Pedestrian bridge is Aesthetic Winner - Tygerburger , 2007/07/11
Engineering consultancy firm Jeffares & Green (J&G) has just completed supervising construction of a pedestrian bridge situated in a local community of the Mkomaas Valley in KwaZulu-Natal (KZN). The company was appointed by the KZN Department of Transport to design and supervise the construction of a pre-stressed concrete ribbon bridge that has never been attempted before. The bridge was built owing to a need expressed by the local community, who live on both sides of the Mkomaas river. There is a trading store situated on one side of the river, and a school and clinic located on the other side. J&G senior partner and bridge engineer Corrie Meintjes explains, “How this division occurred is not clear; however, the river poses a danger to the community and many children have lost their lives when swimming to and from school during the rainy seasons.” The construction of the bridge began in November 2005 and was completed in April this year. The estimated cost of the project upon completion was about R3,5-million. There were many challenges that occurred during the design of the bridge. Meintjes said the design and construction of the bridge were unique, as such a project had never been attempted previously in Africa. “An innovative design was required to prevent the swaying motion associated with several types of popular structures such as suspension and hybrid bridges capable of providing a single span for the required 150 m,” he adds. However, to avoid fears expressed by the community of crossing a bridge that was in motion, Meintjes researched design options and decided on a pre-stressed concrete ribbon structure. This design resulted in Meintjes receiving an Achiever of the Year award from the KZN Concrete Society. The structure is made of a ribbon of pre-cast concrete elements that are suspended from cables tensioned between abutments. The element are joined and compressed in the longitudinal direction of the bridge by tensioning a second set of cables placed in ducts through the elements. The compressible stress provides continuity in the deck and compensates for the tensile stress resulting from live loads. One of the challenges faced by designers was the location on which the bridge was to be built. This resulted in the use of pre-cast elements in the construction process, which reduced the amount of construction required on the site. “It is an innovative all-concrete design, aesthetically pleasing and environmentally responsible and functional. It does not interfere with the flow of the river and does not have stabilisers, towers or large abutments that might have imposed on the rural landscape,” says Meintjes. The company will also be celebrating its eighty-fifth anniversary this year, establishing itself as one of the oldest consultancy firms in structural and civil engineering in the province of KZN. J&G has offices in all major centres in South Africa and employs about 300 staff members. In 1997, the firm began an ongoing process of transformation and has reached a level of 33,8% black ownership.
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