Masonry Infrastructure: the Seismic Improvement Project of a Bridge – CRM and FRP systems

Author: Eng. Alessio Pierdicca (CapStudio Srl) in collaboration with Fibre Net.

The article describes a structural reinforcement intervention using fiber-reinforced composite materials carried out on a masonry bridge that was weakened, deteriorated, and structurally inadequate according to current regulations. The designer’s choice was to adopt innovative intervention solutions proposed by Fibre Net and based on composite materials – CRM and FRP – ranging from the technique of reinforced plaster with fiberglass mesh to the use of pultruded bars and reinforced repointing of joints with steel wire.

The structural and material pathologies detected on the bridge in the province of Ancona required the definition of appropriate consolidation measures to halt the ongoing deterioration and ensure the structural efficiency and safety of the bridge in compliance with current regulations. The performed diagnostics and finite element modeling allowed for the calibration of the most suitable solutions for the bridge piers and arches, integrating differentiated and targeted reinforcement techniques.

The project was approached through three distinct and complementary phases: understanding the structure, numerical modeling using finite element analysis, and seismic risk assessment and mitigation through structural rehabilitation interventions. In addition to common diagnostic investigations for characterizing the construction materials, dynamic monitoring of bridges plays a crucial role in structural knowledge by experimentally determining structural parameters (natural frequencies, damping, and mode shapes) and correlating them with finite element analysis (Finite Element Method, F.E.M.). Based on a calibrated F.E.M. model, structural analyses were conducted, and consolidation interventions were designed with the aim of preserving the original characteristics of the structure. Given the historical significance of the bridge, consolidation interventions were designed to preserve the exposed masonry walls and masonry arches without altering their architectural features.

STRUCTURAL CHARACTERISTICS OF THE BRIDGE

The bridge has a total span of 34 m (distance between the abutments). The arches are semicircular with a radius of 3.50 m, and the piers have a maximum height of 10 m from the springing level of the arch to the ground level of the foundation.

PIERS: The piers are constructed with solid brick masonry for the outer portion (with three faces) and a central core made of disordered stonework.

ARCHES: The arches consist of a three-faced masonry arch with loose and non-cohesive fill material above, which serves as backfill.

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During the knowledge phase, the photogrammetric survey carried out with a laser scanner allowed for the detailed identification of the bridge’s geometry and the detection of masonry structural damages. The axonometric views and the integration of point clouds in the extrados planimetry reveal significant material and structural information. (drawings by Eng. A. Pierdicca)

A dynamic investigation campaign was conducted with the aim of identifying the main natural frequencies of vibration, damping factors, and modal shapes. These data allowed for the calibration of the FEM models for structural analysis, ensuring a reliable model for the interpretation of the results.

The acquisitions were performed by continuously recording ambient acceleration data derived from the natural excitation of the structure caused by external phenomena such as wind, microseisms, and noise from human activity (e.g., vehicular traffic). The accelerometers were mechanically fixed using metal anchors directly to the reinforced concrete slabs located on the sides of the roadway, removing the upper portion of the asphalt pavement. The frequency contents of the time histories were acquired using the FFT (Fast Fourier Transform) algorithm. The structural dynamic parameters were obtained by processing the data using the SSI (Stochastic Subspace Identification) algorithm. The dynamic monitoring campaign allowed for the evaluation of the structural responses of the investigated masonry bridge: a global structural behavior was observed, as the corresponding modal shapes mobilize the entire structure and there are no local modes present.

NUMERICAL MODELING

The preliminary FEM model was created by incorporating the main structural elements, and thanks to the conducted dynamic investigations, a calibration of the models was performed to obtain a reliable computational tool for structural analysis. The calculation model is based on the representation of the bridge as a system composed of: arch, abutments, buttresses, spandrels, backfill, and filling. The mesh was refined to a high level of detail (39938 nodes and 39148 brick elements), where masonry and filling were considered as an equivalent continuum, and individual components (such as bricks, stones, and mortar joints) were not individually represented. The material behavior is assumed to be linearly elastic. The material characteristics were determined, where carried out, through semi-destructive in-situ tests that allowed for the determination of elastic moduli and stresses acting on the masonry (flat jacks in single and double configuration).

PRE-OPERAM ANALYSIS OF STRESSES AND DEFORMATIONS

In the analytical phase, the bridge was evaluated under two different loading conditions: static (defined according to regulations as the presence of two parallel load lanes placed longitudinally on the bridge with a traveling tandem load at the key of the central arch) and dynamic, with seismic action. Preliminary analyses have shown that the bridge is unable to withstand the static and seismic actions required by current regulations. Therefore, it is necessary to define a reinforcement intervention that addresses the identified issues and strengthens the bridge’s abutments and arches in accordance with the parameters dictated by current norms.

MULTI-SYSTEM CONSOLIDATION OF THE BRIDGE

The structural reinforcement interventions concern both the arches and the abutments and specifically address the material and structural degradation issues identified during the diagnostic investigations conducted on the bridge. The project has identified the most suitable methods provided by Fibre Net to meet the specific reinforcement requirements.

The planned interventions, currently under construction, will ensure the required structural performance while respecting the constraints imposed on the historical bridge. Consolidation, reinforcement, and preservation will be achieved in perfect harmony to achieve a single objective: safety and compliance with the imposed constraints for a protected historical asset.