Out-of-plane behavior of stone masonry walls built with earthquake resistant techniques
The main aim of this thesis, by the author Antonio Murano, is to attain a better insight related to the out-of-plane behavior of stone masonry walls built with earthquake-resistant techniques, which can be seen in different countries where local seismic culture is active.
Stone masonry is a traditional construction material widely employed in the building practice throughout history. Its mechanical properties highly depend on the overall quality and mutual arrangement of both masonry units and mortar layers. Historical constructions/buildings, both monumental complex and vernacular architectures are often characterized by oversized structural components, mainly due to the need to effectively deal with seismic events.
Out-of-plane behavior of stone masonry walls
Despite the technical approach of stone masonry resulted in the realization of buildings having a considerable endurance, on the other hand, it is possible to notice that some constructive details negatively affected the seismic response of these constructions. The constructive flaws, such as, high percentage of voids in masonry panels and lack of effective connections among structural components, as well as low-quality stone units and mortar used in the building process, can lead to one of the most common and dangerous local mechanisms affecting historical buildings which is, of course, the out–of–plane failure of masonry walls.
Seismic hazard and local seismic culture
Traditional earthquake resistant techniques are usually diffused in seismic prone areas. They can be considered as the result of practical expertise addressed to the mitigation of seismic disaster risk. Even with no specific knowledge about material structural behavior, these construction techniques proved to be extremely effective, reducing the impact of seismic events both in terms of building damages and economic losses.
To improve buildings’ structural performances under seismic actions, the most common construction materials, namely stone masonry and timber, were combined. Timber elements can often be embedded in the masonry components to strengthen some critical/weak points of the structure such as corners and load-bearing wall intersections or, in the most advanced solutions, timber frames were realized.
According to the geographical area in which the technique is applied and developed, it is possible to identify several different configurations.
1. Kashmir and Central America
In the city of Srinagar (Kashmir), traditional buildings can be divided depending on their constructive system:
– Taq technique, consist of load-bearing masonry piers and infill walls with 'runner' at each level, used to connect walls and floors;
– Dhajji-dawari technique, consists of a braced timber frame with masonry infill.
- Central America
A variation on the infilled timber-frame system is common in Central America and it is known as Taquezak system (Nicaragua) or Bahareque system (El Salvador). In these structures, a heavy timber frame is realized and, the wall set within the frame is made by a row of 5 X 10 cm studs. The timber frame consists of hardwood posts placed at the corners and intersections of walls. Wood laths or bamboos are then nailed across the studs to form a sort of basket, and the resulting pockets are filled with layers of small stones or adobe. The wall is finally plastered with mud or lime plaster.
2. Humush constructive system (Turkey)
Laced bearing wall construction on the ground floor level and infill-frame (humush) for the upper stories was the typical structural configuration of Turkish houses before the advent of concrete. This constructive system is characterized by very thin timber boards laid into the wall at one-meter intervals, placed so that they overlap the corners and bind the stone layers together without interrupting the building’s structural/constructive continuity. The overhanging timber and brick bays above the ground floor, thanks to the joists that cantilever over the walls below, steadily hold the lower story walls. The Turkish – Ottoman typical houses upper story is almost always built using the humush system (Langenbach, 2007).
3. Timber framed systems in ancient Greece
Numerous structural systems have been discovered as a result of the work of archeologists, engineers, architects in several archeological sites throughout Greece (Vintzileou, 2011). The oldest discoveries date back to the late Neolithic era. Furthermore, there are two Ages that represent the techniques used by the Greeks.
- Bronze Age is marked by intensive building activity, especially in Crete. The available documentation points out the use of timber as both horizontal and vertical load-bearing elements. Many of these timber elements’ existence, location and dimensions are evidenced by vertical, horizontal or diagonal holes in the masonry. In Akrotiri, a city on the island of Thera – Santorini destroyed by a volcanic eruption in 1500 BC, local buildings are mainly made of rubble stone masonry, except for their corners, opening and walls free extremities. In those locations, larger stones are used, and rubble masonry is reinforced using timber ties.
- On the Byzantine Era, the so-called 'imandosis' technique is a tying system based on the connection of timber elements embedded in the buildings. One of the most remarkable examples related to the use of timber in Greek historical construction is the structural system developed and applied in the Ionian Island of Lefkada (settled in the most earthquake-prone area of the country). The peculiarity of this system lies in the timber elements, forming a sort of secondary frame, which can sustain vertical loads, in case the masonry of the walls between the ground and first floor is severely damaged or collapsed after a seismic event.
