The Our Lady of Peace Cathedral in N’Djamena in Chad was originally consecrated in March 1965, with a timber structure supporting the soaring roof.
During civil uprisings in this Central-African country, the cathedral was destroyed by a fire in 1980. In 2014, a conceptual design to restore the cathedral was started, with the intention of following the original design of the roof but replacing the timber structure with steel.
Rising from the ashes, the project was named as the winner of the light steel frame building category at the Southern African Institute of Steel Construction (SAISC) Steel Awards 2023. It has received international media attention, including the Vatican News.
The structural steelwork in progress.
Design challenges
The design required anchoring a lightweight steel structure to a concrete ring beam. A specialised shell structure design solution was implemented to address the wind-load considerations, which included the use of custom-made brackets to securely anchor the steelwork.
With a steel structure clad in steel tiles, there was a concern about the interior of the cathedral being very hot, especially given the hot tropical climate. A robust insulation solution and underlay were employed to keep the building cool.
Challenging fabrication
The choice of Ultra-Span was made following a feasibility study. Given the remote project location, on-site fabrication was mandated. Although the lightweight sheeting panels presented a problem with wind load for the structure, it was easier to transport and Ultra-Span’s precision cut-to-length capability proved to be the ideal solution.
The structural frames are 21m high and the distance between the sides of the frame at the base is 18,6m. These frames were laid out on a jig placed on a concrete slab adjacent to the cathedral. Two frames were placed on top of each other, with bracing placed between them, and cross-bracing added to ensure a stable structure while it was hoisted.
Additionally, a removal brace was used between the bottom points so that when lifted by the crane, they didn’t move inwards. Once lifted into place onto the concrete ring beam by the crane, additional braces were placed between each pair of trusses, creating a shell structure for the whole cathedral.
The curved end at the back of the cathedral was critical, with difficult geometry to overcome. By reducing the size of the frames and decreasing the spacing between the frames, the distance that the roof purlins would span was maintained.
Isolated location
The location of the cathedral posed its own challenges, as the nearest port was over 1 000km away and it was impossible to transport prefabricated spans to the site.
Transportation was another unique challenge of the site, whereby goods from South Africa were shipped via Spain to Cameroon and then tackled challenging road conditions to reach N’Djamena in Chad. During the project’s execution, this complex transportation was successfully navigated through a period of political instability in Chad.
Fortunately, once on site, there was a working crane available and permission was granted to use it for erecting the trusses. This was a critical element in the completion of this project.
Project name: Notre-Dame de la Paix de N’Djamena Cathedral.
Location: N’Djamena, Chad.
Client: Catholic Church, Chad.
Architects: Groupement BEX.
Main contractor: Sotieri Construction.
Structural engineer: MiTek Industries.
Steelwork contractor: MaxSpan Roofing.
Steelwork detailer: MaxSpan Industries.
Tonnage: 975kg per truss, 120 ton overall.
Full acknowledgement and thanks go to https://www.saisc.co.za/ for the information in this editorial.