Numerical Assessment for the Structural Improvement of Palladio’s Forni-Cerato Villa in Vicenza (Italy)

Numerical Assessment for the Structural Improvement of Palladio’s Forni-Cerato Villa in Vicenza (Italy)

Abstract

 

The Forni-Cerato Villa in Vicenza, attributed to the renowned architect Andrea Palladio, stands as a prime example of 16th-century Venetian architecture and is a UNESCO World Heritage site. Over time, the villa has suffered from a long period of abandonment and insufficient maintenance, leading to significant structural deterioration. This study assesses both the structural and seismic vulnerabilities of the villa and proposes targeted interventions to improve its stability.

 

Three numerical models of the villa were developed using the DIANA FEA software: the first without floors modeled (AsB-NF), the second with floors modeled (AsB-F), and the final with a retrofitted structure (Retr-F). These models underwent various structural analyses, including linear static, eigenvalue, and pushover analyses, along with a kinematic analysis to investigate potential collapse mechanisms. The proposed intervention, which focuses on reinforcing the timber floors, was found to improve the building’s global structural behavior, particularly in terms of seismic resilience. Comparative results between the models are discussed, along with recommendations for further refinement of the strengthening methods and areas for future research.

 

 

1. Introduction

 

Preserving historical buildings is essential for maintaining our cultural heritage. Historic structures are often symbols of past civilizations, architecture, and craftsmanship. Buildings made of load-bearing masonry, one of the oldest and most widespread forms of construction, are particularly vulnerable over time due to material degradation, environmental exposure, and seismic activity. In regions where earthquakes are common, these buildings are especially at risk, as they were not originally designed to withstand seismic forces.

 

This study focuses on Villa Forni-Cerato, a Palladian villa located in Vicenza, Italy. The villa, attributed to the renowned architect Andrea Palladio, is a 16th-century structure that exemplifies classical architecture. Despite its historical and architectural importance, the villa has suffered from decades of neglect, leading to severe structural deterioration. This has left the building vulnerable to partial collapse, especially in the event of an earthquake.

 

The objective of this research is to assess the current structural and seismic vulnerabilities of Villa Forni-Cerato and to propose interventions aimed at strengthening the building without compromising its historical integrity. Using modern numerical analysis methods, specifically finite element analysis (FEA), this study will simulate the villa’s structural behavior under various conditions, including its present state and after retrofitting. The analysis will allow us to evaluate the effectiveness of the proposed interventions and identify further improvements to ensure the building’s long-term stability.

 

The scope of this work includes the following objectives:

• To identify and characterize the existing structural damage in Villa Forni-Cerato.
• To simulate the villa’s structural and seismic behavior, both with and without effective horizontal diaphragms.
• To propose a retrofitting strategy focused on reinforcing the timber floors.
• To assess the impact of these interventions using numerical simulations.
• To identify additional areas where the structure could benefit from further intervention to improve its seismic resilience.

 

 

2. The Forni-Cerato Villa

 

2.1. Case Study

 

Villa Forni-Cerato is located in Montecchio Precalcino, within the province of Vicenza, Italy. The villa is a quintessential example of Renaissance architecture, attributed to Andrea Palladio, a highly influential architect of the period. Built in the late 16th century, the villa represents Palladio’s efforts to blend classical design with functional, rural architecture. Although the villa is relatively modest in size compared to other Palladian works, it incorporates many of the architect’s key design principles, such as symmetry, proportion, and the use of classical orders.

 

Over the centuries, the villa has changed ownership multiple times and has undergone periods of neglect. Despite this, it remains a valuable cultural asset and is listed as a UNESCO World Heritage site due to its historical and architectural significance. However, its current state of disrepair presents serious risks to its structural integrity.

 

The villa’s walls are constructed from a combination of brick and stone masonry, which has been severely weakened by decades of exposure to the elements. The timber floors and roof are also in a deteriorated state, having suffered from moisture damage and decay. These issues have left the villa vulnerable to collapse, particularly during seismic events. As such, the building requires immediate intervention to stabilize its structure and prevent further damage.

 

 

2.2. Palladio’s Architectural Orders

 

Andrea Palladio was heavily influenced by classical Roman architecture, and this is reflected in his use of the architectural orders—Doric, Ionic, and Corinthian—throughout his works. Palladio’s designs are characterized by their emphasis on symmetry, proportion, and balance. Villa Forni-Cerato follows this design philosophy, with a symmetrical facade and carefully proportioned spaces arranged around a central axis. The villa also features a loggia with Palladian serliana, an element commonly found in Palladio’s works, which adds a sense of grandeur and harmony to the building’s exterior.

