Modelling the fire resistance of prestressed concrete floors using multi-spring connection elements
Despite big advances in analytical modelling of the performance of structures exposed to fire, there has been difficulty in modelling the fire performance of precast prestressed concrete floor slabs in multi storey buildings. The fire resistance of these floor systems is heavily influenced by the end connections and the stiffness of the surrounding structure, both of which must be considered in any analysis. Previous “traditional” studies have modelled the floor slabs with beam or shell elements in which the end nodes share the nodes of the beam elements representing the supporting beams. This is acceptable for cast-in-situ or precast flooring system without prestressing, but leads to a major problem for precast prestressed flooring systems where the steel tendons terminate at the end of the flooring units, because the approach of sharing nodes of the supporting beam and floor assumes that these tendons are anchored into the supporting beams. In order to solve this problem, a “multi-spring” connection element has been developed. The multi-spring connection element consists of several parallel axial springs sandwiched between two rigid plates. Each spring represents either a steel reinforcing layer or a segment of concrete in the floor cross-section. The concrete springs have compression-only properties. This multi-spring connection is placed between the end nodes of the floor and the nodes of the supporting beam. With this element, it is possible to terminate the prestressing tendons at the end node of the floor elements and to anchor only the topping reinforcement into the supporting systems predicted using the traditional approach and the newly developed multispring connection, with applications to different forms of precast concrete floors in multi storey buildings.