4. Gaiola Pombalina (Portugal)
After the most destructive earthquake in the history of Portugal that destroyed large areas of Lisbon in 1755, a complex reconstruction process was organized by the Marquis of Pombal, introducing new architectural and structural concepts to be applied in the building practice. The most relevant seismic-resistant provision was a three-dimensional braced timber frame structure (Gaiola Pombalina) embedded in the building masonry components. The Gaiola is a resistant and flexible cage composed of horizontal, vertical and diagonal timber elements with different geometries, usually filled with rubble/brick masonry and plastered (forming the so-called frontal walls).
Frontal walls can be considered as shear walls. They allow the building to resist horizontal loads, dissipating a significant amount of energy. Furthermore, their bracing function reduces out-of-plane collapse mechanisms. Timber elements are connected using traditional solutions, such as dovetail or mortise and tenon joints and sometimes nail or other metal reinforcement elements are used.
5. Casa Baraccata and military architectures (Italy)
After a disastrous earthquake in 1783 that tore apart a huge portion of Calabria territory that was under the control of Borbone dynasty.
The Borbone government issued what can be considered one of the first examples of anti-seismic codes of constructions.
The prototype of casa baraccata (mixed structure with wooden frame and masonry walls) has a regular plan and height development, characterized by a double symmetry. This is due to an attempt to decrease torsional motions, giving to the structure a uniform stiffness. The lateral buildings have the same function of buttresses and they should decrease acceleration due to dynamic actions. The ground floor does not touch the soil to create a ventilated area and to prevent moisture degradation. The foundation structures are made with wooden piles. Inclined members are close to the framing corner to improve the overall stiffness and to contrast in-plane seismic actions. A floor beam interrupts the pillars of the lateral buildings, whereas the horizontal members are continual wooden elements.
Common features for earthquake resisting construction techniques
- Use of local materials (timber/stones) due to the need to simplify and accelerate the transport activities;
- Use of timber elements based on the assumption/belief that the high tensile strength of wood enhances masonry bearing capacity;
- Seek for symmetry both in plan and elevation; attempt to reduce the amount of participating mass that can be potentially activated due to a seismic event. On the other hand, timber frames have the function to improve the overall structural stiffness.
- Seek for an enhancement of the structural box behavior, reinforcing the weakest points of the building (corners, intersections).
- Seismic upgrading consists of all interventions in the structural systems aiming at improving their overall strength and ductility.
- Rehabilitation can be defined as the repair of a building before an earthquake occurs.
- On the other hand, retrofitting activities are mainly related to all those interventions characterizing post-earthquake strengthening actions. Hence, if the occurred damages in a structure after a seismic event is a consequence of energy dissipation processes predicted by design, only a repair is required to restore its original seismic resistance. Conversely, if the damage level experienced by a building is vastly beyond the expected threshold, then strengthening interventions are necessary to obtain the desired level of seismic resistance.
- Anchoring ties and confining techniques
To fully utilize the resistance and the dissipation capability of masonry structures, the achievement of effective monolithic behavior should be pursued. The possible modes of vibration of a masonry building during an earthquake are strictly related to the quality of connections/anchoring between walls and horizontal structural elements (floors and roof).
If timber joists are not anchored and walls are not tied, vertical cracks develop along the joints, in walls corners and intersections. The lack of connection affects the vibrations of the walls in such a way that they become uncoupled and collapse may occur due to out-of-plane forces. On the other hand, if walls are connected with rigid horizontal floor diaphragm and tie beams, the building vibrates as a monolithic box.
- FRP laminates
The most common FRP laminates used in the building practice are usually carbon, aramid, glass fiber-based. When these strengthening solutions are applied to both modern and ancient masonry structures, some key aspects, namely the effectiveness of the supporting surface and debonding effects, must be taken into account, to accurately predict the interaction between masonry and FRPs materials.
- Interventions to reduce floors and roofs deformability
Poor seismic behavior of existing masonry buildings is often due to the lack of proper horizontal diaphragm action of floor and roof structures. In modern masonry constructions, reinforced concrete beams should be placed along each bearing wall and at each story level. Tie-beams should connect the floors to the walls to attain a monolithic behavior under seismic loading. If adequately anchored to the walls, rigid floor diaphragms, and tie–beams prevent the out–of–plane vibration and possible collapses of walls.
The application of reinforced cement coating (jacket) on one or both sides of the wall is a logical way to improve the lateral resistance and energy dissipation capacity of the structural component. This method is very easy to apply and, at the same time, has good efficiency, for this reason, it has been widely used to strengthen masonry walls in different countries all over the world. Lately, the possibility to use ferro-cement and FRP coating instead of ordinary steel has been experimentally studied.
