1. Overview of masonry buildings

1.1. Introduction

Masonry buildings are those made from bricks, stones, or concrete blocks materials we've used for ages to build strong structures. Case studies, on the other hand, can provide detailed analyses of specific buildings that either survived or suffered damage during earthquakes, along with discussions on the effectiveness of the structural solutions employed. Such studies can offer valuable insights into the development of design and construction practices aimed at enhancing building resilience to earthquakes. In this short introduction, we'll look at the basics of what makes masonry buildings strong and how they deal with different forces.

Figure. Examples of masonry buildings

Technology of masonry buildings changes a lot over time. On the photos, you can see two examples of masonry construction. The first one is a building made in 2017 where modern construction technologies like steel supports were used. On the second photo, there is the first masonry building in the world. It was made around 12,000 to 10,800 years ago in the city of Jericho.

1.2. What a masonry is?

Masonry construction is a method that combines natural elements like stone or artificial ones like brick, held together by mortar or simply overlapped, known as "dry-stone masonry." These structures serve as load-bearing systems, supporting both vertical (such as permanent loads and snow) and horizontal loads (like wind and earthquakes).

There are two primary types of masonry: natural stone elements and artificial brick elements. Masonry buildings must adhere to specific standards, which vary depending on the region.

For instance:

-clay, 
-calcium silicate,
-aggregate concrete,
-autoclaved,
-manufactured stone
-dimensioned natural stone
are in accordance with european norms.

Figure. Example of masonry types

1.3. The "box" behavior

In general, a good seismic behavior can be expected when the "box" effect is guaranteed:

• Proper connections between vertical walls (by means of stones or tie-rods); A good connection between intersecting walls is a staggered disposition of the masonry units, such that the friction forces are maximized;
• Suitable connections between parallel walls (steel tie-rods);
• Connections between floors and walls.

Response dependent on diaphragm types and connection, which type we have.

7. The rigid floor is not essential

It is not recommended with walls of similar stiffness and different loads, not recommended if the plan is quite longer in one direction (beam behavior). It is essential to distinguish the tie capacity of a floor with respect to its stiffening capacity.

The rigid floor is not essential:

It is not recommended with walls of similar stiffness and different loads, not recommended if the plan is quite longer in one direction (beam behavior). It is essential to distinguish the tie capacity of a floor with respect to its stiffening capacity. 

Building with stiff floors:
Seismic forces are distributed according to the walls stiffness. 

Building with flexible floors:
Seismic forces are distributed according to the tributary areas.

Narrow building plan:

If the floor is stiff, the seismic action over the lateral walls is higher, as for the compatibility conditions reads: 

Seismic action: F = K v (where K is stiffness of the wall)

Narrow building plan with flexible floors. If the floor is flexible, the distribution of the seismic action can be designed by the engineer. In these conditions, separation into smaller buildings structurally independent can be a solution:

1.5. Conclusions

To summarise, in masonry building it's important to pay attention on the following topics. Structural Integrity: the strength and resilience of masonry buildings heavily rely on proper connections and construction techniques. Ensuring a "box" behavior through adequate vertical and horizontal connections is paramount for seismic resistance. Importance of Regularity: Both in plan and elevation, regularity in masonry construction aids in distributing loads effectively and mitigating potential weaknesses. Symmetry and consistent distribution of masses contribute to structural stability. Quality Assurance: Maintaining high standards of masonry quality, including the selection of materials and execution of construction, is essential. Uncracked blocks, sturdy mortar joints, and resilient roofing are pivotal for long-term structural integrity. Understanding Floor Dynamics: The choice between rigid and flexible floors significantly impacts seismic response. Engineers must carefully consider the distribution of seismic forces based on the building's layout and floor stiffness to optimize structural performance. Addressing Deterioration: Many structural issues stem from neglect or modifications over time. Preserving the original integrity of masonry buildings by avoiding alterations and enhancing material quality is crucial for longevity and seismic resilience. Attention to Detail: Constructive details, such as well-connected lintels, properly built openings, and secure ties, signify a commitment to quality construction. Attention to these elements ensures overall structural stability and durability.

2. Seismic vulnerability assestment of masonry building 

2.1. Seismic vulnerability assessment and knowledge level of masonry buildings 

The process to evaluate the seismic vulnerability of masonry buildings is:

• Collecting information about the building
• Preliminary evaluations based on engineering experience
• Global and local analyses, that give a quantitative assessment
• Possible structural interventions that upgrade or improve the aim

The interesting information regards: geometric data, masonry elements types (material type, voids, past interventions, foundation system and soil features, existing damage.

Before doing whatever qualitative and quantitative evaluation, there is the need to deeply understand how the structure could behave in case of  a  seismic  event  depending  on  the  available information.  This  crucial  step  concerns  a  critical interpretation of the data and is fundamental for a correct seismic risk assessment.

