Seismic performance of masonry buildings enlarged with additional stories in Barcelona urban centre

Seismic performance of masonry buildings enlarged with additional stories in Barcelona urban centre

The main aim of this thesis, by the author Francesca Marafini is to analyze the impact on the seismic performance of the historical buildings of the Eixample district in Barcelona of the upward extensions, also known as remuntes.

 

THE EIXAMPLE DISTRICT

 

The Eixample district is the second oldest area of the city of Barcelona and at the same time plays a significant role today in the metropolis.

 

Designed between the end of the XIX century and the beginning of the XX century by the engineer Ildefons Cerdà i Sunyer, the Eixample historical and cultural value is recognized not only for including in its boundaries some of the most important monument of the city, such as the famous still-unfinished Sagrada Familia, the Casa Batllò and the Pedrera, all UNESCO recognized World Heritage Sites, work of Antoni Gaudi, but also and above all for being a modernist architecture ensemble unique in all Europe.

 

The buildings in the Eixample, not only the most relevant but all in their complexity, largely consist of unreinforced masonry structures, originally designed without any seismic criteria, which makes them particularly vulnerable to earthquake actions. This aspect is particularly relevant given the high population density of this area of the city, despite Barcelona being an area subject to low to moderate seismic hazard. These considerations on seismic vulnerability take on a broader meaning when considering the historical, cultural, and economic value of this area of the city, also in the light of the above.

 

Location and demography

 

Barcelona is the second largest city in Spain after Madrid considering number of inhabitants, and the political and economic capital of the North-Eastern region of Catalonia. It is located on a plateau on the Mediterranean coast, in an area bounded between two rivers on two sides, the Llobregat on the South and the Besós on the North, to the east by the Mediterranean Sea and to the west by the Collserola mountain range.

 

 

Administrative boundaries and building stock

 

The Eixample district is located in the center of the city, positioning itself as an emblematic area, both for its urban design and remarkable extension, as it presents the highest population density among all 10 districts of the city (355 hab/ha0F), and encompasses some of the most well-known cultural and historical landmarks of Barcelona.

 

 

Almost the 65% of the residential building stock of the district have been constructed before the 1960s, when the use of concrete was introduced in construction in the city, therefore it is possible to assume that more than half of the buildings of the Eixample district, up to 70%, consist of unreinforced masonry structures. The average ages of the Eixample constructions is 78,5 years old, therefore it can be assumed that many of these buildings exceeded the life span they were designed for, and a part of them have been substituted by newer reinforced concrete (RC) buildings.

 

 

In addition, the highest number of buildings with more than 10 stories among all districts is recorded precisely in this neighborhood, for a total of 1.026. Excluding the latter, the average height of the rest of these residences is between 5 and 6 stories, adding the element of high-rise URM structures to the district characterization.

 

On its almost 750 ha, it is possible to observe a squared grid of roads and building blocks, sized of about 113×113 m, with roads of generally 20-25 m width. These blocks have rarely been constructed as individual elements and are mainly occupied with numerous buildings juxtaposed one next to the other, creating an urban network of large aggregates, at times interrupted by singular elements, for example parks or monuments.

 

 

Interventions in the Eixample district are limited and regulated on an architectural level by the ordinance on the protection of the historical and artistic architectural heritage of the city of Barcelona of 1991.

 

 

SEISMICITY

 

The city of Barcelona is positioned in a region characterized by low-to-moderate seismic hazard. According to the European Micro seismic Scale EMS-98, the seismic intensity expected in Barcelona is between VI “Slightly damaging” and VII “Damaging” grades MSK, as defined by the probabilistic seismic hazard map associated with a return period of 500 years.

 

Nonetheless, the high population density, the diffusion of historic buildings with high vulnerability and the presence of specific soil characteristics, have been found to produce considerable amplification of the seismic effect.

 

 

It is important to notice how almost 80% of the historical building stock in the Eixample district was constructed before the first Spanish building code, in 1968, which started introducing seismic design provisions.

