# 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
#### 2.1.3 Damage Type 3: Damage Associated with Shear and Flexural Capacity of Elements  
In areas of moderate/high seismic activity, the structural design for gravity and wind loads usually results in an insufficient shear design, resulting in strength capacities lower than the forces that the element is subjected to during the earthquakes. Furthermore, the current seismic design procedures suggested by Eurocode 8 [8] are based on the ductility  
Figure 5: Damage type 2—example of post-earthquake damage in RC columns: (**a**) Bond and inadequate spacing of the bars and stirrups; (**b**) inadequate stirrup detailing and spacing; and (**c**) inadequate stirrup detailing and use of plain smooth bars.  
concept. Thus, the shear limit (brittle failure) states must be avoided in seismic-resistant structures. For that, the shear demand needs to be limited, or the shear strength capacity needs to be improved. The shear requirements are controlled through design procedures according to the capacity design approach. Since the introduction of the capacity design philosophy, many designers still did not adopt this strategy. Some countries do not have modern seismic design codes like Eurocode 8 [8]. Some countries are only demanding standard guidelines for designing and constructing typical building structures.  
Consequently, many existing structures are not correctly designed and present low shear strength capacity. Therefore, one of the most common failure modes observed over the last earthquakes is the structural elements' shear failure. The designers adopted global safety factors that reduce the seismic loadings, which affects the flexural forces (ductile behaviour) and the shear ones (fragile behaviour) equally. Subsequently, the shear strength capacity is reached before the yielding starts, so the energy dissipation is restrained.  
The following strategies need to be respected to improve the shear strength capacity, namely:  
* Definition of the proper amount of stirrups and respective detailing (according to recommendations like those of Eurocode 8 or other up-to-date codes) to improve the strut mechanism;
* Use of enough transverse reinforcement to ensure the concrete integrity and improvement of the aggregate interconnection;
* Avoid the combination of tensile plus shear loading;
* Use of concrete with better quality;
* Use of diagonal bars to prevent the shear/sliding failure in deep elements.  
The shear strength and confinement problems are prevalent in corner columns, especially if the building structure has eccentricity between the mass and stiffness centres. The corner columns should be designed with the highest confinement requirements. Figure 5(a),b shows examples of damaged columns after the L'Aquila and Chiapas earthquakes, respectively.  
Figure 6: _Cont._  
Nowadays, the seismic design relies on achieving a ductile behaviour for the structural elements (e.g., columns and beams). This ductile behaviour can be ensured if some necessary procedures are satisfied. The main goal is to prevent the fragile concrete crushing from occurring before the yielding onset in the tensile reinforcement. With this aim, the strategy can be:  
* Limitation of the axial loading or assuming an increase of the cross-section area;
* Limitation of the tensile reinforcement area. The force in the reinforcement should be balanced with the compression forces (and the external axial load);
* The element's capacity under compression should be improved by using concrete with better quality, compressive reinforcement and adequate confinement.  
Traditionally, the focus of the designers was related to the structural elements strength and rarely with their ductility. Therefore some of these principles have not been adequately understood and applied. Due to that, some fragile failures were observed in elements subjected to flexural loadings (particularly parts from old existent buildings), as shown in Figure 6c,d.