It may no longer be a tourist attraction, but after 50 years Sentech Tower is still one of Johannesburg's most familiar landmarks, writes Geoff Bottom

Towards the end of the '50s, the South African Broadcasting Corporation turned its thoughts from coaxing broadcasting out of the steam radio mind-set into the new world of very high-frequency (VHF) transmissions, and belatedly introducing South Africans to the even more exciting mysteries of the television.

The fledgling consulting engineering practice of Ove Arup and Partners in Johannesburg was commissioned to design some of the initial VHF transmission masts - essentially steel towers designed to accommodate the antennae equipment.

In keeping with the then international vogue for major cities to have landmark radio towers, the SABC's chief architect, Leslie Durston, briefed Ove Arup's Jack Zunz (now Sir Jack Zunz) and Michael Lewis on the corporation's concepts for a similar iconic structure on the Brixton ridge. Douglas Mills, the SABC's chief engineer, provided the technical requirements.

The aesthetics of the design were a major concern for the Arup team from the outset. The objective was to achieve the technical and architectural requirements of the brief in a functional and pleasing solution. This was achieved by opting for a cantilevered reinforced concrete tower for the main structure, topped with a structural steel mast to accommodate the transmission equipment. The gentle tapering vertical curve of the concrete tower reflected a design used in the Stuttgart tower in Germany.

An observation deck was located at the top of the concrete shaft at a height of 178m, with an additional two floors for equipment and administration requirements below. One of the first window-cleaning gondolas in the country was designed specifically for the building. The lifts installed to reach these decks were, at the time, alarmingly fast.

The observation deck was initially open to the public and it was a popular expedition for visitors to the city, but it was closed in the '80s for security reasons - as was a restaurant on the ground floor.

Design work proceeded during 1960. Christiani and Nielsen SA were the successful tenderers for the construction of the project. Excavation work started on February 1 1961 under Paul Gotze, who was the contractor's site engineer. Basil Isaacs was the Arup partner responsible for the project on site. The tower reached its full height of 237ms on December that year and the first transmission took place on December 22. The 53-tonne structural steel mast at the top was erected in just three weeks.

The tower is built to withstand winds of 186km/h and gusts of up to 200km/h. On windy days, the top of the steel mast has been known to sway up to two metres.

The foundation was constructed as a ring six metres wide and two metres deep with an overall diameter of 26m and used 1342 tonnes of concrete. A further 5648 tonnes were required for the tower. On completion, the project was the tallest man-made structure in Africa, but was eventually overtaken by the Hillbrow Post Office Tower.

The contractors devised an ingenious shuttering system that allowed for the forms to be adjusted and reused for each additional pour of concrete. About two-thirds through the construction phase, one of the forms became displaced which gave the tower its notorious "kink". After some soul-searching and checking of the designs, it was confirmed that this "slip" would cause no structural damage and work proceeded to the top.

The resulting kink provided endless cause for public speculation and was a heaven-sent inspiration for Bob Connolly, the Rand Daily Mail's cartoonist at the time.

To restore the aesthetic symmetry of the tower, it was decided to gunnite the exterior of the structure to smooth out the kink. Gunniting is a sprayed cement mortar which necessitated the erection of an additional external scaffold walkway around the perimeter of the tower to allow for its application.

It was one of my duties as the consultant's resident engineer for the latter part of the project to test the strength of the gunnite daily with a Schmidt hammer. Being prone to vertigo, this cat-walk exercise brought on some very premature grey hairs.

The tower was officially opened in 1962 as the Albert Hertzog Tower. With the known reluctance of the then minister of posts and telecommunications to introduce TV broadcasts, the tower soon became known just as the Brixton Tower.

Towards 2001, naming rights for the tower were sold to Sentech, one of SA's first wireless internet providers. Sentech now broadcasts 18 FM programs and seven TV stations daily from the tower.