 

THE EIXAMPLE CONSTRUCTION SYSTEM

 

Origin and development

 

The building blocks of the Eixample district were completed over an extended period of time. The evolution of these buildings, in terms of typology, morphology, and implementation have been defined, among other factors, by the substantial number of updates to the town planning regulations that have followed the original design.

 

The original project from 1855, was modified already twice, in 1856 and 1859, before the laying of the first stone in 1860. The initial limitations became more and more “tolerant”, responding, among other needs, to the desire of owners to maximize the buildings profitability.

 

 

Structural elements features

 

Referencing in particular only URM buildings, as they represent the focus of the present research, as disregarding specifically the RC structures with waffled slabs, three structural patterns can be identified in the configuration of the load-bearing walls in the typological buildings of the Eixample district.

 

⦁ The first pattern presents the main load-bearing walls oriented in parallel with the external façade, some of interior and lateral walls and the internal façade (Figure 21 (a) and (b)).
⦁ The second pattern presents the load-bearing walls, that are perpendicular to the façade, because of the narrower width of the block (Figure 21 (c)).
⦁ A third pattern can be found in the case of corner buildings, in which the internal and external façade walls are load-bearing, together with a wall offsetting internally from the exterior boundary and a few other transversal internal walls (Figure 21 (d)).

 

 

The typological building can be divided in three structural units, the ground floor, the upper levels and the additions, each of these units contributes to the structural response of the building, given it constructive and morphological characteristics.

 

Different structural elements can be identified for all of these three structural units:

 

⦁ The load-bearing walls.
⦁ The flooring system.
⦁ The pillars or columns at the base level.

 

Structural vulnerabilities

 

– The existing structural system is defined to withstand the vertical loads due to permanent and variable actions, and the horizontal loads only in terms of wind loads, while no seismic specific design elements are in place.
– Notable slenderness of the load-bearing masonry walls.
– Presence of large openings in considerable number in the load-bearing walls, often nonaligned between the lower structural unit and the upper levels, causing great discontinuities, and weakening even the larger sections at the ground levels.
– The wall sections on top of the openings represent a specific weakness points in which cracking can easily occur, although they are reinforced with wood, steel or relieving masonry arches to support them.
– Long extension of load-bearing walls with no perpendicular stiffening, as horizontal elements present poor stiffness both in-plane and out-of-plane, and low strength to bending and axial compression .
– Poor interlocking between the bearing walls and the secondary perpendicular walls.
– Geometric and volumetric discontinuities provided by the presence of internal courtyards and patios, empty volumes which weakens the structure lateral stiffness capacity.
– Presence of columns at the first two stories of the building, providing ground for the soft storey mechanism to be triggered, and even lowering the stiffness of the internal load-bearing walls.
– The aggregation of buildings in the block with differences in the height and number of stories, excluding the possibility of a collaborative response to horizontal loading.
– The additional floors subsequently added to the original structures generate increase in the loading conditions and the geometric irregularities both in height and in plan.

 

Construction materials

 

The construction materials which mainly constitute the Eixample buildings are the ones composing the base elements of masonry, being solid clay ceramic bricks, and lime mortar.

 

 

In reference to the traditional solid brick masonry, generic material properties were studied and compiled in previous studies. An account for such knowledge is provided at Table 3.

 

BUILDING ENLARGEMENTS: THE “REMUNTES”

The successive increase in building capacity set by the municipal ordinances and the revaluation of attics, both responding to progressive densification of the city, gave rise to the proliferation of building enlargements (Figure 36), in particular in terms of additional stories on top of the original buildings, in the range of the permitted buildability.

 

 

The remuntes development

 

A total of 836 remuntes have been identified.

 

 

The typologies of remuntes developed over time, starting from the beginning of the 20th century when the concept of the attic came to be valued, until 1976, when the General Metropolitan Plan regulations ended the speculative construction in the district. The evolution was governed by a large set of parameters: the architectural style, the alignments, the number of stories, the plant occupancy, etc.

