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A structural engineer typically designs new structures or renovations to existing structures. This involves consideration, not only of the stability but the economy of building the structure. However, when disaster hits, the things to consider become more complex. Safety concerns arising from the damaged structure itself and the economic impact of business interruptions have to be added into the already complex equation of safety and economics. What happens when accidents happen to a manufacturing facility? Now, the engineer must worry about the economics of a manufacturing shutdown, often of much higher losses than the repairs themselves.
The owner of a manufacturing facility, NC Moulding, in North Carolina experienced these losses and concerns firsthand when a dumpster truck struck and damaged part of their facility. In August 2023, a truck carrying dumpsters left its back (tilting-frame) lifted and struck a 2nd-floor pedestrian bridge that provided an essential path for product transport and storage between two manufacturing stages located in neighboring buildings. The impact destabilized the bridge by deforming a lower chord on the bridge structure and partially rotating the trussed bridge (Fig. 1). This also displaced the dust collector system equipment, important for reducing breathing and fire hazards, due to the damage to the chord that supported them. The damage created a significant safety hazard, thus shutting down the bridge. This crippled the manufacturing facility, disrupted traffic through and below the bridge, and disrupted the use of the dust collectors. The stairwells were too narrow to allow efficient travel of the product to street level and back up and therefore alternative routes were not feasible. Upcoming on their peak season through December 2023, the immediate losses were building up. Without a quick solution, a prolonged shutdown would exacerbate these losses and would begin to affect employee livelihood and retention as well as relationships with key customers, potentially leading to damage to the business’s long-term stability.
Given the severe consequences of delay, the Owner and insurance representatives brought in disaster recovery specialists, including Collins Structural Consulting, PLLC (CSC) as the structural specialist, to figure out how to get the plant up and running quickly and safely, with the ultimate goal of a complete repair and as few losses to the business as possible. When CSC arrived on site, it was quickly determined that the bridge, structurally, needed a complete replacement. However, the typical time frame for replacing an enclosed pedestrian bridge easily includes several months of construction, not counting the design and planning phases. Therefore, CSC recommended a temporary reinforcement of the existing bridge which could allow production while the long-term solution was generated.
CSC focused the temporary design on providing additional support at the point of impact (Fig. 2) near the bridge’s midspan, using shoring posts anchored to shallow concrete footings near the bridge’s midspan and shoring posts connected to longitudinal beams that were moment-connected to the intermediate bridge girders. However, the nature of the manufacturing plant meant that flammable, chemical-treated wood dust blanketed the site; therefore, field welding was not a safe option. Yet the tight overhead clearances also meant that standard bolted connections were not feasible. Therefore, CSC incorporated an innovative product, Shuriken nut covers (Fig. 2), into the design. This product was designed to allow welding without loss of strength. In this case, the nuts were welded onto plates offsite, which fixed the nut from rotating, allowing bolts to be installed in tight spaces where wrenches do not fit onto the nut side of the bolt. Because CSC designed the structure from field measurements, despite that complex geometry and deformed structure from impact, CSC incorporated Lindapter girder clamps onto locations where lack of exact measurement would impact the ability of the bolts to align. These clamps connect to the flange of a beam instead of requiring an exact bolt-hole location. This temporary repair design allowed a quick return of the bridge’s functions to provide transport across the bridge, safe passage for traffic below the bridge, and stable support to at least one of the dust collectors.
Once the temporary repairs were in place and the plant had returned operational (albeit at a slightly lower pace because the bridge could not be used to store product), the problem still required a permanent solution to replace the bridge without a long plant shutdown. Knowing that the plant would have a scheduled two-week shutdown over the last two weeks of December, the disaster recovery team had approximately three months to develop a plan, have all the parts and pieces ready for installation, and come as close to a two-week install as possible. Meeting these goals depended on the accuracy and precision of the survey, measurements, and fabrication, decreasing manufacturing and installation times, and incorporating preassembly before the facility shutdown.
The first step, before design or construction, was to ensure the information on the existing buildings, bridge, and support equipment was as complete as possible. This was complicated by the complex geometry generated due to the differential heights and skewed angles of the existing buildings and their openings for the bridge. The survey team used a Leica BLK 360 scanner to produce a thorough and accurate three-dimensional scan of the area. The scanner had an enclosed LiDAR sensor that captured the data on the surrounding environment at 680,000 points per second to produce accurate 3D scans. The surveyor recorded data points from six locations: the inside of the bridge at each end, the inside of the bridge at midspan, and the outside of the bridge at each of the four corners from the ground level. The resulting data produced a clear image of the bridge’s structure, including not only the dimensions of the bridge at all locations and distances between the two buildings but also the degree of damage (Fig. 2). This included a rotation of the bridge structure due to the impact, with a full nine inches of relative displacement to the bottom chords, as compared to the top chords. It also showed the installation locations of existing mechanical, electrical, and dust collector equipment within and outside the bridge. The design and construction teams quickly shifted into design mode, knowing the design must prioritize efficiency and reduction of construction errors that would affect speed or quality.