 

Palladio’s villas typically follow a tripartite plan, with the ground floor reserved for storage or service areas, the main floor (or piano nobile) used as the living quarters, and the attic providing additional storage space. This functional arrangement is also present in Villa Forni-Cerato. The use of simple, classical elements gives the villa a timeless quality, reflecting Palladio’s mastery of combining practicality with aesthetic elegance.

 

 

2.3. Description of the Building

 

Villa Forni-Cerato consists of three main levels: the ground floor, the main floor, and the attic. Each level serves a specific purpose, in line with Palladio’s typical villa designs. The ground floor, with its lower ceilings and more utilitarian design, was likely used for storage or as a service area. The main floor, known as the piano nobile, is the most elaborate part of the building, featuring larger rooms and more detailed architectural elements. This level served as the primary living space for the villa’s residents. The attic, which is less ornate, provided additional storage.

The villa’s external walls are constructed from a combination of brick and stone masonry, bound with lime mortar. Over the years, these walls have become damaged due to moisture infiltration, resulting in cracks and areas of weakened masonry. The roof is supported by timber beams, many of which have rotted due to exposure to moisture and lack of maintenance. As a result, the villa’s structural integrity has been significantly compromised.

 

 

The villa is part of a larger estate, which includes a barchessa (a rural service building typical of Venetian villas) and a dovecote. However, these structures have also suffered from neglect and are in need of restoration.

 

 

3. Inspection and Investigation

 

3.1. Visual Survey

 

A comprehensive visual survey was conducted to assess the current condition of Villa Forni-Cerato. The survey revealed extensive damage to both the interior and exterior of the building. The masonry walls were found to have numerous cracks, particularly around window and door openings, as well as at the corners of the building. These cracks are indicative of structural instability and have likely worsened over time due to the lack of regular maintenance.

The exterior plaster has also deteriorated, with large sections having peeled away, exposing the underlying brick and stone masonry to the elements. This has allowed moisture to penetrate the walls, further weakening the structure. Water damage is particularly severe in the loggia area, where rainwater has caused significant deterioration to both the masonry and the timber beams supporting the roof.

Inside the building, similar signs of deterioration were observed. The timber floors have weakened significantly over time, with many beams showing signs of rot and cracking. In some areas, the floors have collapsed altogether, leaving gaps where the beams have failed. The visual survey confirmed that these structural issues present a serious risk to the building’s stability and will need to be addressed as part of any restoration effort.

 

 

3.2. Material and Structural Characterization

 

Non-destructive (NDT) and semi-destructive testing (MDT) methods were used to assess the material properties and structural condition of Villa Forni-Cerato. These tests included resistograph analysis of the timber beams, petrographic analysis of the lime mortar, and thermographic imaging to detect areas of heat loss, which often indicate structural problems such as moisture infiltration or detachment of plaster from the underlying masonry.

 

 

Resistograph tests were conducted on several timber beams, particularly those embedded in the masonry walls. The tests revealed significant decay in many of the beams, particularly at the points where they connect to the masonry. In some cases, the beams had lost more than 50% of their original strength, leaving them unable to support the loads they were originally designed for.

 

Petrographic analysis of the lime mortar confirmed that the mortar had degraded significantly over time, contributing to the loss of strength in the masonry walls. The mortar was found to be brittle and prone to cracking, making it less effective at binding the masonry units together. Thermographic analysis revealed several areas where moisture had penetrated the walls, particularly around the windows and roofline. These areas of moisture infiltration corresponded with the locations of the most severe structural damage.

 

 

4. Methods of Analysis

 

4.1. Numerical Approaches

 

The finite element method (FEM) was chosen as the primary tool for the structural analysis of Villa Forni-Cerato. FEM allows for the simulation of complex structures, making it particularly well-suited for assessing the structural behavior of historical buildings. Given the villa’s irregular geometry and the varying properties of its materials, a macro-modeling approach was selected, treating the materials as continuous, homogeneous elements.

 

The DIANA FEA software was used to create three different models of the villa, each representing a different structural condition:

• AsB-NF (As-Built, No Floors): This model simulates the villa in its current deteriorated state, without the contribution of effective horizontal diaphragms (i.e., timber floors).
• AsB-F (As-Built, Floors): This model includes the structural contribution of the existing timber floors, representing a more stable condition.
• Retr-F (Retrofitted): This model simulates the villa after the proposed retrofitting interventions, focusing on reinforcing the timber floors to improve their structural performance.

