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Not all historic bridges are equal. In fact, 20
years ago the Federal Highway Administration asked the state departments of
transportation to inventory and identify which of their historic bridges are
eligible for the National Register of Historic Places.
Today, DOTs report 1,669 bridges are listed in the
Register and 6,014 are eligible to be listed.
According to a report from the Workshop on the
Preservation and Management of Historic Bridges, about 25% of historic
bridges have been lost in the past 20 years. Some states have higher figures
— about 50% in Colorado, for instance, according to Eric DeLony, a
consultant with Engineering and Industrial Heritage, and Terry H. Klein,
executive director of the SRI Foundation. DeLony and Klein prepared the
workshop report.
Such losses are accelerating according to Jim
Cooper, a preservationist from Indiana. Metal-truss bridges go first, he
says — with over two-thirds of those in use 20 years ago now gone. About 24
National Register-eligible concrete bridges are gone, Cooper says, while
only a very few covered timber-truss structures were lost.
What’s being done
State departments of transportation recognize the
importance of historic bridges and all have rehabilitated one or more in the
past 20 years.
Some states report that such rehab is economically
sound as well as aesthetically pleasing. Vermont reports substantial savings
using rehab.
At the county level, Frederick County, Maryland
rehabs for the same reason, says Ken Harwood.
Rehabilitation often involves use of flexible design
standards.
A few states fund rehab with adopt-a-bridge
programs, usually sponsored by citizens’ historical groups.
Cutting-edge technology and historic bridges make a
good team in some locations. Arkansas developed a Geographical Information
System to manage, market, and mitigate impacts to historic bridges, DeLony
and Klein say.
California’s program includes seismic retrofits.
Fifteen historically significant bridges designed by Merrill Butler were
rehabbed in Los Angeles, for example.
Connecticut uses design guidelines to protect
rustic, art deco, and other bridges on its Merritt Parkway.
Ohio started an historic bridge program 27 years ago
— probably the oldest in the nation.
Oregon uses an in-house SWAT team for historic
bridge maintenance, according to DeLony and Klein. This team is responsible
for cathodic protection of the McCullough coastal bridges, historic bridges
on the Columbia River Gorge Scenic Highway, and the state’s covered bridge
program.
Pennsylvania operates a stone-arch bridge program.
Texas uses an in-house team for historic bridge
maintenance and has rehabbed 10 historic bridges.
Virginia’s maintenance program covers 63 National
Register-listed and eligible bridges.
In all, a dozen state DOTs report they have
engineers on staff with expertise in historic bridges.
Community interest in saving historic bridges is the
driving force behind most programs.
Workshop results
Development of historic bridge management plans are
needed at all agency levels, the group found.
Some workshop participants volunteered to help
create a National Historic Bridge Task Force to work with the Transportation
Research Board and other venues to further its goals.
Those attending also want to develop a National
Cooperative Highway Research Program to set out decision-making processes
agencies can use when deciding whether to rehab or replace historic bridges.
 Deer
Isle Bridge (1939), spanning Eggemoggin Reach, Deer Isle, Maine.
Designed by David B. Steinman, one of the premier suspension bridge
engineers of the 20th Century, this modest 200-foot span incorporated
several money-saving details in its fabrication and construction to make it
affordable to this small coastal Maine ÿresort community.
One of the money-saving features was the
pre-stressed, twisted strand cables, invented by Steinman, which he used in
both the main cables and suspenders. Deer Isle Bridge remains in service.
 Gothic
Arch (1862), spanning the bridal path south of the tennis courts at the
NW edge of the Reservoir, Central Park, New York City.
Another of the elegant cast-iron arch bridges in
Central Park, this one features Art Deco gothic design motifs that didn’t
become common until 20 years later.
 Campbell’s
Bridge (1907), Allentown Road (State Rt. 4027) spanning Unami Creek,
Milford Square vicinity, Bucks County, Pennsylvania. Though scheduled for
replacement, the bridge is an outstanding example of an early reinforced
concrete arch bridge that engineers were experimenting with during the first
decades of the 20th century of which many examples survive in Bucks County.
Prior to the era of the county engineer, the span was designed by A. Oscar
Martin, a consulting engineer to Bucks County. During demolition, Bucknell
University professor of civil engineering, Stephen Buonopane, and his
students hope to learn something of the type and displacement of the
reinforcing, concrete characteristics and other construction details that,
for concrete, is only revealed during erection or demolition.
 Rush’s
Mill Bridge (1869) spanning Plum Creek at the north end of the Union
Canal Bike & Hiking Trail, Sinking Spring vicinity, Pennsylvania. This is
another of the rare composite cast and wrought iron trusses, of which only
70 remain.
Up until the late-1970s, the bridge carried
pedestrian and vehicular traffic over the West Branch of Perkiomen Creek in
Hereford Township, Berks County, when it was relocated to the trailway, thus
preserving this nationally-significant structure.
 Roundup
Bridge (1894), spanning the Musselshell River, just south of U.S. Route
12, 12 miles east of Roundup, Montana. This Camelback truss, distinguished
by an upper chord of five slopes, is typical of the hundreds of
pin-connected, pre-fabricated metal truss bridges that are being replaced in
the U.S. Recent statistics suggest that more than half of America’s historic
bridges have been destroyed in the last 20 years, with metal trusses
disappearing at the highest rate. While not all historic bridges must be
saved, representative examples of all types should be preserved for
posterity.
 Bow
Bridge (1885) spanning the Sacandaga River, Hadley, New York. Another
product of the Berlin Iron Bridge Company of East Berlin, Cconnecticut, this
is a half-deck lenticular where the deck is located at mid-truss level. This
bridge won a highly-competitive Transportation Enhancement grant under the
TEA-21 program. The only problem was that county officials and state
engineers insisted that this 120-year old structure be rehabilitated to an
H-15 load capacity — an interstate standard. It is extremely difficult to
rehabilitate a light metal truss of this vintage to this capacity without
significantly altering its visual characteristics — something to be avoided
if possible. A compromise was worked out by setting two 36-inch-deep
built-up girders in from the sides of the trusses so they will not be easily
visible. The only unresolved design issue is an elegant railing that doesn’t
overwhelm the light, airy scale of the bridge.
Reprinted from Better Roads Magazine
February 2005 |