 

From a stylistic point of view:

 

⦁ Initially the additions traced the style of the existing buildings, using chromatic analogy and avoiding the use of specific ornamental elements (Figure 38 (a)).
⦁ Later on, the modernism influences brought up the use of decorative motives and additional elements, which are still visible today in some of the most eclectic Eixample’s façades.(Figure 38 (b)).
⦁ Finally the stylistic configurations became more diverse and varied, adapting to a contemporary style by the second half of the 20th century, with a greater volumetric and variability (Figure 38 (c)).

 

 

Volumetrically, the boundaries set by the municipal ordinances for enlargement over the maximum allowed height changed greatly throughout the 20th century:

 

⦁ Initially, the roofing of the historical buildings of Eixample was bound to not be visible from the street level, and this was enforced with the introduction of the maximum regulatory height.
⦁ Over time this rule transformed into a limitation in terms of roofing inclination.

 

 

Classification of typological configurations

 

⦁ alineades (a), literally “aligned”, these are configurations in which the additional stories’ façades are aligned with the ones of the original building on which they subsist.
⦁ parcialment alineades (ad), literally “partially aligned”: the façades of these configurations are partially aligned to the original building.
⦁ enretirades (e), literally “retracted”, in these configurations the additional stories’ façades are designed in a backward positioned in reference to the original building.
⦁ combinació de parcialment alineades amb enretirades (ad+e), literally “combination of partially aligned and retracted”, are configurations which consist of both façades, which are partially aligned with the original building and positioned backward in respect to it.
⦁ volades (v), literally jutting out: the configuration in which the façade of the addition is placed outwards from the original façade plane.

 

 

Structural and construction features

 

The structural and construction features of the remuntes developed, as the overall construction typology of the historical buildings of the Eixample district, influenced by external factors, such as:

 

⦁ The availability of materials.
⦁ The evolution of construction techniques.
⦁ The building regulations.
⦁ The interests and trend of private ownership decision-making.

 

Different construction expedients were used to modify the original distribution plan in the new configuration. It is relevant to note how the addition of extra stories on top of existing structures posed a series of structural concerns, like the overload provided by the additional volumes.

 

Other issues could be caused by the changes in the stability and setup of the structural system, or even the potential presence of differential foundation settlements. Moreover, the juxtaposition of different materials and element with different stiffness altered the uniformity of the overall building.

 

CASE STUDY – SEISMIC ANALYSIS OF THE BUILDINGS ENLARGEMENTS

 

In order to perform a structural characterization of the typological building of Eixample, the finite element macro-modelling common approach was selected. This method represents one of the most established methods for the analysis of the response of complex masonry structures and present numerous advantages in its application, providing a convenient compromise between computational demand and accuracy of the results obtainable.

 

Seismic performance assessment based on the selected macro-modelling approach was performed through nonlinear static (pushover) analysis, choosing a mass proportional lateral load distribution pattern. The nonlinear analysis was performed in two directions with the objective of characterizing the horizontal capacity of the structure.

 

REPRESENTATIVE MODEL ORIGINAL BUILDING

 

The representative model of a typological URM Eixample building was developed as part of Sara Dimovska ongoing doctoral research at UPC, with the aim of investigating the influence of its specific structural features on the seismic performance.

 

The model had already been developed in terms of assumptions, geometry, material properties, loading and support conditions. Moreover, a sensitivity analysis had already been carried out for the definition of the mesh size and shape.

 

The representative model geometry had initially been developed in AutoCAD, and then moved to FX+ for DIANA for the properties definition and mesh assignments (see Figure 63). As part of the present research, the model was imported in DIANA, and mass proportional pushover analysis in two perpendicular directions were performed.

 

 

 

Pushover analyses

 

 

REPRESENTATIVE MODEL REMUNTES

 

After having analyzed and described the representative model of the original building, three different variations of such model have been defined, and have been given the denomination of Type A, B and C. They have been developed into FX+ for DIANA both in terms of geometry and mesh definition and then imported in DIANA, for the material properties, data and loading condition setting.

