Technical Preservation ServIces PreservatIon Tech Notes METALS NUMBERS Rehabilitating a Historic Truss Bridge Using a Fiber Reinforced Plastic Deck WILSON'S BRIDGE Chad Randl Chester County, Technical Preservation Services Pennsylvania National Park Service Introduction Iron truss bridges were an essential ele­ the National Register of Historic ment of America's Steam Age. In the Places. decades fo llowing the Civil War, the The bridge features a single-span combination of strength, durability and Pratt pony truss, approximately sixty affordability made iron bridges a feet long and sixteen feet wide (see fig­ favored means of traversing a ravine or ure J) . When first constructed in the river. They were a symbol of industrial late 19th century, the bridge included an power, a testimony to engineering effi­ oak plank deck supported by timber ciency, and a source of local pride. stringers running parallel to the road­ Over the years, however, the increasing way. Stringers, in turn, rested upon load capacity and widths required by four iron floor beams that hung from succeeding generations of vehicles, as the superstructure by large U-shaped well as inadequate maintenance and bolts. Fieldstone abutments supported wholesale replacement programs, have the bridge with wingwalls extending reduced their number dramatically. As down to the ground line. a result of this decline, an increased The bridge had undergone numer­ emphasis is now being placed on iden­ ous repair campaigns over the past tifying and implementing methods to hundred years, some of which included preserve as many of the remaining iron substituting new materials and compo­ truss bridges as possible. One such nents. Between 1930 and 1940, the effort involves Wilson's Bridge, a timber stringers were replaced with wrought iron bridge constructed in steel I-beam stringers. The deck had 1886 to cross Valley Creek in been replaced on four occasions, most Tredyffrin Township, Pennsylvania. recently (in the mid-1960s) with a nail­ Also known as County Bridge laminated wood deck and asphalt wear­ #166, this span now lies within the ing surface. boundary of Valley Forge National At the time of its rehabilitation in Historical Park. It was fabricated by 1998, Wilson's Bridge carried a dead the Phoenix Iron Company and con­ end road linking several residences and structed by John Denithorne & Sons, a park maintenance facility with the both of nearby Phoenixville, rest of the national park and county Pennsylvania. Because of its location, road system. The dirt road had a low Substitute materials may be used connection to local industrial history, average daily traffic level of approxi­ to replace deteriorated or missing and the scarcity of wrought iron mately thirty-five vehicles; it was more elements of a historic bridge bridges still in use, Wilson's Bridge is commonly used by hikers and horse­ recognized as a valuable park and com­ back riders travelling along the park's provided the character of the munity resource, eligible for listing in trail system. A four-person bridge historic resource is preserved. Deck and Wearing Roadway Surface Masonry Profile /Wingwall --------------- Floorbeams \ Masonry Abutment Creekbed~ Stringers Figure 1. Drawing of Wilson's Bridge showing major structural and site components. Drawing: Chester County Engineering Department. crew within the Chester County extensive cracking and was losing would need to undergo significant Engineering Department was responsi­ adhesion with the deck below (see fig­ repair work, ways of increasing the ble for maintaining Wilson's Bridge, ure 2). The thirty-five year old nail­ allowable loads on the bridge would along with the county's other one hun­ laminated deck, made up of individual also be explored. In addition to these dred spans, one third of which were timber boards placed on edge and immediate concerns, the bridge was considered historic. Recognizing that spiked together to form a 4-1/2" thi ck due for cyclical maintenance work the deterioration of Wilson's bridge continuous surface, was uniformly soft including repainting the truss and had reached a point where the structure and unable to support the wearing sur­ repairing and repointing the stone abut­ would soon be unable to support a face above. The steel stringers were ments and wingwalls (see figure 4). minimum required load capacity, the also in poor condition (see figure 3). Any repair program also had to pro­ bridge team sought a means of retain­ Though the full extent of their deterio­ tect the creek flowing beneath Wilson's ing the bridge in place, while upgrad­ ration was not clear until the deck was Bridge. Pennsylvania's Department of ing its structural capacity. Appropri­ removed, preliminary examination Environmental Protection identified ately rehabilitated, the span could then indicated that the I-beam stringers were Valley Creek as an "Exceptional Value continue to function both as a vehicular severely corroded, particularly over the Stream," a designation that meant all bridge and as a link to equestrian and abutments where they had suffered work on the bridge had to be carefully hiking trails, with little effect on its approximately fifty percent section designed to exclude the chance of historic appearance. loss. water contamination or disturbing the At this time, the wrought iron truss creek bed and banks. Problem had a posted weight load limit of seven tons. This restricted the size and type Solution By the late 1990s, there were numer­ of vehicles (including emergency vehi­ Due to the bridge's historic signifi­ ous indications that Wilson's Bridge cles) able to reach the residences and cance and the several hundred thou­ required immediate intervention. The maintenance facility on the opposite sand dollar cost of a new span, the current asphalt wearing surface showed side of Valley Creek. Since the bridge Figure 2. Before rehabilitation, Wilson's Bridge had an asphalt wearing Figure 3. A view of the underside of Wilson's Bridge before rehabilita­ surface that was delaminating from the deck below. Extensive cracks tion. The timber nail laminated deck sat on steel "I-beam" stringers and numerous potholes were also present. 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 only option seriously considered was to from bridge applications, the ecological rehabilitate the existing bridge. A plan importance of Valley Creek led the coun­ was developed to repair the superstruc­ ty to look for a different solution. ture and abutments following standard A nail-laminated bridge deck was practices, along with the replacement another treatment option. This type is of the deteriorated 1960s deck and usually assembled and attached to the wearing surface. Several conventional structure piece by piece. Such a process choices were identified for the replace­ would have required closing the dead end ment deck: a glue-laminated timber road for approximately six weeks - one deck, a nail-laminated timber deck week to remove the old deck, one week similar to the existing surface, or a to install new steel stringers, and four steel and concrete deck. Each was weeks to build the new deck in place. ruled out in tum before a new material Because the bridge was the only means that met all of the bridge rehabilitation for residents to reach the main roads, requirements was identified, evaluated, either a temporary span would have to be and selected for the job. constructed or the residents and the park With a typical lifespan of over thir­ maintenance facility would have to be ty years, glue laminated timber decks relocated for the duration of the project, are reasonably cost effective and would both at a prohibitive expense for this pro­ be in keeping with the historic charac­ ject. This deck system also offered no ter of Wilson's Bridge. Chester improvements in load capacity. Though County engineers had used "glulam" comparable in cost to a glulam deck, nail decks on three other recent bridge laminated decks have a significantly rehabilitation projects. The decks are shorter expected life span, and corre­ usually pressure-treated with chemical spondingly higher costs. This alternative wood preservatives. In the previous was therefore not attractive. county bridge projects, some of the Typical steel and concrete deck sys­ preservative leached out of the decks tems were also considered. As with the and into waterways below. In all three timber-laminated decks, this system Figure 4. The Wilson Bridge wingwalls had projects, remediation was then under­ would have required replacing the steel loose and crumbling mortar joints. Much of taken. While at this time there is not a stringers with heavier new stringers due the wall surface required removal of this dete­ consensus as to the environmental to the increased weight of the deck. riorated material and significant repointing. impact of such leached preservatives Their weight, combined with that of a Existing Bridge Rehabilitated Bridge Bituminous Oak Plank Wearing Surface Wearing Surface New Timber Curb~ I 13" ,/' U-bolt \ Replaced Hanger U-bolt Steel Stringers Floor Beam FRP Deck 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 relo­ cation of the park facility and make the passage of emergency vehicles and delivery of heating fuel to the resi­ dences virtually impossible. Because the load capacity would be reduced rather than increased, this alternative was also ruled out.