Technical Preservation Services 

Preservation 

Tech Notes 

METALS 

NUMBERS 

Rehabilitating a 

Historic Truss Bridge 

Using a Fiber Reinforced 

Plastic Deck 

Chad Randi 

Technical Preservation Services 

National Park Service 

Substitute materials may be used 

to replace deteriorated or missing 

elements of a historic bridge 

provided the character of the 

historic resource is preserved. 

WILSON'S BRIDGE 

Chester County, 

Pennsylvania 

Introduction 

Iron truss bridges were an essential element of America's Steam Age. In the 

decades following the Civil War, the 

combination of strength, durability and 

affordability made iron bridges a 

favored means of traversing a ravine or 

river. They were a symbol of industrial 

power, a testimony to engineering efficiency, and a source of local pride. 

Over the years, however, the increasing 

load capacity and widths required by 

succeeding generations of vehicles, as 

well as inadequate maintenance and 

wholesale replacement programs, have 

reduced their number dramatically. As 

a result of this decline, an increased 

emphasis is now being placed on identifying and implementing methods to 

preserve as many of the remaining iron 

truss bridges as possible. One such 

effort involves Wilson 's Bridge, a 

wrought iron bridge constructed in 

1886 to cross Valley Creek in 

Tredyffrin Township, Pennsylvania. 

Also known as County Bridge 

#166, this span now lies within the 

boundary of Valley Forge National 

Historical Park. It was fabricated by 

the Phoenix Iron Company and constructed by John Denithome & Sons, 

both of nearby Phoenixville, 

Pennsylvania. Because of its location, 

connection to local industrial history, 

and the scarcity of wrought iron 

bridges still in use, Wilson 's Bridge is 

recognized as a valuable park and community resource, eligible for listing in 

the National Register of Historic 

Places. 

The bridge features a single-span 

Pratt pony truss, approximately sixty 

feet long and sixteen feet wide (see figure 1). When first constructed in the 

late l 9'h century, the bridge included an 

oak plank deck supported by timber 

stringers running parallel to the roadway. Stringers, in tum, rested upon 

four iron floor beams that hung from 

the superstructure by large U-shaped 

bolts. Fieldstone abutments supported 

the bridge with wingwalls extending 

down to the ground line. 

The bridge had undergone numerous repair campaigns over the past 

hundred years, some of which included 

substituting new materials and components. Between 1930 and 1940, the 

timber stringers were replaced with 

steel I-beam stringers. The deck had 

been replaced on four occasions, most 

recently (in the rnid-l 960s) with a naillaminated wood deck and asphalt wearing surface. 

At the time of its rehabilitation in 

1998, Wilson's Bridge carried a dead 

end road linking several residences and 

a park maintenance facility with the 

rest of the national park and county 

road system. The dirt road had a low 

average daily traffic level of approximately thirty-five vehicles; it was more 

commonly used by hikers and horseback riders travelling along the park's 

trail system. A four-person bridge 
Roadway 

Profile 

Floorbeams 

Deck and Wearing 

Surface Masonry 

/Wingwall 

---------------

Creekbed,,,ir Stringers \ Masonry Abutment 

Figure l. Drawing of Wilson's Bridge showing major structural and site components. Drawing: Chester County Engineering Department. 

crew within the Chester County 

Engineering Department was responsible for maintaining Wilson's Bridge, 

along with the county's other one hundred spans, one third of which were 

considered historic. Recognizing that 

the deterioration of Wilson's bridge 

had reached a point where the structure 

would soon be unable to support a 

minimum required load capacity, the 

bridge team sought a means of retaining the bridge in place, while upgrading its structural capacity. Appropriately rehabilitated, the span could then 

continue to function both as a vehicular 

bridge and as a link to equestrian and 

hiking trails, with little effect on its 

historic appearance. 

Problem 

By the late 1990s, there were nmnerous indications that Wilson's Bridge 

required immediate intervention. The 

current asphalt wearing surface showed 

extensive cracking and was losing 

adhesion with the deck below (see.figure 2). The thirty-five year old naillaminated deck, made up of individual 

timber boards placed on edge and 

spiked together to form a 4-1 /2" thick 

continuous smface, was uniformly soft 

and unable to support the wearing surface above. The steel stringers were 

also in poor condition (see figure 3). 

