BW 5-2 – B173 Michelau-Zettlitz

In the course of the four-lane expansion of the B173 federal highway between the A73 (Lichtenfels junction) and the KC/LIF district border, structure 5-2 will be required at construction km 0+259.442 for the overpass of the relocated LIF 13 district road. The structure is designed as a three-field prestressed concrete hollow box (longitudinally prestressed, transversely reinforced without prestressing). In the first edge field, the structure crosses a farm track between A10 and A20, in the inner field the two-track Bamberg-Hof railway line and in the second edge field (A30-A40) the four-lane B173 federal highway. The substructures (piers and abutments) are founded using large bored piles. The abutments in A10 and A40 are box-shaped with accessibility to the superstructure, to the underside of the roadway transition and to the bearings in reinforced concrete construction. The superstructure is manufactured in an elevated position using a ground-supported falsework, which dissipates the loads from the superstructure production into the subsoil by means of creep foundations. The elevated manufacturing position is necessary to ensure sufficient headroom during manufacturing, which is why the superstructure is temporarily supported on stacking towers and the final bearings can therefore only be installed retrospectively after destacking. In the final state, the superstructure is supported on spherical bearings and a bracket for wind force dissipation is provided in the end supports (axis A10 and A40). A longitudinal fixed point is formed in axis A30, as the vertical load is sufficiently large here.The structure is strongly curved in plan and elevation.The alignment of the LIF 13 for the overpass structure in the area of axis A10 to approx. 10 m before axis A30 is in a circle with R=250 m, thento approx. 15 m before the abutment axis A40 with a clothoid A = 100 and to the end of the superstructure with a straight line. The gradient runs in a crest with a rounding H = 2400 m. The high point is near the pier axis 30. The longitudinal slopes are 6.0% in the direction of the Michelau abutment and 4.25% in
the direction of the Zettlitz abutment.
The crossing angles to the B173 and the DB are 49.5gon and 36.6gon, respectively. Due to the trailing cuts, the possibilities for pier arrangement in the floor plan are very limited. The pier axes are parallel to the traffic routes below and thus strongly rotated to the abutment axes. On the piers axis 20 and 30, only individual spherical bearings can be arranged centrally under the crossbeams. The pier dimensions are minimal, so that the space for the bearing stacks during the elevated production and later for the presses for the bearing replacement is correspondingly tight and the bearing axis formed by the press application points is rotated to the crossbeam and pier axis. This results in different bearing axes and bearing conditions both for the planned final state and for the construction state in an elevated position and the state of a later bearing replacement.

Manufacturing in an elevated position on falsework:
The superstructure is manufactured using a ground-supported falsework. The load from the fresh concrete is dissipated via the formwork construction into the longitudinal beams of the supporting structure. A horizontal load from the concreting of the superstructure is not introduced into the falsework, as the formwork is anchored through. The longitudinal beams are laid vertically. For this purpose, wedge-shaped centering strips are arranged on the yoke head beams, as these are arranged in a cross slope. The yoke head crossbeams are placed on shoring supports. The shoring supports carry the load downwards. Depending on the yoke axis, the foundation is either on the already manufactured foundations of the final piers and abutments or on temporary foundations. The dissipation of the horizontal load in the longitudinal and transverse directions for the falsework takes place largely via corresponding horizontal stiffening of the shoring supports.
The yokes in the abutment axes A10 and A40 are tensioned to the abutment in the longitudinal direction.
The superstructure is concreted in an elevated position (1.0 m above the target height) so that the clearance gauge for the railway line can be maintained during the superstructure manufacture. First, the first superstructure section from A40 to 12.0 m beyond the pier in A30 is manufactured. After the superstructure has been prestressed, it is dismantled. Subsequently, the second construction section from axis A10 to the coupling joint is concreted, whereby the roadway slab is manufactured in two sections on the hardened trough. After completion of all prestressing work (internal and external), the falsework is lowered or removed and then the superstructure is destacked to the final target height by means of hydraulic presses.

Due to the confined location and the flat crossing angles to the traffic routes below, the falsework construction had to be heavily adapted. The individual yokes differed greatly in terms of construction method (scaffolding tower, pendulum yokes, number of uprights, height of the yoke beams) and foundation (with their own foundations, partly on final foundations, mixed, deep and shallow).The axial directions varied greatly in some cases over a short length, so that very unfavorable support conditions resulted for the longitudinal beams. The determination and adjustment of the superelevation for the structure was complex and required extensive computational consideration of the interaction between the falsework and the structure.