An innovative pipe bridge conversion for eThekwini

A perspective of the current steel pipe bridge crossing a section of the Mdloti River

Using building information modelling (BIM) software as a design platform, Naidu Consulting has value-engineered a sustainable solution for the upcoming Canelands Pipe and Pedestrian Bridge project. The client is eThekwini Water and Sanitation’s Design and NRW Branch.

Completed some 100 years ago for South African Railways & Harbours, the existing Canelands pipe bridge crosses the Mdloti River in Verulam and features five 33 m spans of 5.5 m deep structural steel trusses. These are simply supported on reinforced concrete piers and abutments.

The original bridge was built to support a 400 mm diameter ductile iron sewer pipe, and 350 mm diameter and 400 mm diameter steel water pipes, together with an open cable tray accommodating electrical cables. The bridge further serves as a pedestrian crossing for the surrounding community.

A condition assessment investigation of the entire bridge confirmed that the structural steel trusses were at an advanced stage of corrosion and required immediate replacement. However, the existing substructure is still in good condition and

An innovative pipe bridge conversion for eThekwini shows no signs of distress. Based on a costbenefit analysis, it was therefore proposed that only the current steel superstructure be replaced. “The final design recommendation accepted by the client will see the construction of a 165 m long precast concrete bridge deck that will accommodate water and sewer line infrastructure, as well as improved and safer access for pedestrians,” explains Josh Padayachee, director: Bridges & Buildings at Naidu Consulting.

Design benefits

The design solution passes on the following key benefits for the client: • Since substructure demolition and replacement with new abutments, piers The use of the and piles is not necessary, this reduces existing steel trusses to the environmental impact and results in substantial cost savings. launch the new bridge • An environmental authorisation and waterdeck means that no use licence is not required. cranes are required in • The use of the existing steel trusses to launch the new bridge deck means that no

the river to place the prestressed beams between the supports.”

The new Canelands Pipe and Pedestrian Bridge modelled utilising BIM software

Final deck cross section subsequent to demolition A view of the internal layout of the existing bridge showing the water and sewer lines, and the narrow pedestrian walkway

cranes are required in the river to place the prestressed beams between the supports. • An intrusive geotechnical investigation was not needed, which would in general have had negative environmental impacts. • There will be no construction in the river channel, thereby avoiding negative environmental impacts and the risk of project delays due to inclement weather and flooding. • The engineer’s estimated construction period duration was reduced by approximately seven months.

BIM simulation

Utilising industry-recognised software programmes, the existing and new portions of the bridge were modelled in 3D incorporating BIM principles. This provided a key visual aid to the design team so they could make informed decisions regarding the new design, sequencing of construction, risks, and limitations. Although the new bridge superstructure is comparatively simple in form, the correct sequencing of the demolition and new construction works is integral to the success of the project. Therefore, an intricate demolishing and construction sequence process was developed and will include the following: - construction of temporary pipe and pedestrian walkway structures attached to the sides of the existing bridge - diversion of the existing sewer, water and cable infrastructure services to the temporary structures - extension of the height of the existing substructure - construction of a temporary launching system comprising two steel A-frame structures that slide on a steel rail track - launching of the new precast beams in a sequential manner to their respective positions - laying of new 400 mm diameter HDPE water

and sewerage pipes on the new bridge deck and encasing them in concrete - jacking of a new 600 mm diameter steel water pipe through the void of the new bridge deck - transfer of the water and sewer services from the existing lines to the newly constructed lines - demolishing of the existing bridge steel truss superstructure, temporary structures and existing services in a sequential manner by utilising the A-frames to transport the various steel elements off the bridge. “The utilisation of BIM is often marketed for the long-term benefits it offers a project. However, it can also be utilised as a powerful tool at the initial stages of a project,” adds Padayachee.

“The Canelands Pipe and Pedestrian Bridge project demonstrates that thinking out the box and embracing technology can provide innovative solutions in an everchanging world to ensure that the service delivery of infrastructure is prioritised,” Padayachee concludes.