Though the full extent of their deterioration was not clear until the deck was 

removed, preliminary examination 

indicated that the I-beam stringers were 

severely corroded, particularly over the 

abutments where they had suffered 

approximately fifty percent section 

loss. 

At this time, the wrought iron truss 

had a posted weight load limit of seven 

tons. This restricted the size and type 

of vehicles (including emergency vehicles) able to reach the residences and 

maintenance facility on the opposite 

side of Valley Creek. Since the bridge 

would need to undergo significant 

repair work, ways of increasing the 

allowable loads on the bridge would 

also be explored. In addition to these 

immediate concerns, the bridge was 

due for cyclical maintenance work 

including repainting the truss and 

repairing and repointing the stone abutments and wingwalls (see figure 4). 

Any repair program also had to protect the creek flowing beneath Wilson's 

Bridge. Pennsylvania's Department of 

Environmental Protection identified 

Valley Creek as an "Exceptional Value 

Stream," a designation that meant all 

work on the bridge had to be carefully 

designed to exclude the chance of 

water contamination or disturbing the 

creek bed and banks. 

Solution 

Due to the bridge's historic significance and the several hundred thousand dollar cost of a new span, the 

Figure 2. Before rehabilitation, Wilson's Bridge had an asphalt wearing 

surface tbat was delaminating from the deck below. Extensive cracks 

and numerous potholes were also present. 

Figure 3. A view of the underside of Wilson's Bridge before rehabilitation. The timber nail laminated deck sat on steel "I-beam" stringers 

which, in turn, were supported by wrought iron floor beams. Failure of 

the wearing surface had allowed water to penetrate the laminated deck 

planks causing further deterioration to both the deck and the stringers 

below. 

2 
Existing Bridge Rehabilitated Bridge 

/ 

U-bolt 

Hanger 

Bituminous 

Wearing Surface 

Oak Plank 

Wearing Surface 

New Timber 

Curb~ 

Steel Stringers Floor Beam FRP Deck 

I 13" 

\Replaced 

U-bolt 

Figure 4. The Wilson Bridge wingwalls had 

loose and crumbling mortar joints. Much of 

the wall surface required removal of this deteriorated material and significant repointing. 

only option seriously considered was to 

rehabilitate the existing bridge. A plan 

was developed to repair the superstructure and abutments following standard 

practices, along with the replacement 

of the deteriorated 1960s deck and 

wearing surface. Several conventional 

choices were identified for the replacement deck: a glue-laminated timber 

deck, a nail-laminated timber deck 

similar to the existing surface, or a 

steel and concrete deck. Each was 

ruled out in turn before a new material 

that met all of the bridge rehabilitation 

requirements was identified, evaluated, 

and selected for the job. 

With a typical lifespan of over thirty years, glue laminated timber decks 

are reasonably cost effective and would 

be in keeping with the historic character of Wilson's Bridge. Chester 

County engineers had used "glulam" 

decks on three other recent bridge 

rehabilitation projects. The decks are 

usually pressure-treated with chemical 

wood preservatives. In the previous 

county bridge projects, some of the 

preservative leached out of the decks 

and into waterways below. In all three 

projects, remediation was then undertaken . While at this time there is not a 

consensus as to the environmental 

impact of such leached preservatives 

from bridge applications, the ecological 

importance of Valley Creek led the county to look for a different solution. 

A nail-laminated bridge deck was 

another treatment option. This type is 

usually assembled and attached to the 

structure piece by piece. Such a process 

would have required closing the dead end 

road for approximately six weeks - one 

week to remove the old deck, one week 

to install new steel stringers, and four 

weeks to build the new deck in place. 

Because the bridge was the only means 

for residents to reach the main roads, 

either a temporary span would have to be 

constructed or the residents and the park 

maintenance facility would have to be 

relocated for the duration of the project, 

both at a prohibitive expense for this project. This deck system also offered no 

improvements in load capacity. Though 

comparable in cost to a glulam deck, nail 

laminated decks have a significantly 

shorter expected life span, and correspondingly higher costs. This alternative 

was therefore not attractive. 

