**2. Background and literature review**

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

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

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

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.

specified in the building code.

30 New Trends in Structural Engineering

to substantial increase in construction cost.

properly and advantageously employed in lateral force resistance.

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 wall interaction with structural frame.

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

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

in Appendix B. The current code offers two sets of prescriptive design guidelines, one for masonry walls not considered participating in lateral force-resistance, and the other for walls that are expected to take part in lateral force resistance. For the latter group, masonry standards joint committee (MSJC) 2013 code requires these walls to be designed as shear walls, which necessitates use of sufficient reinforcement and detailing to satisfy in-plane and out-ofplane flexural and shear effects. According to the code, for the former group *B2.1.1: In-plane isolation joints shall be designed between the infill and the sides and top of the bounding frame., B2.1.2: In-plane isolation joints shall be specified to be at least 3/8 in. (9.5 mm) wide in the plane of the infill, and shall be sized to accommodate the design displacements of the bounding frame., B2.1.3: In-plane isolation joints shall be free of mortar to contain resilient material, provided that the compressibility of that material is considered in establishing the required size of the joint.* In practice, sometimes gaps of different sizes are provided (most likely not by design but because of construction issues) between the infill and the frame as shown in **Figure 5** in a building.

structural frame. The fuse element is placed as a masonry unit (or part of it). Depending on the fuse element material and mechanism design, it can have stiffness and damping proper-

Finite Element Modeling of Masonry Infill Walls Equipped with Structural Fuse

http://dx.doi.org/10.5772/intechopen.77307

33

Pilot tests were carried out to investigate the concept and the feasibility of using such a fuse for infill walls [50–52]. A few different materials and mechanisms were studied to develop a potentially acceptable fuse element. The concept of a disk and a punching or penetrating rod was developed. In this concept, a disk of concrete or wood will be used as the breakable fuse element as shown in **Figure 7**. The fuse element types shown in **Figure 7** perform in a rigid manner up to their punching capacity, beyond which, the interaction between the frame and the infill wall stops. In other words, when the fuse is installed between the top of the infill wall and the frame columns, the infill wall is engaged in lateral load resistance, but when the fuse breaks at the threshold design load of the fuse, the infill wall no longer offers resistance to lateral movement of the frame. Another pilot study used rigid wood disks on a one-fourth scale three-story two-bay frame as shown in **Figure 8**. The pilot study investigated a rigid fuse element that worked only under compression. **Figure 8** shows how a wood disk breaks when its ultimate capacity is reached. The size of the gap between the infill walls and the frame shown in **Figure 8** was chosen for convenience of experimental study. For real applications in buildings, the fuse-holding mechanism will be placed in the location of an edge masonry unit, and therefore, normal gap

**Figure 7.** Rigid fuse element with concrete and wood disk alternatives: (a) concrete disk; (b) wood disk; (c) engagement

of steel rod on concrete disk; (d) engagement of steel rod on wood disk [53].

ties and it could be a single rigid-brittle element or a rigid-ductile element.

## **3. Development of a fuse element for masonry infill walls**

In search of ways to find an alternative solution so that the infill wall can participate in lateral load resistance and provide additional stiffness for wind loading and low-to-moderate seismic events, but to disengage (be isolated) under major events, a fuse concept was introduced [49]. **Figure 6** shows the concept of a structural fuse placed between the infill wall and

**Figure 6.** Schematic representation of fuse elements in a masonry infill wall [53].

structural frame. The fuse element is placed as a masonry unit (or part of it). Depending on the fuse element material and mechanism design, it can have stiffness and damping properties and it could be a single rigid-brittle element or a rigid-ductile element.

Pilot tests were carried out to investigate the concept and the feasibility of using such a fuse for infill walls [50–52]. A few different materials and mechanisms were studied to develop a potentially acceptable fuse element. The concept of a disk and a punching or penetrating rod was developed. In this concept, a disk of concrete or wood will be used as the breakable fuse element as shown in **Figure 7**. The fuse element types shown in **Figure 7** perform in a rigid manner up to their punching capacity, beyond which, the interaction between the frame and the infill wall stops. In other words, when the fuse is installed between the top of the infill wall and the frame columns, the infill wall is engaged in lateral load resistance, but when the fuse breaks at the threshold design load of the fuse, the infill wall no longer offers resistance to lateral movement of the frame.

Another pilot study used rigid wood disks on a one-fourth scale three-story two-bay frame as shown in **Figure 8**. The pilot study investigated a rigid fuse element that worked only under compression. **Figure 8** shows how a wood disk breaks when its ultimate capacity is reached. The size of the gap between the infill walls and the frame shown in **Figure 8** was chosen for convenience of experimental study. For real applications in buildings, the fuse-holding mechanism will be placed in the location of an edge masonry unit, and therefore, normal gap

**Figure 7.** Rigid fuse element with concrete and wood disk alternatives: (a) concrete disk; (b) wood disk; (c) engagement of steel rod on concrete disk; (d) engagement of steel rod on wood disk [53].

