Project Planning for Metal Truss Bridges

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


PANEL POINT CONNECTIONS


DECORATIVE FEATURES & AESTHETIC RAILINGS

Features of Metal Truss Bridges That Are Usually Not Significant

DECKS, STRINGERS & FLOORBEAMS


SUBSTRUCTURE UNITS & BEARINGS


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.



• 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

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Project Planning for Metal Truss Bridges