For several years, the connection between the towns of Olot and Vic was one of the main priorities for the counties of Garrotxa and Osona, in Catalonia, Spain. Due to its unique importance and singular surroundings – Olot Volcanic Zone Natural Park as well as several other protected landscape areas and villages – the environmental impact of this project had an extreme importance and was followed closely by the area’s residents and environmental organizations.
The solution was the construction of a structure with minimal visual and environmental impact on the surrounding areas. As a result, the Vall d’en Bas Viaduct was required to be as transparent as possible, with moderately big spans, a sober structure and a sustainable and environmentally friendly type of construction. With these premises in mind, the decision towards a composite steel-concrete solution was compulsory.
The structure is formed by 9 spans up to 80 m (262.5 ft) long, with an overall length of 565 m (0.351 miles). The total width of the deck is 14.5 m (47.6 ft), with 3 traffic lanes. The deck is a tub steel girder which has variable depth along the first six spans and with constant depth over the last three shorter spans. The box girder depth varies from 5 ft 9 in (1.75 m) at the center of the spans to 12 ft 7 in (3.85 m) at the intermediate supports. The variable depth and the constant web slope results in a variable bottom flange width of 19 ft 8 in at the pier and 22 ft 5 in at mid span. The steel section includes two 11.5 ft long cantilever outriggers, that complete the overall width, spaced at 13 ft (4 m).
The top concrete slab thickness is a constant 10 in and was designed to act compositely with the main box girder and the transverse diaphragm and cantilever outriggers. Precast reinforced concrete panels (3 in deep) have been used as a form and as a part of the slab.
Once the drilled piles and the abutments had been built, the steel girder, including the cantilever outriggers was assembled on site. Splices are located to produce the length of 3/4 of one span and 1/4 of the following span. All connections at the site were welded. An exhaustive quality control process was undertaken. All the welds, both off and on site, were checked using techniques such as ultrasonic testing, X-rays and magnetic particles. The construction was unshored.