South African steel bridges commerce and community in Skibbereen, Ireland
30 September,2008
The recent export of South African steel and engineering skills of leading structural steel fabrication company the Cosira Group, has resulted in the successful erection of the Skibbereen Bridge over the Ilen River and associated flood plain, connecting commerce to community, in Cork, Western Ireland.
This is according to Cosira Managing Director John de Silva.
Da Silva says: “We are proud to have been sourced from a variety of global players, and were commissioned to design, fabricate, supply and erect supervision of the Skibbereen Bridge in Ireland by property developers Levis Quay Partnership in 2006. We were selected for our proven quality, and our design and project implementation skills. In particular, the developers admired the design of the Desmond and Leah Tutu Bridge, a project Cosira completed locally, in 2003.”
Da Silva elaborates: “Skibbereen is a sought-after dormitory town in County Cork, Western Ireland. The labour force it houses feeds mostly into the nearby business hub of Killarney. By joining the actual town of Skibbereen to the four-storey apartment and shopping development on the other side of the river Ilen, the bridge has increased the desirability and convenience of the development, and contributes to the growth of property development in Skibbereen.”
The project was challenging, as the 45m bridge had to be designed on a completely modular basis, to facilitate containerised transportation by sea from South Africa, to Cork Harbour in Western Ireland.
Da Silva explains: “Everything had to be worked out to the nth degree before fabrication could commence, at the Cosira Plant 2 premises in Heriotdale, Johannesburg. Accuracy in design, construction and with regard to the civil work implications (for example, dimensions, levels, buttresses and holding bolt positioning) was obviously critical, as the fabrication site and the erection site were separated by literally thousands of kilometres. “
“We had to work very closely with the developer’s construction team in Skibbereen itself. Cosira’s in-house draughting facility used Tekla Structures software for all bridge design, enlarged details and detailing, as well as the design of the bridge pylons and the exact dimensions of the MacAlloy suspension cables. These cables were specially imported from England, and are widely used for suspension applications of this type,” explains Da Silva.
To facilitate the modular design, two single girders were used that, when assembled, formed a triangular, tubular girder 2.2 m wide at the top and 1.5 m deep. The girders were manufactured in segments 10.5 m long to enable them to fit into the shipping container. Double bottom cords were also used to make shipping simpler, and to make corrosion protection easier and more effective.
All jigs were created with a precise, built-in radius, to ensure the correct cambering, or ‘hump-backing’ to stabilise the final structure. The entire bridge, including the balustrade framing, was trial-erected at Cosira’s South African works prior to corrosion protection and shipping. Finally, a specialised marine paint was applied to the 20 tonnes of steelwork before shipment, to ensure protection from Ireland’s famously damp climate.
According to Cosira Project Manager Ken Smith, before any erection took place, the South African team worked closely with the Irish erectors and crane suppliers to establish and agree on an erection method statement, or EMS, and risk assessment. Says Smith: “Once the EMS and risk assessment were in place, all was ready for the bridge components, which arrived in their 12m container, and were then unloaded onto the flood plain.”
Continues Smith: “We chose October 2007 to erect the bridge, as this is the driest season of the year, and would allow us maximum ground stability for the cranes, as there is a high clay content on the flood plain surface. We would also have good available working space on the flood plain itself.”
Smith elaborates: “The erection process was problem-free and took just five days. First we pre-assembled the latticework, and then the team attached the suspension cables to the sides of the bridge, facilitating later installation onto the pylons. The pre-erected civil work had been meticulously done, so when the 100-tonne crane lifted the pre-assembled latticework into place, matching of items such as holding bolts was trouble-free and everything slotted in easily, Cosira-style.”
“The pylons were then assembled and put into position, using cherry pickers to lift the extension cables. A specialist technician from MacAlloy in England was brought out to supervise the tensioning of the suspension cables to pre-determined loads, ensuring that the requisite design loads and deflections would be achieved” Smith noted.
“Cosira’s previous experience on the construction of the Desmond and Leah Tutu Bridge stood them in good stead, and ensured that our reputation for South African steel industry design, fabrication and erection excellence and expertise, was well-maintained. The finished bridge was well within budget and was completed inside the designated timeframe, with the developers expressing themselves as exceptionally satisfied with all aspects of the project. Overall, the Skibbereen Bridge project has been extremely successful; and strengthens both Cosira’s and South Africa’s position in the structural steel export market – a ‘proudly South African’ steel export success” concludes Da Silva.