Stiffness for simply supported beams and cross-sectional stresses in cracked state
Steel girders set in concrete as rolled Beam-in-concrete are considered to be an economical solution in bridge construction, to effectively counteract the currently very high demand for renovation of railroad and road bridges throughout Germany. The request for financially feasible solutions for maintenance activities, especially for smaller bridges, which can also be implemented particularly quickly, is growing among public institutions and local authorities at the same time. This is because nationwide investments in bridge construction for rail and road in Germany are expected to be in billions. Renovation or replacement construction often requires short construction times with a most possible minimal impact to ongoing traffic. Therefore, the Rolled Beam In-Concrete Bridge is an appropriate solution for smaller and medium spans. A Bridge type that has often been used in the past for smaller railroad Bridges. Around 20 percent of all railroad bridges in Germany in the support range of 10 to 20 meters are currently already designed as Rolled Beam In-Concrete Bridges and this trend is growing. It also has the advantage that the substructures, once built, can often be reused for the realization of further replacement constructions. This also saves time and money and can further increase the economic efficiency of such construction activities.
A Rolled Beam In-Concrete Bridges can be built much faster than comparable composite concrete or composite steel bridges. Bridges of this type of construction can be easily implemented with commercially available components and materials. At the same time, this type of bridge eliminates the relatively long planning process that must always be taken into account whenever precast elements are used. Further advantages in the design of this rather simple type of structure are that no scaffolding and formwork is required. With a relatively tight schedule and monetary framework, Rolled Beam In-Concrete Bridges are therefore increasingly favored and also increasingly used for road bridges.
Rolled Beam In-Concrete Bridges can be usually manufactured with a high slenderness L/h of from 25 to 30 and are still characterized by a relatively high stiffness. The higher stiffness results in less deformation. Compared to the usual composite steel , there are only cracked cross-sections in frame bridges. In the case of simply supported beams, the concrete cross-section cracks at the bottom of the field region, whereas in the case of frame and multi-span bridge structures the concrete cross-section also cracks at the top of the support area.
The behavior of casted steel girders depends strongly on this crack formation. This special feature requires special requirements for all verifications. Analogous to composite steel, the basic calculation is based on the total cross-section method. The reduction numbers can be calculated either according to DS 804 Annex 8, DIN EN 1992-1-1, or the Ec(t)-method. In the non-cracked area, the composite section is transformed into an equivalent steel section, while in the cracked area the section values are determined by the position of the plastic zero lines. The tensile stressed part of the load-bearing section is usually neglected.
For this bridge, we only placed the longitudinal beams and concreted them directly after installing a constructive, easy to lay reinforcement.
Specialist engineer Stefan Kleffel
The Engineering office of Stefan Kleffel in Rippershausen-Solz in Thuringia has currently carried out the structural analysis for an skewed Rolled Beam In-Concrete overpass bridge for the river Hasel in Dillstädt/Schwarza with a span of 12,40 meters and a variable width of 6,50 to 9,13 m meters. The superstructure of C35/45 consists of nine steel Girders set in concrete, each with a HEM 400 profile and the material S355. The longitudinal girders are fixed into an end transverse beam, which in turn are supported on deep-founded bored piles. A local traffic load of category 4 was assumed as a traffic load.
„With this assignment from the public administration sectors we have designed meanwhile the third Rolled Beam In-Concrete Bridge“, reported specialist Engineer Stefan Kleffel. „From the beginning, the structure was intended to be this type of bridge. Otherwise we wouldn´t be able to realize this structure with such a small cross-sectional height in comparison with any other construction, which was required by the underflowed River. The high flowing potential of the river made it particularly difficult to construct supporting scaffolding, which meant that a reinforced concrete bridge had to be ruled out in principle. With an optional prestressed concrete bridge, we would not have been able to meet the budget of the commissioning municipality,“ the expert continues.
The engineers especially appreciate this type of bridge variant for contract work like this. Not only does it offer a particularly high load-bearing capacity with small cross-sections, but also the construction work on a Rolled Beam In-Concrete Bridge can be carried out without too much effort and above all very quickly. „With this Bridge, we simply placed the longitudinal girders and concreted them directly“, said Stefan Kleffel. A major advantage is that local construction companies can also be involved in such projects, as it was the case in Dillstädt. Because Rolled Beam In-Concrete Bridge require neither any comprehensive steel construction nor welding of any transverse beams or complete support structures, construction companies that don't have this highly specific expertise can also participate in the tendering. As a result, the projects can be handled with local companies. „We can also do without steel construction companies for this type of structure, because the girders can be bought quasi off the shelf “he adds with a smile.
The FEM-Analysis of the entire structure was carried out in 5 construction stages including a secondary effect consideration:
The composite girders were designed for all elastic and plastic ultimate limit states. The fatigue and serviceability limit state design includes the deformations in the cracked state.
The software PONTI of RIB, on which our office has relied for many years, has always made our work much easier. We can work not only more accurately, but also much faster.
Stefan Kleffel, specialist engineer
Calculation and design of the Rolled Beam In-Concrete Bridges were initially carried out by hand in a classical manner. Due to the availability of the new bridge module PONTI compositsteel WIB, it is now possible to represent an integral bridge construction also for Rolled Beam In-Concrete Bridges with finite elements in an overall model. A subsequent designing of the municipal bridge as an overall integral model with the new PONTI module showed even more saving potentials, which cannot be achieved by a manual design. Rolled Beam In-Concrete Bridges can now be displayed in high quality and designed economically with a complex structural model. Stefan Kleffel concludes: „The PONTI software of RIB, on which our office has relied for many years, has always made our work much easier. We do work not only more accurately, but also much faster“.
> Faster and more accurate working
> Economically dimensioned WIB bridges
> Modelling of the integral bridges with finite elements in an overall model
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