Typical steel and concrete deck systems were also considered. As with the 

timber-laminated decks, this system 

would have required replacing the steel 

stringers with heavier new stringers due 

to the increased weight of the deck. 

Their weight, combined with that of a 

Figure 5. A section drawing of the Wilson 's Bridge. The left side shows the construction of the span before rehabilitation, while the right side shows 

removal of the stringers and the installation of an FRP deck with a plank wearing surface. Both systems were approximately thirteen inches thick. 

Drawing: Chester County Engineering Department. 

3 
new concrete deck, would have 

decreased the load capacity of the 

bridge to as little as three or four tons. 

Such a change would then require relocation of the park facility and make the 

passage of emergency vehicles and 

delivery of heating fuel to the residences virtually impossible. Because 

the load capacity would be reduced 

rather than increased, this alternative 

was also ruled out. 

After most conventional solutions 

were assessed, Chester County engineers sought an alternative approach. 

Recent articles in technical publications and presentations at several conferences suggested the possible application of plastic composite decks for 

bridge rehabilitation work. The material is essentially a very high strength 

structural fiberglass commonly known 

as fiber reinforced polymer (FRP). 

Though in existence since the end of 

World War II, new technological developments have made its use in large 

structural sections such as bridge decks 

increasingly practical. 

The characteristics of an FRP deck 

matched those sought by Chester 

County for Wilson's Bridge (see figure 

5). Composite decks are chemically 

stable and resistant to deterioration 

caused by moisture, freeze-thaw 

cycles, chemicals and other forces that 

act on bridge surfaces. They also have 

a higher strength-to-weight ratio than 

conventional decks. This strength 

makes the use of steel stringers unnecessary. The reduced dead load of an 

FRP decked bridge allows a corresponding increase in the live loads the 

bridge is able to carry. Resins used in 

the FRP deck harden to a solid state so 

they can not leach or deteriorate, thus 

avoiding the release of any potentially 

harmful materials into the surrounding 

area, especially the creek below. 

Though the cost of a composite deck is 

somewhat higher than that of a naillaminated timber deck, the estimated 

life span of the former is more than 

twice as long. Additional cost savings 

are usually realized in the short construction times and low level of 

required maintenance. 

The installation of a fiber reinforced polymer deck on Wilson 's 

Bridge would be part of a thorough 

rehabilitation program. Repointing the 

masonry abutments and wing walls, 

replacing steel U-bolt hangers, and 

stripping and repainting the superstructure would be undertaken prior to the 

delivery of the composite deck. 

4 

Figure 6. Taken at the composite manufacturer, this photo shows layers of fiberglass arranged 

around the deck core and placed in the mold. The mold was then infused with a resin that sealed the 

layers into a one-piece panel. Photo: Hayati Catbas, Hardcore Composites. 

Deck Fabrication 

The county worked with a manufacturer of fiber reinforced composites that 

had extensive experience in developing 

structural applications for the material. 

Using fiber-reinforced polymers, the 

company had fabricated a dozen traffic-bearing bridges or bridge decks in 

the previous four years, as well as 

numerous piers, pilings, and other 

marine structures. Plans were made 

for fabricating the deck at their facility 

in New Castle, Delaware, then shipping it as a single panel to the Valley 

Creek site where it would be installed. 

