ENGINEERING STUDYENGINEERING STUDYBURR ARCH-TRUSSTOWN LATTICE-TRUSS

PINE GROVE BRIDGE—LANCASTER/CHESTER COUNTIES, PABROWN BRIDGE—RUTLAND COUNTY, VT

DYLAN LAMAR, HAER ENGINEERING TECHNICIAN, SUMMER 2002BENJAMIN W. SCHAFER, PH.D., ASST. PROFESSOR, JOHNS HOPKINS UNIVERSITY

OVERVIEW

BURR ARCH-TRUSS

• ARCH VS. TRUSS BEHAVIOR

TOWN LATTICE-TRUSS

• GENERAL BEAM BEHAVIOR• STRUCTURAL EFFICIENCY

BURR ARCH-TRUSSOF PINE GROVE BRIDGEAS BUILT BY CAPTAIN ELIAS MCMELLEN

LET’S CONSIDER THE ARCH AND TRUSS SEPARATELY…

ARCH BEHAVIOR

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Forces of Arch Elements under Dead Load

TRUSS BEHAVIOR

Force Diagram of Truss Elements under Dead Load

ARCH-TRUSS BEHAVIOR

Arch carries significantly higher forces than truss.

Forces in lower chord of the truss significantly reduced.

Forces in Arch-Truss under Dead + Mid-span Live Load

Arch behavior

Deflection under Quarter Point Load

Deflection under Mid-span

CONCLUSIONS

BURR ARCH-TRUSS IS A SYNERGY OF THE TWO ELEMENTS.

THE ARCH IS STRUCTURALLY DOMINANT.

HOWEVER, TRUSS IS NECESSARY FOR STABILITY.

NO ELEMENT IS OVER-STRESSED BY TODAY’S STANDARDS FOR EXPECTED LOADING.

TOWN LATTICE-TRUSSOF BROWN BRIDGEAS BUILT BY NICHOLS M. POWERS

TRUSS BEHAVIOR FOLLOWS BEAM BEHAVIOR

Diagonals follow Global ShearChords follow Global Flexure

Axial Force Diagram of Truss Elements under Dead Load

STRUCTURAL EFFICIENCY

Elements placed where most necessary?

Member sizes proportional to loads carried?

•Bolster Beam•Secondary Chord Placement•Uniform Diagonals•Member Sizing

EFFICIENCY OF BOLSTER BEAM?

BOLSTER BEAM REDUCES STRESS CONCENTRATIONS• Decreases Max Stress by 22% (in diagonal)• Decreases bottom chord stress by 53%

EFFICIENCY OF SECONDARY CHORD PLACEMENT?

IN EQUIVALENT SINGLE CHORD MODEL, DEFLECTION DECREASED 20%, BUT MAX STRESS INCREASED 20%.

FINAL PLACEMENT PROBABLY DECIDED BY CONSTRUCTION ISSUES.

Shear Stress DistributionStress Distribution due to Flexure

EFFICIENCY OF DIAGONALS?

MAX STRESSES NOT INCREASEDDEFLECTION INCREASES ONLY 3%

EFFICIENCY OF MEMBER SIZING?

CONCLUSIONS

STRONG BALANCE OF STRUCTURAL AND CONSTRUCTIONAL EFFICIENCY.

POWERS’ UNDERSTANDING WENT BEYOND BEAM-ANALOGY.

MAX STRESS IS SIGNIFICANTLY OVER-STRESSED BY CURRENT DESIGN STANDARDS.

MAX THEORETICAL DEFLECTION OF 0.76” (L/1600) IS IMPRESSIVE.

End.

[ADDITIONAL] Arch-truss behavior

Arch’s weakness for concentrated loadings is evidenced by increased proportion of load carried by truss.

Chords of the truss resist the moment which would otherwise greatly deform the arch.

Axial Forces in Arch-Truss under onlyMid-span Concentrated Live Load

DEVIATION FROM BEAM BEHAVIOR

STRESS CONCENTRATIONS AT SUPPORTS

Axial Force Concentration Bending Moment (Flexure) Concentration