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Bethlehem Steel, a name synonymous with big business and molten metal, helped shape our country. From shipbuilding and munitions during World War II to supplying the steel for just about every tunnel and bridge built connecting New York and New Jersey, Bethlehem Steel reined as America’s steel king for most of the 20th Century.

Today, the site owned by Bethlehem Works Corporation is one of the largest redevelopment projects in Pennsylvania. Currently home to the SteelStacks Campus, Sands Casino, Sands Event Center, and ArtQuest Entertainment Center, the overall project upon completion will encompass 120 acres of connecting venues, thousands of commercial and residential units, and is destined to become a multi-dimensional cultural and entertainment district dedicated to the arts, community, and educational programming throughout the year.

Acres of aging steel blast furnaces and structures remain as an artifact of the past. Named for the original design firm, the Hoover-Mason Trestle was commissioned in 1905 by American steel magnate Charles M. Schwab. The trestle formerly enabled electrically powered rail cars to deliver 90 tons of iron ore daily to each of the blast furnaces that still stand majestically beside it. The focus of a joint effort between the City of Bethlehem and the Bethlehem Redevelopment Authority, the trestle restoration is part of a $15.5 million adaptive reuse plan to create a unique elevated pedestrian walkway on the trestle that connects the SteelStacks campus, Sands Casino Hotel Bethlehem, and other developments on the site. The walkway was designed to offer a physical environment to these destinations while integrating a sense of history about the immense, looming furnaces.

The project architect, Wallace, Roberts & Todd, retained Maser Consulting P.A. to perform the initial preliminary inspections, which eventually led to a complete structural survey using 3D high-definition laser scanning and geotechnical inspections. The initial examination of the trestle assessed its current condition to determine the possibility of the proposed pedestrian walkway. The trestle area was modified in 1960-1970s to allow a new rail system to go underneath the trestle system. The last known set of plans, which were at least 40 years old and likely the result of this rail modification, were used as a guide for this field inspection. Once the feasibility of the project was confirmed, the team would move forward to garner more detailed information assessing the condition of the structure and physical components with laser scanning, which captures as many as a million points of data per second.

Structural inspection and design

The original trestle was constructed using a system of bents. The existing structure was evaluated in detail to ensure that the large reactions from the walkway could be safely transmitted through the trestle framework.

In general, the structural steel members for the trestle exhibited numerous modifications during the years the trestle was in service. Additional structural support members had been added at various locations, possibly in response to structural movements due to the heavy ore/coal railcar loads, and structural members were modified to accommodate the rail system that ran underneath the trestle. Numerous structural repairs were also performed on the trestle to replace various deteriorated steel members. The trestle exhibited slight to extremely heavy rusting of structural steel members throughout the entire structure and a few structural components exhibited possible distress due to previous induced heavy railcar loads and from possible ground level vehicle impacts.

Although the original trestle design was modified throughout the years to accommodate heavy railcar loads, and the proposed use of the trestle as a pedestrian walkway would impose lighter vertical loads on the structure, it was still evident that repair work on the trestle was necessary. Additionally, accommodations for installation of large poured-in place concrete planters had to be added into the equation as well as bringing the structure up to modern codes for thermal, wind, and seismic loads. This meant the structure needed to also be evaluated for lateral loads, with the high potential of exceeding the original design.

At the time the trestle was built, seismic analysis and design was not performed. Today’s seismic analysis showed the trestle required upgraded bracing in several areas, so the new walkway supports were stabilized accordingly. Because the condition of each member varied greatly from leg-to-leg, the recommended measures included a mixture of restoration or reinforcement, which was achieved by strengthening or adding bracing to specific areas to address the lateral loads.

Now that the use of a pedestrian walkway atop the trestle structure was deemed feasible, a more in-depth structural inspection was performed to fully assess and quantify the amount of repair work that would be required and to address the structural steel components.

Typically for these types of structures, hands-on in-depth inspection includes multiple stringers, floor beams, bracing, columns, and column caps located underneath the rail bed and performed on ground level. But the elevated project site made it difficult to inspect. Additionally, the structure also supported a system of large elevated steel bins countersunk beneath the railroad tracks that received the materials the railcars formerly dumped into them from the rail.

The trestle structure supported a system of large elevated steel bins that received the materials from the railcars. The bins were left filled with iron ore, coal, and limestone, as well as dirt, vegetation, and other materials.

When the blast furnaces were shut down and the plant closed, the workers literally walked away leaving behind the bins within the railway filled with iron ore, coal, and limestone. Over the years, the bins also filled-in with dirt, vegetation, and other materials, which limited the extent of what could be inspected. Getting to the bottom of the bins to unearth the structure below was the cornerstone in allowing the team to fully evaluate the existing structure’s condition and capacity, and allow them to determine how the walkway support system would be designed going forward.

Excavation of the bins would take months and inspections required specialized equipment including lift trucks and other hoisting devices to protect against fall hazards within an old structure. Maser Consulting developed and implemented its own extensive safety plan as well as adhering to safety measures required by the Sands organization.

