St. Stephen Church Deanery is one of the biggest churches in Africa.
The complex building shows its modern features thanks to its unique architecture. The unusual cover consists of doubled-curved gridshell domes supported by structural arches. With this solution, modern character was skillfully merged with the solemn traditional formality of religious buildings.
The Architect’s designs focused on the dramatic shell roof form that covered the entire Cathedral. The purpose was to achieve a roof structure that was buildable, economical and sustainable.
Life cycle assessment revealed that steel would be both economical and sustainable.
Roof structures are discretized by means of a grid pattern and resist loads through their geometric shape.
Shell structures, as opposed to the traditional system, allow to have a double-curved surfaces. Therefore, their stiffness and ability performance to carry vertical and horizontal loads to the foundation minimizing stresses of members that compose the roof. The grid, built up by beams, forms an approximately evenly spaced net of triangles. The necessary stiffness to carry the external loads is provided to the shell by its natural three-dimensional shape.
The optimal roof structural geometry and the three-dimensional distribution of the elements were defined through Kangaroo and Grasshopper, parametric and algorithmic plug-in of the modelling software Rhinoceros.
The final solution for the roof consists of three steel gridshell domes, which permeate one to each other, with different geometry and height. The highest dome is about 40 meters and the largest one is 50 meters. Between domes, there are two planar middle facade with steel truss structures.
Each dome has three-dimensional arches that improve global lateral stiffness carrying loads to the foundations.
Arches’ sections have a triangular shape to increase out-of-plane stiffness and to avoid out-of-plane buckling failure. The circular section of arches elements optimizes the structural design for modelling, calculation and assembly of modules.
The internal architectonic scheme is simple: a wide main nave with a system of complex overhead galleries for faithful and choir. The main nave carries the faithful's sight to the altar, where a stately steel arch gives a solemn importance to the sanctuary.
The maximum plan dimensions of the internal building, also including the foundations, are 68.20 m × 52.74 m and the total height is 32.15 m above the bottom foundation level.
The internal structure has a main concrete skeleton, where steel galleries lean on.
The concrete structure, both walls and columns, is divided into three portions for the presence of the technical joint realized by disconnection between elements.
Steel galleries have a three-dimensional truss structures with secondary transversal ones. This solution allows to obtain stiff and light structures and it guarantees design performance for resistance, deformability and comfort.
A steel frame of double T members (HEA120) with welded joints composes the sanctuary arches. This structure is connected to each concrete slab in order to avoid out-of-plane buckling failure.
Foundations are two-dimensional reinforced concrete slabs with pile structure. Thickness of internal foundation is 95cm and the roof one is about 200cm.
