1. 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.eduschafer@jhu.edu www.ce.jhu.edu/bschaferwww.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 Bridge Chester/Lancaster County, PA Burr truss (arch-truss) Captain Elias McMellen builder 1884 double span, two at 93 ft HAER PA-586 (2002)

5. Brown Bridge Shrewsbury, VT Town lattice Nichols M. Powers builder 1880 single span, 112 ft HAER VT-28 (2002)

6. Pine Grove Bridge

7. 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 35 k 9 k

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

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

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

16. Live Loads?

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

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

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

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 11 in.) (3 x 9 7/8 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 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