There are several weaknesses that may cause the failure of unreinforced masonry structure during earthquakes. These weaknesses are design and construction errors which should be considered and addressed during the design and construction of such structure.
The main weaknesses in unreinforced masonry design and construction that lead to extensive seismic damage are inadequate brick unit, poor mortar, irregularities in plane and vertical directions, weak load bearing walls, lack of vertical confining elements, weak first storey, wall openings, long; slender; and unsupported walls, insufficient shear base, heavy and stiff structure.
Causes of Earthquake Damages in Unreinforced Masonry Buildings
1. Inadequate Brick Unit
Commonly, natural stone, solid brick, solid concrete block, and brick with vertical holes are used in the construction of load bearing walls. if the strength of these brick units is less than that of designated strength, then these units would become weak points of the structure and may cause extensive failure during earthquakes.
2. Poor Mortar
Poor quality mortar used in load bearing walls can cause the disintegration of masonry units and loss of support to floors. This is because masonry walls with poor mortar would have very low tensile strength and shear strength which create weak points in the structure. Consequently, the masonry wall failure during earthquake is highly likely.
Masonry buildings in mud mortar or lime mortar are prone to severe damage due to lack of bond strength. Use of rounded stones in wythes without through-stones can further aggravate the problem. The failures of such structures are essentially due to out-of-plane flexure
3. Irregularities in Plane and Vertical direction
Overall unsymmetry in plan and elevation of building is another weakness of masonry building that cause failures during earthquakes. Load-bearing walls of masonry buildings must be arranged in plan regularly and symmetrically in respect of the two main axes.
4. Weak load-bearing Walls
Based on the aspect ratio of unreinforced masonry elements, excessive bending and shear can produce in-plane failures. Many masonry buildings had sustained very significant damage to walls, in the form of X (double-diagonal shear) cracking which is a common weakness of unreinforced masonry walls in shear. when a full X crack occurs during an earthquake, the triangular sections of the X crack become unstable and it may lead to collapse.
5. Lack of Vertical Confining Elements
Lack of vertical confining elements lead to the formation of vertical cracks and failures at the corners of an unconfined masonry building in which the wall begins to form a hinge from the swaying. Vertical confining elements should be located at the end of the load-bearing walls, at the both sides of the doors and windows opening in order to prevent such failure during earthquakes.
6. Weak First Storey
The first storey collapse is occurred in the masonry structures as a result of limited ductility capacity and poor strength of masonry unit materials.
7. Connections of Crossing Walls
In a seismic-resistant masonry building, crossing walls have to be interlocked properly to improve out-of-plane strength. The problem of unconnected intersecting walls is very common. Due to bad connection detail of the intersections, safety of the connections relied on tensile strength of the mortar used for connection.
Unconnected walls were more prone to out-of-plane failures. These types of deficiencies were not limited to external wall–partition wall intersection. At the corners of the buildings, where two external wall intersects, masonry units were not overlapped sufficiently so as to ensure an earthquake-resistant connection.
8. Wall Openings
Unsymmetry due to imbalance in the sizes and positions of openings in the walls may cause failure of unreinforced masonry walls during earthquakes. That is why wall openings should be regular and minimized to improve earthquake resistance of unreinforced masonry buildings, which have lateral load resisting mechanisms provided by walls only. Generally, the distance between the two openings, distance between an opening and a corner is determined based on applicable code requirements to address wall opening problems.
9. Improper Wall Section and Dimension
This involves long; unsupported; slender walls (Fig. 8 and Fig. 9) and improper wall section (Fig. 10) formation such as unconnected inner and outer wythes. These deficiencies of unreinforced masonry walls lead to out-of-plane failure which is an important component of complex nonlinear masonry behavior. Light roofs with insufficient in-plane stiffness are also important reasons for this type of damage.
10. Insufficient Base Shear Capacity
The use of low-quality mortar such as mud between wall bricks decrease the cracking strength and base shear capacity of the wall. As a result, wall blocks were separated from each other under very small shear forces
Heavy Roofing Heavy and stiff buildings is among the factors that results in the failure of masonry structure due to earthquake forces. This is because heavy and stiff buildings attracting large seismic inertia forces.
