The elevated metro structure in concrete, UHPC and composite

Abstract

In some large cities, the infrastructure is elevated high above the ground. An elevated metro system has the advantage that its construction is cheaper compared to an underground metro system. The construction time is relative limited and the physical barrier is small. The large elevation has a positive influence on the visual hindrance of an elevated metro system as it creates a more open and lighter space below the structure. In the future, Rotterdam wants to extent its existing metro system. An elevated metro system high above the city is one of the possible concepts. The engineering office of Rotterdam Public Works is interested in this concept and moreover in whether there can be gained profit on the elevated metro structure by applying Ultra High Performance Concrete (UHPC) or Fibre Reinforced Polymer (FRP) instead of conventional concrete. The objective is to determine the dimensions and normative structural verifications of the elevated metro structure when this is made of conventional concrete, UHPC or FRP and to compare these designs with each other. For the designs of the elevated metro structure made of conventional concrete, UHPC or FRP the focus is on the lightest railway girder and not on the minimum depth of the girder. The suitable elevation and span of the elevated metro structure are respectively 15 and 45 metres. The best concept for the concrete and UHPC railway girder is the precast segmental box girder with external prestressing tendons. The optimal concrete box girder has 6 prestressing tendons and a dead load of 102.02 kN/m. This optimal design is found by means of an optimisation process where the behaviour of the box girder is examined by changing several parameters. The normative structural verification of the optimal concrete box girder is fatigue of the concrete. By closer examination, it turns out that fatigue of the concrete is not normative when studied in more detail. As a consequence the verification of the ultimate resistance moment at t=0 is normative. The optimal UHPC box girder has also 6 prestressing tendons and a dead load of 69.4 kN/m. The normative structural verification of the optimal UHPC box girder is the ultimate resistance moment of the box girder at t=0. The design of the railway girder made of FRP is a sandwich girder and is based on a bridge concept. The normative structural verifications of the FRP sandwich girder are deflection of the girder and buckling of the core triangles. The dead load of the FRP sandwich girder is 34.48 kN/m. The difference in dead load between the three designed railway girders is quite large. The application of a lighter railway girder does however not result in a large reduction of the number of piles. This is due to the small weight contribution of the railway girder to the total vertical load at the piles and the large contribution of the bending moments at the foundation to the pile forces. The normative structural verification of the columns is stiffness of the viaduct. Applying UHPC or FRP instead of conventional concrete for the railway girder thus has a small impact on the substructure. The direct construction costs for the elevated metro structure with a concrete box girder are about €450,000 per span of 45 metres. When the unit price of UHPC is lower than € 450/m3, the UHPC box girder becomes a serious competitor of the conventional concrete box girder from a financial point of view. For the FRP railway girder holds that FRP is currently far too expensive to compete with the (UHP) concrete box girder.