So collecting informations about the building is the start point to attribute the correct knowledge level and the relative confident factor (according to italian code). The level of knowledge is based on surveys, investigations of structural details and tests on materials, and determines the values ​​of the confidence factors to be applied and suggests the most appropriate analysis method. The following table shows the 3 level of knowledge according to the italian Norme Tecniche delle Costruzioni (NTC 2018) and the relative confidence factor. Note that the confidence factor decreases when the knowledge increases.

Figure. Knowledge levels and confidence factor according to italian NTC 2018

2.2. Types of cracks in masonry buildings  

In masonry building, materials have a really good resistance to compressive actions, but very little to traction actions. In these cases, low tensile stress states are sufficient to cause local cracking, generating danger to the structure. There are different types of cracks in masonry facade due to different reasons. 

• Vertical cracks: this class of cracks are one of the most dangerous, and they are due to vertical load of compression.

Figure. Example of vertical crack 

• Arched cracks: also frequent on masonry are arched cracks on load-bearing wall structures or internal infills. Each crack has an arch shape, with deviations in shape and transition at the corners of doors and windows. The phenomenon is symptomatic of a failure of the underlying load-bearing part, which can be the ground, beams or floors. 

Figure. Example of arched crack 

• Diagonal cracks: they are due to shear forces. They usually have an inclination of 45° for simmetric buildings, but it can vary depending on the size of the wall partitions, their consistency and the more or less regular dislocation of the openings on the facade. They are located and formed from the opening corners, that represent a weak part of the facade. This type of crack can born after a structural failure of fondation. If there’s no simmetry respect to an axis, cracks have all the same inclination and are similar, as we can see in the picture. This pattern of lesions is associated with the presence of a rotation of a portion of the building due to a non-uniform and concentrated failure of the ground.

Figure. Example of diagonal crack 

• X cracks: they are a typical phenomenon of consequence of an earthquake. They are due to oscillatory motion and have an inclination variable from 30° to 45°. This type are studied as a seismic effect and they are dangerous when they cut all the continuous vertical partitions, as they create discontinuities in the main load-bearing elements of the building.

Figure. Example of X crack 

2.3. Failure mechanism of a masonry building

• Outward rotation of the corner: this is caused by the combined action of forces acting against the corner. The block rotates out ward with the formation of a hinge at the lower part.

Figure. Outward rotation of the corner

• Partial out-of-plane overturning: the  mechanism  is caused  by  the  roof pounding.  Only the upper portion of the facade is subjected to overturning, thanks to effective connections of its base with the floors.

Figure. Partial out-of-plane overturning

• Outward overturning with effective connections with roof: the mechanism is similar to the previous one, but a restraints is available at the roof intersection. This mechanism also occurs    when the masonry quality is bad (multylayers)

Figure. Outward overturning with effective connections with roof

• Global facade overturning: If the connection between the facade and the transverse walls is not proper, the crack is vertical and the hinge is formed at the base of the facade.

Figure. Global facade overturning

3. Seismic damage on masonry buildings - case studies

3.1. Unreinforced Masonry Structures in Historical Centers of L'Aquila and Castelvecchio Subequo after The Abruzzo 2009 Earthquake

 The earthquake happened at the Abruzzo Region located in the central part of Italy (April 6th, 2009) had caused extensive losses and the recorded ground accelerations, velocities, and displacements reached significant levels. Approximately 18,000 unusable buildings were recorded in the epicentral area. Among them, the civil and heritage buildings are heavily damaged in L'Aquila historic center.

 In the Abruzzo region, much like the rest of Italy, the historic city centers serve as the heart of the built environment, predominantly comprised of Unreinforced Masonry (URM) structures, constituting approximately 20% of larger cities such as L'Aquila and 50% of smaller towns. Generally, URM buildings experienced significant damage.

Figure. Damage / collapse in L'Aquila area.

Out of the roughly 300 digital strong-motion stations in the Italian Strong Motion Network (RAN) overseen by DPC, 56 captured the main shock, along with 42 broadband stations. The Abruzzo region houses 14 stations, with the rest spread across the Apennines, primarily northwest and southeast toward L'Aquila. This distribution results in one of the most thoroughly recorded earthquakes of the 2009 Abruzzo event. The recordings acquired for the main shock, from the four foremost stations (AQG, AQA, AQV, and AQK), all positioned on the hanging wall of the rupture. 

Figure. The elastic response spectra of the AQG, AQA, AQV and AQK stations accelerograms for 5.0% damping (source: Italian Institute of Geophysics and Volcanology [INGV]).

