**1. Introduction**

Masonry bearing wall systems were the predominant structural systems prior to the use of framed structures. Once steel and concrete framing systems replaced bearing wall systems, especially for multistory buildings, masonry walls were then used as infill walls (**Figure 1**) to provide the envelope and partition wall functions. Besides such functions, the role of masonry infill walls in seismic resistance of buildings has long been well recognized. In fact, because of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

participation of masonry infill walls in resisting lateral seismic loads, infill walls and/or their framing systems can sustain damage with life-safety hazard potential [1, 2].

frames could possibly lead to premature column failure as a result of short column effect or to increased levels of ductility demand in columns. Furthermore, because these tight-fit infill walls essentially behave as shear walls, their distribution in plan could increase torsional moments and create structural irregularities, if not placed symmetrically. The manner infill walls resist lateral loads is much like a compression brace, and the cyclic interaction of this effective brace with structural frame connection may lead to either the failure in the masonry and/or damage to the beam-column connection. Tight-fit construction of infill walls, whether of partial height or full height can lead to extensive damage to walls, columns, or beam-column joints. Besides the life-safety hazard that such damage will pose, in terms of financial loss, infill wall damage and subsequent repair/replacement work can seriously challenge building owners and tenants. In order to avoid damage to infill walls, columns, or joints, the use of gaps between the infill wall and the frame is one alternative as shown in **Figures 3** and **4**. Providing gaps between the infill wall and the confining frame is a building code requirement (e.g., [7]) if the infill wall is not designed as part of the lateral force-resisting system, that is, if it is a nonparticipating infill. On the other hand, if the in-plane isolation joints are not large enough to satisfy the conditions

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**Figure 3.** Example of partition infill wall isolated from the frame with small gaps: (a) beam-column joint area and (b) gap

**Figure 4.** Use of large gaps between infill backup wall and frame in a Seattle building: (a) view of several stories of the

building and (b) close-up view of gaps between column and infill walls [Photo by Ali M. Memari].

between column and infill wall [Photo by Ali M. Memari].

Traditionally, masonry infill walls are specified by architects as exterior envelope walls, backup walls for veneer systems, or interior partition walls. Such construction does not carry gravity load from floors and only carries its own weight. Depending on the details of joints between the edges of infill walls and infilled frames, the interaction between the infill wall and frame can adversely affect the seismic behavior of the structure (e.g., [3, 4]). In most cases, the small gaps between the infill wall and the structural frame are infilled with caulking and in some cases with mortar [5]. This tight-fit construction (**Figure 2**) engages the infill wall in in-plane lateral load resistance [6]. Depending on whether the wall is solid (complete infill) or the existence of large openings that make the infill wall partial infill, the wall's interaction with confining

**Figure 1.** Examples of masonry infill wall in a reinforced concrete frame building [Photo by Ali M. Memari].

**Figure 2.** Example of a tight-fit masonry infill wall construction [Photo by Ali M. Memari].

frames could possibly lead to premature column failure as a result of short column effect or to increased levels of ductility demand in columns. Furthermore, because these tight-fit infill walls essentially behave as shear walls, their distribution in plan could increase torsional moments and create structural irregularities, if not placed symmetrically. The manner infill walls resist lateral loads is much like a compression brace, and the cyclic interaction of this effective brace with structural frame connection may lead to either the failure in the masonry and/or damage to the beam-column connection. Tight-fit construction of infill walls, whether of partial height or full height can lead to extensive damage to walls, columns, or beam-column joints. Besides the life-safety hazard that such damage will pose, in terms of financial loss, infill wall damage and subsequent repair/replacement work can seriously challenge building owners and tenants.

participation of masonry infill walls in resisting lateral seismic loads, infill walls and/or their

Traditionally, masonry infill walls are specified by architects as exterior envelope walls, backup walls for veneer systems, or interior partition walls. Such construction does not carry gravity load from floors and only carries its own weight. Depending on the details of joints between the edges of infill walls and infilled frames, the interaction between the infill wall and frame can adversely affect the seismic behavior of the structure (e.g., [3, 4]). In most cases, the small gaps between the infill wall and the structural frame are infilled with caulking and in some cases with mortar [5]. This tight-fit construction (**Figure 2**) engages the infill wall in in-plane lateral load resistance [6]. Depending on whether the wall is solid (complete infill) or the existence of large openings that make the infill wall partial infill, the wall's interaction with confining

**Figure 1.** Examples of masonry infill wall in a reinforced concrete frame building [Photo by Ali M. Memari].

**Figure 2.** Example of a tight-fit masonry infill wall construction [Photo by Ali M. Memari].

framing systems can sustain damage with life-safety hazard potential [1, 2].

