Hello everyone. In this sequence we will analyze from experience feedback, the behavior of structures encountering earthquakes. Earthquakes don't kill people, buildings do. This famous sentence recall us the necessity to design robust structures that may face earthquakes. Indeed, in case of bad design, structural elements or even structures may collapse, leading so unfortunately to people loss. In order to design a safe structure, an engineer, should know how a structure behaves and avoid dangerous design choice. The most part of the cases we will see in these sequence, are extracted from database produced during post earthquakes mission. More particularly, a large part of the pictures have been produced by AFPS, the French Association for Earthquake Engineering. What we can learn from past earthquakes is of main importance to avoid reproducing the same mistakes. Let's first consider the most dramatic case, the total collapse of the structures. These pictures are extracted from mission performed after the Kocaeli Earthquakes in Turkey in 1999. Earthquake may produce large horizontal loadings, that are not classical for simple design considering gravity loads. You see here however, earthquakes can produce gigantic horizontal forces. If your structure is not designed to carry this load, it will just collapse. We can say here the structures shown were not designed to face earthquakes. Let's see now the specific case of bridge pier. This type of elements, if not well-designed, is generally a witness of strong earthquakes. Horizontal loading, produced in panel like element as shear wall or bridge pier, large shear loading. With the development of cracking in concrete, the longitudinal rebars that carry flexural and gravity loads observe some unconfinement. Stirrups play then an important role to keep together the longitudinal rebars. For too large spacing between stirrups or in extreme case, for lack of stirrups, nothing maintains the longitudinal rebars. In that case, with compression loading, we observe a buckling of rebars. The effect of horizontal loadings due to earthquakes are also observed in structural element like masonry walls or short colunn. With the two example for earthquake in United States, Northridge and Loma Prieta, we observe classical degradation of masonry walls with cross-shaped cracks. The cracks tend to follow the weakest part of the masonry wall that are the joint and the cross shape is linked to stress states observed that is mainly shear stress state. We observe so a stair like crack pattern. With the two examples for earthquake in France, we observe degradation due to shear in short columns. This type of failure is often observed when infill masonry is used in combination with opening. The free length of the column is largely reduced leading so to an increase of the shear force developing in the column. Without enough stirrup, a failure can be observed. We see here the effect of a weak storey on the seismic response of the structure. This building design is often observed for the first floor. Indeed, floor can be used to park cars or as a shop. It leads so to a distribution of the structural elements different from the one we can have for the upper floors. In the first floor, to get some larger area, the structural elements are more spaced and the horizontal resistance may be sometimes neglected. As a consequence, the motion of the structure is mainly carried by this weak floor, whereas the rest of the structure tends to have a rigid motion. For Kobe earthquake, these failure modes have been observed for an upper storey than the first one. In this case, the storey, has been weakened by the impact of the footbridge that connected the building to an adjacent building. We have here an illustration of the previous response with a weak storey. In order to obtain an irregular stiffness in elevation, a reinforced concrete frame is considered with masonry infill from the second storey to the top and for one direction. From the modal analysis, we can see the first bending mode along the regular direction tends to have a distributed deformed shape. In contrary, for the irregular direction, the deformation is concentrated in the first storey while the upper part of the structure has a rigid motion. Another effect linked to the distribution of the resisting elements in the structure is a torsional effect. In that case, torsional effect is linked to an irregular distribution of the resisting elements in plane. The global torsion of the structure brought additional solicitation to the structural element and bring larger damage than with regular buildings. The Benchmark SMART was dedicated to the investigation of the effect of an earthquake on irregular building, like the ones we can found in nuclear power plants. A mockup of a part of an electrical building was testing on the shaking table of the CEA Saclay. For very large earthquakes, cracks developed in walls due to combined effect of lateral and torsional loadings. For the examples of Kocaeli Earthquake, structures were designed with a very stiff core and a soft structure around or with an irregular shape in play. This type of design leads to large loaging for the structural elements furthest from the stiff zone of the structure. For a stronger earthquake it may lead to a failure of these structural elements and even to the total collapse of the structure. We have here an illustration of the torsional effect due to the irregular distribution in plane of the stiffness. To explain qualitatively the phenomenon. We can define a center of mass where the equivalent seismic force apply and an elastic center associated to distribution of the stiffness. When these centers are merged, only horizontal loaning are applied to the structure. Otherwise, an additional torque developed in the structure. The motion due to torsion is also observed with mode shapes. Finally, last but not least point discussed in this sequence, the equipment and secondary elements. This part may be seen as insignificant, but it's generally an important cause of collateral damage and people loss. We can see, for instance, the failure of cladding panel fixation leads to their fall. Fortunately in the example, it is only the destruction of a car. But imagine if people were below these panels. We have already discussed the effect that has had this footbridge. Don't forget also the objects that can fall from the ceiling. You may be now a designer of safer structures for the future.
