Spada Bridge Competition, Lugano
The structure selected for the bridge is of the stress-ribbon type which resists the loads by the combined action of supporting cables located inside the section and of a slab with a bending stiffness created by a co-action.
The concept is simple: it is a catenary in tension between two end points and stiffened by a concrete monolithic section built without form works and intermediate supports.
This concept founds an excellent use in the design because features a simple and evolutive construction method which does not require important equipments and limits to a minimum the impact on the site; there is no need for form works, provisionl suspensions, launching devices and so on.
In addition the structure has not joints and therefore its durability is increased and the maintenance is reduced.
The structure developes three beahaviour phases:
- The catenary action alone: the prefabricated concrete elements not connected yet are launched over the bearing cables
- Inverted arch action: it is subjected to upside forces which generate compression in the whole system. These upside forces are created by the stressing of post tensioning cables anchored to the buttresses and constitute superimposed loads; limited size cast in place unite the concrete elements
- The compressed catenary: bending stiffness is added and effects the distribution of the live loads and yields a correct dynamic behavior.
Because of the site morphology the bridge design was developed with two spans in order to better meet the existing ground elevations; the result is an higly asymmetrical structure.
The basic need of this type of structure is the existence of end anchors capable of resisting large tension forces.
In the bridge position a close to surface rock layer allows for the construction of direct anchors which results in a feasible and suitable solution.
