Steel as the main material for bridges is often associated with very large spans due to its favorable relationship between strength and weight. But even with small and medium spans of up to 60 m, steel combined with concrete as a composite bridge can lead to economically justified structures. Small bridges are used for crossings over two-lane urban roads with spans of up to 10 or 15 m, for example. Medium-span bridges cross highways and reach spans of about 40 m, for instance, but sometimes even 60 m. Based on hot-rolled steel sections, there are several composite solutions that are well known all over Europe. Multiple I-section composite decks and filler beam decks have been standard bridge designs for decades – the latter for more than 100 years.
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For designers it is common practice to optimise the steel sections depending on the loads and construction phases, resulting in composite bridges with asymmetric steel sections having narrower, thinner upper flanges and wider, thicker lower flanges. Occasionally, the cross-sectional depth is also varied by changing the web depth. In this way, the amount of steel used is minimised.
With rolled sections, variation of the cross-section along the longitudinal axis can generally only be achieved by adding reinforcing plates on the flanges or by welding plates into the web. In general, however, the designer is bound to the geometric properties of the standard rolled products. In direct comparison, rolled sections may be a few percent heavier than optimized built-up sections, for the same steel grade. Nevertheless, it is beneficial to use rolled sections with a maximum depth of 1138 mm for bridges with spans of up to about 40–45 m (slenderness L/30). In Europe, welded beams for small- and medium-span bridges are usually made from standard plates in steel grade S355. The use of plates with a higher strength is not common for small- and medium-span bridges. On the other hand, the use of hot-rolled sections in grade S460 is well established in European practice and is advantageous for road bridges, since the high strength can lead to weight-savings in the steel structure. Further, when the higher strength sections are thermomechanically rolled, such as HISTAR® with further advantages, the low carbon equivalent of the steel beams results in improved weldability of the material. For welded built-up sections, in addition to the fillet welds between the flanges and the web, butt welds are required between the plates or plated beams every 12–18 m (either in the workshop or on the construction site). For long products such as rolled sections, neither fillet welds nor butt welds are required up to a component length of 40 m. In addition to the reduction in processing costs, any capacity bottlenecks in steel fabricators’ workshops can be circumvented and delivery times thus shortened.
Owing to the benefits of weight-savings and simplified manufacturing, rolled sections in high-strength steel (HSS) grades are being used more and more frequently as the more economical alternative to welded built-up sections.
Almost all short- and medium-span bridges are constructed as beam bridges. Only in exceptional cases are other structural systems used, e.g. arch or truss bridges. Most structures are in the form of T-beams with steel and/or concrete webs with an overlying reinforced concrete deck slab. Assuming a distance between the main girders of about 2–3 m, crossbeams are only required in exceptional cases for road bridges, since the concrete slab is sufficient for the transverse distribution of the loads. Filler beam bridges, where the steel beams are integrated into the deck slab, represent an exception to this design principle.
The table below contains a selection of common and wellknown designs for small- and medium-span composite bridges using rolled sections. The specified spans refer to the usual and economic ranges for single-span systems. With multiple-span and integral systems, correspondingly longer spans are possible. Deviations from the specified values are thus not an exclusion criterion for the application.
|Main applications||Usual spans (single span)||Special properties|
Railway and road bridges, highly suitable for multi-span road bridges
Steel sections with overlying reinforced concrete slab, different composite bridges with rolled sections.
Concrete slab either cast on site or steel beams with prefabricated concrete slab (VFT) as formwork and for stability, thus less traffic disruption.
|Filler Beam Bridges|
Railway and road bridges, highly suitable for bridges with construction depth restrictions
Little welding work necessary by using closely spaced rolled sections; spaces filled with concrete on site.
Very low construction depth
Railway and road bridges, highly suitable for multi-span and integral bridges
Prefabricated Composite Beam
Combines filler beam and VFT construction methods. Use of prefabricated composite beams reduces traffic disruption.
The new system for spans in the range 50–60 m is based on using rolled I-sections at mid-span and T-sections with additional plates at support regions in between, where, additionally, double composite action applies. The system was developed as a result of R&D work carried out by ArcelorMittal with PONTES. The development and design of the system and a detailed calculation report are described in detail in the freely available R&D report.
The bridge Niemodlin´ska Street) in Opole/Poland (completed in 2017) is the first implementation, see photo below.
The bridge at Niemodlin´ska Street in Opole (PL) during loading test
Network arch bridge with 75 m span using rolled I-sections in western Poland
Network arch bridges incorporating rolled I-beams in their arch structure have been constructed for some time in Poland. The main reason for using rolled sections instead of traditional welded box sections are cost-savings and shortening the construction time. Owing to the availability of grade HISTAR®460 as standard and a wide range of heavy rolled sections (HD sections up to 140 mm flange thickness and with a maximum weight of 1299 kg/m), less steel is needed to resist the high compression forces in the arches. Additionally, rolled sections are less costly in terms of fabrication and the connection of the hangers is much easier. Gusset plates can be easily welded between the webs of the HD sections.
Gusset plate to connect hanger with arch of HD section
The positive experiences gained with this construction method and the cost-effectiveness have allowed this solution to spread and its significance is increasing now. Several road and railway bridges with spans up to 120 m are already constructed.
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Rolled sections in bridges - see the advantages in our videos
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