Bridge Renovation: Success with Innovative Engineering Design


The BORAPA engineering company is responsible for the design and structural engineering of the new Schwarzbachtal bridge in the town of Zweibrücken.
The construction of the new bridge deck took about 18 months and cost a total of almost 5 million euros. The concrete carcassing work of the new deck was completed in 2009.

The specifications for today’s bridge constructions can only be met with the support of integrated Finite Element calculation software. And information technology assumes even more importance in engi-neering when totally new types of design methods are used, as was the case with the renovation of the Schwarzbachtal bridge. Here, the BORAPA engi-neering company even used two FEM systems simultaneously to be on the safe side.

Founded in 1987 by Prof. Dr.-Ing. Helmut Bode and Prof. Dr.-Ing. Wieland Ramm, the engineering com-pany has over 20 years of experience in all types of structural engineering in the fields of reinforced con-crete, prestressed concrete, structural steel, compo-site and timber design. BORAPA is increasingly rely-ing on high-performance IT systems that help the engineers to cover practically all structural engineering applications. While preparing structural engineering and inspection reports and on going supervising of construction work on site, the Kaiserslautern-based company has worked on a large number of greatly varied steel, concrete, composite and timber construction projects. It has also written many expert appraisals for courts of law and private clients. The range of the company‘s services extends from recording of the state of repair, the assessment and valuation of the cause of damage, the execution of load tests and the experimental measurement of bearing loads to the elaboration of suitable renovation concepts. The engineering company’s portfolio also comprises the tendering and supervision of repair and rehabilitation measures.

New Superstructure for Schwarzbachtal Bridge

One of the engineers‘ recent projects was the design and structural engineering for the new Schwarzbachtal bridge in the town of Zweibrücken. The nine-span bridge with a total length of 244 metres over the river Schwarzbach and the railway line from Contwig to Zweibrücken was originally built in 1966. After the valuation of an appraiser, the decision was made to renew the bridge’s superstructure due to damage at the coupling joints. As the inspection proved the sub-structure of the bridge to be in a relatively good state of repair, the new superstructure now rests on the pillars and abutments of the existing construction. As there are only point supports in axes 5 to 9, the new cross section had to have a high torsional ridigity in this 130-metre section. 

Innovative Cross-section Design

To determine the ideal cross section, the structural engineers performed some extensive examinations in advance. Common cross section variants such as a single- or double-webbed slab and beam cross sec-tion could not be used for static or economic reasons. A variant with composite steel cross section turned out not to be optimal either. There was not enough space for a walk-through box-type construction. Together with the owner, the Mobility Department of the State of Rhineland-Palatinate, it was therefore decided to use a totally new design. This consists of a conventional double-webbed slab and beam cross section between axes 0 and 4, and a torsion bracing in steel between axes 4-6 and 7-9, which have to bear large torsion loads at these points. This combination results in a box section with an idealised “lower slab” as braced truss framework. This solution is particularly economical, as the steel bracing is only required in the areas that need adequate torsional rigidity.

The deck height was increased to 2.20 metres above the existing construction by raising the corresponding bridge gradient in the bridge’s ramp band. From this a maximum span of 36.55 metres over the railway and a slenderness of 16.6 were calculated. In order to avoid stress such as creep and shrinkage or temperature-related effects on the torsion bracing, the company selected a bracing design of rising and falling diagonals.  

For the construction work, this means that the pre-stressed slab and beam section had to be built first. After completion, this was given steel torsion bracing in the lower area between axes 4-6 and 7-9. To do this, the construction company, Max Früh Bauun-ternehmung from Achern, used a ground-based sup-porting structure in five construction sections begin-ning with axis 9. 

The construction of the new bridge deck took about 18 months and cost a total of almost 5 million euros. The concrete carcassing work of the new deck construction was completed in 2009. Last year, the construction company worked on the steel bracing, which was scheduled for completion before the end of 2010.  

Focus on IT

“Bridge engineering projects of this complexity cannot be realised without the support of integrated FEM software,” explains Dr. Rolf Wörner, Manager of the BORAPA engineering company. The engineers from Kaiserslautern work on this and other challenging projects at a total of fourteen completely interlinked IT workplaces. Since 1996 the company has used RIB’s PONTI und TRIMAS software for construction and bridge engineering projects.  

Double Modeling

“In this special case we wanted to be sure of getting a correct model right from the beginning“, reports Dr. Wörner. No wonder – after all, the renewal of the Schwarzbachtal bridge marked the première of this special method. While the engineering company had calculated the construction statics with the PONTI FEM system for bridge engineering from RIB, a com-parative calculation was made in parallel to determine the load-bearing effect of the integrated steel bracing with the second FEM system from the RIB company installed at BORAPA, namely TRIMAS. “We had to use two independent modeling systems to guarantee that the model for the interaction of the slab and beam section with the steel bracing is a hundred per cent correct and the structure delivers on its promises later – both in terms of load-bearing capacity and construction cost,” relates Dr. Wörner.

Inspection engineer Heinz Steiger from Darmstadt is also able to confirm that the results of the Kaiserslau-tern engineers are correct. Und last but not least, the new deck of the Schwarzbachtal bridge is not only economical but also attractive. All in all, an impressive engineering achievement.

Industries>Architecture, Engineering & Construction>Architects and Engineers>BORAPA Engineering Company

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