SHOWCASES
Bolintxu viaducts
BILBAO, SPAIN
Structural designer: ARENAS & Asociados
Years: 2021–2023
Source of photos: ARENAS & Asociados
PROJECT DESCRIPTION
The Bolintxu Viaducts were built as part of the Supersur bypass project in Bilbao, Spain. The viaducts have main span lengths of 162 m and 142 m. The main goal of the project was to reduce heavy traffic and congestion on the city’s old road network. To solve this problem, the regional government decided to build the Supersur bypass so that through-traffic could avoid the city center.
One of the biggest challenges of the project was crossing the Bolintxu Valley, a protected natural area with important ecological and landscape value. Because of the sensitive environment, the bridges had to be designed and built with as little impact on nature as possible. From the beginning, the engineers wanted the viaducts to be elegant structures that fit naturally into the landscape while protecting the valley below.
The final design consists of two slender steel arch viaducts with a light and shallow appearance. The bridges use an innovative composite arch system made of weathering steel box sections filled with high-strength self-compacting C60 concrete. This solution gives the structure high strength, good stiffness, and long-term durability while reducing maintenance needs. The arches are very slender, with a span-to-rise ratio of about 11, which improves both structural performance and visual appearance.
An important goal of the project was to avoid any construction work on the valley floor or near the river ecosystem. To achieve this, the engineers developed a special swivel-arch construction method. Instead of using temporary supports inside the valley, each bridge arch was built in two separate halves, one on each side of the gorge.
The semi-arches were first assembled vertically on the abutments and supported by temporary steel towers. They were connected to large swivel mechanisms that allowed them to rotate during installation. After both halves were completed, they were slowly rotated and lowered at the same time using a controlled strand-jack system until they met exactly at the center of the span.
This construction method avoided the need for temporary supports inside the protected valley and greatly reduced environmental impact. The Bolintxu Viaducts are therefore known not only for their elegant design and technical innovation, but also for their successful combination of sustainability, environmental protection, and advanced bridge engineering.
