Zambesi Mall Project 2009

Transcription

1 Zambesi Mall Project 2009 A double-quick roofing solution What do you do when the developer of a shopping mall sees an opportunity for more revenue at a late stage in the project? The original plan was a concrete slab for a rooftop parking area. The new idea: A cinema complex, restaurant area and indoor soccer stadium all covered under long span steel roof structures. The problem most of the concrete structure has already gone up and the building site is going full steam ahead. The solution use light steel frame roof trusses, roll them in Mitek s factory, transport the sections to site, assemble and erect with the main contractors tower cranes. This is what happened on Phase 5 of the Zambesi Mall. First it was a challenge for the project team and then it became an excellent case study for the flexibility and speed of erection for the light steel frame building method. So how did the quantity surveyor decide on a light steel frame solution? He consulted the architects yellow pages Classidex and considered his options. Light steel frame building was the solution. This was not a huge marketing exercise from the LSFB industry the product sold itself. Max-Span, a light steel roof fabricator and Mitek licensee, was appointed as the steelwork contractor and approached Mitek to assist with the design and pricing of the roof structures. Scope The roof structures over Phase 5 of the Zambesi Mall includes a cinema roof that spans 22.5 metres (with a 950mm overhang at high end); a restaurant area with a skylight in the middle with 15 metre spans on both sides (the skylight roof span is 7.5 metres) and an indoor soccer

2 stadium with a 30 metre span. So far, the roof structures of the cinema and restaurant have been erected. The total roof area to be covered by light steel roofing is m² with a total of 78.5 tons of steel. Structure The 22.5 m span roof structure covering the cinema is a simple truss structure with a spacing between each truss of about 1400 mm. There are horizontal bracing frames between every 4th and 5th truss running parallel to the trusses, one in the plane of the top chord and one in the plane of the bottom chord. Each truss weighs 220kg. The roof structure of the cinema was designed to carry the load of the acoustic panelling. The 12m high concrete columns on the external side of the cinema required structural support to withstand the wind forces from the north. The roof structure was hence designed to act as a stabilising structure and by connecting it to the opposite concrete gutter, the wind force on the concrete column was accounted for. The cold-rolled members were formed in the Mitek factory in Midrand, where they were specially rolled to fit onto a 12.5 metre truck, saving the project some costs. Mitek s usual stock lengths are 5.8 metre long sections to fit into a 6 metre container for export purposes. The sections were transported to site and the trusses were fabricated on site in 20 days prior to erection. This was also an advantage for the project since the trusses could easily be adapted to small tolerances/ deviations in the concrete structure. Between the 12m high concrete columns along the north side of the mall light gauge girders spanning 8.5m were fitted. These were designed to be supported on their extended top chords, and were thus simply lowered into place on top of the concrete columns. Similarly the trusses spanning at right angles to these girders were supported on their extended top chords on these girders, and again could simply be lowered into place without the need for cleats. Another reason for using LSFB instead of hot-rolled steel was the reach of the contractor s tower cranes. Since the weight of each lightweight truss is about 220kg in comparison with several tons for a hot-rolled truss, the tower cranes were able to lift the trusses into position. The cinema and restaurant area s roof structure (5300 m²) took a mere 17.5 days to erect. Speed of erection was one of the deciding factors in using LSFB in this project. Savlock 700 will be used as the cladding material on all the roofs. The spacing of the lightweight trusses offer more support for the sheeting and a thinner sheeting can be used (0.5 instead of 0.58mm) which also saves the project some costs. Teamwork Max-Span, light steel specialists, was appointed as the only steelwork contractor, but some of the steelwork, such as the skylight steelwork as well as the columns in the cinema area were specified as hot rolled steel. Max-Span then subcontracted this portion of the steelwork to another steelwork contractor namely Tass Engineering specialising in hot-rolled steel, thereby turning the age-old way of doing things on its head. Max-Span has been in the business for the past 3 years and has done various other projects including a control room for Sasol and a church in Wendywood. The erection of the trusses was subcontracted to All-Africa Trusses, also a Mitek licensee, with a team of 20 erectors. They are a reputable company and are regarded as one of the best light gauge steel roofers in the Gauteng area.

3 This is the biggest roof that has been done using Ultra-Span, and hence any light gauge steel, in South Africa. However further abroad in Africa some large jobs have been successfully tackled, including the Molepolole Mall in Kwaneng, Botswana which is square metres, and a market in Asu in Nigeria of square metres. Showcase This project is a showcase for light steel roof structures. It mainly illustrates LSFB s flexibility and speed of erection, but also its ability to save the project money. One thing that s clear from this project is that light steel frame building is not seen any more as a strange new building method on the market, but as a possible competitive solution for any project.

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