*Effects of Earthquakes on Buildings in the Ibero-Maghrebian Region DOI: http://dx.doi.org/10.5772/intechopen.94739*

*Natural Hazards - Impacts, Adjustments and Resilience*

the progressive and slow landslide.

**Figure 15.**

effects can be explained by the instability and slope of the ground, as well as a bad combination between the weight of the structure, the insufficient foundations, and

*Baelo Claudia, roman ruins (theatre): bowing of a* vomitorium *wall (left) and expulsion of voussoirs near the* 

A single shear crack only proves the existence of a one-way displacement force.

*Imzourem, X-shaped damage completely surrounding the load-bearing walls on the ground floor of a four-story* 

*residential building of vulnerability class B (grade 3). Source: P Murphy.*

However, X-shaped cracks are unmistakable signs of lateral and reverse forces that result in a very characteristic diagonal crack pattern. The best candidates are earthquakes due to the effect of S-waves and surface waves, particularly Love waves, which transmit loads in opposite directions. In Al Hoceima, this type of damage is found in any vulnerability class buildings (A, B or C). **Figure 16** shows a very striking case, where the X-shaped damage completely surrounds the load-bearing walls on the ground floor of a four-story residential building of vulnerability class B in Imzourem (grade 3). There are no shear cracks on the upper floors, which is a clear indication

parascenium *(right), which show a tensile stress in the direction of the slope. Source: J A Aparicio.*

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**Figure 16.**

of a soft-story damage, which will be discussed later. X-shaped cracks are sometimes blurred by the loss of outer leaf or toppling of the external walls (**Figure 17**).

The effects of this diagonal tension cracking are also common in infill walls of RC structures where, except in case of collapse, are clearly visible. They spread from lower to upper floors, with a more severe impact on the ground floor after receiving the loads of seismic shaking. In this case, the positive aspect is that these infill walls are not structural elements, unlike in buildings of class A or B, and must be assessed as damage of grade 3 in buildings of vulnerability class C, unless the RC frame structure has been seriously damaged. When X-shaped cracks cross external infill walls located between discontinuities or openings arranged for windows and doors, and the location and depth of these cracks coincide with a RC frame column that ends up being damaged, it can result in a very characteristic type of damage called "short column" or "captive column" (**Figure 18**). This effect is caused by the modification of the expected proportional distribution along the column body of its deformation ability under the influence of lateral loads [20], due to a partial confining of RC frames and a lesser stiffness of a free portion of the column less supported by partitioning brickworks. The consequence is a shorter column that concentrates most of the shear stress, i.e., a major part of the column ductility is lost.

#### **Figure 17.**

*Examples of X-shaped damage clearly visible in a single-story class A house (above) and blurred by loss of outer leaf in a four-story residential block (below). Source: P Murphy.*

#### **Figure 18.**

*Imzourem, short column effect in a three-story residential block of vulnerability class C (grade 4). On the right, a detail of the damage is shown. Source: P Murphy.*

#### **Figure 19.**

*Alhucemas, soft-story failures: (a) tilting of RC frames on the ground floor; (b) soft-story damage with clear X-shaped crack on the infill wall; (c) collapse of ground floor due to a previous soft-story damage. Source: P Murphy.*
