BRIDGE RESCUE Treated like family - Amazon Web … · Treated like family H ... the bridge features...
Transcript of BRIDGE RESCUE Treated like family - Amazon Web … · Treated like family H ... the bridge features...
42 November 2015 • ROADS&BRIDGES
Treated like family
How do you rehabilitate a
100-year-old masonry arch
bridge without impacting its
natural aesthetic?
Factor in that the bridge is the only access
point for a 50-acre family property. These
were the challenges of the Thompson family,
owners of the Furness Bridge and descendants
of Philadelphia architect Frank Furness and
influential Shakespearean scholar, Horace
Howard Furness.
The 60-ft Furness Bridge spans Ridley Creek
on the charming grounds of the former Upper
Bank Nursery in Media, Pa. With one of the
bridge’s three arch spans partially collapsed,
the Thompson family required immediate
engineering expertise to ensure their safe pas-
sage, as well as that of any emergency vehicles.
Additionally, the family needed a design that
BRIDGE RESCUE
By Joseph Spadea, P.E., and William Cameron, P.E.Contributing Authors
ROADSBRIDGES.com 43
would preserve the appearance and legacy of
the bridge and its picturesque setting.
Moving fast for emergency vehicles
Pennsylvania is home to one of the largest
inventories of stone-arch bridges in the U.S.
The arch form is both functional and aesthetic
and was the natural bridge of choice for early
settlers, remaining popular into the early 20th
century. The Furness Bridge, built in 1916 by
Fairman R. Furness, the Thompson family’s
great uncle, was no exception. The bridge con-
sists of two 16-ft spans and one 20-ft center
span. With an overall length of just under 100
ft, the bridge features elliptical concrete-arch
barrels and stone masonry spandrel walls,
piers and abutments. The use of unreinforced
concrete for the arch rings, instead of stone
masonry, reflected the country’s shift in
building-material preference during the
period. Fairman Furness, founder of the
Upper Bank Nursery, was a charismatic artist,
horticulturist and lecturer. After serving as
Third Secretary to the U.S. Embassy in Russia,
he would return home in 1910 to purchase
the 50-acre farm which he described as “the
promised land.” Amongst perfectly manicured
topiaries, Fairman’s bridge also served as a
focal point of the nursery entrance to receive
his distinguished client base. Among them
was Pierre duPont. Furness forged a close
relationship with DuPont, and would supply
Designers go extra mile to rehab old Pa. span
44 November 2015 • ROADS&BRIDGES
exotic plants for his personal estate now
known as the world-class Longwood Gardens
in Chadds Ford, Pa.
Within the last two years, members of the
Thompson family sought to develop three
of the five lots at the exclusive location. But
before this plan could progress, the family
had to be sure that the nearly 100-year-old
bridge could support the weight of construc-
tion vehicles needed to install 600 ft of access
roadway to the lots. In November 2014,
Pennoni was contacted to perform an in-depth
inspection and load rating of the bridge. The
findings from the preliminary inspection
revealed settlement and rotation of the east
abutment leading to transverse cracking and
failure in the adjacent Span 3 arch ring. A
partially paved bridge roadway also allowed
excessive water penetration and lateral spread
of the fill material during winter months. The
repeated freeze-thaw cycle had accelerated the
rotation and movement of the abutment and
north spandrel wall. Scour and undermin-
ing of the piers and abutments also were of
significant concern. To assist with recommen-
dations for repair, Pennoni next completed a
geotechnical investigation at the bridge piers
and abutments. With no existing bridge plans
available, high-definition laser scanning was
utilized to capture the essential 3-D bridge
geometry and site topography. The end result
of the HD laser scan was a 3-D point-cloud
scale model, yielding accurate and detailed
design drawings. Not only did this reduce time
recording dimensions in the field, but also it
served as a historical record for the bridge.
An analysis of the bridge using the Modi-
fied Military Experimental Establishment
(MEXE) Method resulted in a load posting of
5 tons, a capacity insufficient to sustain mod-
ern trucks and emergency vehicles. Developed
in England, the MEXE Method provides an
empirical basis for the evaluation of stone
masonry and concrete-arch bridges. This
approach takes into consideration parameters
such as arch geometry and material, type
of fill, mortar condition, and the size and
orientation of observed cracks and defects.
As the long-term strength of such bridges is
often difficult to assess, the MEXE Method is a
practical and cost-effective alternative to finite
element analysis.
In addition to the three parcels planned
for development, the family also occupied
two existing homes on the land. They too
relied on the bridge for access. Given the poor
condition of the bridge and inability for fire
and medical vehicles to access the occupied
homes, the focus shifted to developing an
accelerated bridge rehabilitation scheme.
Additionally, the design would need to
account for pedestrian access during construc-
tion. And last but not least, all of the above
would need to be accomplished without
impacting the aesthetics of the bridge.
