In the heart of Rwanda’s remote Nyamagabe municipality, a groundbreaking project unfolded in summer of 2023.
Bridges to Prosperity (B2P), in collaboration with TYLin and ARUP, joined forces with local residents to construct the Bishenge (Masuma) Bridge, a corporate-sponsored hybrid suspension bridge spanning the Gasumo River. TYlin’s engineers are accustomed to designing structures daily. However, this endeavor provided a different perspective as they experienced the technical details and challenges first-hand while creating field solutions to complete the construction of this 223-foot-long structure.
The Design and Materials
The Gasumo River expands considerably during the rainy season, necessitating an extended bridge span to accommodate fluctuating water levels. B2P opted for a light cable-supported structure to minimize costs and construction time. A hybrid suspension bridge was ideal as it addresses the elevation differences of the project site’s rugged terrain. One braced steel tower stands at the lower elevation, while a concrete pedestal tower graces the higher side, ensuring a user-friendly pathway for travelers. Load-bearing cables are suspended from saddles on top of these towers and efficiently distribute weight to the foundations on each side of the river.
Nearly all construction materials for this project were donated. The main suspension cables were initially designed and used as crane cables, while the suspender bars were originally intended for concrete reinforcement. The towers, railing, and decking were fabricated locally to stimulate the economy.
To improve the usable life span of its bridges, B2P is transitioning from timber plank decking to locally sourced steel grids, significantly enhancing durability with only a marginal increase in construction costs.
Let’s Get Building!
Before the team’s arrival, Rwandan counterparts constructed much of the substructure, including the concrete tower on the higher elevation. They organized the construction materials across the site and set up temporary facilities, including a shaded lunch area, a shed for tools and materials, and washrooms.
The project kicked off with a safety orientation and a materials inventory. Traditional Rwandan songs and dances often accompanied each day’s construction briefing and safety instructions. After the experienced Rwandan team erected the scaffolding, it was time to construct the tower and prepare the decking.
Tower and Cable Installation
With only small hand tools onsite, lifting the tower legs, comprised of 6-meter-long steel pipes and each weighing 900 pounds, required teamwork and brute force. Team members greased and clamped the legs to create a pin connection so they could be rotated into the final position as construction proceeded. The towers were painted and braced together. Tower rotation was controlled using winch lines and pulleys.
The weight and length of the 1 1/8-inch-diameter cables, combined with the site’s topography and elevation, made cable installation one of the most challenging aspects of the build. The team hand-pulled four lengths of 400-foot-long cable, each weighing 1100 lbs., across the river between the two anchorages, lifted them onto the tower saddles, and pulled them taut before using a winch line and hoisting loop to tighten them.
Using a predetermined amount of cable slack, the team released the cables in a controlled manner to a hoisting sag. A timber pole was used to crudely pound the cable for a slow release until the hoisting sag was met. The team cinched the cables at the live anchorage with a torque wrench and Crosby clamps to match the dead anchorage at the opposite end of the bridge. Once the deadloads were applied, it was adjusted to a final slag.
Dead load = 68 lbs / ft
Live load = 85 lbs / ft
Design windspeed = 100 mph
Tar Cable Protection and Abutment Work
With the cables set, tar was brewed over an open fire and applied to protect the cables and clamps from corrosion. This process was rudimentary but effective. In an arduous task, several of the crew broke up rocks sourced from onsite boulders and placed them at the abutments. Local team members topped the area with concrete to create a walking approach path to the bridge.
Suspenders and Swing Assembly
The team fabricated long bundles of donated #3 rebar onsite using angle grinders, one of the few power tools available, into 142 suspenders. An experienced community member set up a jig to bend the bars to the suspender dimensions noted on the plans. The assembly of ‘swings’ — two suspenders linked by a crossbeam and used to support the decking — was another sizable portion of the project. A production line approach expedited the process, and quality control measures ensured the swings were assembled correctly.
