Repovesi National Park is the gem of hiking in Lapinsalmi and the entire Kouvola area. If you want to get there, you first need to cross the Lapinsalmi strait safely. The old bridge broke down in summer 2018, but fortunately none of the people on the bridge at the time were seriously hurt in the accident.
Safety and durability are the most important things in the design of the new steel suspension bridge that was opened in November 2019, Sweco’s expert Timo Heloaro knows.
The new suspension bridge in Repovesi is in place and in heavy use. The decking was previously made of wood, but now the wooden decking hides a steel frame underneath. Sweco served as the bridge’s lead engineer and specified the dimensions of the joints outlined by the contractor.
Metsähallitus wanted the new bridge to be safe and have a long service life, so that it would also support year-round hiking in Repovesi National Park and its development. ‘When people are guided to nature sites and trails, both bridges and other elements that support hiking must be absolutely safe and functional. Under particular examination are the bridges that cross our waters as well as observation towers. Steel is a durable and maintenance-free material, and it supports our strong life cycle thinking,’ master builder Pasi Ikonen from Metsähallitus says. Ikonen, who is happy with the end result, thinks that the painted and wood-decked bridge also blends in well with the national park landscape, and people will no longer have to queue to cross the bridge.
The bridge’s location was challenging to the engineer
‘In principle, the Repovesi suspension bridge involved basic steel structure engineering, but many external factors guided our work. The bridge’s location in the forest is inaccessible by basic equipment. The terrain, with its steep hills, requires the equipment to be as lightweight as possible. This, in turn, means that the bridge had to be made of pieces that were as small as possible, and the anchoring to the bedrock had to be implementable with lightweight equipment,’ Timo describes.
Compromises are constantly being sought in bridge engineering. In addition to feasibility and safety, financial and aesthetic perspectives are also taken into account, and nothing can be subpar.
‘Within a specific budget frame, we had to engineer a suspension bridge that is safe and will last for the next hundred years. The conditions vary in the wilderness, and we found the best possible option for this purpose – one that will endure both load and time.’ For example, the anchor bolts used in the foundations are stainless steel.
Another criterion set by the client was for emergency responders to be able to cross the bridge by ATV or snowmobile when necessary. ‘The load from pedestrian and bicycle traffic is distributed more evenly across the bridge, but motor vehicles are another matter entirely. We are talking about a so-called point load, which makes strain a more significant factor in a rope structure than in a beam structure. This brought its own challenges to the engineering of a deck structure assembled from small pieces, in which the decking distributes the point load more evenly across the rope structure.’ Because of the strain caused by a snowmobile, the grating is protected with boards.
Full of innovative steel joints
‘The frame of the suspension bridge, with its railings and ropes, is completely made of steel and full of innovative steel joints. The group examined fatigue in steel joints and sought structural limit values pertaining to durability. It is clear that the bridge must also withstand large numbers of people, so we calculated the uniformly distributed load closely and engineered based on that.’
‘We had to get any slack out of the ropes to prevent the bridge from sagging. We studied strain by setting a pre-camber and test load, which allowed us to minimise structural elongation.’ The ropes can also be adjusted afterwards, if necessary.
Another thing that deviates from normal structural engineering is so-called love locks. The engineers wanted to eliminate places from the structures where love locks could be attached. Love locks place an additional load on structural parts, but, above all, hikers may tear their backpacks on them.
Made with emotion by hand
Today, engineering and analysis are carried out by utilising calculation software, but the suspension bridge was, for the most part, calculated by hand. ‘In order to understand what is going on with the structure, you also have to be able to calculate by hand. I’m able to understand from a few photos how the forces work in the rope structures and how the calculation process progresses. This engineering process, which was carried out manually, is close to my heart, but it’s good to doublecheck more demanding engineering with software.’
Timo is also father to a young Girl Scout, so they find themselves visiting suspension bridges every now and then. ‘A suspension bridge allows people to move about the natural environment safely, and it must also last for generations to come. This steel bridge certainly will.’