 

Linear analyses have been carried out on the three models in order to verify geometrical continuity and higher modes contribution. Successively mass proportional pushover analyses in two perpendicular directions were performed.

 

Choice of configuration

 

 

Pushover analyses

 

Pushover analyses were performed on all remuntes models, with an application of equivalent acceleration in both the X and Y positive direction, and a pattern proportional to the distribution of mass, in the same way it was done for the original building model.

 

 

Only partial considerations can be made, given the absence of data for type C. Nonetheless, consistent decrease of displacement demand, and increase in damage is recorded from type A to B.

 

 

Overall, five analysis on each of the four models developed were performed. A summary of the main results of all of these analyses is provided at Table 34.

 

 

CONCLUSIONS

 

Firstly, considerations on the seismic performance of the typological Eixample’s building, mainly validating already consolidated knowledge:

 

– The typological building of the Eixample’s district presents specific structural vulnerabilities which impact the seismic performance, as shown by the different response obtained in the two directions, highlighting the influence of large openings, lateral patios, and different plan configuration on the ground floor.

– The minor degree of asymmetry present in the X-direction influences the collapse mechanism of soft-storey behaviour which results to be more intense in the front façade, while the symmetry in the Y-direction leads to a balanced reaction concentrated in the lateral walls.

 

Secondly, specific conclusions on the analyses operated on the remuntes models, and their comparison to the original building model:

 

– The upward extensions of these buildings resulted to negatively impact the seismic capacity of the structure.
– As a general rule, a direct proportionality between the negative impact on the seismic performance and the number of storeys added to the structure, or similarly the vertical overload considered was found.
– The pushover analyses carried out on the three additions models yielded results providing evident increase in base shear and displacement demand proportional to the number of storeys added.
– Consistency was found in the damage mechanisms developed in each direction, while the numerical results did not present sufficient variation to define specific trends in terms of total strains and crack widths.
– The asymmetry in the overload (as in the retracted typologies B and C the alignment of the remuntes was chosen to move towards the rear façade) was reflected in the elements which resulted to be subject to the highest damage, especially in type C where the increase of vertical load was maximum.
– Specific structural features of the remuntes retracted typology represented a modeling challenge and hindered the direct results comparison, given the presence of singularities in the outputs.

 

Lastly, additional points can be discussed on the chosen modelling and analysis strategy:

 

– The numerical results provided satisfactory predictions of seismic response and the influence of the building’s structural characteristics, considering the compromise operated between required computational cost and the desired level of accuracy.
– The accurate modelling of the geometry and mesh generation through FX+ for DIANA provided the minimization of importation problems in DIANA, for a smooth and direct analysis setup.
– Further improvements can be applied to the analysis setup, in order to reach an optimum balance between accuracy and computational cost.

 

FUTURE RESEARCH

 

Numerous ideas and suggestions can be formulated for potential future research developments:

 

– Nonlinear static analysis could be carried out on all the original model in the negative Ydirection, given the asymmetry between the front and rear façade, and the plan distribution at the ground floor, to evaluate if more unfavourable results are found in such direction.
– The soft-storey behaviour relation with the upward extensions could be analyzed, perhaps with the plot of inter-storey drift profiles, possibly including a focus on the impact of the storey height on the displacement demand.
– Refinements could be introduced in the developed numerical models, with the objective of maintaining the conservative approach and a high level of accuracy but reduce considerably the high computational cost.
– Further analysis could be carried out on retracted remuntes typologies, and specific structural modelling solutions could be formulated to deal with the attic retracted façade singularity.
– As research lines with greater scope, parametric analyses could be developed, studying the impact on the seismic performance of all factors involved in the upward extensions.
– The impact of the structural slenderness could be evaluated with the comparison with a reference model with larger elements.
– The finite element modelling of the representative model could be extended to the corner building typology, and the study of the impact of the urban configuration of these constructions in aggregates.