Turning it inside outBased on the findings and needs of the
family, Pennoni proposed a unique design—
rehabilitate the bridge from the inside out by
constructing a new bridge within the existing
structure. Pennoni’s design would call for the
removal of the roadway and earthen fill over
the existing concrete-arch rings and masonry
walls. The non-structural fill would then
be replaced with a reinforced concrete slab
made integral with two of the existing arch
spans and masonry façade. The compromised
third span arch ring required complete
replacement. Additionally, design experts
called for general masonry repointing and
Rehabilitation of the bridge was predicated upon the need for construction vehicles to safely cross it in order to build 600 ft of access road.
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the restoration of the adjacent abutment and
wingwall, which was progressively displaced
from its initial geometry. Pennoni’s design
also included a stream ford-style crossing
to maintain pedestrian access for the family
during construction. The crossing utilized
standard PennDOT-approved details and
doubled as an access route for construction
vehicles and workers.
There were other design considerations
as well. Pennoni’s bridge experts needed to
determine if the existing stone masonry piers
could support design vehicles. The increased
superstructure load resulting from the use
of a reinforced concrete slab also would
garner attention. Geotechnical test borings
were ordered and taken through the center
of the masonry piers. The borings were
used to calculate the strength of the piers,
which were found to be adequate. They also
revealed that competent bedrock underlaid
most of the bridge.
A waterproof membrane to be placed on
the proposed slab was included in the design
to minimize water intrusion over the bridge
and abutments. Additionally, the roadway
design included the addition of a rolled
asphalt curb to channel water away where there
was previously no protection from intrusion.
To facilitate utility installation for the new
development, Pennoni included provisions for
future gas, electric and communications lines
to be encased beneath the new bridge deck.
Undermining of the piers and abutments
was observed during the initial inspection.
To address this challenge, Pennoni designed
waterline concrete aprons for the piers and
abutments as an effective long-term scour
countermeasure. The apron design called for
keying into the relatively shallow bedrock,
with removed material to be repurposed as
additional scour and channel protection. Work
within Ridley Creek, including cofferdams and
center-bracing for the third span replacement,
would require close coordination with the
Pennsylvania Department of Environmental
Protection (PADEP). Pennoni’s environmental
scientists and engineers provided the PADEP
with detailed drawings. Design elements were
arranged to minimize and avoid regulated
resources. Erosion and sediment pollution-
control plans also were developed for the
agency’s review. Regular communication and
strong relationships with regulators proved
to be a key element in meeting the project-
permitting schedule. The end result was
PADEP approval of a GP-11 permit in less than
30 days. Permitting for the temporary ford
crossing also was approved prior to contract
bid preparations. These permit approvals
also ensured that the project would hit the
necessary construction timetable and avoid
in-stream time-of-year restrictions associated
with Ridley Creek, a trout-stocked fishery.
Pennoni remained actively engaged
with potential contractors throughout
design. Three contractors were invited to
review Pennoni’s design and to express
interest in the construction. The design
was then refined based on the contractor’s
constructability feedback and the need to
Above: Based on the findings and needs of the family, Pennoni proposed a unique design—rehab the bridge from the inside out by constructing a new bridge within the existing structure.
Below: A rent in the third span arch ring discovered following earth fill excavation.
46 November 2015 • ROADS&BRIDGES
stage construction to maintain pedestrian
access for the Thompson family. Special
provisions to maintain bridge aesthetics also
were included in the contract documents.
Details for cleaning and repointing the stone
masonry were developed based on PennDOT
standard specifications. Pennoni also called
for a mortar analysis and field test panel
to ensure matching of new stone masonry
elements with those existing.
Although price was considered, final
contractor selection was based on qualifica-
tions, responsiveness during the invitation-to-
bid period and approach to completing the
project. Pennoni’s contract bid review phase
culminated with the selection of specialty
bridge contractor J.D. Eckman Inc. of Atglen,
Pa., to perform the rehabilitation work. Eck-
man provided value in a competitive bid, and
its approach also included pedestrian-friendly
amenities such as temporary pick walkways
for direct access over the bridge.
Final design was completed in under three
months thanks to close collaboration with
the Thompson family and the continuous
collaboration between Pennoni’s bridge
professionals, reviewing agencies and the
contractors. Construction commenced in July
2015 and the bridge was on track to reopen
by November 2015—approximately one year
after initial concerns arose. Upon completion,
the aesthetic of the Furness Bridge will have
been preserved. The bridge will be able to
support the weight of modern construction
and emergency vehicles and will be capable of
providing another century of service. R&B
Spadea is a senior engineer and project manager at Pennoni, Newark, Del. Cameron is a senior engineer and project manager at Pennoni, Mechanicsburg, Pa.
For more information about this topic, check out the Bridges Channel at www.roadsbridges.com.
Top: The concrete arches revealed.
Above: Workers set a rebar skeleton for pier and abutment “aprons.”