The manufacture of the FRP deck 

was made possible by a number of 

recent innovations in composite material production. The Wilson's Bridge 

deck was fabricated using a adaptation 

of the technique known as vacuum 

assisted resin transfer molding, or 

VARTM. In this technology, a plastic 

honeycomb core is sandwiched 

between fiber layers in a mold (see figure 6). The mold is sealed and a vacuFigure 7. After the panel had set into a solid mass, workers removed it from the mold and sanded 

and finished all surfaces. The last steps included painting the deck and installing the oak plank 

wearing surface. Photo: Hayati Catbas, Hardcore Composites. 
Figure 8. U-bolts connected each of the four floor beams to the superstructure above. Prior to rehabilitation, some bolts, like the one in the photo, showed signs of impact damage. The engineers 

determined that all of the wrought iron bolts were near the threshold of their stress level and called 

for replacement with high-strength steel U-bolts of the same dimensions. 

um removes air while drawing a resin 

into the mold and through the 

fiber/core sandwich. It sets into a uniform, seamless panel that is then sanded and finished (see figure 7). Closed 

vacuum infusion molding processes 

such as VARTM allow for the creation 

of three-dimensional structural elements that are extremely light yet 

durable. 

Fabricated as a single piece, the 

deck measured approximately sixty 

feet long by fifteen feet wide by thirteen inches thick. After fabrication, it 

was painted with a primary coat of 

epoxy paint that bonded well to the 

plastic substrate, followed by a 

polyurethane top coat matching the 

bridge superstructure color that further 

protected the composite from the 

degrading effects of ultraviolet light. 

The white oak wearing surface was 

then attached by drilling lag bolts 

through the planks and into the top of 

the deck. Low, timber curbs were also 

installed on either side of the deck. 

Figure 9. A worker applied adhesive to the floor beams where each of the bearing pads would be 

positioned. Four pads were installed on each floor beam to cushion the deck, evenly distribute loads 

to the floor beam and provide a secure fit between beam and deck. 

When the entire assembly was complete, the deck was loaded onto a tractor-trailer and shipped forty miles to 

the bridge site. Because of the size of 

the deck, it was necessary to obtain 

special hauling permits similar to those 

required for transporting double-wide 

manufactured homes. Approximately 

one month passed between the order 

date and the ship date. 

Truss and Abutment 

Rehabilitation 

While the deck was being fabricated in 

Delaware, the Chester County bridge 

team began work on the truss and abutments. Both the masonry abutments 

and wing wall approaches required significant repointing. Existing mortar 

from earlier patch jobs had separated 

from the joints in many locations. This 

deteriorated mortar was gently raked 

out with hand tools and a new mortar 

pointed into place. The existing 

asphalt wearing surface, nail-laminated 

deck, and steel stringers were all 

removed. Wrought iron U-bolt hangers, several of which had suffered 

impact damage, were known to be 

weak links in the structure. As part of 

the rehabilitation they were replaced 

with high-strength steel substitutes that 

had a higher structural capacity than 

the original hangers but were identical 

in appearance (see figure 8). 

After structural work was completed, the entire truss was repainted with 

a three-coat system. The base coat was 

an inorganic, zinc-rich primer that 

inhibits oxidization. Epoxy paint was 

selected for the intermediate coat. 

Though the epoxy bonded well to the 

primer, it was not sufficiently durable 

for a finish coat, so a hard and ultraviolet light resistant polyurethane paint 

was used. To protect the creek, all surface preparation was undertaken with 

shrouded vacuum power tools that significantly reduced airborne dust, chips, 

and debris. Work along the outside 

and underside of the bridge took place 

from a floating platform. Additionally, 

all vehicles and machinery were kept 

on the roadway and away from the 

creek banks. 

Installation of New Deck and 

Wearing Surface 

Work on repairing and repainting the 

truss was completed just prior to delivery of the deck. A six-person team of 

engineers and workers from the composite manufacturer assisted the county 

bridge crew. First, four bearing pads 

5 
Figure 10. When the deck arrived at the site, the trailer was backed onto the bridge approach and crane 

cables secured to the deck. Although the road beyond that side of the bridge is a dead end, the truck driver 

was able to maneuver across a vacant site to get the deck on the opposite side of the creek. 

Figure 11. As the deck was lifted off the trailer and onto the floor 

beams, workers wedged timbers and bars along the sides to prevent the deck from damaging the truss and to help place the deck 

in position. 

were glued at regular intervals to the top 

surface of each floor beam (see figure 9). 

A twenty-ton crane then lifted the deck 

off the trailer bed and, with guidance 

from the workers, slowly lowered it into 

position (see figures 10 and 11). Lag 

bolts with steel clips were inserted into 

predrilled holes along the underside of 

the deck, securing the deck to each of the 

floor beams (see figure 12). 

