A collapse induced by shortening in a multispan viaduct

Sudden collapses call for timely surveys performed by experts in order to detect their causes and prevent analogous drawbacks in other similar situations.

Frequently, the collapse shape strongly conditions the judgment criteria and this may lead to wrong conclusions.

This paper presents the case of a recent collapse occurred at an intermediate pier of a long viaduct. The deck of the viaduct had a so-called “tied-deck-slab” or “kinematic chain” scheme. A kinematic chain viaduct is composed of a train of simply supported decks, longitudinally connected through short and flexible continuity slabs. By avoiding joint devices between the spans, such scheme makes the carriageway continuous and improves the riding comfort. Figure 1 Failure mechanism evolution: (a) Situation at the maximum displacement; (b) spalling of the concrete cover; (c) contact surface reduction due to spalling; (d) buckling of the bars and rotation of the upper plate. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig209_1.tif"/>

The surveys carried out after the aforementioned collapse showed a visible bent of the pier towards the remaining span. This attitude was interpreted as the final consequence of a rotation of the foundation shaft, with all the consequences in terms of planning the repairing works.

A deeper exam of the structure led to a different conclusion: in fact, in a long viaduct, creep, shrinkage and temperature variations may induce large longitudinal displacements at the extreme ends. Sometime such displacement exceed the allowable excursion of the bearing supports and cause damages at the head of the supporting structure or even the fall of the deck. When these effects happen at the top of a high pier, they may be difficult to detect and the final collapse may develop through a sequential and complex mechanism.

The paper retraces the origins of this collapse and studies, in particular, the singular breakage that happened in the diffusion zone of the bearing supports.

Being the kinematic chain scheme rather common, the outcome of this experience may be of interest for all those involved in inspection and bridge maintenance.