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 ?

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

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

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

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

Cost Analysis 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 Cost Parameter = Steel Weight Constructability Factor CDOT Staff Bridge Communication Day – September 27, 2004

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

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

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

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

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

Wind Loading Statistics - Probabilities 25 bins for 25 wind velocities Area of BIN = Probability of Occurrence (P oi )

Dynamic Analysis - Loading 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 XoXo

Stress – Time History Standard deviation of combined stress = s i

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

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

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

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

Combining Utilities Weighting factors  Adjust emphasis of cost (a c ) and performance (a p ) CDOT Staff Bridge Communication Day – September 27, 2004

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

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

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

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

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

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

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

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

CALCULATED STRESSES Stress Resultants Stress Resultants –P x, P y, P z, M x, M y, M z Connection Properties 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 Michigan Type C Cantilevered Michigan Type C Cantilevered –Base-to-Column Bolts CDOT Staff Bridge Communication Day – September 27, 2004

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

RESULTS

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

Thank You! My Contact Information: John W. van de Lindt Associate Professor Colorado State University Department of Civil Engineering Fort Collins, CO Ph: or Fax: Both reports are available in PDF at: CDOT Staff Bridge Communication Day – September 27, 2004