Sandra Bullo

Self-healing capacity of advanced lime mortars

Abstract The addition of new additives and materials to traditional lime mortars may engineer their self-healing capacity, i.e. make them able to repair an occurred mechanical damage. The phenomenon of self-healing may take place both at microscopic and macroscopic level and, in case, can be initiated by a dispersed phase that becomes available as a result of a mechanical damage. The idea underlying this work is hence twofold: first, a mortar is created with a part of the lime protected by an impermeable coating surface, able to make it inert during the phases of setting and hardening; the mechanical characteristic of this material are thus compared to those of a standard mortar. In a second stage, when the material is subjected to an external stress, the protective layer of the treated lime granules, should undergo a rupture which will trigger another step of setting and hardening in the material made dormant by the same treatment. The sealing of the cracks induced by the aforementioned applied stress will hence be made possible by the delayed reaction of hydration. The purpose of the work is to establish whether this kind of material can be employed in structural application with reference to its durability.

The Footbridge over the River Adige at Legnago in Italy Part 1: The Role of Modal Analysis in Design:

Weight reduction, achieved through the use of materials with high mechanical strength and, in terms of shape, by adopting visually light structures, gave rise to the design of constructions with low self-weight, which were therefore sensitive to the effects of live loads. This sensitivity is particularly acute in the case of footbridges, where pedestrian traffic translates into dynamic forces that can lead to resonance, since pacing frequencies would be similar to the natural frequencies of the structure itself. In general, the problem is one of serviceability. The footbridge over the River Adige at Legnago (Northern Italy), designed by some of the authors(1) of this paper, showed a marked sensitivity to repeated horizontal forces that tended to cause transversal and torsional vibration around its longitudinal axis. This paper examines how an analysis of Natural Frequencies (NF) and modal response revealed the excessive deformability of the piers to be the key reason for the structure to behave in this way. As a result, a design solution was adopted that involved a more sound connection between the heads of the new and existing piers. In regard to the serviceability behaviour with respect to vibrations caused by human-induced dynamic loads, the NF analysis showed that the added links improved the behaviour of the structure. However, some natural frequencies remained close to the range of typical human pacing frequencies (walking, running or jumping). A dynamic analysis was therefore performed, the results of which are presented in the second part of the work - with the aim of ensuring that comfort conditions are verified. Finally, the experimental frequency determination enabled the design solution to be checked, demonstrating the excellent ability of the Finite Element Model used to predict dynamic structural behaviour.

The Role of Vibration Analysis in the Design of the Pedestrian Walkway Extension of the Principe Umberto Bridge in Legnago, Italy

This paper describes the design and construction of a new pedestrian walkway extension to the Principe Umberto bridge in Legnago (Italy), testing of vibration, and work carried out to improve its performance in serviceability condition. This walkway showed a marked responsiveness to repetitive horizontal forces, which tended to excite transversal and torsional vibration around the longitudinal axis of the structure. To overcome this problem a new static condition was created. This was based on analysing the response of the structure to human-induced dynamic loading by pedestrian movement (walking, running or jumping). Vibrations derived from such human-induced loads can cause disturbance to pedestrians (the serviceability problem) or, in more severe cases, resonance of the structure. According to technical literature and various design codes, there are two factors to be considered in overcoming these phenomena: the natural frequencies of the structure must be made to fall outside the range of human-pacing frequencies, and the maximum acceleration at any point of the structure must be not greater than a threshold value. On that basis, both modal and dynamic analyses were performed on the Legnago Bridge and the results made it possible to identify the cause of the excessive deformability. These analyses were then used to develop a design solution and to re-verify the structural project, which consisted of a variation to the static design.

The Footbridge over the River Adige at Legnago in Italy Part 2: The Assessment of Human-Induced Vibrations

Pedestrian bridges are usually characterised by moderate dead loads and high deformability; such conditions make them particularly sensitive to dynamic loads, which can induce significant vibrations. The footbridge over River Adige in Legnago (Northern Italy) is a triangulated space-lattice truss made using steel tubes; after its construction, the structure proved to be particularly sensitive to horizontal actions, that can produce an oscillation involving lateral translation and the deck torsion on its longitudinal axis. The effectiveness of the stiffening work - carried out in order to reduce such movements — has been assessed through analyses that were able to reveal the structures response to dynamic loads, such as those induced by pedestrian traffic. In particular, the study was broken down into two successive analyses, following the current procedures for vibration control in similar structures. A modal analysis for the evaluation of natural frequencies and mode shapes (with experimental tests for validation) — constituting the first part of the work [1] — enabled us to verify whether the structures natural frequencies are included within the typical frequencies range of human-induced dynamic load; and a dynamic analysis — whose outcome is illustrated below - aimed to assess the entity of the vibrations and maximum accelerations due to the transit of one or more pedestrians, with view to verify the emergence of uncomfortable conditions for the users.