Spectacular deconstruction of the old Lahntal bridge near Limburg

Movable scaffolding system from thyssenkrupp Infrastructure forms the hub

Piece by piece, the workers from ADAM HÖRNIG Baugesellschaft mbH & Co. KG and the demolition subcontractors Frank Beißner and KRAKBAU Sp. z o.o have demolished the 397 meter long superstructure of the old Lahn valley bridge near Limburg over the past few months. For the spectacular project commissioned by DEGES Deutsche Einheit Fernstraßenplanungs- und -bau GmbH, the contractor developed a special proposal involving a movable scaffolding system in cooperation with the shoring systems unit of thyssenkrupp Infrastructure. The basis for this successful demolition project were the main supports of the movable scaffolding system, made from the HV21 modular support system developed by thyssenkrupp Infrastructure and which were designed and used in this size for the first time. Equally noteworthy: The movable scaffolding system was supported throughout the demolition solely by a structure that rested on the abutments and piers of the bridge, covering the up to 70 meter spans between the piers. In this way the old bridge and the movable scaffolding system formed a joint support system and the interaction of the elements was calculated separately for each phase of the work. The role of the scaffolding system was both to support the old superstructure and collect the dismantled material. The bridge was demolished segment by segment. Once one segment had been removed, the equipment was moved hydraulically to the next.

On the basis of the special proposal, the work was completed as planned in fall 2017.

“Structural analysis of the old Lahn valley bridge on the A3 freeway near Limburg showed that the structure was no longer capable of bearing future traffic loads and needed to be replaced,” says Axel Keck, head of the design engineering P2 department, DEGES Deutsche Einheit Fernstraßenplanungs- und -bau GmbH. Like the new Lahn valley bridge that was opened to traffic in December 2016, the old pre-stressed concrete bridge east of Limburg comprised two superstructures which spanned the River Lahn and the parallel Lahn valley railway as well as the L3020 road at a maximum height of 57 meters. The superstructures comprised two single-cell box girders with a center-to- center distance of 14.60 meters, each 15.00 m wide and longitudinally and laterally pre-stressed at a constant height of 4.00 meters. The spans of the 7-segment bridge were 46.00, 53.00, 59.00, 68.00, 63.00, 57.00 and 50.50 meters.

Planning, supplying and installing

For Michael Däbritz from the scaffolding systems unit at thyssenkrupp Infrastructure GmbH, the movable scaffolding system formed the hub of the demolition project. In terms of load-bearing capacity it is the largest system used in Germany in the last 40 years. Together with construction engineer Peter Wagner, ADAM HÖRNIG Baugesellschaft mbH & Co. KG, and the engineering offices Marx Krontal GmbH and saul ingenieure gmbh, the head of movable scaffolding systems developed a viable concept for the bid phase that met all the customer’s requirements. Everyone involved agrees that this resulted in an innovative and completely new type of demolition process. “When formulating the concept we did not calculate using type testing and usual standards, but rather developed our own standards and a new technical solution which we then implemented,” says Däbritz. The main component of the structure was the HV21 scaffolding support which has a height of up to 4,100 millimeters as standard. For the demolition of the Lahn valley bridge the modular main girder was increased to a height of 5,000 millimeters, making a crucial difference in terms of increasing bending stiffness.

World premiere

The main segments of the bridge superstructure with spans of up to 68 meters were then demolished on a movable scaffolding system to minimize intervention and trans- portation in the valley below. The old bridge was demolished segment by segment in the direction of construction and not – as is usually the case – counter to the direction of construction: the first time that this approach has been used worldwide. “The prerequisite for this was to find a solution for separating the old structure between the prestressed tendon anchors,” says Hörnig department manager Peter Wagner. During the demolition the bridge piers were surrounded with scaffolds as load-bearing and auxiliary structures. Then heavy construction machinery fitted with demolition cutters and hydraulic breakers was positioned on the movable scaffold. “After breaking out a roughly 3 meter wide section the tendons were cut in the coupling joints,” says Wagner, “while the stress was retained in the rest of the structure.” Demolition of the superstructure could then continue with the debris falling into the formwork surrounding the scaffold and being transported away via the remaining sections of the bridge.

A total of around 11,100 m3 of concrete debris from the superstructure and approx. 12,500 m3 of concrete debris from the piers, abutments and supports highlight the scale of the project. Complex calculations and examinations of the structure preceded the work. “It was necessary to examine the geometry of the structure and quality of the superstructure in advance to check whether it was suitable for such an approach,” says Däbritz. “Through appropriate testing of the pre-stressed tendon anchors outside existing standards it was possible to determine the residual load-bearing capacity of the cut tendons and thus the remaining superstructure.” This approach also helped meet the safety requirements for the work in full. The movable scaffolding system over the Lahn was dismantled after the demolition of the penultimate segment and the final segment was demolished from the ground in the conventional way.