Overhead Sign Support Structures: Meeting AASHTO 2001

1
Overhead Sign Support Structures Meeting AASHTO
2001

• John W. van de Lindt

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

CDOT Staff Bridge Communication Day September
27, 2004
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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
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Structure Modeling - FEA
Simplified FEA model to determine dynamic
properties
MDOT Cantilever Simplified
Model
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Modeling -Connections
Connections modeled according to cross-sectional
properties
MDOT Cantilever Base
MDOT Cantilever Arm - Pole
CDOT Staff Bridge Communication Day September
27, 2004
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Wind Loading Statistics
Wind speed distribution over the contiguous USA
in the 1980s (NOAA)
CDOT Staff Bridge Communication Day September
27, 2004
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Wind Loading Statistics Data Fitting
Lognormal
NOAA
Comparison of NOAA to lognormal PDF
CDOT Staff Bridge Communication Day September
27, 2004
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Wind Loading Statistics - Probabilities

• 25 bins for 25 wind velocities
• Area of BIN Probability of Occurrence (Poi)

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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
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Stress Time History
Standard deviation of combined stress si
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Fatigue Constants S-N Curves

• Stress category from AASHTO 2001 Sign
  Specification
• S-N curve from AASHTO 1994 LRFD Bridge
  Specification

CDOT Staff Bridge Communication Day September
27, 2004
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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
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Cost Utility Functions

• Convert cost and performance into common units

Cost Utility
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Performance Utility Functions
Performance Utility
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Combining Utilities

• Weighting factors
• Adjust emphasis of cost (ac) and performance
  (ap)

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Ranking Results Excluding Cost
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Ranking Results 25 Cost
CDOT Staff Bridge Communication Day September
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OH Signs Selected

• 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
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GENERAL OVERVIEW

• Design Check Procedure
• Structural analysis method
• Elastic method
• Allowable stress design (ASD)
• Structural properties

CDOT Staff Bridge Communication Day September
27, 2004
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GENERAL OVERVIEW (CONT.)

• Design Check Procedure
• Serviceability requirements
• Not considered
• Wind and ice loads
• Michigans location
• Steel and Fatigue design checks
• Fatigue not considered for bridge types

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

• Michigan Type C Cantilevered
• Group II-Case 2
• Visual Analysis 4.0

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

• Recommendations
• Inelastic method vs. elastic method
• More accurate and detailed FEMs
• Future Work
• DOTs
• 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
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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 [phone] E-mail
jwv_at_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