Mpumalanga Fresh Produce Market

The required climatic conditions within the completed building envelope dictated the roofing system to be used comprising a two layer sandwich configuration of 0,47 Widedek ceiling sheets , a 135mm Starlite Insulation blanket and 0,53 SAFLOK external weather sheeting. The roof is split into three equal segments for sheeting separated by gutters with the 68m long SAFLOK roof sheets being rolled on site and pushed up a scaffold ramp onto the roof. The area of the roof is approximately 30,000m2 with a side cladding area of 6,200m2. The sides of the building are clad with 0,47 Widedek sheets with a 50mm layer of Lambdaboard for insulation. The Widedek ceiling sheets were installed in various lengths lapped over the purlins since they do not form part of the weather envelope. Doubler plates were introduced at all the toblerone truss connections to strengthen the walls of the truss chords against buckling but more importantly , from a fabrication point of view , it simplified the jigging of the chord members by enabling the connections to be fully fabricated in simplified jigs separate from the chords then added to the chords in a single line jig setup. Since the roof profile had such a large radius it was also decided , and agreed , to fabricate the trusses with segmented chords and the splice locations were carefully chosen to limit the number of different connection configurations in the truss segments. The roof consists of 11 trusses in its length , 3 truss spans over its width and 7 truss segments in each span which amounts to 231 truss segments all told. As such it was extremely important to ensure quite early in the fabrication process that when assembled the full trusses would provide the correct dimensional profile to accurately fit into the roof on site. Similarly the configuration of the support column / girder / truss intersection was incredibly complicated and the trial assembly of this section of the roof at the workshop was seen as being critical to the success of the Project. Assembly of the toblerone trusses on the site necessitated the provision of a number of assembly frames which had to be set out , lined and levelled to ensure the accurate assembly of each truss before hoisting. A major concern throughout the installation phase of the Project was that the holding down bolts at the two concrete abutments of the roof would talk to each other such that 210 meters of roof truss would fit snugly between the steel buttress frames once they were mounted on them. Even though accurate surveys were provided by the Main Contractor and we had allowed some tolerance in the buttress frames base plate holes it was not a given that with such a long length of steelwork we would end up with a perfect fit. As it turned out the building works were fit for purpose and the Main Contractor could be commended for a job well done. The trial assembly at the workshops proved invaluable to the installation on site. At 70 meters long tandem lifting was the order of the day.

The main challenge on this Project was the shear size of the roof structure and it’s structural framing system requiring a major consideration of how to transport it to the site 330 km’s away from the fabrication shop. This was overcome by detailing it as piece small allowing the transport loads to be maximised which then added further challenges as to how to fabricate it like a meccano set while ensuring all the pieces went together seamlessly. These challenges were overcome by careful consideration of the jigging systems that would be used for the fabrication of the components and then trial assembling the major structural components and interfaces between them.