Replacement of an ageing and narrow footbridge in Norway will make it easier and more pleasant to cross the capital city’s ring road at a busy intersection. When it is completed next autumn, the new Ullevaalskrysset footbridge will provide more space for cyclists and pedestrians to co-exist, and will make the journey safer and more pleasant for all.
The new crossing is a 400m-long and 7.2m-wide bridge structure which is served by four ramps feeding foot and cycle traffic into a central circulation area. It will provide a new elevated route across the Ring 3 highway, which carries 60,000 vehicles per day, as well as the smaller Sognsveien road which carries 12,000 vehicles per day. The bridge is designed to link and interweave separated cycle and pedestrian lanes from four directions at the elevated integration zone, resolving the current problem of insufficient capacity, and improving traffic safety in the area.
The site is next to Norway’s national football stadium, Ullevaal Stadium, which also hosts concerts and other crowd-gathering events. The surrounding area is heavily used by motorists, pedestrians, and cyclists as they commute to and from the facilities in and around the district. The primary aim of this project is to improve the accessibility, safety and capacity for pedestrians and cyclists by replacing the narrow and ageing bridges at the site. It will also serve as an important link in the ongoing expansion of the city’s cycling network, of which the cycle route along the Ring 3 highway is a vital component.
The existing bridges are narrow, at only 2.5m wide, and this not only restricts the capacity, it increases the risk of collisions between pedestrians and cyclists. Another issue is that the bridge ramps slope at 1:10 and have no landings, making them unsuitable for the disabled. When footfall is high due to events at Ullevaal Stadium, the bridges tend to oscillate, making users feel unsafe. The cycle route across the smaller street, Sognsveien, is currently via a zebra crossing which is very close to the Ring 3 slip road. This gives rise to a danger of collisions between motorists and cyclists crossing the road due to limited visibility and the high speeds involved. Using the existing bridge to cross Ring 3 on a bicycle is not much easier, which effectively creates a gap in cycling provision along Sognsveien at this point.
EFLA Consulting Engineers and Brownlie Ernst & Marks Architects were assigned the project design by the bridge owner, the Norwegian Road Administration, after the team had carried out a conceptual design phase in early 2015. This resulted in the definition of plan layout, elevation and bridge type. Tender design was completed in the spring of 2017. The total cost of the project is US$35 million.
The bridge site poses a series of technical challenges, and the chosen layout and structural system very much reflect the site restrictions. The narrow boundaries of the available area led to a bespoke planar layout, made up of ramps from four directions connecting to the main spans of the bridge and an integration zone where the traffic streams weave between each other.
Another challenge is the large depth to bedrock, which ranges from 20m to 45m, with the soil mainly consisting of sensitive clay. To address this, drilled, concrete-filled steel tube piles were chosen for the bridge foundations, along with lightweight access ramps made of expanded polystyrene that connect the bridge superstructure to ground level. Furthermore, there is a large quantity of services in the ground at the site, some of which cannot be moved, and this directly affected the bridge alignment.
One of the main design criteria was to minimise traffic disruption during construction, particularly on the Ring 3 highway. Discussions with the bridge owner and road authorities at the early stages of the design process resulted in steel as the construction material of choice. The decision to use steel for the superstructure allowed the construction period for the whole bridge to be minimised, as the superstructure could be fabricated at the same time as foundation works were being carried out. With the exception of two road closures of one or two-day periods, necessary for the erection of the superstructure over the road, traffic disruption could be kept to a minimum.
The bridge layout, along with the client’s stated requirement for an elegant aesthetic, led the EFLA/BEAM design team to the chosen bridge type; a steel box girder with cantilevered ribs stretching out from a curvaceous, flowing spine and supported by pairs of V-shaped steel columns. Tuned mass dampers are necessary at the two longest spans of 27.5m and 37.5m, to achieve a dynamic behaviour appropriate for pedestrian comfort. The use of mass dampers also allows for a relatively slender and lightweight construction of between 1.4t and 2.4t per metre and a transparent structural system.
The construction of the bridge is being carried out by the main contractor NRC Group Norge, supported by steel fabricator Western Constructions, Hallingdal Bergboring for the pile installations and Fastlane Technologies for on-site erection works. Construction site supervision is run by the project owner, the Norwegian Road Administration, with the support of Axess for steel inspection.
Construction work began last year, in November 2017, and the first phase included relocation of the local services along with site preparation. This was a substantial undertaking, as cables and pipes at each of the 19 bridge pier locations had to be diverted and relocated.
Installation of the steel tube piles started in February this year, when more than 2km of piles with diameters of 500mm or 600mm were installed. The longest piles were up to 50m, pushing the installation process to its limit with the casting of concrete into the steel tubes being the most challenging part. The piles were excavated using reverse-circulation drilling with the excavated soil being removed by suction up to the ground level. This method is particularly favourable for installing very deep piles in sensitive soil. In conjunction with the piling, construction of the pile caps and abutments started in spring, and all the bridge foundations were cast in place by September.
Fabrication of the steel superstructure started in Klaipeda in Lithuania also in spring this year – most of the bridge is being brought to Oslo by road, with just the four largest elements being shipped to the site. The structure is emerging from the fabrication plant in 19 main elements that range from 14m to 18m in length and between 20t and 36t in weight. Due to their substantial width, 15 of the main segments had to be divided in two to enable their transport by truck from Klaipeda to Oslo.
The largest challenge of the fabrication and erection process is the complex geometry of the bridge, since it has a range of different curvatures in plan and elevation, and the geometry is further complicated by the four adjacent ramps that come together with different cross-slopes at the integration zone. To make the bridge fabrication easier, a detailed 3D model of the bridge superstructure was created in Revit by EFLA to support the 2D tender drawings. The model not only defines the bridge geometry, but also the material type, plate thicknesses and various details; it formed the basis of the shop drawing modelling done by the steel fabricator.
The first elements of the bridge arrived on site in August; most of the 19 segments arriving by road are being erected soon after arrival on site, due to the limited space available at the construction site for storage.
At the beginning of the process an integration zone was erected on rigid foundations to create a stable backbone for the bridge structure, followed by the mounting of the four ramps onto their hinged columns.
The two scheduled road closure periods are for the mounting of the bridge’s main spans. The 37m, 62t Ring 3 span was erected on to its column and welded onto the adjoining section on 28 October, during a 48-hour weekend road closure. This involved strategically mounting two large cranes with a lifting capacity of 500t and 250t so that they would not conflict with the existing bridges, which remained open during the operation. A similar operation is scheduled for the Sognsveien bridge span next month (December).
By the end of last month (October), eight of the 19 bridge elements were in place and erection work was set to continue throughout the year. The bridge access ramps were under construction, and this phase will be followed by linking the bridge to the cycle and pedestrian networks in the area and associated landscaping. The whole project is scheduled to be completed in autumn 2019.
Andri Gunnarsson is structural engineer and Magnús Arason is project design manager at EFLA Consulting Engineers