Study of the seismic vulnerability of masonry building aggregates in the city of Barcelona

Study of the seismic vulnerability of masonry building aggregates in the city of Barcelona

Abstract

 

Barcelona, located in a region of low-to-moderate seismicity, holds a rich architectural heritage within the Eixample district, notably composed of unreinforced brick masonry buildings. Most of these were constructed without seismic design, leading to growing interest in evaluating their vulnerability. This thesis investigates the seismic vulnerability of masonry building aggregates in the Eixample using two complementary methodologies: the Vulnerability Index Method (VIM) and kinematic limit analysis.

 

A representative case study was selected, and both techniques were applied to assess structural behavior, damage scenarios, and potential collapse mechanisms. The findings contribute to the ongoing research on seismic risk in historical urban fabric, particularly emphasizing the impact of aggregate behavior and structural heterogeneities. The study concludes with proposals for future enhancements in seismic analysis methods and strategies for mitigation.

 

 

1. Introduction

1.1 Background

The seismic vulnerability of masonry buildings has long been recognized as a significant risk, particularly in Southern Europe, where many cities have historic centers composed predominantly of unreinforced masonry (URM) structures. These buildings often lack the ductility and energy dissipation capacity needed to withstand seismic events. The problem is compounded when buildings are connected to each other, forming aggregates, a configuration that alters their structural response in complex ways.

 

Although research into the seismic behavior of individual masonry buildings is extensive, fewer studies address the performance of these structures when considered as part of an aggregate. Building aggregates may benefit from lateral confinement and mutual support, but they also present irregularities and discontinuities in geometry, materials, and structural systems, which can lead to localized weaknesses or stress concentrations.

 

The city of Barcelona offers an ideal case study due to its moderate seismic hazard, extensive stock of masonry buildings, and characteristic urban morphology, especially in the Eixample district. The potential for even moderate earthquakes to cause serious damage, combined with a dense and historically valuable building fabric, makes seismic vulnerability assessment in this context particularly important.

 

 

1.2 Objectives

The thesis aimed to:

1. Study heterogeneities among buildings and classify typical collapse mechanisms.
2. Apply the Vulnerability Index Method to evaluate a pilot aggregate.
3. Use kinematic analysis to assess seismic damage potential.
4. Draw conclusions on typical vulnerabilities of Eixample aggregates.

 

1.3 Methodology

The study combines three methodological approaches. First, an empirical evaluation using the EMS-98 vulnerability index method is applied to assess a representative aggregate in Barcelona. Second, nonlinear static (pushover) analyses are conducted on both isolated and aggregated models using finite element software (DIANA FEA) and equivalent frame modelling software (3Muri). Finally, a parametric study explores how various parameters—geometrical, mechanical, and topological—affect seismic response. These three approaches are integrated to provide a comprehensive understanding of the issue.

 

 

2. Barcelona’s Eixample

The Eixample plan, devised by Ildefons Cerdà, represented a pivotal urban intervention in 19th-century Barcelona. It expanded the historic city, integrating surrounding villages into a rational grid structure.

 

 

 

2.2 Seismicity of Barcelona

Barcelona is located in a zone of moderate seismicity, with an expected peak ground acceleration (PGA) of around 0.07g according to Eurocode 8. While this is relatively low, the high vulnerability of the building stock and the concentration of cultural heritage increase the potential impact of even moderate earthquakes. The last major earthquake affecting the area occurred in 1428, and the city’s current infrastructure and building regulations have evolved in a context where seismic risk has often been underestimated.

 

 

Barcelona Response Spectrum

The city’s spectrum is based on standard soil characteristics and peak ground accelerations, providing input for evaluating displacement demands in seismic analyses.

 

2.3 Historical and urban development of the Eixample

The expansion, initiated by Ildefons Cerdà, was revolutionary. His grid layout introduced uniform urban blocks with planned heights and open courtyards. Despite later deviations from the original plan, the Eixample has remained a model of 19th-century planning, blending functionality with beauty.

 

 

Cerdà’s plan: an urban revolution

Cerdà’s 1859 plan introduced blocks of equal dimensions, chamfered corners, and wide boulevards to optimize light, air, and traffic. His design principles were visionary, aiming to balance urban density with livability.

 

 

The Eixample today

As of 2021, the district covers over 746 hectares and houses nearly 270,000 residents. The building stock, mostly constructed between 1860 and 1960, consists mainly of URM and some reinforced concrete buildings. Post-1960 remuntes (vertical additions) altered many original structures, introducing irregularities that impact seismic behavior.

