At Roark, structural engineering embraces a flexible approach, combining RCC and steel based on each project’s needs for optimal suitability, efficiency, and performance. This methodology was tested on a project in Kinshasa, Democratic Republic of Congo, where a seven-storey building required four floors to cantilever out by 25 m. Each cantilevered floor covered an area of 450 sq m.

Initially, the design team considered using RCC for the side walls, effectively turning them into deep beams (four floors deep) extending from long shear walls. However, the sheer weight of the cantilevered floors made this option impractical. The next option involved cantilevering RCC beams supported by RCC or steel diagonals, but this approach still faced challenges with excessive deflections due to the high self-weight.

The final solution adopted was a cantilevered diagrid system, featuring steel beams trussed with steel diagonals. This system also employed load-bearing steel decks with minimal concrete topping, significantly reducing the overall weight and bringing the design within acceptable limits. For the rest of the building, RCC was used to provide the necessary counterweight to balance the overturning forces caused by the extensive cantilevers. Despite testing steel as an alternative, it did not generate sufficient counterweight. Long RCC shear walls were utilised to support the cantilevering steel diagrid, which helped reduce overturning moments and foundation costs.

This hybrid approach proved to be highly effective and construction-friendly. The construction sequence involved installing two welded trusses of floor height depth-first, followed by bolting the connecting diagonals between these trusses to create a four-floor deep diagrid on both edges. This method facilitated the installation of framing steel beams and decks for each floor, overcoming the difficulties faced with the initial options.

The success of this project highlights the advantages of combining steel and RCC, demonstrating that each material can be utilised for its strengths. Without the steel components, the project would have likely been unfeasible or excessively costly if executed entirely in RCC. The project’s success not only led to cost savings but also satisfied the architectural vision, with the diagrid providing a visually striking design language. The structural design, adhering to American codes and accounting for seismic considerations, ensured the stability and integrity of the building, making it a remarkable example of innovative material use in engineering.

“A structural steel diagrid is a beautiful structural mechanism. When deployed rightly, it can be an extremely efficient structural system capable of transferring loads in a predictable, functional, and precise manner without being obstructive and bulky, unlike most structural elements. In addition, it can create a very aesthetic architectural language, which most architects are happy to work with, especially as a facade element. This project embodies the blending of both the structural and architectural requirements, seamlessly and harmoniously through a diagrid.”
– Pankaj Gupta, Managing Partner, Roark Consulting Engineers LLP