The prevalent structural system of the buildings situated in the historic center of L'Aquila (as well as in nearby villages) comprises Unreinforced Masonry (URM), with ancient constructions dating back to the 14th century. Typically, these buildings are one to three stories tall and feature relatively lower heights for the upper levels, often adorned with finely crafted decorative marble posts, sills, lintels, balconies, and architraves. However, these non-structural elements, if inadequately bonded, were susceptible to detachment and subsequent collapse, as evidenced by instances such as chimney falls and the dislodging of roofing clay tiles.

Figure. Several surveyed buildings reveal decorative marble posts, sills, lintels, balconies, and architraves, which were insufficiently bonded, leading to subsequent collapse.

The urban fabric comprises noble palaces, luxurious residences, and lower-income dwellings. Many buildings feature well-textured masonry, at least when viewed from the outside. However, in numerous instances, the walls consist of two external layers of low-quality masonry—composed of riverbank pebbles and weak mortar, typically lime—encasing an inner core filled with uneven and disaggregated materials.

Figure. The example of a good masonry texture.

In L'Aquila's URM buildings, intersections and crossing walls lack proper connections due to missing bond stones. While horizontal steel ties are found in about half of the buildings, their installation is inconsistent, and they often fail partially or completely due to weak masonry. However, effective tie insertion can improve structural behavior by preventing wall separation and overturning. Additionally, some buildings feature timber beams inserted within the masonry for added stability, though these are susceptible to decay.

Figure. Various types of steel tie configurations, discovered in URM buildings in L'Aquila, exhibited varying performance levels, ranging from effective to partially functional or entirely ineffective.

The vulnerability of URM structures, exacerbated by the earthquake's consistent actions and the underestimation of seismic hazards, led to widespread devastation. Strengthening measures, such as stretcher bond stones and steel ties, demonstrated better resilience. Addressing this requires updated seismic hazard methodologies, harmonized assessment tools, and rehabilitation techniques compatible with conservation goals. Moreover, investing in proactive investigation programs and seismic retrofitting of old masonry structures in the Mediterranean Basin is crucial for disaster prevention. In conclusion, shifting focus from post-emergency response to prevention is essential. This necessitates international collaboration and resource allocation towards comprehensive seismic risk reduction measures and heritage preservation.

3.2. The 24 August 2016 Amatrice earthquake: macroseismic survey in the damage area and EMS intensity assessment

One of a good examples and one from we can learn a lot about Seismic behavior of masonry building was the 2016 earthquake in central Italy. The capital of pizza as no one is know for its masonry building. Between august 2016 and January 2017, nine earthquakes with power between 5,0 and 6,5 stroke middle of the country, region near Rome. Scientists expected 2 or 3 times weaker seismic movement. The damage was severe with many buildings ruined as a consequence of this events.  Shaking of ground alone cant explain alone the damages, even as significant. Big parts of blocks completely collapsed, this shown need to make a research on vulnerability factors and construction features of old, reinforced masonry buildings. The most common in high buildings are made out of two layer walls, not connected to any other part, this type of structure is known to be very vulnerable. Also a lot of people reinforced their homes old wooden floors with heavy concrete ones, without changing their vertical walls and making proper connections between floor and wall.

Figure. Consequences of changing old wooden floor with heavy concrete without reinforcing the vertical walls.

Another big factor was a bad state of maintenance. Many people used their houses only as vacation homes or were abundant because of depopulation in the last 50 years in that region. Not taking care of these buildings on a daily basis increased the risk of collapse and, as a consequence the risk to other buildings surrounding. What is interesting, historic buildings made by square stone masonry with horizontal layers of bricks and ones witch was been recently reinforced survived earthquake much better. Soon after the earthquake, survey team started doing their research with aim to define the damages. In total, they have inspected more than 150 localities, hamlets included. For each of them the focus was on establishing the number of buildings, their typology, the elements of specific vulnerability, grade of damage. Results were separated into 10 degrees, with the 10th representing the biggest damages.

Degree 5

According to EMS guidelines (Grünthal, 1998), we include areas with sporadic, minor damage (e.g., hairline cracks in plaster) in buildings of vulnerability classes A and B. However, due to challenges in observing such negligible effects, especially inside buildings, and the widespread nature of the affected area, the inventory of these locations remains incomplete.

Degree 6

Slight damage, such as plaster cracks and partial chimney collapse, shows a distinct distribution. Many affected areas are situated west of Amatrice, within 4-6 km of the degree 9 or 10 zone. Another cluster appears in the northern part of the surveyed region, seemingly an anomaly within the degree 5 area.

Degree 7

Moderate damage, including partial collapses of class A buildings, stretched over a 30 km area along the same north-south strike. Notably, Fornisco, within the slight damage zone east of the epicenter, stands out.