 

 

2.4 A unique construction typology: the “illes urbanes”

Buildings were typically constructed with unreinforced brick masonry, timber floors, and cast iron columns. They exhibit uniform façade patterns and often underwent vertical expansions called “remuntes,” further complicating seismic behavior.

 

 

Building enlargements: the “remuntes”

Remuntes, often added in the 20th century, contribute to vertical irregularities and seismic vulnerability. These additions may not align with original structural schemes, creating discontinuities in mass and stiffness.

 

 

3. State of the art and methodology

3.1 Introduction

Masonry aggregates in historic cities are typically the result of organic growth. However, in Barcelona’s Eixample, aggregates were systematically planned. Despite their coherent layout, ongoing modifications—including height extensions and rear expansions—have introduced heterogeneity that compromises seismic performance.

 

3.2 Unreinforced masonry

Earthquake-resistant building configuration

URM buildings lack ductility and energy dissipation. Earthquake-resilient design requires symmetry, regularity, and good wall-to-floor connections—all typically absent in Eixample buildings.

Local collapse mechanisms

Common failures include wall overturning, out-of-plane bending, and detachment at wall intersections. These mechanisms are exacerbated in aggregates with varied heights and discontinuities.

Aggregate behaviour

When buildings are attached, interactions affect load transfer and failure propagation. Aggregate behavior can amplify or reduce seismic response depending on geometrical and material uniformity.

 

 

3.3 Methodology

This study combines two approaches:

• Vulnerability Index Method (VIM): a semi-quantitative tool assessing seismic risk via weighted parameters—constructive features, configuration, and aggregate influence—yielding a numerical vulnerability index.
• Kinematic limit analysis: this technique evaluates failure through identified collapse mechanisms, considering horizontal acceleration needed to trigger displacements. It simplifies the structure into rigid blocks and calculates damage thresholds based on displacement and acceleration.

 

 

4.  Seismic Vulnerability Assessment

The chapter applies the methods to a real case in the Eixample, progressing from urban to building scale.

 

 

Urban scale

An initial analysis identified areas with dense concentrations of URM buildings. The selected site, enclosed by Gran Via, Passeig de Sant Joan, and Plaça Catalunya, is highly representative of early Eixample development.

 

Aggregate scale

A specific block—Block B—was selected based on available documentation and typological representativity. It comprises 17 buildings, mostly traditional masonry.

 

 

Building scale

Three buildings from the aggregate (B2, B7, B11) were further analyzed for kinematic assessment. Selection criteria included geometric heterogeneities and documented vulnerability indicators.

 

 

4.3 Vulnerability Index Method

Data collection and processing

Data were collected via existing architectural plans and site observations. Parameters included wall slenderness, openings, floor types, aggregate interaction, and height differences.

 

Discussion of results

The analysis differentiated vulnerability across directions (longitudinal vs. transverse). Buildings B2, B5, B7, B11, and B14 were classified as highly vulnerable due to poor seismic coefficients and vertical discontinuities.

 

 

4.4 Kinematic limit analysis

Discussion of results

Mechanisms analyzed included out-of-plane wall overturning. Displacement spectra revealed high damage grades (Ds3–Ds4) for walls with significant height contrasts. Overturning could cause partial or total collapse, also affecting adjacent buildings. Results highlight the fragility of lateral walls, especially when flanked by shorter neighbors.

 

 

5. Conclusions

5.1 Summay

The thesis evaluated the seismic vulnerability of masonry aggregates in Barcelona’s Eixample. Combining VIM and kinematic analysis provided a multi-scale perspective on risk. The research affirmed the critical role of aggregate configuration and inter-building dynamics in seismic behavior.

 

5.2 Main Findings

• VIM showed aggregate effects lowered vulnerability by 30% in transverse direction.
• The most vulnerable buildings featured excessive height, poor wall-floor connections, and remuntes.
• Kinematic analysis confirmed severe to extensive expected damage for key walls.
• Aggregate-induced amplification of damage was evident.

 

 

5.3 Proposals for future research

• Expand collapse mechanism types in kinematic analysis.
• Use dynamic simulations to complement static models.
• Integrate floor-wall friction effects into assessments.
• Study reinforced buildings to compare vulnerability evolution.
• Develop fragility curves to guide post-earthquake interventions and cost assessments.