Post on 25-Jun-2015
Integrating Assessments of Historical Significance into Project Planning for Metal Truss Bridges
Patrick Harshbarger
TranSystems | Lichtenstein
Establishing Significance – Why All the Fuss?Truss Bridges – A Place of Honor in American History
U.S. was the world leader in truss development in the 19th century Trusses chronicle the transition from wood to iron and steel Incredible diversity of design 400-500 truss bridge patents! Bridge engineering as a profession came of age with the truss Most surviving metal trusses (post-1900) are not from the “golden age”
of truss development but later standard solutions Sound scholarship and historic context is the basis for defining
significance, not that it’s merely old and standing
The Great Timber Truss Bridges, 1790s-1830s
Timothy Palmer’s Permanent Bridge – Philadelphia 1804
Theodore Burr’s Arch-braced Truss, 1806
Evolution from Wood to Metal Truss Bridges, 1800s-1840s
Ithiel Town’s Lattice Truss, 1820
William Howe’s “Combination” Truss, 1840 Richard Osborne’s All-Iron Howe Truss, 1846
Squire Whipple’s Bowstring Truss, 1849
Normans Kill Bridge, Albany, N.Y.
Cast & Wrought Iron Bridges, mid 1840s-1870s
Reading Halls Station Bridge, near Williamsport, Pa., ca. 1846
The Great Era of Competition Among Bridge Builders, 1870s-1901 – Hundreds of Patented Designs & Details (A Few Great Ideas, Some Good, Many Bad, and Most Short Lived)
Innovation and competition had risks …. by modern standards structural theory and metallurgy were poorly understood
THE MATURATION OF PROFESSIONAL BRIDGE ENGINEERING, 1890s-1900s – in part due to concerns over the safety, economy, and durability of metal trusses.
Pratt Truss – the American standard truss, circa 1880-1905
Warren truss – the “new” American standard truss, circa 1905-1960
Cantilever and Long-Span Trusses – Poughkeepsie Bridge, 1888… and longer and longer into the 20th century
Features of Metal Truss Bridges That Are Usually Significant
TRUSS TYPES & WEB PATTERNS
Features of Metal Truss Bridges That Are Usually Significant
PANEL POINT CONNECTIONS
Features of Metal Truss Bridges That Are Usually Significant
DECORATIVE FEATURES & AESTHETIC RAILINGS
Features of Metal Truss Bridges That Are Usually Not Significant
DECKS, STRINGERS & FLOORBEAMS
Features of Metal Truss Bridges That Are Usually Not Significant
SUBSTRUCTURE UNITS & BEARINGS
Features of Metal Truss Bridges That Are Usually Not Significant
LOCATION
Henzey’s Bridge, relocated from Wanamakers, Pa., to Central Pennsylvania College
Case Study 1: Using Significance to Help Define Project Outcomes – A Rare & Technologically Significant Truss (Criterion C)
• Urban setting with high ADTs
• Failing deck system
• Concern for impact damage
• Lower panel points encased in concrete with trapped moisture promoting deterioration of iron.
• Originally a railroad bridge, relocated to carry vehicular traffic about a century ago.
Case Study 1: Using Significance to Help Define Project Outcomes – A Rare & Technologically Significant Truss (Criterion C)
Case Study 1: Using Significance to Help Define Project Outcomes – A Rare & Technologically Significant Truss (Criterion C)
• Relocate trusses to museum for restoration and display
• HAER documentation prior to removal
Case Study 2: Using Significance to Help Define Project Outcomes – Technologically Significant Pony Truss (Criterion C)
• Pratt pony truss, built in 1884, significant for hand-riveted panel points by prominent regional builder
• Low ADT and no history of accidents, but substandard width and under capacity for school buses and emergency vehicles
• Community valued appearance and sound/feel of wood plank deck
• Shipped truss lines and railings off-site, cleaned, repaired, and painted to be attached to sides of new superstructure
• Widened by about 6 ft. maintaining scale
• HAER documented and prepared a small historical exhibit for display in county courthouse.
Case Study 3: Using Significance to Help Define Project Outcomes – Truss That Is Historic for Connecting Point A to Point B (Criterion A)
•A conventional ca. 1910 Warren deck truss
• High ADTs and traffic study shows need for extra lane for turning movements
• Significance is scale and overall character, not truss details. Public values “see through” look of bridge
• Adjacent to a historic district
• Alternatives under consideration
• replacement with preservation of truss lines
• total replacement on same alignment but with open spans (not fill) and bents (not solid piers)
Case Study 4: Using Significance to Help Define Project Outcomes – Contributing to a Historic District
• Standard Pratt thru truss, built in 1900, contributing to rural historic district
• Low ADT, good sight lines, and no history of accidents
• Took widening off table to maintain character & scale. Public also identified keeping existing width and alignment as their primary concern
• Truss members have numerous welded repairs, some have section loss, and most lower chord bars bent and warped
• Rehab by replacing and strengthening existing members – no adverse effect
Case Study 5: Poor Understanding of Significance Contributed to Project Delays
• Significance was misstated and not well-established at project’s outset.
• Rare design?
• Contributing to Historic District?
• A fine example of a standard type?
• The best answer was none of the above, but the cat was out of the bag and it was too late
• Truss panel points irreparably altered by welding, waterway opening inadequate, width inadequate
• Truss lines will be saved and reattached to sides of new bridge
SUMMARY: Why Is Understanding Significance Important?
NR Criteria Recognize Many Ways To Be Historic All Bridges Identified as Historic Must Be Treated Equally Under the Law But Not All Historic Metal Truss Bridges Are Equally Significant Section 4(f) and NEPA Processes Stress “Holistic” Approaches to Decision
Making – Cannot Avoid or Minimize Harm without Knowing What It Is That Makes the Bridge Significant
Significance Helps to Analyze What Are Prudent & Feasible Alternatives – Some Bridges May Deserve Greater Effort than Others Prudent = $ and safety Feasible = technically possible
A Clear Statement of Significance Educates Decision Makers and the Public
• BALANCING HISTORIC SIGNIFICANCE WITH ENGINEERING AND OTHER ENVIRONMENTAL ISSUES TO DETERMINE PRESERVATION POTENTIAL
• MANUAL OF BEST PRACTICES• BRIDGE-BY-BRIDGE BASIS
• IDENTIFYING & PRIORITIZING BRIDGES WITH GOOD PRESERVATION POTENTIAL EITHER TO REMAIN IN SERVICE OR TO FIND ALTERNATIVE USES
• IDENTIFYING THOSE METAL TRUSSES THAT ARE MOST SIGNIFICANT AND MAY DESERVE GREATER LEVELS OF EFFORT
• PRO-ACTIVELY IMPLEMENTING BEST MAINTENANCE PRACTICES SO THAT BRIDGES IDENTIFIED FOR PRESERVATION DON’T REACH “CRISIS” CONDITIONS
MANAGEMENT PLANS
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM PROJECT 25-25, TASK 19
– HISTORIC BRIDGE REHABILITATION OR REPLACEMENT GUIDEANCE
http://www.trb.org/NotesDocs/25-25(19)_FR.pdf
THANK YOU