Guyed masts are one of the most complex non-linear structures that one can encounter as a structural engineer. Due to the nature of the non-linearity of the cable elements, the solving of the cable forces is a highly iterative procedure. Appurtenances attached to the mast are generally modelled as applied dead and wind loads in the model, whilst thick feeder cabling can also effect the mast solidity ratio and add significant wind and dead load.
In 2012, the telecoms giant Sentech SOC, embarked on a countrywide network expansion project which included the upgrading of some 180 sites from the existing Analogue transmission to Digital Terrestrial Television (DTT).
The DTT works package are broken down into two main components:
- The design and construction of the core infrastructure for seven new guyed mast greenfield sites, including the construction of the associated new access roads, transmitter buildings, sub surface copper earth mat systems, and satellite dish antenna foundations for 4.5m/2.4m diameter satellite dishes. These seven sites are situated at Burgersfort, Harrismith, Holy Cross, Lady Frere, Cofimvaba, Ngqeleni and Qokama.
- The installation and addition of an extra guyed level and Kathrein UHF Antennae (approx. 10m high, 2 000kg in mass) above the existing FM Antennae cantilever section of the mast for some 19 existing mast sites. This work typically included modifications to the existing RC anchor blocks to accept the additional guy ropes as well as the fabrication and installation of a new platform and mast adaptor section. This component of the DTT work proved to be particularly challenging as new structures had to match the existing with a high degree of precision.
The broadcaster transmitter stations are more often than not located on remote mountain tops or high ridges, in unmanned areas and are only visited on a scheduled maintenance or call-out basis. Some of the 4-leg “Liebherr” type masts that were considered for this project are in excess of 280m in height.
The addition of the new transmitter sites will ensure maximum digital terrestrial coverage in the far lying areas of South Africa.
Twistdraai Thubelisha Mine
The 18 km2 Twistdraai Thubelisha Colliery is a 160m deep underground coal mine that is operated by Sasol Mining, producing 8.7 million tonnes per annum, near Secunda, in Mpumalanga, South Africa. The mine sustains jobs for over 1600 workers at the Twistdraai Colliery. The shaft consists of surface buildings, coal conveyance infrastructure (such as conveyors and conveyor bridges), workshops and a full shaft system. The total conveyor belt length is approx. 20 km, making it one of the longest conveyors in South Africa.
In 2012, Sasol "inaugurated" the Thubelisha Shaft at the Twistdraai Colliery. The Thubelisha shaft supplies coal to Sasol Synfuels, for the export and domestic market. The new shaft is expected to extend the life of the Twistdraai Colliery beyond 2039. Due to the size and sale of the project, at one stage there were a maximum number of people on site of 2 050. The above surface infrastructure consists of structural steelwork crane bay buildings and storage sheds, RC Reservoirs, brickwork offices, parking bay areas, Pollution Control Dams (PCD’s), canals and reinforced concrete silt traps. Over 4 000 tons of steelwork was therefore fabricated and erected and over 34 500 m3 of concrete was poured during the construction of this enormous facility.
This prime industrial warehouse area is located within the Corporate Park South precinct, Midrand, Gauteng. The park offers the discerning owner or tenant a secure and modern business environment and the buildings themselves grant direct highway exposure, with approx. 370 m2 of office space and 1 940 m2 of warehouse per building, of which there are five buildings. Included in the building space is approx. 560 m2 prestressed Echoslab mezzanine which allows for open office space together with a large kitchen and entertainment area.
The warehouses are designed from lightweight steelwork trusses and the sheeting design includes allowance for translucent sheeting which augments the lighting design.
The site has it's own stylishly designed cantilever roof guardhouse and there is parking available for approximately 72 vehicles in total. Each warehouse has one ramp and one dock leveller included as part of the available goods and materials handling system. The facility currently includes roof sprinklers which have been installed as part of the fire protection system.
Bel Air Mall Centre is a convenient community centre located in North Riding, Johannesburg. The Mall has something for everyone – from retail areas to food and restaurants and even a large gym.
From a structural point of view, there are several items of interest, including the 15m high RC Column structure in the middle of the mall – this is often used for the display of banners and advertising. The roof design incorporates a stylish multi-tiered design which is visible from afar and is a defining feature of the Mall. A relatively high, stylistically shaped brickwork façade greets the prospective shopper as they enter the mall’s front entrances and tension type canopies are included for covering the open deck areas on the First Floor. These open deck areas are supported by pinned architecturally styled castellated columns.
The mall includes an underground parking area as well as an above ground post-tensioned parking deck slab which is separated by an expansion joint from the coffer slab deck used inside the mall.