Case Study

Design of the Novi Sad Friendship Bridge

  • cable stayed swing footbridge
  • global analyses of structure
  • local analyses of critical components

Novi Sad Friendship Bridge is an asymmetric cable stayed swing footbridge which spans the River Wensum in Norwich to provide access to a new commercial development and helps regenerate a historic but run-down part of the city.  The structure is named in recognition of the twinning ties between Norwich and Novi Sad in Serbia. Buro Happold Consulting Engineers Ltd in association with architects, Yee Associates, used LUSAS Bridge to generate global and local models of the steel superstructure to assist with the design and analysis of the crossing. Norfolk County Council were co-consultants on the project.

The bridge carries a footway/cycleway and comprises two approach spans of 5m and 14m on the eastern bank, a 28m central span which swings about the main tower and a 5m span on the western bank. The main bridge is cable stayed and it is also supported through a steel shroud, beneath which the rotating machinery is housed. The deck cross section is made of two main 400mm by 400mm rolled sections, rotated at 45 degrees to give a diamond shaped beam. The deck consists of a 10mm plate spanning between the main beams, with transverse ribs at 2m centres and with longitudinal stiffeners to reduce the local deflections.

Aerial viewAccording to Karim Yngstrom, the nature of the structure made the global analysis quite challenging. He said, 'The asymmetric layout of the cable stayed spans required kentledge on the shorter (back) span to counteract the overturning moment arising from the self weight of the main span. The initial design had considered using a thick steel plate but this was abandoned as it required relatively thick (113mm) plates and increased the overall weight of the structure considerably. The final solution adopted was to provide a permanent steel shutter under the deck plate filled with steel ingots'.

LUSAS modelTo investigate the response of the structure three separate global models were created in LUSAS. The first was used to determine the behaviour of the bridge under dead load – where the structure is in effect three different structures with the east and west ramps and the main swing bridge behaving independently from each other.

The second model was identical to that used for the dead load analysis, but was subject to displacement loading at the interface between the ramps and the swing bridge. The results of this analysis allowed Buro Happold to determine the additional stresses arising from possible mismatch caused by the effects of temperature, high uplift wind and snow loading.

The third model represented the structure at its operational condition where the edge beams provide continuous shear transfer along the structure through a connecting pin between the ramps and the main bridge. The joining pins were modelled with their true degrees of freedom allowing only vertical force transfer between the ramps and the main bridge.

Deck anchorage and tower shroudThe results from each analysis were then combined and enveloped in separate spreadsheets and the maximum load effects for each member were determined.

The deck anchorage detail and the steel shroud to the tower were singled out for more detailed investigation with LUSAS. Local models of these critical regions were generated and in-service stresses and displacements were obtained.  Stresses due to local effects at other members were calculated from first principles. As Karim explains: 'Using LUSAS was the only viable way of analysing the structure for the multitude of loadcases that had to be considered - in effect, we couldn't have done it any other way'.

 

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