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Overhead Sign Support Structures: Meeting AASHTO 2001

Overhead Sign Support Structures: Meeting AASHTO 2001. John W. van de Lindt. CDOT Staff Bridge Communication Day – September 27, 2004. Motivation. AASHTO Standard Specifications for Highway Signs, Luminaires, and Traffic Signals, 2001 Fatigue problems. Where to begin ?.

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Overhead Sign Support Structures: Meeting AASHTO 2001

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  1. Overhead Sign Support Structures: Meeting AASHTO 2001 John W. van de Lindt CDOT Staff Bridge Communication Day – September 27, 2004

  2. Motivation AASHTO Standard Specifications for Highway Signs, Luminaires, and Traffic Signals, 2001 Fatigue problems Where to begin ? CDOT Staff Bridge Communication Day – September 27, 2004

  3. Project Objectives Develop a method to analyze and evaluate overhead sign support structures in order to determine a metric accounting for both cost and performance. Apply the method to overhead sign support structure designs currently in use in the U.S. Check the identified structures for compliance with AASHTO 2001 CDOT Staff Bridge Communication Day – September 27, 2004

  4. Nationwide Survey • State of the Art / State of the Practice Survey • Brief and Simple (9 Questions / 15 min.) • What types of sign structures are being used? • Are there any that are having problems? • Is the AASHTO 2001 Sign Specification being used? CDOT Staff Bridge Communication Day – September 27, 2004

  5. Survey Response 38 Responses = 76 % CDOT Staff Bridge Communication Day – September 27, 2004

  6. Survey Highlights Great variety in different types of sign structures 20 states claim to be using AASHTO 2001 already Steel is the dominant material used CDOT Staff Bridge Communication Day – September 27, 2004

  7. Cost Analysis Cost Parameter = Steel Weight Constructability Factor • Cost data is not easily available and highly variable • Steel weight is controlling factor in production cost • Variability in construction / fabrication methods • Constructability factors • Tubular = 1.00 • Monotube = 1.15 • Truss = 1.20 CDOT Staff Bridge Communication Day – September 27, 2004

  8. Performance Analysis • Estimated fatigue life of structural connections subject to natural wind gust loading • Fatigue design was the focus of AASHTO Sign Specification update • Existing work identified structural connections as susceptible to fatigue problems • All types of structures are susceptible to natural wind gust loading • Random vibration approach – Crandall and Mark (1961) CDOT Staff Bridge Communication Day – September 27, 2004

  9. Structure Modeling - FEA Simplified FEA model to determine dynamic properties MDOT Cantilever Simplified Model

  10. Modeling -Connections Connections modeled according to cross-sectional properties MDOT Cantilever Base MDOT Cantilever Arm - Pole CDOT Staff Bridge Communication Day – September 27, 2004

  11. Wind Loading Statistics Wind speed distribution over the contiguous USA in the 1980’s (NOAA) CDOT Staff Bridge Communication Day – September 27, 2004

  12. Wind Loading Statistics – Data Fitting Lognormal NOAA Comparison of NOAA to lognormal PDF CDOT Staff Bridge Communication Day – September 27, 2004

  13. Wind Loading Statistics - Probabilities • 25 bins for 25 wind velocities • Area of BIN = Probability of Occurrence (Poi)

  14. Dynamic Analysis - Loading Xo • Convert wind velocity to force (AASHTO 2001/ASCE7) • Initial conditions for free vibration Initial Position Initial Velocity Initial Acceleration • Stiffness and Damping Matrices Rayleigh Damping CDOT Staff Bridge Communication Day – September 27, 2004

  15. Stress – Time History Standard deviation of combined stress = si

  16. Fatigue Constants – S-N Curves Fatigue Constants # of Cycles to Failure Magnitude of Stress • Stress category from AASHTO 2001 Sign Specification • S-N curve from AASHTO 1994 LRFD Bridge Specification CDOT Staff Bridge Communication Day – September 27, 2004

  17. Fatigue Life • Damage from each stress time history (25) • Fatigue life from all damages Performance Parameter = Estimated Fatigue Life CDOT Staff Bridge Communication Day – September 27, 2004

  18. Cost Utility Functions • Convert cost and performance into common units Cost Utility CDOT Staff Bridge Communication Day – September 27, 2004

  19. Performance Utility Functions Performance Utility CDOT Staff Bridge Communication Day – September 27, 2004

  20. Combining Utilities • Weighting factors • Adjust emphasis of cost (ac) and performance (ap) CDOT Staff Bridge Communication Day – September 27, 2004

  21. Ranking Results – Excluding Cost CDOT Staff Bridge Communication Day – September 27, 2004

  22. Ranking Results – 25% Cost CDOT Staff Bridge Communication Day – September 27, 2004

  23. MDOT Optimal OH sign support structure OH Signs Selected Optimization of Cost and Performance of Overhead Sign Support Structures (Ahlborn et al, 2003) CDOT Staff Bridge Communication Day – September 27, 2004

  24. GENERAL OVERVIEW • Design Check Procedure • Structural analysis method • Elastic method • Allowable stress design (ASD) • Structural properties CDOT Staff Bridge Communication Day – September 27, 2004

  25. GENERAL OVERVIEW (CONT.) • Design Check Procedure • Serviceability requirements • Not considered • Wind and ice loads • Michigan’s location • Steel and Fatigue design checks • Fatigue not considered for bridge types CDOT Staff Bridge Communication Day – September 27, 2004

  26. LOADS Excerpted from AASHTO Standard Specifications for Highway Signs, Luminaires, and Traffic Signals, 2001. CDOT Staff Bridge Communication Day – September 27, 2004

  27. LOADS (CONT.) • Michigan Type C Cantilevered • Group II-Case 2 • Visual Analysis 4.0 CDOT Staff Bridge Communication Day – September 27, 2004

  28. LOAD COMBINATIONS Excerpted from AASHTO Standard Specifications for Highway Signs, Luminaires, and Traffic Signals, 2001. CDOT Staff Bridge Communication Day – September 27, 2004

  29. CALCULATED STRESSES • Michigan Type C Cantilevered • Base-to-Column Bolts • Stress Resultants • Px, Py, Pz, Mx, My, Mz • Connection Properties • A = area of pattern • c = distance from centroid to point* • I = moment of inertia of pattern • J = polar moment of inertia of pattern CDOT Staff Bridge Communication Day – September 27, 2004

  30. ALLOWABLE STRESSES • Calculated according to 2001 AASHTO design code • Anchor bolts • Other bolts and all welds • References • AASHTO Standard Specifications for Highway Bridges • AWS Structural Welding Code D1.1-Steel CDOT Staff Bridge Communication Day – September 27, 2004

  31. RESULTS CDOT Staff Bridge Communication Day – September 27, 2004

  32. CONCLUSIONS • Recommendations • Inelastic method vs. elastic method • More accurate and detailed FEM’s • Future Work • DOT’s • Adopt typical plans • Adopt 2001 AASHTO design code • Monitor OH sign support structures • Design and research • Serviceability requirements • Include fatigue in design checks for bridge types CDOT Staff Bridge Communication Day – September 27, 2004

  33. Thank You! My Contact Information: John W. van de Lindt Associate Professor Colorado State University Department of Civil Engineering Fort Collins, CO 80523-1372 Ph: 970-491-6605 or 970-491-8691 Fax: 970-491-7727 E-mail: jwv@engr.colostate.edu Both reports are available in PDF at: http://www.engr.colostate.edu/~jwv/reports.htm CDOT Staff Bridge Communication Day – September 27, 2004

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