Degree 8

 Accumoli, Arquata del Tronto and other localities near were affected by severe damage and some collapses of class A buildings.

Degree 9

Many hamlets of the territory north of Amatrice suffered total or near total collapse.

Degree 10

Some locations in he middle of the epicenter of earthquake. All buildings with low quality and high variability, with most of them masounary suffered total destruction

Figure. Intensity of EMS in part of Italy effected by 2016 earthquake

Figure. Example of damages made by the earthquake

During this event most of the ancient vulnerable building collapsed while the remaining part of masonry structures suffered from heavy to very heavy damage from small cracks to  collapse of whole walls. However, the high concentration of damage in specific zones depends not only on purely engineering factors like poor materials, slipshod workmanship, state of repair and maintenance, but also on site effects.  Furthermore, it should be stressed that the effects of past seismicity in the localities of the epicentral area were not properly considered to improve building vulnerability over time.

4. Improve the resistance of masonry structure: the case of the armed arch 

4.1. The technique 

A recent case study in Italy proposes a new active technique capable of significantly improving the breaking resistance of an arch and its seismic behavior; it consists in disposing on the extrados or intrados of the curved element a series of metal cables, placed in tension, in capable of preventing the formation of hinges with extradosal or intradossal opening and of simultaneously impose an increase in axial force, with an increase in ductility. 

The arches and vaults reach collapse when, as the loads increase, the pressure curve is tangent in several points to the external profiles of the arch, giving rise to localized rotations between the segments (hinges) in a number that generate a collapse mechanism. It can be seen, based on numerous tests and in situ observations, that during the collapse phase the hinges give rise to alternating cracks between the extrados fibers and those of intrados of the arch.

Figure. Collapse mechanism of an arch

If we are able to prevent at least one of the two families of hinges, the structure would not form any collapse mechanism. The structure, originally continuous, could at most degrade to a "three-hinged arch", which therefore does not collapse due to kinematics. To achieve this result, a diffused, tensile-resistant armor was designed, to the extrados, or, in a dual way, to the intrados. If instead of limiting themselves to a simple juxtaposition between masonry and cables, the latter were even placed in traction (making them work as "active tie rods") you would obtain a distribution of forces applied on the arch in a radial direction, causing a beneficial axial compression and, of consequently, the centering of the pressure curve. To create adequate forcing between the ropes and the arch, simply fix the ropes at the ends of the arch and move them away from the extrados using spacers. A similar result is obtained with common tensioners, placed for example at the ends of the cables, as long as they allow sliding between the cable and the masonry along the contact line. 

Figure. Example of armed arch

We note that the proposed technique is able to significantly increase the breaking load of the arches and vaults when the collapse mechanism is predominantly flexural. The proposed method turns out less efficient when the collapse mechanism is shear, which is a much rarer case. The advantages of using post-tensioned reinforcement rods can be summarized in: small size, low costs, lightness, large resistance, high global ductility of the masonry-cable assembly, immediate recognisability and possible reversibility of the intervention. With the use of cables extended to the ground, the resistance increase not just for vertical loads but even for horizontal loads from earthquakes, working in pressure-bending. 

4.2. Conclusions 

Compared to the traditional systems, which use concrete layers, reinforced arches method (RAM) shows an higher ductility and a lower residual inelastic deformation.
In case of vertical concentrated loads, the increase of load capacity of the reinforced arches reached more than 1400%, compared to non reinforced ones. 
Furthermore, different collapse mechanisms is pointed out. In particular, for the un-reinforced arches and for the ones reinforced using a concrete layer, the hinges were more opened and localized, while adopting RAM the cracks were smaller and more diffused.
Some results of an experimental campaign are shown in the following. The collapse load and the load-displacement curves are compared for 4 different configurations tested.  

Figure. Collapse load for configuration of arches 

As we can see on the table, the technique of RAM shows considerable increases in resistance and ductility towards than the arch simple and the traditional technique of the reinforced concrete hood. Its application in zones characterized by seismic events appears interesting, especially taking into account the negligible increase in the masses involved. 

5. References

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Indirli, M., S. Kouris, L. A., Formisano, A., Borg, R. P., & Mazzolani, F. M. (2013). Seismic damage assessment of unreinforced masonry structures after the Abruzzo 2009 earthquake: The case study of the historical centers of L'Aquila and Castelvecchio Subequo. International Journal of Architectural Heritage, 7(5), 536-578.

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http://quest.ingv.it/index.php/rilievimacrosismici. 

Gasperini, P., Vannucci, G., Tripone, D. and Boschi, E. (2010). The location and sizing of historical earthquakes using the attenuation of macroseismic intensity with distance. Bull. Seism. Soc. Am., 100:2035–2066

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Norme Tecniche per le Costruzioni, 2018