The bridge’s structural design consisted of an upper span of the bridge built from two Warren trusses with alternating compression and tension diagonal members supported by an A-frame design with four new columns and footings, two located at each end of the bridge, adjacent to the respective buildings. The 3D scan depicted the skew between the two buildings which do not run parallel to each other, producing one truss length spanning 51 feet, 3 inches and the other at 62 feet, 2 inches. The truss’s vertical and diagonal members were designed as Hollow-Structural-Steel (HSS) tubes shop-welded onto the upper and lower chords of the truss. The floor beams, roof beams, upper chords, and lower chords are all W-shaped steel beams that were mostly detailed as bolted connections, allowing for full shop fabrication of the two trusses and therefore, quicker field installation.
CSC designed the bridge such that the structural steel and light gauge steel-framed walls could be preassembled as three main pieces: two support pieces (A-frames consisting of the columns and cross beams to be installed onto the footings) and the full elevated length of the trussed bridge (to be placed onto the two A-frames at each end). This design, which creates a bridge structurally independent of the adjacent buildings, was selected to eliminate uncertainty of the unexposed structural elements of the existing building and to decrease the assembly time required during the plant shutdown. Not including the existing building as part of the bridge’s structure eliminated the chance of discovering existing issues within the building that would have required repairs or corrections during installation. Also, a bridge bearing on the existing structures would need to be longer than the space between the buildings to reach the support beams in the existing walls, eliminating the ability to maneuver the full span of the bridge into place after assembly. By designing new supports and ensuring the full span of the bridge was slightly shorter than the clear span between the buildings, the full span of the bridge could be assembled in a nearby lot before the shutdown and lifted into place as one piece, decreasing assembly time needed during the plant shutdown. This design, however, created additional design considerations, as the structure had to meet requirements for two load-path design scenarios: 1) the construction loads as the pieces were being lifted into place suspended by the crane and 2) the in-place, in-use loads of the pieces, each being supported on the new columns and supporting the main gravity live and dead loads of the bridge.
The structural team worked collaboratively with the steel fabricator and contractors to ensure that each decision in the design did not slow down the process. CSC consulted with North State Steel (NSS), an experienced fabricator for time-sensitive projects, and generated iterations of connection designs to be used only for connections with a quick fabrication process in the final design. NSS used imported Revit drawings directly from CSC to ensure accurate shop drawings. This close collaboration allowed for the swift generation and approval of the drawings. CSC also consulted with the contractor (Belfor Property Restoration) to ensure the design incorporated high-early-strength concrete that would not require extended cure times.
The two-week shutdown was scheduled for December 21, 2023, to January 2, 2024. So, when the manufacturing workers started their break, the disaster-recovery teams picked up the pace and initiated the installation. All shop-fabricated steel elements of the bridge arrived on site on December 18, 2023, three days before the shutdown, to begin pre-assembly in the site’s parking lot. Demolition of the existing bridge began as soon as the shutdown allowed. The demolition was completed in five days, allowing for the pouring of the concrete footings on December 26, 2023. The high-early-strength concrete allowed the footings to be ready for loads the next day. With five days left before the manufacturing workers returned, the footings and the pre-assembled bridge were ready for installation on the morning of December 29, 2023 (Fig. 3). With members of the construction and design teams on site to ensure accuracy and speed of problem-solving, the construction team installed the A-framebridge support frames onto the footings and rigged, lifted, and bolted the full elevated length of the trussed bridge into place. The team’s preparation paved the way for the installation of the main structure in just a few hours. This left more than four days for the mechanical and equipment contractors to reconnect existing equipment onto the bridge structure, which was guided by the detailed scans of the original placement. While some architectural and finish work was performed after the workers returned, full operation of the bridge and therefore the manufacturing facility was returned in less than the aimed two weeks, allowing the manufacturing plant to reopen on schedule on January 2, 2024 (Fig. 4).
The team used intense collaboration throughout the design and build process, an integral piece to achieving the speed required for success on this repair and replacement project. The owner’s needs drove the design process and dictated the construction schedule. By prioritizing this speed, the design team managed to protect the livelihood of the business and the employees while preventing additional losses from the owner and insurance company. The need to put the manufacturer back into production quickly to avoid loss of business meant shifting design economics from the standards used for new construction to a focus on reducing downtime. By providing quick temporary repairs to allow the business to function until a scheduled shutdown and using the intervening time to carefully plan and pre-assemble as much as possible, the demolition and construction of the permanent repair required less than the two weeks of the scheduled shutdown time, thus achieving the goal of repairing the damage with as few disruptions to the owner and their employees as possible. ■
About the Authors
Ken Kennardi, PE, is a structural engineer for CSC working out of Durham, North Carolina. (kkennardi@collinsstructural.com)
Scott A Collins, PE, SE, is the principal engineer for CSC and a Structural Specialist for the NC Emergency Management teams. (scollins@collinsstructural.com)
Ann L Collins, Ph.D, is the Director of Finance for CSC and serves as the
primary technical writer for the company. (acollins@collinsstructural.com)