Structural Stability: the Alamillo Bridge - Case Study Example

Structural Stability: the Alamillo BridgeThe paper discusses the structure of the Alamillo Bridge and its action under various loading conditions. The bridge is one of the long-span bridges crossing the Guadalquivir River built on the occasion of Expo '92 held in 1992 in Seville (Spain). It is a cable-stayed structure spanning 200 m without any intermediate supports, and its originality is in the lack of back stays and the balancing of the front stays through the backward inclination of a massive pylon.

Although the bridge has a steel deck, the pylon is composite element, with an external steel formwork structurally connected to the filling concrete. The uniqueness of this bridge lies in sophisticated and complex design and construction stages and its unique manner of structural behaviour and load transfer, which makes it a structure only of its kind in the world. The structure is complemented by accurate monitoring of the real forces (in deck, pylon and stays) and movement. The single pylon inclines away from the river, and supports the 200m span with thirteen pairs of cables.

As Alexander Tzonis explains," Rather than use back stays, Calatrava allowed the weight of the concrete-filled steel pylon to counterbalance the deck. By substituting one set of stay cables with the weight of the pylon inclined at 58 degrees, he created a new kind of cable-stayed bridge" (114). These cables connect the road deck to the pylon and remain in tension transferring the load to the pylon which in turn transfers it to the ground The subsequent reactions are generated which are in larger magnitude near the base of the pylon and in considerable low magnitude at the opposite end.

The chief loads that are generated are resultant of the dead loads of the bridge and with some effect from the live loads of the moving vehicles. The weight of the concrete and steel pylon provides a counterbalance for the bridge deck. The single plane of cables supports a beam down the middle of the road, maintaining the bridge's image of a harp. The roadway itself is cantilevered out from the beam with no support provided at the bottom of the bridge. The central beam is made out of steel box and it experiences concentrated loads at an interval of twelve meters at the junctions of the elastic directional support.

With less than 5 meters of distance between every pair, across the 32 m wide bridge, the cable cannot take any load by adjusting the axial force on each cable. Thus the torsional load generated from the live load is withstood by the torsional strength of the steel box. The pylon is loaded with its dead weight and the forces of the cable stays and with its own weight. The resultant of those two forces is inclined and the force is transferred through the pylon. However under the conditions of live loading of by the vehicular movements over the deck, the axial forces acting though the cables change in its magnitude and ultimately resulting in creation of a bending moment in the pylon.

The external loads of any structure are transferred to the foundation. Equilibrium under changing loading conditions is reacted by developing the needed reaction forces at the supports. The Alamillo Bridge relies on a single support at the foundation of the pylon to withstand any loading changes.Varying loading conditions resulting from the live load and wind load are expected to change the magnitude of the axial forces of the cable stays, thus creating eccentricities that caused bending moments at the foundations of the pylon in all directions.

Furthermore transverse loading of the pylon will cause bending moments in the other direction – in the same direction as the torsional moment that the steel box carries to the foundation. The pylon foundation then, should constrain all six degrees of freedom, countering reaction forces and moments that are expected to be quite large. Furthermore a strong vertical support is needed for the weight of the pylon and the vertical loads along the steel box, transferred through the cable stays to the pylon.

The eastern abutment is expected to experience significantly smaller reactions than the foundation. References1. Molinari, Luca, Santiago Calatrava, New York, NY: Abbeville Publishing Group 19992. Tzonins, Alexander, Santiago Calatrava, The poetics of Movement, New York, NY: Universe publishing, 19993. Frampton, Kenneth, Calatrava Bridges, Basel; Boston; Berlin: Birkhauser, 19964. Werner Blaser, ed., Santiago Calatrava Engineering/architecture, Basel; Boston; Birkauser Verlag, 1988.5. Schodek, Daniel, Structures 5th ed.

, Upper Saddle River, NJ: prentice Hall 2004