- Corners and wall intersections repairing techniques
According to traditional building practice, in highly seismic prone areas, experienced builders used to employ cut stones accurately refined to improve the quality of the corner connections. Vertical confining elements are often combined with stones or metals stitchings to strengthen brick masonry structures.
Two U-shaped walls, reinforced with steel elements (wall 1) and timber elements (wall 2) at the connections between the façade and transversal walls, were constructed and, successively tested to assess the effectiveness of the embedded elements regarding the enhancement of the out-of-plane load capacity.
The construction process of the stone masonry walls
The construction of the prototypes was followed step by step to control construction deviations from the original plans and to further characterize the real constructed walls from a geometrical point of view. At the same time, the construction quality was evaluated also based manufacture of the mortar. The first prototype has been reinforced with steel bars connecting the front and transversal walls, whereas timber braces have been installed at the corners of the second one.
- Non-destructive techniques (NDT)
- Sonic Tests: Sonic testing is an NDT based on the elastic wave method and consists of measuring the velocity of the wave propagation within a certain volume under evaluation
- Dynamic Characterization tests: Dynamic characterization tests allow to estimate the dynamic characteristics of a structure in terms of natural frequencies and vibration modes
- The out– of–plane tests
- Quasi-Static testing: The Quasi-static tests consider the load applied in a static way through hydraulic actuator or often by airbags in small increments. The airbags represent more realistically the uniform out-of-plane load corresponding to inertial forces
The results of the out-of-plane tests obtained for the two walls tested include:
- Force-displacement diagrams
- Damage and deformation patterns
- Definition of the drifts corresponding to different limit states
- Comparative analysis between the walls with steel and timber ties and unreinforced masonry
Both results obtained from the non-destructive tests and the out-of-plane tests allowed calibrating a FE numerical model realized to simulate the stone masonry walls. The numerical models, prepared using a macro-model approach, were calibrated with the results of the experimental tests.
Moreover, this information can potentially represent a good reference for all those practitioners involved in preservation and restoration activities requiring inspection of masonry.
The main outcomes from the sonic tests confirmed that the overall quality in both masonry bonds was similar. The natural frequencies related to the undamaged conditions in walls 1 and 2 were similar and the first mode shape identified an out-of-plane displacement. On the other hand, natural frequencies related to the damaged condition (after the out-of-plane test) showed a reduction, which is the result of stiffness decay due to the out-of-plane loading action.
The quasi-static cyclic out-of-plane loading tests performed on both walls allowed a quantitative assessment of their out-of-plane behavior. Both walls showed a markedly linear behavior until the peak load was reached. The post-peak behavior in wall 1 was characterized by a softening branch with increasing levels of displacements for relatively constant loads. On the other hand, wall 2 showed a relatively smooth softening in the post-peak branch, characterized by a decrease of the force for increasing lateral displacements.
The numerical pushover curves obtained from the numerical analysis showed a good correlation with the force-displacement envelopes obtained from the out-of-plane tests. A good correlation was obtained in terms of maximum load capacity, stiffness, deformation and damage pattern. Wall 1 post-peak behavior showed a slight difference if compared to the experimental data, whereas wall 2 post-peak behavior was accurately captured.
To conclude, it can be said that this thesis highlights the importance of a good characterization of the walls typology to correctly understand their structural behavior. The approach followed represents a meaningful reference example of how to address this issue. Moreover, non-destructive methods and numerical approaches proved to be very helpful tools to achieve a good characterization of the structure help, at the same time, in making decisions on a proper reinforcing or strengthening intervention.
Results provided also contribute to a better understanding of traditional stone masonry walls, which are representative of the vernacular architecture in many countries.
Present work could be further extended performing a parametric analysis to assess the influence of the mechanical properties in the out-of-plane behavior of the masonry walls. It could be interesting to assess how variation in Young modulus, compressive strength, and tensile strength affect the numerical capacity curve results from the pushover analysis.
More importantly, to have a deeper insight regarding the response of the reinforcing elements, their non-linear properties should be introduced in the numerical model. The influence of geometrical parameters, such as the span and height of the wall, can also be investigated and assessed through numerical analysis. Also, the influence of the reinforcing elements geometrical configuration needs to have, a numerical and experimental analysis, trying to identify how it affects the global response of the wall.
Therefore, the outcomes, both numerical and experimental, provided by this work will represent a useful contribution to an ongoing work that can allow achieving an aware and effective approach regarding all those activities aimed at the conservation of vernacular architectures both from a structural and a technological point of view.
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