28 New Trends in Structural Engineering

In order to avoid damage to infill walls, columns, or joints, the use of gaps between the infill wall and the frame is one alternative as shown in **Figures 3** and **4**. Providing gaps between the infill wall and the confining frame is a building code requirement (e.g., [7]) if the infill wall is not designed as part of the lateral force-resisting system, that is, if it is a nonparticipating infill. On the other hand, if the in-plane isolation joints are not large enough to satisfy the conditions

**Figure 3.** Example of partition infill wall isolated from the frame with small gaps: (a) beam-column joint area and (b) gap between column and infill wall [Photo by Ali M. Memari].

**Figure 4.** Use of large gaps between infill backup wall and frame in a Seattle building: (a) view of several stories of the building and (b) close-up view of gaps between column and infill walls [Photo by Ali M. Memari].

for nonparticipating infills, then the infill wall is considered as part of the primary lateral forceresisting system, that is, participating infill, and it must be designed as a shear wall, which complicates design and construction and is not typically desirable by designers. For isolation of infill walls, small gaps (e.g., 9.5 mm–12.7 mm) are usually provided, which are then filled with caulking or other deformable fillers. **Figure 3** shows an example of an infill wall construction with isolated joints from the frame with small gaps. In more seismically active areas, larger gaps are usually provided, as shown in the example of **Figure 4**, which shows a large gap between concrete masonry unit (CMU) infill wall and reinforced concrete frame. This particular infill wall was, however, intended to function as the backup wall for brick veneer exterior skin. In general, when such a gap is to be provided, the gap size should exceed the expected interstory drift, which is determined either by structural analysis, or as the maximum allowable value specified in the building code.

In this chapter, preliminary studies on this concept are reviewed. Initially, the concept of the masonry infill structural fuse is explained. This is followed by discussion of experimental tests on different types of fuse elements. Next, the pilot experimental studies employing a one-fourth scale frame and infill walls with fuse are reviewed. The results of computermodeling studies are then presented followed by recommendations for additional follow-up

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There has been over 60 years of research on infill walls. The following references are mentioned as chronological representative examples of the experimental and analytical studies done over the past six decades: [2, 5, 9–48]. Even after such extensive international efforts, there is still room for enhanced understanding and design considerations of masonry infill

The consensus among researchers is that it is wise to use the beneficial properties of the infills in design if their strength and stiffness characteristics can be relied on. The masonry design code, ACI 530–13/ASCE 5–13/TMS 402–13 [7], provides design guidelines for infill walls

studies that need to be undertaken.

wall interaction with structural frame.

**Figure 5.** Use of different size gaps in a building [Photo by Ali M. Memari].

**2. Background and literature review**

Providing large gaps for partition wall applications will cause its own challenging issues with respect to fire safety and sound transmission issues for which the architect and designers should recommend appropriate solutions. Providing small gaps in general will not have the scale of the problems of large gaps. However, under moderate-to-strong earthquakes, the gap openings will likely approach the upper limit for story drift ratios of building codes such as ASCE 7–16 [8]. For instance, for a Risk Category II building with allowable story drift ratio of 2%, the upper limit will be nearly 75 mm for a 3750 mm story height. In that case, once the gap opening is overcome by the frame story drift, the columns will then bear against the infill wall, and under cyclic-type oscillations, the infill wall and/or frame members can sustain damage. It is the responsibility of the designer to assure the sufficiency of the gap size, and if it is desirable to keep the gap size small, the designer will have to increase the size or number of frame members designated as part of the lateral-force-resisting system, which could translate to substantial increase in construction cost.

By isolating the infill wall from the frame and avoiding their interaction in buildings with moment-resisting frames as their primary lateral force-resisting system, damages to infilled frames, failure of infill walls, and potential life-safety hazards can be avoided. However, in that case, the building is deprived of the potential benefit from the strength and stiffness that masonry infill walls can offer even if they are not designed as shear walls. It should be noted that even unreinforced masonry walls inherently possess considerable stiffness that can be properly and advantageously employed in lateral force resistance.

One shortcoming of this isolation option is that the beneficial effects of the masonry infill in stiffening and strengthening the structural frame system will not be employed. In general, since the masonry infill walls are heavy and greatly increase the effective seismic weight of the building, it would be logical to engage them also in lateral load resistance. However, it is the potential damage to these brittle components that designers wish to avoid. The compromise solution seems to be a controlled engagement of the masonry infill walls by employing a structural fuse concept. Such an idea is based on desirability of employing beneficial effects of strength and stiffness of infill walls to reduce story drifts during seismic events up to certain controlled levels. Under strong shaking, when the interaction force between the infill wall and the frame exceeds a certain level, it is desirable to isolate the infill wall from the frame in order to avoid damage to the wall or the frame. This function is provided by using a structural fuse.

In this chapter, preliminary studies on this concept are reviewed. Initially, the concept of the masonry infill structural fuse is explained. This is followed by discussion of experimental tests on different types of fuse elements. Next, the pilot experimental studies employing a one-fourth scale frame and infill walls with fuse are reviewed. The results of computermodeling studies are then presented followed by recommendations for additional follow-up studies that need to be undertaken.