, Evaluation 

The wrought iron truss is 

the most historically significant component of 

Wilson's Bridge. In rehabilitating the span, priority 

was given to preserving 

and upgrading the truss 

without introducing additional members, cables or 

other strengthening systems. The deck and wearing surface present at the 

time of the rehabilitation 

were fourth generation. 

Replacing them with a new 

composite deck was an 

innovative solution that 

kept the previously altered bridge 

in service without adversely affecting its historic character. Other 

work, including replacing the floorbeam hangers in kind and repainting the abutments, was also sensitive to the historic appearance of 

the bridge. 

When originally constructed, 

iron bridges were valued for their 

structural efficiency and ease of 

replication. Today, those same 

benefits are attributed to composite 

materials. Fiber reinforced polymer technology provided what is 

anticipated to be a long-tem1 solution to the challenge of rehabilitating, maintaining, and continuing 

the use of Wilson's Bridge. Only 

when viewed from the stream bed 

directly beneath the bridge is the 

change to a FRP deck readily 

apparent. From all other perspectives, Wilson's Bridge retains its 

historic appearance with its iron 

superstructure preserved and a 

renewed plank wearing surface on 

top. 

The composite deck's light 

weight/high strength combination 

permitted the bridge to support 

greater live loads. The structural 

capacity of the bridge was more 

than doubled to fourteen tons, 

enough to support emergency vehicles, with no major change to the 

overall visual appearance of the 

superstructure (see figures 14 and 

I 5). With a one-piece plastic construction and anticipated lifespan of 

fifty to seventy years, the deck 

should also require less intervenAfter the crane cables and fasteners 

used to lift the deck were removed, the 

remaining oak planks were attached to 

the deck with lag bolts (see figure 13). 

The use of a timber plank overlay was 

unusual for an FRP deck. Often these 

decks have a thin epoxy resin and aggregate surface that is susceptible to damage 

by snowplows. The timber deck used on 

Wilson's Bridge, however, is easily 

replaced when damaged or deteriorated. 

It also recreates the historic appearance 

of the bridge from the roadway. Figure 12. To secure the deck to each floor beam, lag bolts with steel clips were inserted into 

predrilled holes in the deck surface and fastened tight against the floor beam's upper flange. 

6 
Figure 13. After the deck was locked to the floor beams and 

the crane cables removed, remaining oak planks that made 

up the wearing surface were bolted to the composite deck. 

tion for maintenance and repair than a 

traditional deck system. 

Solutions developed for Wilson's 

Bridge were tailored to the specific 

requirements and conditions of the 

structure. The relative infrequency of 

traffic and live load cycles permitted 

its continued use as a single lane 

bridge without the need to increase 

structural capacity to a point that 

would require altering the 

tru itself. Infrequent live 

load cycles also allowed for an 

economical approach to connecting the FRP deck. 

Applications that involve 

higher traffic volumes may 

require more substantial 

attachment mechanisms than 

the steel clips and bearing 

pads used on Wilson's Bridge. 

Where necessary, heavy-duty 

connections would provide 

continuous support and prevent loosening over time. 

Composite decks are a relatively recent development in 

bridge engineering. Though they 

have been tested extensively in the 

laboratory and their performance in 

the field is encouraging, a complete 

evaluation of the material's durability 

will only come with the passage of 

time. The long term viability of FRP 

decks is especially important because 

the higher material cost is justified by 

a much longer anticipated lifespan 

than conventional deck systems. 

Factors that may affect the long-term 

performance of the FRP deck, including eventual surface cracking and ultraviolet light degradation if not regularly 

painted, remain areas to monitor and 

assess. 

Conclusion 

The area around Wilson's Bridge is virtually unchanged from its late nineteenth century appearance. The onelane road is still unpaved and the bills 

above have experienced little new construction. Wilson's Bridge forms an 

important component of this landscape. 

