office of Research and Engineering Gusset Plate Inspection Issues Mark Bagnard Presentations Presentations 1. 2. 3. 4. 5. 6. 7. 8. Bridge description and collapse Construction activities on bridge at time of collapse Gusset plate inadequacy Finite Element Analysis Design and review process Bridge load rating and bridge load analysis Bridge inspections Gusset plate inspections Board Meeting HWY07MH024 Issue Issue Areas • Visual inspections may not detect or accurately quantify corrosion on accurately quantify corrosion on gusset plates • Lack of guidance for inspecting and evaluating gusset plates for distortion (bowing) Board Meeting HWY07MH024 Board Board Meeting HWY07MH024 I-35W Bridge, source: UMN Board Board Meeting HWY07MH024 I-35W Bridge, source: URS Corporation Gusset Gusset Plate Distortion • At least one Mn/DOT inspector had observed bowing observed bowing • He concluded bowing resulted from original construction • Because of his training, he believed of his training he believed gusset plates were overdesigned Board Meeting HWY07MH024 Summary Summary of Distortion Issue • Bowed gusset plates on I-35W bridge were not addressed through inspections • Bowing may indicate out-of-design condition and should be subjected to further engineering analysis • Lack of specific training references to bowing could cause bridge inspectors to give inadequate attention to this condition Board Meeting HWY07MH024 Board Board Meeting HWY07MH024 ...
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This note was uploaded on 09/12/2011 for the course ENG 221 taught by Professor P.sannuti during the Fall '09 term at Rutgers.

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