The conservation area ‚Die Burg‘ south-east of the North-Rhine-Westphalian town Marl is a popular conservation area thanks to its old forests and winding streams with impressive banks, and is open to the public for ‚gentle recreation‘. At the same time it offers protection for domestic endangered species, such as kingfishers, sculpins, river trout or black woodpeckers. Visitors are guided through the area on footpaths and walkways, over a swamp and streams. A wooden walkway from 1998 had become rotten as time went by and needed to be renovated. The main criteria for the new surface were that it be non-slippery and that it has building construction approval. The regional association responsible for the conservation area Ruhr (RVR) decided to use a wood composite material. 145 square metres of ‚Popular massive‘ in the colour brown from Franconian producer Naturinform’s range were laid.
Walkway in conservation area
Popular massive in brown
Type of construction
Walkway renovation in conservation area
area ‚Die Burg‘, 45570 Marl
The name ‚Die Burg‘ comes from a mediaeval castle, the walls of which are completely overgrown nowadays. The Nieringsbach and the Silvertbach, two streams, flow through the area, shaded for the most part by trees, making them cool and full of oxygen. A designated conservation area since 1991, the approximately 143 hectare large forest area was recorded in the European wide net of protection areas ‚Natura 2000‘ in 2001, according to the FFH guidelines (Flora, fauna-habitat) by the European Union.
Extensive renewal of wooden construction parts
‚The state of the 21-year-old walkway across the swamp area, including a small bridge over the stream, required extensive renewal of the wooden building parts. Adding to that, there was quite a lot of damage to the walkway by storms recently,‘ explains Christiane von Recklinghausen, who works for the RVR and Ruhr Grün in construction and building maintenance. As a constructional engineer she coordinates building projects and is responsible for diverse tendering, planning and monitoring tasks. The regional association Ruhr, located in Essen, is the largest communal forest owner in Germany. The RVR’s own company Ruhr Grün farms forest conservation and recreation areas in the association’s region, therefore also the conservation area ‚Die Burg‘. ‚The walkways were badly damaged, especially on the surface made of oak wood, as well as on the handrails,‘ adds Prof. Dr.-Eng. Rüdiger Harnach from the authorised engineers office for structural engineering in Bochum. ‚The longitudinal carriers (made of wood and steel), as well as the steel frame, were only slightly damaged. Damage to the wooden decks of the walk surface was – mainly due to humidity – tears, cracks, bursts and decay. In some parts, the old deck surface had already been replaced by a covering deck surface. Especially on the sides there was algae and moss. The striation on the top of the decks had frequently been rubbed off or at least affected. Some binding agents were no longer connected to the wood and some decks were warped.‘
Low maintenance strengthening and slip resistance required
‚As the surface with its comparatively thin cross section is exposed to weathering throughout the year, it is usually the first item that needs to be replaced. With this walkway the surface, as well as the wooden parts next to the ground, were already very much weathered and partially had already been repaired when the renovation was decided,‘ says Christiane von Recklinghausen. ‚Therefore we were looking for a possibility to strengthen the construction during and with the renovation in such a way that it would survive for a long time, requiring little maintenance, despite being situated in a forest and swamp area, meaning there is always a high amount of humidity to be expected.‘ And safety also is extremely important due to the high amount of expected visitors. This meant that slip resistance also played a central part in choosing a suitable material.
Building inspectorate approval as a criterion for tenders
In this context the responsible parties at the regional association Ruhr discussed the different options of using material other than wood with the engineer’s office for structural engineering Harnach. ‚There was a phase when we contemplated using a purely artificial product, as products are now being developed that permit a far larger support gap, due to steel rods integrated in the material,‘ explains Rüdiger Harnach. This would have meant needing less additional carriers. But the RVR insisted that, due to the situation of the walkways in the conservation area, the walk surface must contain wood. Due to the high requirements for publicly accessible constructions only products with building inspectorate approval could be possible options. ‚RVR Ruhr Grün sees itself as a role model regarding sustainability, so that the decision was made for a wood composite material, whereby the underlying construction and handrail were made completely out of wood,‘ says Christiane von Recklinghausen.
