Overview
The balanced cantilever method is an economical construction method for prestressed concrete bridges with large spans in a challenging topography. In Austria, but also internationally, a few prestressed cantilever bridges were detected within bridge maintenance which show contrary to expectations that, on the one hand, the actual deformation is significantly larger than the predicted one, and on the other hand, this deformation does not seem to stabilize even after decades. In former investigations it was assumed that this is mainly due to the uncertainties of the creep and shrinkage models used (CEB or ACI). This argumentation, however, cannot be accepted since the two phenomena mentioned do not occur in many other prestressed cantilever bridges.
The proposers assume that the structural characteristics such as slenderness, extent of prestress, static system etc. are of substantial importance in this context. Furthermore, the influence of a possible crack formation was not adequately taken into account by the former investigations. A numerous successful prestressed cantilever bridges as well as the very successful history of prestressed concrete in bridge constructions support this estimation. Besides, if the uncertainty of the creep models used is the predominant reason, the extent of the crack formation in prestressed concrete bridges would have to be significantly greater than what is previously known, since the conventional creep models were also used for the determination of loss of prestress.
The innovative content of the present proposal is a comprehensive assessment including all relevant parameters of deformation of prestressed concrete. Of course, the international experience is considered, especially regarding WP 2.1.1 of the fib as well as TC 242-MDC of RILEM, however, also structural conditions as well as cracking state were also always adequately taken into account. The reliable reasoning is finally enabled by a multilevel approach consisting of
- an analysis of historical data of existing bridges with respect to deformation, structural characteristics and cracking state as well as their conventional recalculation with standard beam models taking into account relevant models for creep and shrinkage,
- a monitoring of the TÜ Schottwien, with which both the creep and shrinkage deformations during the observation period as well as the present structural stiffness can be determined,
- a parametric study with a verified 3D-FE model for the identification of the essential Influence factors on deformation development and
- creep tests on samples taken from TÜ Schottwien with different stress history to quantify the remaining creep behaviour.
On the basis of the results, the causes of the unusual deformation development of the prestressed cantilever bridges are determined and recommendations in the form of limit values for the preservation of existing buildings as well as justified assumptions and constructive rules for the planning of new structures are derived.