Feasibility study on fiber reinforced polymer cylindrical truss bridges for heavy traffic

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Abstract

Considering the recent increase in the use of fiber reinforced polymers in the civil engineering industry in general and in the bridge engineering industry in particular, as well as the recently more and more applied cylindrical truss bridge type, this research focuses on the question whether it is possible to combine fiber reinforced polymers as stand-alone structural material and this bridge type to construct a bridge suitable for heavy traffic as well as bicycle and pedestrian traffic. This research combines an extensive literature study on the use of fiber reinforced polymers for bridge engineering with a theoretical feasibility- and design-study on fiber reinforced polymer cylindrical truss bridges for heavy traffic. During the design study the spatial needs of all bridge users were defined to obtain an initial shape of the bridge. This shape was then optimized in several steps using finite-element-modeling and -analysis, yielding a final shape of the bridge. The behavior of this structure under design loads was then extensively investigated, again using finite element analysis, showing that the bridge could very well meet the self-derived deflection limit for fiber reinforced polymers at relatively low stress levels. Since fiber reinforced materials are a very diverse field of material, with hundreds of different compositions being available, the first result of this study was the choice of a suitable composite for further analysis. For this bridge design very high fiber content (>60%) carbon/epoxy composite was used. The main reason for this choice was the high modulus and -strength of the carbon fibers and the high durability and strength of the epoxy resin. A major reason of the slow implementation of fiber reinforced polymers in the bridge engineering industry are the worries concerning the lack of fire safety of the material. The literature study of this research showed however that it is possible to construct a heavy traffic full-FRP truss bridge, while complying with the known fire safety standards. The virgin FRP material can be adapted by several fire-protection measures; it turned out that a combination of intumescent gel-coating and low volume phosphorous filler systems works best in increasing the fire resistance and thereby providing a fire resistance class of R30 for hydrocarbon fire curve loading. The initial shape of the bridge was optimized in three stages: first several different truss topologies, which were derived with a parametric geometric model, were analyzed and compared using finite element analysis software, yielding the square truss with one diagonal as most efficient topology. In the next steps several grid sizes of this truss as well as several cross section dimensions were compared, again using finite element analysis software. An optimum was found between minimum material usage and minimum deflection, which reduced the material usage of the main load bearing elliptical truss by about 40% compared to the initial variant. The optimized structure was then fitted with the inner bridge deck supporting trusses as well as the cantilever trusses. The elliptical truss bridge performed very well considering the maximum deflections and stresses under Eurocode design loads and load combinations that were derived in finite element modeling software. When comparing the full-FRP bridge design with similar, existing steel structures, the maximum deformations and –stresses were considerably lower for the full-FRP bridge while only weighing about 60% of the steel structure. This research showed that the ‘new’ cylindrical truss bridge type is not only an aesthetically appealing structure but also performs structurally very well when combined with fiber reinforced polymer as structural material. It turned out that fiber reinforced polymers can be used as stand-alone structural material for medium span heavy traffic bridges. Next to that, this research clarified that there is no legitimate structural reason for the fact that fiber reinforced polymers are used relatively scarcely in the civil engineering- and bridge engineering industry compared to traditional building materials such as steel and concrete. Since this research is one of the first researches of its kind, using FRP as stand-alone structural material for a relatively new and complex bridge type, more research is needed in the field of high order connections for fiber reinforced polymer circular hollow sections. Next to that the possibility of the use of differently sized and shaped cross sections for the truss members should be investigated.