 

4.2. Applied Analyses

 

The models were subjected to a variety of structural analyses to assess the villa’s performance under different conditions:

• Linear Static Analysis: This analysis evaluates the load-bearing capacity of the villa under static conditions, allowing for an assessment of the structure’s ability to support vertical and horizontal loads without excessive deformation.
• Eigenvalue Analysis: This analysis examines the natural frequencies of the structure, identifying areas of weakness that could be vulnerable to seismic forces. Low-frequency modes are particularly problematic, as they can lead to resonance effects during an earthquake.
• Non-linear Static (Pushover) Analysis: This analysis simulates the building’s response to increasing lateral loads, such as those generated by seismic activity. The non-linear approach allows for a more accurate assessment of how the building will behave under real-world conditions.
• Kinematic Limit Analysis: This analysis focuses on potential local collapse mechanisms, particularly those related to the out-of-plane behavior of the masonry walls. It helps identify areas where reinforcement is needed to prevent localized failures during an earthquake.

 

 

5. Structural Analysis: Modelling

 

The geometry of Villa Forni-Cerato was modeled in AutoCAD, based on original architectural plans and measurements taken during site visits. These models were then imported into DIANA FEA for further analysis. The materials used in the villa, including brick and stone masonry, timber, and lime mortar, were assigned their respective mechanical properties based on the results of the material characterization tests.

 

 

Three different models were developed to simulate the villa’s structural behavior under various conditions:

• AsB-NF: This model represents the villa in its current deteriorated state, without the contribution of effective timber floors. The analysis revealed significant structural weaknesses, particularly in the upper levels of the building, where the lack of horizontal diaphragms made the structure more vulnerable to lateral loads.
• AsB-F: In this model, the timber floors were included as effective horizontal diaphragms. The inclusion of these floors improved the villa’s performance under seismic loads, as they helped distribute the lateral forces more evenly across the structure. However, some areas of weakness remained, particularly in the corners and upper walls.
• Retr-F: This model simulated the villa after the proposed retrofit, which focused on reinforcing the timber floors. The analysis showed a significant improvement in the villa’s structural performance, with the retrofitted floors providing additional stiffness and load-bearing capacity. This allowed the villa to better withstand lateral loads and reduced the likelihood of localized collapses.

 

 

6. Structural and Seismic Assessment

 

6.1. Simulation of the Existing Layout

 

The initial analysis focused on the AsB-NF model, which simulated the villa in its current condition without effective floors. The linear static analysis revealed that the building was unable to bear significant lateral loads due to the lack of horizontal diaphragms. Eigenvalue analysis showed that the villa was particularly vulnerable to torsional effects, which could lead to catastrophic failures during a seismic event.

 

The AsB-F model, which included the contribution of the timber floors, performed better under seismic conditions. The floors acted as effective diaphragms, helping to distribute the seismic loads more evenly across the structure. However, the building still exhibited weaknesses in certain areas, particularly in the upper levels, where the walls were thinner and more prone to out-of-plane failures.

 

6.2. Simulation of the Strengthening Intervention

 

The retrofitted model (Retr-F) simulated the villa after the proposed intervention, which involved reinforcing the timber floors. The non-linear static (pushover) analysis showed a significant improvement in the villa’s seismic performance. The retrofitted floors provided additional stiffness and strength, allowing the building to withstand higher lateral loads with less deformation. This reduced the risk of structural failure and improved the building’s overall stability.

 

Kinematic analysis confirmed that several potential collapse mechanisms, such as out-of-plane failures of the masonry walls, were mitigated by the proposed retrofit. The addition of new timber beams and the reinforcement of existing beams significantly improved the villa’s ability to resist seismic forces. As a result, the building was better able to maintain its structural integrity during an earthquake.

 

 

7. Conclusion

 

The structural analysis of Villa Forni-Cerato revealed several critical vulnerabilities, particularly due to the lack of effective horizontal diaphragms. The villa’s current condition leaves it highly susceptible to damage from seismic events, with a significant risk of partial or total collapse. However, the proposed retrofitting intervention, which focuses on reinforcing the timber floors, has the potential to significantly improve the villa’s structural performance.

 

The numerical analyses showed that the retrofitted model performed much better under seismic conditions, with the reinforced floors providing additional stiffness and load-bearing capacity. The villa was able to withstand higher lateral loads without excessive deformation, reducing the risk of collapse. Kinematic analysis further confirmed that several potential failure mechanisms were successfully mitigated by the proposed intervention.

 

While the proposed retrofit represents a significant improvement, further work is needed to ensure the long-term stability of the villa. Additional interventions, such as reinforcing the roof structure and further strengthening the masonry walls, may be necessary to provide a more comprehensive solution. Regular monitoring of the building’s condition will also be important to ensure that the interventions remain effective over time.