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Engineering Analysis of Covered Wooden Bridges from the HAER Summer 2002 Project

Engineering Analysis of Covered Wooden Bridges from the HAER Summer 2002 Project. Dylan Lamar Undergrad Researcher, Univ. of Arkansas Ben Schafer Asst. Professor, Johns Hopkins University. schafer@jhu.edu www.ce.jhu.edu/bschafer. Acknowledgments. NPS – HAER and summer 2002 team

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Engineering Analysis of Covered Wooden Bridges from the HAER Summer 2002 Project

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  1. Engineering Analysis of Covered Wooden Bridges from the HAER Summer 2002 Project Dylan LamarUndergrad Researcher, Univ. of Arkansas Ben Schafer Asst. Professor, Johns Hopkins University schafer@jhu.eduwww.ce.jhu.edu/bschafer

  2. Acknowledgments • NPS – HAER and summer 2002 team • Erika Stoddard, Francesca da Porto

  3. Objectives • To use modern engineering analysis to better understand historic covered wooden bridge forms • To better understand the intent of the design details selected by the historic builder through engineering analysis • To provide a context whereby historic engineering structures can be understood and discussed in relation to modern structures

  4. Pine Grove BridgeChester/Lancaster County, PABurr truss (arch-truss)Captain Elias McMellen builder1884double span, two at 93 ft HAER PA-586 (2002)

  5. Brown BridgeShrewsbury, VTTown lattice Nichols M. Powers builder1880single span, 112 ft HAER VT-28 (2002)

  6. Pine Grove Bridge

  7. Pine Grove Bridge architectural rendering from summer 2002 HAER documentation team

  8. Longitudinal System architectural rendering from summer 2002 HAER documentation team

  9. Longitudinal System Model

  10. Understanding Dead Load

  11. Dead Load Response - Separate (shaded bars proportional to axial forces in bridge members) 33 k 47 k

  12. Dead Load Response - Combined 9 k 35 k

  13. Dead Load Subtleties axial (F) 46 k-in. moment (M) (shaded areas indicate magnitude of bending in a member)

  14. Dead Load Member Demand Highlights • Arch (at end)s = -391 (C) psi(allowable 1000 psi) • Truss (post 4)s = -272 (C) psi(allowable 1000 psi) • Truss (post 5)s = 261 (T) psi(allowable 925 psi)

  15. Arch-truss synergy Midspan deflection (dead load) Structural modeltruss alone = 0.96 in.arch alone = 0.91 in. Simple parallel combinationtruss+arch = 0.47 in. Structural modeltruss+arch = 0.25 in. arch truss arch-truss is far stiffer than a simple addition of the two separate systems

  16. Live Loads?

  17. 5 k 5 k Live Load Arch Deflections d = 2.0 in. d = 4.8 in.

  18. 5 k Live Load Results 5 k d = 0.07 in. d = 0.06 in.

  19. Total Load Member Demand Highlights (results for total load = dead load + quarter point live load) • Arch (at end)s = -489 (C) psi(allowable 1000 psi) • Truss (post 4)s = -394 (C) psi(allowable 1000 psi) • Truss (post 4)s = 458 (T) psi(allowable 925 psi)

  20. Pine Grove Bridge Thoughts • Arch is the dominant load carrying member • Arch success w/ live loads depends on truss • Stiffness of system greater than sum of parts • No overstressed members • Tension in some diagonals under live load • Far more enlightening analysis of these forms is possible through further engineering study

  21. Brown Bridge

  22. Brown Bridge Structural System architectural rendering from summer 2002 HAER documentation team

  23. Brown Bridge Longitudinal System architectural rendering from summer 2002 HAER documentation team

  24. Brown Bridge Longitudinal Model

  25. Dead Load Behavior

  26. Dead Load 37 k 25 k 22 k 40 k

  27. Brown Bridge Longitudinal System (3 x 9 7/8 in.) (3 x 11 in.) architectural rendering from summer 2002 HAER documentation team

  28. Dead Load Subtleties moment 170 k-in.

  29. Influence of Bolster Member stresses are typically reduced by ¼ and reductions as high as ½ are possible due to the addition of the bolster.

  30. Stress Demands

  31. Alternate Lattice Forms

  32. Alternate Lattice Forms Alternate lattice forms with greater structural efficiency are possible, but constructional efficiency drives the actual design.

  33. Brown Bridge Thoughts • Structural system = beam behavior • Stiff! L/1600 for our loading cases • Sizing of primary bottom chord member reflects deeper understanding of member demands • Bolster relieves stress concentrations • Form follows construction efficiency more than structural efficiency

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