AGUA POTABLE

Structural damage, based on observation and on correlation with past experience, can be classified according to the possible inferred causes. The in-plane wall failure modes are usually related to an overall global response. Such type of failure can be distinguished from local collapse mechanism, which can occur due to intrinsic vulnerability of the material or unsuitable structural details. Structures with proper connections between the orthogonal walls as well as between the walls and the floors can exploit the in-plane resistance of walls, allowing the building to resist the seismic action as a whole, and the damage associated to this kind of response is generally related to the in-plane response of the masonry walls. Depending on the geometry and position of the walls and on the distribution of the openings, these damage tend to be located in specific portions of masonry such as masonry piers and spandrel beams.

In the presence of low-quality construction and inadequate structural details, the response of the masonry building tends to be governed by local phenomena and damage mechanisms. In poor quality masonry walls, made of two leaves of irregular stones and no transverse connection offered by the through elements, a typical damage in URM buildings is summarized in Figure 2.6.

Figure 2.6. Various types of failures observed in unreinforced masonry buildings under seismic actions.

2.2.2.1 In-plane failure mechanisms

The main structural elements that resist earthquakes in masonry buildings are the unreinforced masonry walls which are designed to resist mainly gravity loads.

Under seismic loading, the principle in-plane failure mechanisms of unreinforced masonry walls can be summarized as shown in Figure 2.7. These failure modes are as follows (ElGawady et al., 2007) [3]:

Shear failure: Walls with low aspect ratios and high axial loads tend to develop a diagonal cracking failure. Diagonal cracks developed in the wall either follow the path of the bed and head joints for relatively strong bricks and weak mortars or may go through the masonry units in case of relatively weak bricks and strong mortars, or both. Depending on the level of drift demand, the damage can be moderate and easily repairable, or severe to the extent that the buildings are usually unfit for further use. However, the consequences of such failures to the residents using the buildings are significantly less serious than in the case of the out-of-plane wall overturning.

Sliding failure: In the case of low vertical loads and/or low friction coefficient, which may be due to poor quality mortar, horizontal cracks in the bed joints can form a

sliding plane extending along the wall length. This causes the upper part of the wall to slide on the lower part of the wall.

Rocking and toe crushing failure: In the case of high moment/shear ratio or improved shear resistance the wall may be set into rocking motion or toe crushing depending on the level of the applied normal force. Numerous conventional techniques (e.g., ferrocement, shotcrete, grout injection, external reinforcement, posttensioning, center core, etc.) are available for retrofitting of existing masonry structures. Pier flexural-rocking failure cracks are produced in case of slender piers (portion of the wall between two openings). Failure initiates with large flexural cracks developing at the bottom and the top of the pier. As the displacement increases, the pier deforms as a ‘rigid body’ rotating about the compressed toe. Rocking may occur in piers having relatively higher aspect ratios (i.e., height-to-length ratio) with lower magnitudes of compressive stresses acting over piers.

Figure 2.7. In-plane failure modes of a laterally loaded URM wall: (a) two-whythe stone wall with a rubble core; (b) stones displacement due to vibrations; (c) internal lateral

pressure due to rubble fill increases.

2.2.2.2 Out-of-plane damage mechanisms

Out-of-plane wall collapse is one of the major causes of damage in masonry buildings, particularly in presence of flexible floors and roofs. As previously discussed, the overall building integrity is critical for the satisfactory seismic performance of

masonry buildings and the connections between structural components are crucial for maintaining building integrity. Integrity is absent or inadequate when the walls are not connected at their intersections and there are no ties or ring beams at the floor and roof levels. As a result, each wall vibrates on its own when subjected to earthquake ground shaking and is therefore likely to collapse. In multi-story buildings, this type of collapse usually takes place at the top floor level due to the significant earthquake accelerations there.

Depending on the intensity of earthquake ground shaking, this failure mechanism is characterized either by vertical cracks developed at the wall intersections, or by tilting and collapse of an entire wall.

When cross walls parallel to the direction of earthquake shaking are far apart, the central areas of long walls are subjected to significant out-of-plane vibrations and may collapse (Figure 2.8). The inadequacy of connections between the cross walls and long walls is one of the key factors influencing out-of-plane wall collapse. When connections are inadequate, long walls are more susceptible to the effects of out-of-plane vibrations and the chances of collapse are higher (Figure 2.9).

Furthermore, out-of-plane wall collapse is common in buildings with flexible roofs and floors, and where wall-to-roof connections are inadequate. Buildings with pitched roofs have gable walls. These are taller than other walls and tend to vibrate as freestanding cantilevers during earthquakes, unless they are tied to the roof structure. These walls are often inadequately connected to the roof.

Figure 2.8. Out-of-plane collapse mechanisms with and without ties.

Figure 2.9. Overturning mechaninsm without wall connection, with good wall connection, in presence of ties.

2.2.2.3 Lateral thrust from roofs

Out-of-plane lateral thrust at roof level due to inclined roofs, added to inertial forces, can become a significant cause of collapse of masonry structures. In addition to the roof lateral thrust, another observed reason for out-of-plane failures is the lack of connection between the walls and the supporting roof, as often observed when wooden roof trusses are just resting on the walls, thus providing no out-of-plane restraint. The supporting walls thus fail in the out-of-plane bending as the building is unable to develop ‘box action’ against the lateral vibrations induced by the earthquake.

Also, the wedge separation at top of the wall junctions due to lateral thrust from the supporting roof truss can be observed. Such types of localized failures normally occur in the masonry walls supporting roofs inclined in the both horizontal directions. Masonry gets separated in the form of wedges below the roof level due to thrust from roof purlins, added to the inertial forces. Such failure mechanisms are mostly observed in case of openings close to the corner.

Regarding the wall-to-roof connections, it was observed that such type of damage occurs as a result of the forces transmitted between the walls and roof, mostly due to out-of plane horizontal excitation of the walls but also due to vertical ground accelerations, which could be very high in localities very close to the fault

rupture. The top of the wall may slip out from underneath the roof and/or crush under the dynamic loading. Such types of failure particularly occur in the case of poor quality masonry (rubble stone) and relatively heavy and rigid roofs, not connected properly to the supporting walls. Also, in the case of light roofs, the friction under the roof bearing may not be sufficient to avoid slippage.

3 REVIEW OF EXPERIMENTAL TESTS ON MASONRY