Complexity in Design – Capitec Project Ikhaya

The design solution was predominantly a concrete framed structure however structural and composite steel and concrete construction were used in a complimentary way. Additionally, bridge links and the floor over the main entrance required light, long span structures with limited headroom implications and the parking structure required the use of a solution that would be removable in the long term to allow conversion of the building for a different use.

A variety of structural framing was used to bring this innovative building design to life, namely:

• Macsteel/Westok Cellular Beams with Bond-Dek composite flooring were used for the Headquarters Walkway Bridges
• Macsteel/Westok Cellular Beams with Bond-Dek composite flooring and Macsteel/Westok Ultra-Shallow Floor Beams with Voidcon permanent formwork floors spanning between floor beams were used for the main entrance floors in Capitec Bank Headquarters
• The use of I-Section portal frames with transfer systems supporting rectangular hollow section roof light pods were implemented for the roof at headquarters
• I-Section roof framing; supported on slanted circular hollow section columns for the entrance canopy at headquarters
• And lastly the parking structure infill floors which were comprised of H-Section columns supporting I-Section composite floor beams with Bond-Dek composite flooring

Schalk Marais PrEng, Aecom Practice Area Lead says, “Whilst the project was impressive from a technical perspective, the overall design of this building was geometrically complex due to the curve-linear shape of the superstructure. The shape combined with the height restrictions on the site and the divergence between the column grid in the lower parking levels resulted in a complicated structural frame.”

Moreover, there was a requirement to have long-span pedestrian bridges crossing the central building atrium and the need for a very open reception area and combining this with a shallow ceiling system and exposed concrete soffits forming a specific design language throughout the building posed challenging.

The pedestrian bridges crossing the atrium had more ceiling space available and also served as links for services crossings. The use of composite cellular beams provided a cost-effective long span system that could cater for the dynamic performance requirements and allow services to cross without paying the penalty in terms of weight.

The ceiling requirements from the architect above the main entrance resulted in a very shallow available floor depth and a long span. The total span in this area between columns is 18m. Bridging this span was done by using a thickened reinforced concrete floor cantilevering over the support columns, supporting a simply supported Ultra-Shallow Floor Beam (USFB) system spanning almost 13m.

The USFB system comprises of asymmetric steel sections with circular voids in the webs. The top flanges are narrower to allow permanent formwork to be dropped in, to span from beam to beam, resting on the bottom flanges. Composite action is achieved by using conventional reinforcing bars placed in the web openings (bars perpendicular to the beam span; plug composite action). Due to the long spans a propped construction methodology was adopted to limit the final deflection.

To add an additional layer of complexity, the tips of the concrete edges supporting the USFB floor system on the building edge, had to be column free at ground level to create a weather canopy. This was accommodated by suspending the perimeter of the 1st and 2nd floors using high capacity rods from a cantilever transfer beam at roof level.

“This project was a benchmark in demonstrating the benefits of using steel as a material. Some structural systems and configurations necessitate the use of steel with its long-span and overall weight reduction capabilities; however, this project displays how steel can be used successfully in a combination of ways, complimenting conventional concrete construction”, concludes Marias.