Increasing numbers of South African architects and structural engineers, many of whom were slow to adopt hollow-core technology, are now using it as a means to safe-guard quality, accelerate construction time and save costs. Three recent projects serve to illustrate the point. Two involved expanding existing structures, both of which could not have taken place without the lighter mass of hollow-core, while the third entailed fast-tracking a new office development.
MTN Mobile Data Centre Expansion
Extensive use was made of prestressed slabs for Phase 2 of the MTN Group’s data centre in Centurion, a town situated between Johannesburg and Pretoria. MTN is a South African-based multi-national mobile telecommunication company operating in African, European and Middle Eastern countries.
Phase 1 comprised a ±3 000m² single-storey structure which was roofed with Echo prestressed slabs. Begun in January 2014, Phase 2 entailed the construction of a second storey which was built on the prestressed slab roof of Phase 1. Once again prestressed hollow-core, in this instance covering an area ±2 600m², was used for the roofing.
The new roof was designed in close collaboration with Echo Group engineer, Daniel Petrov. Grinaker-LTA was the main contractor on both phases and DSM Consulting Engineers was the engineering consultancy on the Phase 2.
DSM consulting engineer, Mike Silberman, commented that the amount of propping required for an in-situ slab, a high loading on the existing slab, and the amount of services and penetrations through the new roof, precluded this option.
The prestressed roof slabs used on both phases were 250mm deep and varied between spans of 2,2m and 11,5m. They were designed to carry live loads of 10kN/m² and dead loads of 2,5kN/m² and were mounted on precast inverted T-Beams measuring 420mm deep and 630mm wide. The design and manufacture of the beams were outsourced by Echo. Andreas Rehwinkel of ENCON did the design and Civilcon handled the manufacturing.
The beams and the slabs were installed by Echo using 80 and 200 ton cranes. The former were placed on 6.5m high reinforced cast-in-situ concrete columns. Most of the columns were cast with additional rebar which protrudes ±1,5m through the prestressed slab roof. These may be used at a later stage as stub columns for supporting solar panels or other plant on a suspended steel grid.
After all the slabs had been placed, a lightweight politerm screed-to-falls, ranging between 75-400mm, was installed. It was covered by a second 40mm screed to take a dual layer of water proofing. Once the new roof was semi-sealed, Grinaker LTA was able to remove the screed on the roof of Phase 1.
Commenting on the project, Echo Group marketing director, Melinda Esterhuizen, said precast hollow-core slabs are to the construction industry what cell phones are to modern communications – fast, flexible, multi-functional and cost-effective.
Stanger Maternity Ward Expansion Project
Prestressed slabs are being used to construct two additional storeys, each measuring 1 230m², for a new maternity block at Stanger Hospital in Kwazulu Natal, one of South Africa’s nine provinces. The first floor section, which will house an NVD anti-natal ward, is being constructed on an existing suspended roof. The floor above it, which will accommodate a post-caesar and gynaecology ward, is being constructed with prestressed hollow-core slabs.
The prime reason for using prestressed slabs is that the existing suspend roof was not designed to carry the weight of a new in-situ slab. To strengthen it would have necessitated back propping, either through the elaborate staging of existing load-bearing walls or through the installation of temporary beams at floor level.
Neither of these options were feasible as the wards below were fully functional and occupied by patients. Prestressed slabs, which are 30% lighter and provide favourable span to depth ratios, have provided an alternative solution which has allowed normal hospital activities to continue uninterrupted during construction.
At 150mm and 200mm deep, the slabs were erected using steel cellular beams and in-situ beams. Support for some of the new beams and slabs had to be created by bolting steel sections to concrete columns.
The prestressed option is offering other advantages. Slab installation takes hours as opposed to the weeks it takes to construct in-situ flooring. Second, and most importantly for a hospital environment, there is far less attendant noise to disturb and upset the patients during construction.
The professional team on this project comprised: Elias Mechanicos Building and Civil Engineering Contractors; ZAI, the engineers and architects and ……………………………………………….
Clearwater fast-track office construction
This innovative project involved the fast-track construction of an office development, Clearwater Estate, situated in Boksburg, one of Johannesburg’s satellite towns. Built in two phases using hollow-core in combination with structural steel frames and raft foundations, Phase 1 compromised a gross lettable area of ± 3 100m². Phase 2 was somewhat larger at 3 700m².
Having already signed a lease agreement with Michelin Tyre Company South Africa, local property developer, Krisp Properties, had only seven months in which to have Phase 1, which comprised two
1 550m² double storey buildings linked by a central atrium, ready for occupation.
Echo’s prestressed hollow-core slabs were specified by PDS Civil and Structural Engineers for both phases and were deployed for the first floors as well as the roofs; this reduced construction time by several weeks. Slabs were used for the roofing to allow for the future installation of photovoltaic power generation units and to create catchment areas for harvesting rainwater.
Slabs up to 10m were deployed on the first floor sections and others of between five and seven metres were installed on the roof. Both the first floor and roof panels were topped with steel reinforced screeds and a bitumen coating was used to waterproof the roofs. In addition, Echo also supplied nine metre H-shaped universal steel columns to provide slab support for the internal cantilevering.
The development was designed by Francois Marais Architects and PDS was the structural engineering consultancy.
According to PDS structural engineer, Spiros Sdralis, Echo’s roll involved considerably more than simply supplying the slabs.
“We provided Echo with a set of drawings and they then prepared a layout which took all the servicing requirements into account. Daniel Petrov, was involved from the outset and he was always on site when we needed him, particularly in solving specific on-site challenges,” said Sdralis.
“For example, some of the slabs had to be modified so that the upper-section of steel skeleton could be anchored to the first floor on a solid platform. This meant that the hollow-core end sections of some slabs had to be packed with concrete to provide the solid base. Echo filled in the voids with concrete according to our specifications and this enabled us to drill out the holes for the bolt anchor sleeves in the same way as was done on the ground floor.
“The builders were able to begin the assembly of the upper level steel frame as soon as the slabs had been installed and this was the prime advantage of using hollow-core slabs. If in-situ flooring had been used it would have delayed the completions dates on both buildings by several weeks.
“Extensive use was made of cantilevering on this development, both internally and for the shade overhangs, and Daniel assisted with redesigning some of the cantilever sections on Phase 2. He also helped us with changing the direction of some of the slabs.”
Francois Marais of Francois Marais Architects said the construction of the buildings embraced South Africa’s new energy efficiency legislation rather than being restricted by it.
“The design uses overhangs above all the windows and doors to minimise the impact of the sun and to reduce the escape of heat from the buildings,” said Marais.
Echo also allowed for service ducts based on the architectural plans and suspended ceilings were created 600mm below the soffits of the roofs and first floors to accommodate electrical, plumbing and air-conditioning services.