## 2 The Unreinforced Masonry Infill (UMI)--RC Frame Interaction  
The in-plane UMI-RC frame interaction has been widely studied in the past by many researchers through analytical treatments [11; 12; 13; 14; 15; 16], focused experiments [17; 18; 19; 20; 21; 22; 23], and/or numerical modeling [24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. The in-plane seismic response of UMI frames problem has also been investigated by tests employing scaled models [37; 38; 39]. The influence of UMI is studied by experimentally comparing the response of identical RC frame samples with or without such infills [17; 18; 19; 20; 21; 22; 23]. These comparisons document the influence of the inclusion of UMI on the horizontal stiffness, strength, and various nonlinear mechanisms (damage forms). Observed damage during testing develops at either the UMI or/and at the surrounding RC frame members replicating similar damage patterns in prototype struc  
Figure 4: Damaged short columns from the interaction of unreinforced masonry infills (UMI), built within the bay of an RC frame structure, with the adjacent RC columns. tures. A common feature of most of these studies is the in-plane nature of the applied horizontal loads. Thus, the measured stiffness, strength, and damage forms are related mainly to the in-plane response. The usual UMI damage is either the horizontal shear sliding or the diagonal tension/compression within the masonry volume. In addition, compression crushing at the corners of the infill near the RC column to beam joints of the surrounding frame also appears, which can be accompanied by the crushing of the RC column to beam joints themselves. The prevailing form of damage depends on the diagonal or compressive strength of the masonry infill and the cross-section detailing of the RC members. These forms of damage are documented from in situ and laboratory observations. Initially, [PERSON] [13, 15] proposed an ingenious analytical approach based on the diagonal strut analogy in order to approximate the influence of UMI within an RC frame. Following this basis, many researchers proposed various approximations that numerically simulate with some success these basic behavioural characteristics. Most of these numerical simulations make use of the finite element method, discretizing both the infill and the frame, employing a combination of nonlinear mechanisms for the masonry and the RC structural members [24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36]. A number of researchers base the numerical simulation effort on focused experiments, thus increasing the realism of such numerical predictions [40, 41, 42, 43, 44, 45, 46]. The degree of realism depends on how well the numerical predictions simulate the changes of stiffness and strength caused by the inclusion of UMI, as well as on the realism in replicating the forms of damage to the UMI and/or the RC frame. Very few of these numerical approximations recognize the important role played by the interface between the masonry infill and the surrounding frame. As demonstrated [23, 44], when the in-plane horizontal load is applied, it results in deformations of both the RC members and the masonry infill that lead to a partial separation of the masonry from the RC frame shown in an amplified form in Figure 5a,b. The shape and distribution of this separation depends on the deformation and strength properties of the RC members, of the UMI, and of the peripheral mortar joint that lies between the UMI and the surrounding frame. The state of stress and the subsequent modes of failure of the masonry and/or of the RC members depend on these separation regions between the RC members and the infill. Therefore, the realism of the numerical predictions obviously depends on the realism to successfully simulate the nonlinear stress-deformation mechanism of all the participating media, e.g., the RC members the masonry infill (UMI) and the peripheral mortar joint at the interface.