The "Die Burg" nature reserve in the southeast of the North Rhine-Westphalian town of Marl, thanks to its ancient forests and winding streams with impressive steep banks, is a popular local recreation area open to the public for so-called "gentle relaxation." At the same time, it offers protection to endangered species native to the area, such as the kingfisher, bullhead, brown trout, and black woodpecker. Paved paths and boardwalks guide visitors through the area, through a marsh, and over streams. A wooden boardwalk dating from 1998 had become dilapidated over the years and required replacement. The key requirements for the new surface were slip resistance and building approval. The Regional Association of the Ruhr (RVR), responsible for the nature reserve, opted for a wood composite material. 145 square meters of Popular massive in brown from the Franconian manufacturer NaturinForm were laid.

The area takes its name from a medieval earthwork, whose ramparts are now overgrown. The Nieringsbach and Silvertbach streams run through the area, largely shaded by trees and therefore cool and rich in oxygen. The approximately 143-hectare forest area was designated a nature reserve in 1991 and included in the European Union's Natura 2000 network of protected areas in 2001, in accordance with the European Union's Habitats Directive (Flora and Fauna Habitats).

Comprehensive renewal of the wooden components

"The condition of the 21-year-old footbridge over the marshland, including a smaller bridge over the stream, required extensive replacement of the wooden components. In addition, the footbridge had suffered several storm damages in the recent past," explains Christiane von Recklinghausen, who works in the field of structural maintenance for the RVR and Ruhr Grün. The civil engineer coordinates construction projects and is entrusted with various procurement, planning, and monitoring tasks. The Ruhr Regional Association, headquartered in Essen, is the largest municipal forest owner in Germany. The RVR-owned company Ruhr Grün manages local forest recreation areas within the association's territory, including the "Die Burg" nature reserve.
"The walkways showed the most damage, particularly to the oak decking and railings," adds Prof. Dr.-Ing. Rüdiger Harnach from the Bochum-based engineering firm commissioned with the project. "The longitudinal beams (made of wood and steel) and the steel frames showed only minor damage. Damage to the wooden planks of the walkway included cracks, splitting, chipping, and rot, primarily due to moisture exposure. In some areas, the old plank decking had already been replaced by a new decking layer. Algae and moss growth was evident, particularly at the edges. The ridges on the top of the planks were often worn or clogged. Some fasteners no longer held the wood securely, and some planks were warped."

Low-maintenance reinforcement and slip resistance required

"Since the pavement, with its relatively thin cross-sections, is exposed to the elements year-round, it is usually the first to be replaced. In this case, the pavement, along with the wooden components in contact with the ground, was already heavily weathered and had already been partially repaired when the renovation was decided upon," says Christiane von Recklinghausen. "We were therefore looking for a way to strengthen the structure with the renovation so that it could be preserved for a longer period with as little maintenance as possible, even though its location in the forest and marshland always means high ambient humidity is to be expected." Traffic safety also had to be ensured in this area with its high visitor volume. Therefore, appropriate slip resistance played a key role in the selection of the suitable material.

Building authority approval as an award criterion

In this context, those responsible at the Ruhr Regional Association, together with the Harnach engineering firm for structural engineering, discussed various options for working with materials other than wood. "For a time, we even considered a purely plastic product, as there are products under development that allow for significantly greater support spacing thanks to steel rods incorporated into the material," explains Rüdiger Harnach. This would have required fewer additional central supports. However, due to the location of the walkways in the nature reserve, the RVR insisted that the pavement must contain wood.
Due to the high standards for publicly accessible buildings, only products with building authority approval were considered. "RVR Ruhr Grün sees itself as a role model in terms of sustainability, so the decision was made in favor of a wood composite material, although the substructure and railing were still made of solid wood," says Christiane von Recklinghausen.

