SA: A New Pedestrian Bridge over Markgraaff Street
This project is an extension of a previous project, the Bloemspruit Pedestrian Route, which was completed late last year by Vela VKE Consulting Engineers. The bridge is located adjacent to the original Bloemspruit, i.e. the birthplace of Bloemfontein. This pedestrian bridge will accommodate the pedestrians walking from the Inter Modal Transport Facility (Vela VKE Engineers) towards the stadium as well as everyday pedestrians.
The client’s brief required a unique pedestrian bridge that would serve as a “gateway” to the Bloemfontein CBD. It was also specified that the bridge should complement the surrounding environment and be an experience for the pedestrian to cross.
The project is funded by a combination of funds from National Treasury as well as the Mangaung local municipality. With a total estimated cost of R14. 5 million(1.5 million USD) the project is scheduled for completion by end of February 2010.
Design Concept
The flat terrain along the Bloemspruit necessitates long sloping approach spans on either side of a main span which crosses the busy Markgraaff Street. In order to encourage pedestrians onto the structure the approach spans are set at a gentle gradient. The main span itself covers 32m and is a structural steel butterfly tied arch with a 3.5m wide concrete deck slab. With the two supporting arches leaning away from the deck the soccer fans walking to the stadium will have a clear view of their destination. In all the structure stretches over 152m long.
The whole construction fraternity is busy with preparations for the 2010 FIFA World Cup and Bloemfontein is no exception. VELA VKE Consulting Engineers is currently involved with the design and construction of a new pedestrian bridge over Markgraaff Street that will provide safe access to the newly upgraded Free State Soccer stadium.
The approach spans consist of a continuous composite concrete and steel beam spanning approximately 7.5m. A composite structure was chosen because it allows a 150mm concrete deck slab to run the total length of the bridge. On the approach spans the slab acts compositely with steel I-beams to achieve the required stiffness. Across the main spans the slab spans the short distances between the steel I-beams supported by the arch hangers. This achieves design continuity across the differing sections of the bridge as highlighted in the architectural perspective.
With a conventional tied arch the arches typically lean towards each other and are braced against lateral buckling by transverse elements. In this case the arches lean outwards and are unbraced. This is not a first but requires careful checking of the arches buckling modes under various load combinations. The main work of the arch is done via a set of in-plane high tensile steel hangers. These hangers are not vertical but run diagonally in a truss arrangement.
This is the typical configuration of a network arch and helps to reduce the bending moments that develop due to uneven pedestrian loading. Out of plane stainless steel cables connect to the deck’s CHS curving edge member. These cables provide the necessary stability to the arch and restrain it from buckling laterally. The variation in materials between the in-plane and out of plane hangers are to provide aesthetic value.
In terms of the steel sections used, each arch will be constructed from single radius 273 x12 CHS and the 219x10 CHS deck edge elements bent to a single radius that fits the 3D geometry. The remainder of the sections are standard I-beams. Other interesting features include stainless steel handrail incorporating LED lighting.
The main supports will consist of rectangular tapering reinforced concrete columns, while the pier supports will be Y-shaped 219x10 CHS’s. The concrete deck will be finished off with a 30mm mastic asphalt layer to provide a non-slip finish as well as for aesthetic value.
Management, planning, and technical design
Key challenges faced by the design team have included relocating an existing 525mm diameter sewer line and developing an erection method for the main span itself. Markgraaff Street may not be closed for extensive periods, thus requiring that the main span be assembled on site and lifted into position. This requires an accurate setting out of the substructure elements and close control over the fabrication tolerances.
In this regard the main span will be pre-assembled in the fabrication yard and the dimensional tolerances checked against the onsite measurements.
In the design process a dynamic analysis was undertaken considering a single pedestrian, a pedestrian group and a continuous pedestrian stream load models. The aim was to limit possible deck accelerations that would cause discomfort to pedestrians. Another important design consideration was limiting the fatigue stresses in the steel sections. To this end the steel base plates of the back span struts are pre-stressed against the reinforced concrete base to limit stress reversals.Construction
After the main span has been erected in the fabrication yard and approved, the structure would have to be dismantled and transported 180km from Kimberley in the Northern Cape to Bloemfontein in the Free State. The manoeuvrability of such a long structure presents severe challenges. An application to reassemble the bridge in the adjacent residential street was submitted to minimise the lifting distances.
Conclusion
After the completion of the bridge, it will provide access for thousands of spectators commuting to and from the 2010 World Cup soccer matches as well as serve as a landmark for Bloemfontein. In the long term the bridge will also assist in the rejuvenation of the Bloemfontein inner city as a symbol of modernisation in conjunction with the traditional surrounding landmarks.
Article Courtesy of Vela VKE Consulting Engineers
By: Harald Ronne
PROJECT TEAM
Client Mangaung Local Municipality
Project Management Vela VKE Consulting Engineers
Structural Engineer Vela VKE Consulting Engineers
Civil Engineer Vela VKE Consulting Engineers
Architect GAPP Architects
Electrical Engineer Dirk van Heerden Engineers Practice CC
Main Contractor HSH Construction NC & FS (Pty) Ltd
Steel Sub-Contractor General Engineering and Repairs CC
Plumbing Sub-Contractor ESL Plumbing Services
Electrical Sub-Contractor Pongwana Electrical
Concrete Supplier Procrete
Reinforcement Supplier Steeledale Reinforcing
