Learning from historic timber bridges
For centuries, timber was the preferred material for constructing bridges. The alternative, masonry, was much more difficult to work. Wood, a locally available resource, was easily and cost-effectively processed by neighbourhood carpenters for building bridges with small to medium spans. Timber bridges built in the 17th and 18th centuries and continuously maintained are still in use today. These bridges are almost all roofed or fully enclosed timber trusses or arch structures, such as the old bridge over the Rhine at Bad Säckingen, the oldest and longest covered timber bridge in Europe. With the beginning of industrialisation in the 19th century, steel followed by reinforced and prestressed concrete in the 20th century became increasingly used instead of wood as a bridge-building material. Despite the rapid acceleration of technological development in the last 20 years, for example in the fields of adhesive and threaded fastenings technologies and in industrially manufactured large-format timber-based construction elements, timber continues to lose its earlier important role in bridge construction. Frequent serious defects discovered in the often inadequately designed and built structures of the 1970s and 80s strengthened this trend and confirmed the opinion of many clients that timber bridges were not durable and were expensive to maintain. So, for example, in Baden-Württemberg only 62 out of a total of approximately 9200 existing bridges were constructed with timber as the load-bearing material (data from 2012). A research project, which commenced in 2013, analysed the causes of defects in eleven old but still operating timber bridges in the Stuttgart region. The results of the MPA Stuttgart report clearly identified the reasons for the defects, some of which were serious and extensive. These reasons included continuous moist conditions in the bearing area and beneath failed waterproofing systems. Connections exposed to the weather and unable to dry out adequately after becoming saturated with moisture considerably shortened the service life of the bridges. Based on these findings, the new Stuttgart Timber Bridge is a covered bridge in the sense that its timber structural members are sheltered by a cleverly detailed deck. The dense surfacing material projects beyond and protects the glued laminated timber (GLT) beam from the direct effects of the weather. Furthermore, the tops of the solid timber segments are waterproofed with a vapour-permeable foil waterproofing membrane. A 15 cm gap between the beam and the walking surface material allows adequate ventilation.
First integral bridge with a timber superstructure
In contrast to historical timber bridges, the new bridge type is of integral construction, i.e. a bridge with no bearings or movement joints: the beams and abutments are monolithically connected to one another. Glued-in threaded rods, which are connected by the appropriate lap lengths to the reinforcement in the abutments, transfer the bending tension and normal section forces between the timber superstructure and the reinforced concrete substructure (Figs. F, H). The direct coupling of the solid, block-glued GLT deck and the reinforced concrete substructure carries with it the risk of crack formation because swelling and shrinkage of the timber due to changes in its moisture content are constrained. This form of structural connection had not been used before in bridge construction. Therefore, MPA Stuttgart developed and evaluated an extensive test programme and undertook long-term testing of a prototype to validate the connection concept. Load tests on GLT/reinforced concrete test pieces, which were connected with 16 mm diameter reinforcement bars by gluing with two-component epoxy resin, confirmed the calculated load capacity and a very high residual strength. A master’s degree project in 2014 supervised by Knippers Helbig at the Stuttgart University of Applied Sciences analysed the theoretical and practical issues to be considered for the first adaptation of the “integral bridge” principle involving timber as the load-bearing material. Superstructure, substructure and foundation soils interact with one another; soils and structure must be precisely assessed, and the crucial construction components identified. A prototype with a span of 40 m was used for this purpose. The bridge was based on a single, prefabricated timber segment and investigated using varying parameters. The findings showed that timber had favourable characteristics as a material for use in integral bridges. The coefficient of thermal expansion of timber is relatively small, the ratio of strength to Young’s modulus is good and timber’s tendency to creep reduces long-term imposed stresses.