Optical and technical requirements met

"The walkways are essentially a steel-and-timber construction," explains Rüdiger Harnach. "At intervals of up to 5 meters, the walkway features steel frames whose posts are embedded in individual foundations. The outer longitudinal beams rest on the crossbeams of the steel frames at their ends and are reinforced on the inside by parallel U-profiles. The original wooden decking consisted of six-centimeter-thick wooden planks. The leveling timber attached to the underside of the decking along the longitudinal axis of the walkways does not serve a load-bearing function. Two wooden posts are placed between each steel frame. The handrail with protective board, as well as knee and toe rails, are also made of wood and are screwed to the other elements."
Regarding the special features of this project and the associated structural requirements, Harnach points to the requirements to maintain the original appearance and structural layout, as well as the use of building materials approved for use in a nature reserve. The construction period was also set from the beginning of October to the end of February to avoid disturbing the birds living there during their breeding season. "What was required was a surface that would be more durable than the original wooden surface and would also have increased slip resistance," says Harnach. "In the end, we jointly decided to use the building authority-approved load-bearing floorboard Popular massive as the new flooring." Due to the maximum support spacing of 30 centimeters prescribed in the floorboard approval, additional wooden beams had to be installed as a substructure between the steel frames. The railings were replaced in line with the original design, and the steel girders were reused.
The engineering firm for structural engineering was founded in 1981 by Prof. Dr.-Ing. Rüdiger Harnach. The company operates in virtually all areas of construction (such as bridge construction, open spaces, building construction, new construction, building renovation, construction management, and expert opinions). This includes construction consulting, structural engineering, and project planning. A key focus is the monitoring of local construction activities. Clients include construction companies, public authorities, and private developers.

Many years of practical experience with wood composite materials

Schmees & Lühn from Freesenburg was commissioned to carry out the work. Founded in 1900 as a timber merchant specializing in piles and pit timber, the long-established company, which now has 75 employees and four apprentices, specializes in timber and steel bridge construction and carries out projects across Europe, primarily in Germany, the Netherlands, England, and Poland. Markus Lühn, responsible for timber construction production management and construction management within the company, pointed out the special challenges faced when constructing projects in nature reserves. "The footbridge was in poor condition; the original oak decking and railing were dilapidated," said Lühn. The decision to use wood composite material was made not only for safety reasons but also because of the desire to use natural materials. "At Schmees & Lühn, we have extensive, long-standing practical experience with wood composite materials, which, in our view, are primarily due to their longevity," Markus Lühn continued. His team was on site for around 20 working days to repair the footbridge. A particular challenge during the work was the fact that, due to the nature of the terrain and the adapted substructure of the footbridge, manual joining was required on site.

Solid plank made of environmentally friendly composite material

Popular massive plank, made from an environmentally friendly wood-polymer composite material, is approximately 25 millimeters high and 138 millimeters wide (system width approximately 146 mm). The surface is available with a finely or coarsely grooved side; both are brushed for greater grip and slip resistance, thus achieving the high slip resistance levels R12 and R13 according to TÜV Rheinland. The color palette is classic, with brown, black-brown, gray, stone gray, and anthracite; the profiles are available in lengths of approximately 4 and 6 meters.

Regulated and approved construction product

NaturinForm received general building approval (AbZ) for Popular massive. This marks the first time the German Institute for Building Technology (DIBt) in Berlin has granted approval to a decking made of a wood composite material (also known as WPC) without referring to a composite decking. The DIBt's assessment relates to the applicability of the subject of approval with regard to building regulations and is valid for all federal states. Once approval has been granted, the tested decking is considered a regulated building product within the meaning of the state building regulations. A separate approval for private or public areas is therefore no longer necessary. Unlike ground-level terraces, balconies and terraces over 60 centimeters and higher are generally considered "raised" in state building regulations and must therefore be planned and constructed as load-bearing structures. In these cases, only building products with building regulations approval may be used, or time-consuming and costly individual approvals must be obtained. With Popular massive as a regulated and approved construction product, architects, clients and processors are on the safe side and thus have an advantage in the planned implementation of projects with load-bearing structures in outdoor areas.