A bridge to prosperity - delivering a structure and social value

The social value of civil engineering is frequently overlooked despite societal benefit being the foundational purpose of infrastructure. This case study shows how social value is an intrinsic part of any project, including project purpose, design and construction, and why this is relevant to all engineers.

The finished bridge at Minigo, Rwanda
The finished bridge at Minigo, Rwanda

Project details

Location: Rwanda
Date of completion: 28.06.2018
Duration: 2 weeks
Client: Bridges to Prosperity
Contractor: Mott MacDonald and Balfour Beatty
Project manager: Ben Murphy (BB)
Challenge summary: A footbridge build in Minigo, Rwanda
Challenge solution: Using innovative design changes to increase safety and reduce construction time in a restrictive environment.

More than 4,300 residents of the Gisa, Kabilizi, and Terimbere communities in the region, including more than 800 children, will benefit from the new Minigo suspension bridge.

It will provide safe, year-round access to critical government resources on the other side of the river, including the health clinic.

A 10-person team consisting of five members from Mott MacDonald and five from Balfour Beatty had less than two weeks to complete the suspension bridge across the Sebeya River.

The project works involved construction of the superstructure of a 36m span suspension footbridge.

Key activities included assembling and erecting the steel towers, setting the correct sag for the main cables, installing the deck hangers and installing the timber deck.

The works combined high risk activities such as working at height and using a winch and pulley system to lift the towers.

The team used an innovative construction and temporary works plan to increase safety on site and make significant programme benefits which allowed the project to be completed ahead of schedule even with an ambitious ten-day construction programme.

Throughout both the planning and construction phase of the project, the team encountered multiple challenges requiring innovation and effective team work to achieve safe and successful results.

The weather was a challenge which limited time on site due to the high levels of precipitation we encountered on site. This then affected our programme creating pressure on whether we would complete the bridge build in time.

Bridge foundations
Towers and cables in place

A winch and pulley system as well as a fabricated saddle extension unit which we brought over with us allowed the towers to be erected with the cables in place.

The saddle extension units ensured that the required bend radius for the cables was maintained during the lift and reduced the amount of friction between the cable and the towers. This allowed us to raise the towers with the cables already placed and reduced the amount of time required of the team working at height.

This then removed the need to lift by hand the extremely heavy cables up the scaffolding to place them on top of the towers. Time was also saved on setting the sag, saving the team 1.5 days in the programme.

The towers and support cable
Detail of scaffold

The team worked on developing the scaffold design prior to our trip as we wanted to increase the safety in the design due to the significant amounts of time spent working at height on the scaffold.

The scaffold was positioned much closer to the towers using the abutment as support as seen in the Sketch Up diagram below.

This provided close access to the towers for key activities such as clamping the main cables to the towers and installing the safety line for the decking operation.

3D model of the bridge and river
Sketch Up diagram of the scaffold's proximity to the towers

The scaffold system also allowed us to place all the hangers on the main cables in a safer environment by creating a platform on the top of the scaffold where the work was being undertaken.

All but the three final hangers were positioned across the span by attaching to the main cable at the platform location and pulling along the main cables into their final position.

The final three hangers were installed on the cables inside the scaffold, removing the risk of unstable working at height.

Bridge decking in place
Detail of scaffold platform

Throughout the project it was vital to ensure that skills and knowledge was passed onto the local bridge construction team as they form the Minigo Bridge Committee which are tasked with maintaining the bridge after the UK team depart.

As part of this, we ensured that an excellent safety culture was undertaken throughout the whole construction process.

Safety debriefs were held at the start and end of each day to share any lessons learnt. It also gave us an opportunity to ensure that everyone on site knew what tasks they were undertaking and how, to make sure they were undertaken as safely as possible.

Small innovative ideas such as using bamboo mushroom caps for the rebar which had been put into the ground was a local idea.

Tool box talks on manual handling, working at height and concrete burns were also run through before any major activities commenced.

Conclusions

The project has created a safe crossing for the community around the Minigo footbridge, ensuring that children are able to get to school safely, workers to their jobs and families back home at the end of their day.

It has also made year-round access to local health facilities easier. The innovation in the tower lifts and scaffold design has enabled the charity Bridges to Prosperity to streamline their design and construction process contributing to their development growth scheme.

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