Three prototypes in Remstal
After the design for the project began in 2017 and a restricted tender awarded in 2018, the first three pedestrian and cycleway bridges – Weinstadt Häckermühle, Weinstadt Birkelspitze and Urbach – of the new, integrated Stuttgart Timber Bridge type were completed in May 2019, precisely in time for the start of the Remstal Garden Show organised by a group of local district councils. The clear spans measure 13.60 m and 32.20 m. One of the two bridges with the larger span, Weinstadt-Birkelspitze, provides a good example and crosses a river to connect the residential area of Trappeler to the north of Rems with Birkel-Areal in Weinstadt-Endersbach. The geometry of its elevation is designed to ensure that the timber superstructure is always above the water surface, even when the river is at its maximum flood level. The form of the superstructure follows the bending moment diagram of the bridge beam under a uniformly distributed load: the cross section reduces from the point near where the timber connects with the concrete abutment to a minimum at the zero moment point and increases again towards mid-span (Fig. B, p. 55). The steps in the cross section follow from the method of fabrication: a total of thirteen GLT segments each 0.2 m thick in strength classes GL 28c and GL 24h were formed to varying curvatures on the gluing bed and block-glued together in the side-on position horizontally to create the 2.6 m wide and 0.93 m deep bridge beam with a total volume of 45 m³. Hence there are moisture-susceptible cut fibre ends only on the top, which is protected against standing moisture by transverse and longitudinal falls and a vapour-permeable waterproofing membrane. Other than a blocking coating for the end grain faces in contact with the reinforced concrete abutment, no wood-preserving treatment is required. To ensure that timber delivers the desired durability, MPA Stuttgart monitors the superstructure at eight selected positions using permanent moisture and temperature sensors. The data are read and evaluated at regular intervals. It would show whether there was a persistent increase in the moisture content at the critical areas near the contact joint between the timber superstructure and the concrete abutment, and beneath the waterproofing foil membrane. Fungal attack and other damage can occur above an equilibrium moisture content of 20 %.
Carbon concrete and lead wool
Seventy-eight 20 mm diameter ribbed reinforcement bars between 2.3 – 3.0 m long were embedded up to 1.2 m deep and glued into the 30 m long block-glued beam (Fig. H). Machining the top of the timber created symmetrical cross-falls about the longitudinal axis of the deck. After installing the vapour-permeable foil waterproofing membrane and the handrail attachment blocks, the prefabricated superstructure was transported to site and lifted into position using a mobile crane. Concreting the integral joint areas completed the structural connection of the deck to the reinforced concrete abutments. The walkway surface, which is subject to high mechanical loads, consists of prefabricated carbon-reinforced, fine-aggregate concrete slabs. Because the carbon fibre non-crimp fabric is not affected by corrosion, it requires only 10 mm of concrete cover. If this were steel reinforcement, then the protective concrete layer would need to be four or five times as thick. Not only is the volume of concrete less but the quantity of cement and therefore the energy used in its manufacture are also reduced by more than 50 %. This new material promises very high levels of durability. To ensure the walkway surface is not slippy even when wet, the tops of the 3 m × 3 m × 7 cm thick slabs are grit-blasted, which does away with the need for an additional anti-slip and crack-bridging coating. The joints between the slabs are sealed with lead wool. This traditional technique, which was used for the reconstruction of the Frauenkirche in Dresden, makes the surface durable, low-maintenance and capable of accommodating movement.
Sustainable, unique bridge type
The integral “Stuttgart Timber Bridge” confirms the sustainability of wood, the oldest bridge-building material, and underlines the new possibilities it offers. The monolithic structure without joints or bearings is low maintenance and, because of the high degree of prefabrication, is quick to erect. Climate-damaging carbon dioxide is permanently bound up in the solid, block-glued GLT superstructure. The lower processing stage of this material means that less emissions and energy are involved in its manufacture, sale and distribution compared with other bridge-building materials. The unique architectural language of this bridge type arises from its monolithic construction and the cross-sectional shape, which is designed to facilitate its method of manufacture.
