Masonry walls, when integrated within a reinforced concrete frame, have a different mechanical behavior compared to unconfined masonry walls. The lateral loading on the confined system creates mainly biaxial stresses with compression on the diagonal connecting opposite corners and tension on the perpendicular direction. The walls can also be subjected to high shear demands, failing along the mortar layers. However, the greater changes on the stress demands occur to the frame. The interaction between the two elements changes the behaviour of the assemblage completely. Figure 2a,b supports these remarks by showing the differences both on global and local levels. Analysing the displacements between a building with and without infill masonry walls, it is possible to assess that the bare-frame structure has higher deformations that are more distributed in height than the infilled frame. It shows that, for higher seismic demands, there is a tendency for concentration of deformations at specific levels, especially for the infilled frame. This occurrence leads to the formation of a soft-storey mechanism, a possible consequence intensified by the implementation of brittle masonry walls. Figure 2c shows the influence of the wall at the local level; the typical stress diagram of a bare frame is normally completely different from the presented here, demonstrating that ignoring masonry walls during the design of a structure can lead to errors on the expected stress resultants.

Infill walls are considered non-structural elements in RC structures because they are not intended to carry loads in static conditions. However, in case of higher deformations, lateral or vertical, these elements become active, change the response of the frame structure, and claim a structural influence, typically causing a reduction of the bending moments and an increase on the axial loads on the columns and beams. These aspects become even more important since the requirements for non-linear response evaluation of structures, according to new codes, are also higher. The walls increase the lateral strength of the frames locally, however globally they may reduce the base-shear strength of a structure due to failure on a certain level. At a global level, the infill walls can contribute to an increase of stiffness with the direct consequence of reducing the lateral displacements and can improve the dynamic behavior due to higher energy dissipation through friction on the structural interfaces between infill and frame.

The uncertainties related with the behavior of infilled frames are numerous. For medium-strong lateral loads, the frame tendency to deform creates a detachment between infill and frame. In this process, the contact length is mainly controlled by the relative stiffness between the involved elements. The bond strength between infill and frame on the mortar boundary layers is less significant for the global behavior of the system. Openings affect the behaviour of the infilled frame because the stiffness is not homogeneous which can result, for example, in short-column mechanisms, locally, or in undesirable loss of stiffness in a smaller region of the structure, globally. Hereupon it is noteworthy that infill walls with more than 50% of openings are usually ignored because the behaviour of the system is close to the bare frame. Moreover, as seen in Figure 2, infill walls completely change the load redistribution of the structure, creating, for example, local stresses concentrated near the corner region, which in turn can affect structural stability, causing another source of uncertainties.

Fardis (2009) assesses that the effect of masonry infills is generally beneficial for an infilled building without irregularities [4]. Table 1 summarises the advantages and disadvantages on the integration of infill masonry walls on RC buildings.

By neglecting the existence of the infill walls, a wrong lateral stiffness is indirectly assumed [5,6] for the structure in analysis, introducing potentially dangerous consequences such as unrealistic seismic design actions, prevention from accounting for torsional effects, formation of mechanisms, or local failures. It is worth noticing that although many issues were discussed about the behavior of infilled frames, the capacity of these infilled systems are stronger when compared to unreinforced masonry walls.

Different failure modes may occur depending on the properties of the materials and the stress rate [7]. Several researchers have classified those modes in a slightly different way. The following list is a collection of some definitions; however, it is important to keep in mind that failure is usually attained through a combination of different modes and not due to a single reason. Some failure modes are identified as dominant nevertheless.

Many studies in the literature assess the various masonry infill failure modes. In some research papers [8,9,10,11], the failure modes are categorised into the following groups:

• Purely Flexural. It is identified as the case in which frame and infill wall cooperate as a unique element during a low loading phase. Since it does not reach a failure state from this behaviour, the mode is not considered as a primary failure mechanism.
• Sliding Shear. This failure occurs from the shear failure on the bed joint layer of the masonry infill wall. It is one of the most important failure modes, which decreases the stiffness of the panel and may create local deficiencies on the frame, but not an ultimate failure because the panel maintain some lateral strength capacity for the infilled frame. The mode occurs in masonry walls with a weak mortar joint and a strong frame.
  Multiple Sliding Shear. This mode is a special case of the previous one. While the single sliding shear is usually observed for an individual layer located near the middle of the panel, for the case of multiple sliding, the formations are distributed along the height of the panel, creating fewer local problems in the RC frame.
• Diagonal Compression. Occurs from the crushing of the infill. It may be located on the central region, and it is associated with slender infill walls and out-of-plane buckling.
• Corner Crushing. Crushing by compression located on one or more corners of the masonry panel, with probability of occurrence for weak infills within frames with strong members and weak joints.
• Diagonal Cracking. It is the formation of cracks along the infill compression strut, or also in the horizontal layer along the panel’s half height. It is common to occur together with sliding shear, corner crushing, or frame failure modes.
• Out-of-Plane. It is characterised by a more destructive and spectacular failure compared to in-plane failures. Seismic action does not occur perfectly aligned to the building directions. Therefore, the failure often occurs with a combination of in-plane and out-of-plane forces, with initial cracking caused by in-plane forces that weaken the wall substantially. Masonry walls, as infills for frame structures, due to the interaction with the bounding frame can develop out-of-plane resistance through the arching mechanism, which is mainly dependent on the slenderness of the wall, on the compressive strength of the masonry and on the support conditions. The risk for this type of failure can be greatly reduced by decreasing distances between supports, use of proper floor-to-ceiling supports, and attachment between walls and frames, however the available knowledge in this regard is still limited, especially for non-pure out-of-plane failure considering prior in-plane damage.
• Frame Failure. An obvious possibility for the failure is the formation of plastic hinges in the columns, or shear failure in the beam-column joints or columns. It can be caused by either a weak frame or weak joints with a strong infill. The failure induced by the infill walls may arise from a number of factors. The increase of stiffness and the forces introduced by the infills can elevate the axial stresses on columns (both in tension and compression). The diagonal truss of the infill creates a concentration region where the wall transfers some of the stress near the joints, resulting in increased shear stresses on both columns and the beams. In such a scenario the joints may fail, causing the failure of the system. The flexural and shear demands may increase even more if a short column is formed by an infill shear failure or by the existence of openings.

From this list, considering only in-plane demands, the most dominant types of failure are sliding shear on mortar layers, corner crushing, and cracking by diagonal tension. Table 2 correlates the in-plane failure modes with the infilled frame conditions. The blank spaces are too unpredictable to assume a tendency for the failure.

The interaction of the non-structural elements with the structural elements, specifically infill masonry walls with columns, beams and their joints, has great structural implications. The combination of both responses during an earthquake increases the complexity of the system and may prove crucial for the structural safety.

The infill walls, when distributed irregularly in plan, may introduce dangerous torsion effects on the buildings, which may lead to global collapse. On the other hand, when infills are distributed irregularly in elevation, soft-storey mechanisms (collapse/failure of a building’s level) can occur. Similar consequences can be inflicted if the stiffness of the infill walls is not properly balanced both in elevation and in plan, as a result of concentration of openings, thicker walls, etc., in a smaller region of the building. Weak infill walls, in-plane damage, and out-of-plane damage can induce the same vulnerable mechanisms of soft-storey and torsion. As a direct consequence on a local level, various adverse conditions may occur, such as column and joint shear failure due to strong infill or formation of short-column mechanism. At a global level, infill walls increase the stiffness of frames, which creates higher base-shear demands and also changes completely the redistributions of stresses on the structure.