Masonry Wall Ties and Anchors: Design and Functions in Masonry Construction

Masonry structures often face issues with interlocking between walls, making the building vulnerable to out-of-plane overturning phenomena. To effectively address such structural criticalities, it is possible to intervene using the so-called “masonry wall ties and anchors.” These are structural elements consisting of a metal bar resistant to tension called a “tie,” anchored to the perimeter walls through specific fastening elements. The ties connect opposite walls, providing the structure with greater cohesion and resistance to horizontal forces, such as those caused by seismic events.

In the following article, we will examine in detail the functioning of masonry wall ties and anchors, offering a practical guide on how to effectively design and install these devices for reinforcing masonry structures. We will illustrate the fundamental principles of their sizing, the materials to be used, and the installation techniques, thus ensuring greater durability and safety of the building.

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What are masonry wall ties and anchors?

The term “masonry wall ties and anchors” is used in structural engineering to refer to one-dimensional metallic elements, commonly made of steel, that are used to reinforce masonry structures lacking adequate interlocking between walls.

These elements serve to ensure a box-like behavior in masonry structures through the interaction of the following components:

• the tie: is the main element of the masonry wall tie and anchor, consisting of a steel bar with a circular or rectangular cross-section, which spans the structure to connect opposite walls and prevent overturning phenomena. The tie is designed to resist only tensile stresses;
• the key plate: is the terminal element of the tie, located at the ends of the tie, which functions by contrast and can be of the “stake” or “plate” type. The latter can take on a circular or rectangular shape depending on structural needs. The key plate ensures the correct anchoring of the tie to the masonry, allowing for tension adjustment and ensuring the effectiveness of the reinforcement.

The insertion of masonry wall ties and anchors is a very common structural reinforcement technique for masonry buildings, as it is particularly effective for seismic upgrading and improvement. The ties and anchors help, in fact, to prevent out-of-plane overturning of the walls and to ensure the box-like behavior of the building under seismic action. This technical solution is considered one of the most effective, simple, and economical ways to intervene on existing masonry structures, both temporarily and permanently, with the aim of restoring structural stability and ensuring effective safety of the masonry walls.

Masonry wall tie

What is the function of ties in masonry walls?

Ties play a fundamental role in providing stability and strength to masonry structures, preventing collapse phenomena and ensuring adequate building performance under various stresses. Specifically, these elements are capable of ensuring the following functionalities:

• structural reinforcement: ties give the structure a box-like and monolithic behavior, ensuring greater solidity and resistance against horizontal forces caused by wind or earthquakes;
• prevention of overturning: ties help maintain the vertical position of walls that are not well connected to each other, avoiding out-of-plane overturning phenomena that can lead to the collapse of the entire structure;
• counteracting lateral forces: ties effectively help prevent the displacement or collapse of masonry walls subjected to lateral forces caused by pushing elements such as arches, vaults, or sloped roofs;
• improvement of cohesion: in the absence of beams or other elements connecting the floors to the perimeter walls, ties help increase the overall cohesion of the structure and better distribute forces, effectively connecting the different parts that make it up;
• control of settlements: by providing a box-like behavior to the structure, ties can limit differential settlements of the foundations (i.e., the relative movements between different structural parts), improving overall stability and reducing movements between the various parts of the building.

Where to position the ties?

In order to adequately fulfill the previously illustrated functions, it is essential to ensure both appropriate sizing of the ties and their correct placement, responding to specific static and structural needs. In particular:

• in plan, ties should be positioned adjacent to the spine walls, that is, the walls orthogonal to the perimeter walls to which the ties are anchored. This is because the spine walls act as counteracting elements against the compression exerted by the pull of the ties on the structure. The proximity to the spine walls allows the ties to “properly discharge” the generated tensile forces, preventing these from excessively and uncontrollably stressing the perimeter walls;
• in elevation, ties should be installed at the level of the floor systems, just below the underside of the slab. In this way, the slab, if equipped with a rigid topping, will also help counteract the tensile forces exerted by the ties. This placement allows for better utilization of the structural contribution offered by the slabs, thus achieving a better overall static behavior of the structure.

Correct positioning of ties

6 steps for the installation of masonry wall ties and anchors

To ensure effective structural consolidation, the installation of masonry wall ties and anchors must occur through a series of fundamental steps, outlined below:

1. possible pre-consolidation of the masonry: if the walls involved in the intervention are deteriorated or have low resistance, it is necessary to first proceed with injections of consolidating material (such as cement mortar or resins), in order to strengthen the masonry and ensure adequate anchorage for the ties;
2. creation of holes in the walls: the next phase consists of drilling through holes in the walls to be reinforced, with an appropriate diameter, to allow for the subsequent insertion of the ties;
3. insertion of the ties: once the holes are prepared, the ties are inserted into the walls. These can be made of metal bars or appropriately sized steel cables;
4. creation of the anchorage bulb: to ensure effective locking of the ties within the holes, cementitious or resinous material is injected under pressure, which creates an anchorage bulb at the end of the tie;
5. fixing of the anchorage plates: metal plates are fixed to the ends of the ties, which allow for the distribution of tensile forces over the surface of the wall and counteract the pulling action;
6. tensioning of the ties: the final phase involves applying the desired tension to the ties, generally achieved through the tightening of threaded nuts.

How to design the ties?

To design a tie in order to prevent the overturning mechanism of a masonry wall, the procedure of linear kinematic analysis is used, which includes the following steps:

1. identification of the overturning mechanism: first, identify the type of overturning mechanism (simple or compound) that could occur in the masonry wall;
2. application of the principle of virtual work: apply the principle of virtual work considering the balance of overturning moments (Mr) and stabilizing moments (Ms). The former are caused by external loads that tend to overturn the wall (such as seismic loads), while the latter are due to the action of the ties and the self-weight of the structure that tend to keep the wall in position;
3. determination of the activation multiplier of the mechanism α₀: by setting the balance of moments, the multiplier α₀ is derived, which represents the horizontal acceleration that triggers the overturning mechanism;
4. comparison with the maximum expected ground acceleration: the value α₀ is compared with the maximum ground acceleration expected for the considered limit state, in accordance with the technical regulations. If α₀ is found to be less than this value, it is necessary to size the tie to prevent out-of-plane overturning of the wall;
5. determination of the necessary tension: the balance of moments is then redefined considering also the stabilizing moment due to the tension T of the metal tie. This will be the variable to determine, and its value will be calculated by imposing the equality between the stabilizing moment and the overturning moment, assigning to the multiplier α the value of the expected seismic acceleration at the ground;
6. distribution of the tension among the ties: once the tension T is calculated, this value is divided by the number of ties to be installed on the wall, yielding the tension associated with each individual tie (T_c);
7. sizing of the tie section: having determined the value of the tension T_c for each tie, the sizing for tension is carried out, ensuring that the tie can withstand such force.

The verification of a steel tie consists of comparing the tensile strength of the tie with the axial tensile stress acting on it. However, this is not the only verification to be performed, as it is also necessary to consider the shear strength of the wall according to the possible failure mechanisms and verify the strength of the key by comparing the stress developed in the anchorage system with the design yield stress of the steel.

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