Swing installation began simultaneously from each side of the structure. The team approached this task systematically and resourcefully. The swings were affixed to a restrainer cable at set distances and secured with clamps. The suspenders would slide onto the main cable when the restrainer cable was pulled out. The abutment side offered the advantage of working from a firm platform at grade. The swing installation on the tower side was completed from scaffolding approximately 25 to 30 feet above the ground. As the swings were installed, the bridge began to take shape.
Steel Decking and Fence Installation
The steel decking was added using handmade tools created from extra rebar. Individual team members used sharpened pieces of rebar to align the holes in the decking with the openings in the crossbeams and bolted them into place. The team used torque wrenches to tighten each bolt to the correct specification. The entire process was laborious. There were 216 steel decking sections, and they were heavy, especially when working at heights from partially completed decking. Bolts snapped and had to be replaced. Once again, teams worked from both sides of the bridge. Corporate and community team members signed the last deck panel before placing it. As with every milestone along the way, the team and community spectators celebrated its installation with hooting and hollering.
In the final construction task, the entire team worked together to put up and wrap chain link fence over the top cable. Before the fencing, everyone working on the bridge was attached with a harness. It took nearly 1,800 clamps to secure the fencing.
The Final Touch and Celebration
This project involved over 3,000 nuts and bolts, symbolizing a significant collaborative effort. Construction team members enjoyed a traditional barbecue to celebrate the project's completion. There was singing and dancing as the team celebrated the accomplishment together.
With only nine working days to accomplish the build, the team worked in groups to tackle tasks simultaneously. Careful planning and maximum efficiency were critical. Despite a strict construction schedule, unforeseen challenges arose. A last-minute change in overnight accommodations further distanced the corporate team from the build, increasing daily travel time to and from the site. Several lightning storms forced work to stop, further disrupting the schedule. TYLin employed accelerated bridge construction techniques to improve efficiency and address construction delays. Assembly-line production style methods expedited the crossbeam and suspender assemblies and the deck installation. According to B2P, the construction team installed the bridge fencing in record time.
Additionally, the team discovered discrepancies between the design drawings and the sag calculations while installing the main suspension cables. TYLin engineers determined the differences resulted from switching from the timber deck planking to the heavier steel grid decking. TYLin’s engineers, in collaboration with the B2P construction team, corrected the issue in the field by modifying the sags and installing to the corrected sag.
During construction, the team noted that one of the two main cable pairs displayed significantly more dead load deflection than the other. The defection variance exceeded the difference predicted during design. The team surmised some of the reused crane cables had experienced higher loads over their lifetime than others. The higher-preloaded cables exhibited greater stiffness and, consequently, less deflection. To address this issue, TYLin adjusted the clamping on the cable anchorages to tighten the lower-preloaded cable, reducing its sag and mitigating the differential deflection.
Working in Cooperation
During the planning phase, the construction schedule and resources were carefully strategized, with valuable input from local and international experts. The construction lead made onsite adjustments to adapt to the abilities and strengths of the construction team members, many of whom met for the first time in Rwanda. A critical factor in the project’s success was the emphasis on effective communication among team members. With a shared goal, team members approached communication challenges with empathy for one another. The team used paper drawings and physical demonstrations to convey ideas. When tasks became more complex, bilingual B2P staff stepped in to ensure smooth collaboration.
The Official Bridge Opening
The Bishenge Bridge exemplifies the power of collaboration, innovation, and determination in bringing essential infrastructure to remote communities. This project overcame geographical and environmental challenges and demonstrated the potential for sustainable development through thoughtful engineering and community engagement.
The team decorated the bridge for the official inauguration. Officials from the capital city of Kigali drove five hours to mark the occasion. As community members – young and old – who will use the structure to attend school, go to market, and reach health services crossed the bridge for the first time, clapping and singing in celebration, it was clear that the Bishenge Bridge is not just a physical structure but a pathway to a brighter future for the people of Rwanda.