Bridge repair by external prestress: the Gibe crossing in Ethiopia

The Gibe bridge is located between Addis Ababa and Jima along the homonymous highway connecting the two Ethiopian cities. The bridge is made of a continuous, 4 span, reinforced concrete girder of 120m length and 9.6m width. The structure has 4 longitudinal beams, of variable height, connected by reinforced concrete diaphragms at piers, quarter spans and midspans.

The bridge, built in 1976-1977, suffered major damage from a terrorist bombing in the 80’s. The bombing caused the collapse of a whole section of the deck that had to be re-cast in situ. The bridge has been used since then with load restriction with an alternative crossing available a few hundred metres downstream along the old road built by the Italians in ‘30. This latter crossing was an old steel truss girder built by the English in the ’40 that collapsed in the fall of 2006 when a dozer crashed into the upper bracing of the main steel girders causing the spectacular collapse of the whole structure. Rehabilitation and strengthening of the concrete bridge became, all of a sudden, vital and urgent as it stands as the only connection between the agricultural South Western region and other parts of the country.

The Ethiopian Government contacted Salini Costruttori SpA, a contractor presently engaged in constructing hydropower projects in Ethiopia, with a request for technical assistance. Salini Costruttori SpA decided to perform the bridge strengthening as a contribution to the Country, and asked Integra to devise a fast, reliable and economic method to carry out the operations. The proposed solution makes use of external prestressing to increase the girder strength by closing the extended crack pattern and reducing bending and shear forces in the deck. The works, performed while the bridge was in service, were completed in three months and the bridge opened back to traffic without load restrictions in November 2007.