How Bridges Carry Loads Prof' Ed Dauenheimer Civil and Environmental Engineering Department Newark C

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How Bridges Carry LoadsProf. Ed
DauenheimerCivil and Environmental Engineering
DepartmentNewark College of EngineeringNew
Jersey Institute of Technology
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More on Trusses
Only Type of Superstructure Allowed by
WPBDesigner

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Basic Nomenclature
Components of a Typical Truss Bridge Isometric
View
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Truss Types
By Location of Roadway
By Configuration of Diagonals
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Truss Behavior
Top members in compression
Web members may be in Compression (C) or Tension
(T)
Live load applied at this level
C
T
Bottom members in tension
Simple Span Howe Truss
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Simple Versus Continuous Spans
2 Simple Spans
2 Continuous Spans
Load
Load
T
C T
C
Deflected shape
T C
C T
Load in one span is felt in adjacent spans!
Tension (T) andcompression (C) vary.
Load in one span has no effect in adjacent
spans! Compressionis in top, tension in bottom.
Note The WPBDesigner models both one span and
two span bridges.
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More on Truss Behavior
How to find type of force in diagonal members
Simple versus continuous span trusses
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Structural Stability
The structure is unstable if 2j gt
m 3 where, j number of joints
m number of members
j 12 m 20 24 gt 20 3 Unstable
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Trusses and WPBDesignerDeck at Highest
ElevationLongest Span length
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Deck at High Water LevelShortest Span Length
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Arch Configuration
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Two Span Arrangement
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WPBDesigner 2005 Contest
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Load Testing Bridges May Be the AnswertoHow
Bridges Behave!
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Background

• More than 590,000 bridges in U.S.
• More than 163,000 are structurally deficient or
  functionally obsolete
• Many bridges are more than 50 years old
• Most continue in-service carrying heavier loads
  then for which they were designed
• Load test help explain bridge performance

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Types of Load Tests

• Diagnostic Load Tests
• Measures the effects of a live load on one or
  more members of the bridge
• Generally requires the field installation of
  sensors to measure the effects

• Proof Load Tests
• Maximum safe bridge capacity for both dead and
  live loads is established
• Monitoring of bridge during load test is simpler
  than for diagnostic load tests

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Calhoun Street Bridge, Trenton, N.J.
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Damaged by a car, yet still stood
Note temporary cables and damaged vertical
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Jacking easterly truss back to its pre-collision
position before making permanent repairs
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Results of sensor monitoring of load test after
all repairs were completed
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Mead Avenue Bridge, Crawford County, PA
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A bridge inside a bridge!!
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Results of load test to confirm participation of
truss members
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Broadway Bridge, Newark, N.J.
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For railroad bridges the test load is a little
bit heavier!!!!
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Dynamic test results for engine travelling west
at 20 mph on the westbound track
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Typical data acquisition system
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Typical strain gage sensor installed in the field
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Summary

• Load test can be helpful in understanding bridge
  performance. The appropriate load test method
  for various types of bridges is suggested below
• Truss bridges - Diagnostic
• Arch and rigid frame bridges - Proof
• Concrete slab bridges - Proof
• Multi-girder bridges - Diagnostic or proof
• Two-girder bridges - Diagnostic
• Timber bridges - Proof or Diagnostic
• Long Span bridges - Diagnostic for cables