Optical and technical requirements fulfilled
‚The walkways are basically a steel-wood construction,‘ says Rüdiger Harnach. ‚With gaps of up to 5 metres the walkway has a steel frame, the posts of which are embedded in individual bases. The outerlying longitudinal carriers are on the cross profiles of the steel frame and are reinforced on the inner side by parallel laid U profiles. The original wooden surface was made up of six-centimetre-thick wooden planks. The winding sticks attached to the walkways underneath the walkway surface longitudinally do not carry any loads. There are always two wooden posts between two steel frames. The handrail with a protecting board, as well as knee and foot skirting boards, are also made of wood and are screwed onto the other elements.‘
Harnach points out that everything concerning this project, or respectively the structural requirements, was influenced by the requirements to retain both the original look, as well as the structure, and to use building material that has been approved for use in a conservation area. The construction time was defined to be from the beginning of October until the end of February, so as not to affect breeding birds living there. ‚We were looking for a surface that would be of higher durability than the original wooden surface, as well as one that was more slip resistant,‘ says Harnach. ‚In the end we decided to use the load carrying deck with building inspectorate approval, ‚Popular massive‘, as a new walking surface ‘ Due to the maximum laying gap of 30 centimetres required by inspectorate approval, further wooden beams had to be inserted as an underlying construction between the steel frames. The handrails were renewed analogue to the original setup, the steel carriers were reused.
The structural engineering office was founded in 1981 by Prof. Dr.-Eng. Rüdiger Harnach. The company works in almost all areas of construction (bridges, open-air plants, civil engineering, new-builds, building renovation, building management, certification). This entails construction consulting, structural design and property planning. A main focus is on monitoring local construction work. Clients are construction companies, the public sector and private building owners.
Long-term expertise with wood composite materials
The company Schmees & Lühn from Freesenburg was awarded the contract. Founded in 1900 as a timber trader for posts and pit wood, the traditional company now has 75 employees and four apprentices, specialising in bridge construction with wood and steel, realising projects throughout Europe, with a main focus on Germany, the Netherlands, England and Poland. Markus Lühn, responsible for production management wood construction and supervision in the company, points out the special requirements for construction measures in conservation areas ‚The walkway was in a bad state, the original surface of oak, as well as the handrail, were rotten,‘ says Lühn. The decision to use wood composite material was made not only for safety reasons, but also because it was decided to use material that was close to nature. ‚We at Schmees & Lühn have extensive and long-term experience with wooden composite materials, that, in our opinion, stand for longevity,‘ says Markus Lühn. His team was on site for around 20 workdays to repair the walkway. A special challenge during the work was the fact that it was necessary to handbind on site due to the state of the area and the adapted underlying construction for the walkway.
Massive deck made of environmentally-friendly composite material
‚Popular massive‘, a massive deck made of an environmentally-friendly wood-polymer-composite material, is 138 millimetres wide (system width ca. 146 mm) at a height of ca. 25 millimetres. As a surface you can choose from a finely or widely striated side, each is brushed for higher grip and slip resistance and has been awarded the high slip resistance R12 and R13 by the TÜV Rhineland. The classic colour range covers brown, black brown, grey, stone grey and anthracite; the profiles are available at lengths of ca. 4 and 6 metres.
Regulated and approved construction product
NATURinFORM was awarded the general building inspectorate approval (AbZ) for ‚Popular massive‘. This was the first time the DIBt, the German Institute for structural engineering, awarded a patio deck made of wood composite material (also called WPC) approval, without it being connected to a bonded surface. The evaluation of the DIBt focuses on the utilisation of the approved object in regards to the inspectorate approval requirements and is vaild for all federal states. After certification has been awarded, the tested deck surface counts as a regulated construction product according to the state construction regulations. This means it is no longer necessary to apply for separate approval for use in private or public areas. Contrary to ground-level patios, balconies and patios above 60 centimetres and higher count as ‚raised‘ in the state regulations and must therefore be planned and executed as load-bearing constructions. You are only permitted to use construction products with inspectorate approval for such projects, or you have to apply for individual approval which takes a lot of time and costs a lot of money. Using ‚Popular massive‘ as a regulated and approved construction product, architects, clients and processors are on the safe side and have thereby an advantage when implementing projects with load-bearing constructions outdoors.
Climate-neutral, sustainable production
Climate protection and resource protection are very important for Naturinform. In wood composite production they only use wooden fibres from domestic and sustainable forestry (PEFC-certified wood suppliers), as certified as climate neutral since November by the TÜV NORD. No additional trees are felled, as the wood fibres are a by-product of the woodprocessing industry. This makes the material an extremely environmentally- and climate friendly product that is, in addition, recyclable by 100 %. After utilisation, the decks are taken back by the company if possible and fed back into the production process.