# A Review of the Performance of Infilled RC Structures in Recent Earthquakes
## 2 Seismic Behaviour of Infilled RC Structures: Definition of Damage Type
### Representative Damage Type Observed in RC Buildings Due to Earthquakes
#### 2.1.6. Damage Type 6: Damage Associated with Short-Column Mechanism  
One of the most common failure mechanism observed in RC structures is the short-column mechanism. There are three types of situation where the short-column mechanism can be triggered out:  
* Situation 1: Existence of columns shorter than others in a moment-resistant frame system. It is well-known that the stiffness of shorter columns is inversely proportional to their length. Examples of this situation are beams for stairs support linked to the surrounding columns and, subsequently, reduce the columns' length. This situation is quite commonly observed in Nepal, as shown in Figure 9a. Some examples of short-column mechanism due to this structural configuration have resulted in the collapse of the columns;
* Situation 2: Building structures located in inclined plans, which lead to columns at the same building level with different length, as shown in Figure 9b. It is possible to observe an RC structure severely damaged after the India Earthquake in 2011 [28];
* Situation 3: The third situation in which the short-column mechanism can occur is when the column is adjacent to infill panels that do not fill the total column height. As mentioned before, the infill panels are not considered in the structural and seismic design. However, they have an essential contribution in terms of strength and stiffness, leading to different behaviour than what was initially predicted. In addition, many infill panels do not fill the total floor height in the construction, such as openings such as doors or windows, leaving a short part of the column exposed to higher shear demands. This situation is not considered in the structural design and is likely to potentiate the shear failure mechanism of those columns (Figure 10). The column failure can redistribute additional loadings to the remaining structural elements, thus increment their vulnerability and, consequently, the building structure vulnerability. Figure 9: Damage type 6—example of post-earthquake damage in RC structures: short-column mechanism (**a**) due to the presence of beam (situation 1) and (**b**) due to a sloping site (situation 2). The short-column mechanism can be minimised or prevented through a previous study that combines architectural and structural projects. In other words, the columns' and shear walls' design and/or assessment must be carried out after defining the disposition of the infill panels and the corresponding openings. With this easy strategy, it will be possible to improve the potentially vulnerable zones of the columns subjected to higher shear loadings. 1.7 Damage Type 7: Damage Associated with Structural Irregularities (in Plan and/or in Elevation: Torsion, 'Weak-Storey' and 'Soft-Storey')  
A proper structural conception is essential to ensuring a good performance against any type of loading demand. Simple-, regular- and redundance-resistant systems for horizontal loading demands tend to result in better behaviour. By contrast, complex structural systems usually result in structures in which the dimensions and detailing of structural elements show some deficiencies. Abrupt variations of stiffness, strength, mass and/or other elements' properties in one building, either in a plan or in elevation, can result in horizontal loading distributions and deformations much different from those usually occurring in regular structures. A very common structural irregularity is observed at the buildings bottom storey. The architects usually remove the infill panels for using these spaces for retail stores, garages or only due to architectural reasons. This type of construction practice often results in the deformation of these empty storeys under seismic action, triggering the soft-storey mechanism (Figure 11). Typically, this failure mechanism is characterised by the shear or bending failure of the columns and consequent collapse of the storey. In many situations, the destruction of the soft-storeys was observed while the remaining ones were intact (or just with very slight damage).