**Figure 6.** Schematic representation of fuse elements in a masonry infill wall [53].

in Appendix B. The current code offers two sets of prescriptive design guidelines, one for masonry walls not considered participating in lateral force-resistance, and the other for walls that are expected to take part in lateral force resistance. For the latter group, masonry standards joint committee (MSJC) 2013 code requires these walls to be designed as shear walls, which necessitates use of sufficient reinforcement and detailing to satisfy in-plane and out-ofplane flexural and shear effects. According to the code, for the former group *B2.1.1: In-plane isolation joints shall be designed between the infill and the sides and top of the bounding frame., B2.1.2: In-plane isolation joints shall be specified to be at least 3/8 in. (9.5 mm) wide in the plane of the infill, and shall be sized to accommodate the design displacements of the bounding frame., B2.1.3: In-plane isolation joints shall be free of mortar to contain resilient material, provided that the compressibility of that material is considered in establishing the required size of the joint.* In practice, sometimes gaps of different sizes are provided (most likely not by design but because of construction issues)

In search of ways to find an alternative solution so that the infill wall can participate in lateral load resistance and provide additional stiffness for wind loading and low-to-moderate seismic events, but to disengage (be isolated) under major events, a fuse concept was introduced [49]. **Figure 6** shows the concept of a structural fuse placed between the infill wall and

between the infill and the frame as shown in **Figure 5** in a building.

32 New Trends in Structural Engineering

**3. Development of a fuse element for masonry infill walls**

finite element modeling, initially a single-bay, single-story steel frame with tightly fitted infill wall that has been studied by others was modeled. Once the single-bay, single-story model was validated using existing literature results, the model was subjected to monotonic pushover loading as well as cyclic loading under different load-control and displacement-control parameters. The presentation also includes discussion of a parametric study. Practical design approaches and guidelines for masonry infill walls equipped with the proposed structural fuse element and variation of masonry type for the fuse concept are presented in Refs. [54, 52].

Finite Element Modeling of Masonry Infill Walls Equipped with Structural Fuse

http://dx.doi.org/10.5772/intechopen.77307

35

**4. Development of the finite element model for fuse-equipped infill** 

to model the proposed structural fuse component.

In the finite element model, material nonlinearities were considered because of nonlinear moment-rotation and force-deformation responses of steel frame connections, equivalent infill wall struts, tie-down anchors, and the fuse element. Large deformation and geometrical nonlinearities existed due to movements and contact between infill wall and frame. In this study, ANSYS finite element analysis program [55] was employed. Five different finite element types from ANSYS element library were used for modeling. The uniaxial BEAM3 element with compression, tension, and bending modeling capabilities was used to model the frame members. PLAIN42 element was used to model the masonry infill wall. CONTACT12 element was employed to model the interaction between infill and frame, and COMBIN39 spring element was considered to model the diagonal strut representing the masonry infill and rotational spring representing beam-column joint. Finally, COMBIN40 element was used

To model a bare steel frame, BEAM3 element with three degrees of freedom (two translations and one rotation) at each node was used. PLAIN42 element with four nodes and two translational degrees of freedom per node was used as a plane stress element to model the infill wall. COMBIN39 element with two nodes and with up to three translational degrees of freedom per node can be used as a unidirectional element (e.g., uniaxial compression-tension element or purely rotational spring). The longitudinal option with two degrees of freedom per node was used to model the diagonal struts to represent effective infill and also tie-down rebars. The rotational option was used to represent the frame's beam-column connection. CONTACT12 element with two nodes and two translational degrees of freedom at each node was considered to model a gap between two surfaces, which can be in compression contact or at no contact and may also slide relative to each other considering Coulomb friction. This element was used to model the interaction between infill wall and frame when equipped with fuse. When there is interaction between the two surfaces, the normal stiffness and tangential (shear) stiffness may be active. A negative normal force represents contact between the two surfaces through a linear spring, while a positive normal force means lack of contact. On the other hand, when there is a negative force and the tangential force is less than the product of the normal force and friction coefficient, the two surfaces do not slide freely and are governed by the tangential spring stiffness. However, the two surfaces slide when the tangential force equals that product. COMBIN40 element is a special element to provide stiffness and damping to one side of a gap modeled in series. This two-node element with one degree of freedom per node (e.g., translational or rotational) can be

**wall**

**Figure 8.** Test results on one-fourth scale frame and infill wall with wood disk fuses: (a) one-fourth scale experimental setup; (b) steel rod tightened against wood fuse; and (c) steel rod puncturing wood fuse [53].

sizes can be used. Furthermore, the out-of-plane movement of the infill wall can be restricted using different available mechanisms as appropriate to a given design.

The process of pilot study leading to the test specimen shown in **Figure 8** consisted of initially developing load-deformation relations for isolated disk element to obtain the average capacities. Then isolated masonry walls were tested under in-plane shear loading to determine their capacities. Finally, the fuse disks were chosen such that they will break prior to masonry infill shear capacity. The detail of the experimental study is explained in Ref. [53]. In this chapter, only the computer-modeling aspect of masonry infill walls equipped with rigid-brittle structural fuse elements is presented. The objective of this chapter is to discuss development of a finite element model for the system (infill-fuse-frame) and validate it by using the results of tests on masonry infill walls (without fuse) available in the literature. In the process of developing the finite element modeling, initially a single-bay, single-story steel frame with tightly fitted infill wall that has been studied by others was modeled. Once the single-bay, single-story model was validated using existing literature results, the model was subjected to monotonic pushover loading as well as cyclic loading under different load-control and displacement-control parameters. The presentation also includes discussion of a parametric study. Practical design approaches and guidelines for masonry infill walls equipped with the proposed structural fuse element and variation of masonry type for the fuse concept are presented in Refs. [54, 52].
