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.

ROADSBRIDGES.com 45

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.”