Its historic, functional and aesthetic 

significance framed the search for a 

way to preserve the span while ensuring its continued use as a vehicular 

bridge. The resulting treatment combined traditional repair treatments such 

as repainting and repainting with innovative technology, in the form of an 

FRP deck. The composite system 

offers a longer lifespan, is visually 

compatible with the historic material 

and offers increased load capacity. 

Because of its sensitive rehabilitation, 

~i~:;::s4·iti~e nc:wm:::~te d:ck, p~inted t; match the superstructure, is visually compatible with the historic bridge as it originally appeared with timber 

face. Pho.to: Chad Randt" wearing sur ace are the same thickness as the previous system of steel stringers, nail-laminated deck, and bituminous sur7 
Figure 15. The landscape surrounding Wilson's Bridge has changed little since the span was constructed in the 1800s. The use of a composite deck and plank wearing surface in place of the deteriorated, fourth generation deck was an innovative solution that allowed the bridge to remain in 

place and in use with little change to its historic character. Photo: Chad Randi. 

Wilson's Bridge continues to serve the 

needs of local residents, the park, and 

the thousands of people who each year 

cross Valley Creek on foot, on horseback, and by car or truck. The sensitive approach to preserving and 

upgrading Wilson's Bridge offers considerable promise to other efforts to 

save aging bridges across the country. 

PROJECT DATA: 

Structure: 

Wilson's Bridge / County Bridge 

Number 166 

Valley Forge National Hi torical Park 

Valley Forge, PA 

Owner: 

Chester County 

West Chester, PA 

Project Date: 

1998 

Deck Fabricator: 

Hardcore Composites 

New Ca tie, DE 

Cost: 

The total cost of the Wilson 's Bridge 

project including rehabilitating the 

truss and abutments and the price of 

the composite deck was approximately 

$40,000. The design and fabrication of 

the FRP deck came to $28,000 with 

another $2,000 for delivery and installation of the deck at Valley Creek. 

These figures reflected the close proximity of the composite manufacturer's 

facility to the bridge site as well as a 

modest discount offered by the manufacturer to encourage the use of FRP 

decks in historic bridge applications. 

The remaining $10,000 was attributed 

to truss and masonry work, including 

removing the deteriorated stringers, 

deck and wearing surface, replacing 

the iron U-bolts, repointing the abutments and wingwalls, and repainting 

the superstructure. 

All photos and drawings courtesy of 

Che ter County Engineering 

Department, unless noted. 

Cover Photo: Wilson's Bridge after 

rehabilitation. Photo by the author. 

THIS PRESERVATION TECH NOTE was prepared by the 

National Park Service. Charles E. Fisher, Heritage Preservation 

Services, National Park Service, serves as the Technical Editor of 

the PRESERVATION TECH NOTES. Information on the 

Wilson's Bridge project was generously provided by Richard 

Craig, Township Engineer at West Goshen Township, Ben 

Craddock of the Chester County Office of Facilities 

Management, Hayati Catbas of Hardcore Composites, and 

Audrey Everett of McCormick Taylor & Associates. Special 

thanks are extended to Joe Pullaro of Lichtenstein Associates, 

Gary Klein and Deborah Slaton of Wiss, Janney, Elstner 

Associates, Eric DeLony of the Historic American Engineering 

Record, and Sharon C. Park and Kay D. Weeks of the National 

Park Service's Heritage Preservation Services for their review 

and comments. 

PRESERVATION TECH NOTES are designed to provide practical information on traditional practices and innovative techniques 

8 

for successfully maintaining and preserving cultural resources. 

All techniques and practices described herein conform to established National Park Service policies, procedures and standards. 

This Tech Note was prepared pursuant to the National Historic 

Preservation Act, which direct the Secretary of the Interior to 

develop and make available to government agencies and individuals information concerning professional methods and techniques for the preservation of historic properties. 

Comment on the usefulness of this information are welcomed 

and should be addressed to PRESERVATION TECH NOTES, 

Technical Preservation Services, National Center for Cultural 

Resources, National Park Service, 1849 C Street, NW, (2225) 

Washington, DC 20240. 

ISSN: 0741-9023 PTN46 November 2003