The original trestle was constructed using a system of bents, which are a group of columns, dedicated to supporting each of the bins. Together the series of bins supported at each end with the bents made up the lengthy trestle structure. Across the top of the bins, spanning from bent to bent, are the steel stringers that support each of the trestle’s rails. The trestle had to be inspected from the top to the bottom of the bins to expose the true structural framework and to see if they were worthy of carrying the various loads.

After excavating the bottom of the bents, the steel supports were found to be in a variety of conditions and disintegration from being buried under moist materials for an extended period of time. In combination with the structural condition of each area, the load reactions from the new walkway structure needed to be evaluated to determine the extent of repair work at each walkway support location.

Some support locations contained nominal loadings and little to no work may have been required; however, in other areas the walkway introduced very large loads into the trestle below. Not only large vertical loads, but also large uplift and horizontal loads. In these areas, the existing structure was evaluated in detail to ensure that the large reactions from the walkway could be safely transmitted through the trestle framework.

The existing structure required steel restoration repair work in various support locations that had contained deterioration. In addition to the restoration repair work, special connectors and localized stiffening and bracing were added to the trestle, most notably at the steel stringers that directly supported the legs of the new walkway. The original stringers were designed for the magnitude of uplift and horizontal loads introduced in certain areas of the walkway.

Coordination of the connection details of the new walkway to the existing trestle was an exhaustive process, with various rounds of ideas and value engineering until all parties were confident the design was the most efficient. Each step of the iterative process between designers, contractor, and managers produced an improved product.

Today, the sleek linear elevated walkway sits approximately 40 feet above the ground and is contrasted by the vintage patina of the steel blast furnaces. Narrative plaques and seating areas with vegetative landscaped planters are strategically placed along the promenade, relaying the history of the SteelStacks.

The connectors not only needed to safely transfer the loadings from the new loadings from the walkway, but also needed to accommodate the thermal expansion and contraction of the new walkway while minimizing the impact on the existing trestle. They also had to be connected to the existing trestle at more than 200 locations, with multiple different existing configurations, and safely transfer the loads to the structure.

Laser scanning was performed to produce an as-built of the unique and varying existing structures to allow us to replicate the sizes and configuration of the trestle in structural computer modeling. The survey was done through combining the results of conventional and geospatial laser surveying data. Then using state-of-the-art software, the collective scan information was transformed into comprehensive 3D models that provided dimensional and elevation data, which aided the analysis and repair design of the structures. This set the stage for the exact manner in which the new pedestrian walkway would physically be attached to the trestle.

Conclusion

The overall project upon completion will encompass 120 acres of connecting venues, thousands of commercial and residential units, and is destined to become a multi-dimensional cultural and entertainment district dedicated to the arts, community, and educational programming.

Today, the sleek linear elevated walkway sits approximately 40 feet above the ground and is contrasted by the vintage patina of the steel blast furnaces. Being that close to these massive hunks of steel has shifted their focus from forgotten relics to taking their place in the forefront of the park as a homage to the many years of service they provided in helping to build our great nation. The walkway has three stairways including the Grand Staircase at the Visitor Center end, which also offers an elevator and creates pedestrian connectivity from the Bethlehem Landing Visitor Center to the Sands Casino Hotel Bethlehem. Narrative plaques and seating areas with vegetative landscaped planters are strategically placed along the promenade, relaying the history of the SteelStacks to visitors while enabling them to take-in the entire panorama.

Opened in the summer of 2015, the Hoover-Mason Trestle is a prime example of a successful adaptive reuse project and has already received an array of awards, including ENR’s 2015 Best of the Best Award for Landscape/Urban Development Project; 2015 Lehigh Valley Planning Commission Award, Revitalization/Environmental Project Winner; and a Certificate of Recognition from the Commonwealth of Pennsylvania and Senate of Pennsylvania.

While this section of the project has been completed, the total vision is for the potential addition of another 1,000 feet of walkway to accommodate new plans for the Sands facility expansion and more retail space. Open seven days per week, the Hoover-Mason Trestle project is one more segment within the SteelStacks Park helping to boost Bethlehem’s economy while preserving the city’s history by linking the community to its past century of steelmaking.

C. Richard Roseberry, P.E., PP, CME, AICP, is the regional manager for Civil/Site Development for Maser Consulting P.A. (maserconsulting.com) in Lehigh Valley, Pa. He has more than 24 years of diversified experience in municipal and private development engineering services. Roseberry is a certified instructor for the Pennsylvania Municipal Planning Education Institute, a Certified Public Works Manager, Licensed Wastewater Collection System Operator, and LEED Green Associate.

Richard C. Maloney, P.E., is the discipline leader for Structural Services at Maser Consulting P.A.’s Red Bank, N.J. office. He has more than 25 years of experience providing project management and structural engineering services to a wide variety of public and private clients, including government agencies and owners/operators of various facility types.

Maraliese Beveridge is the senior technical writer and public relations specialist for Maser Consulting P.A., a multidiscipline engineering firm with a network of offices nationwide. With more than 25 years of experience in journalism and expertise writing about technical topics, her work has been published in numerous national publications on a variety of engineering subjects.

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