Masonry structures are most vulnerable during earthquake. Performance and behavior of masonry structures during earthquakes is discussed in this article.
Many human fatalities have depended on masonry constructions from the past. The condition is same at the present. As the main problem concerned is earthquakes, it is important to improve the seismic behavior of masonry buildings.
The most common materials used for the construction of masonry buildings are brick and hollow concrete block. The types of materials used for construction of masonry buildings are:
- Brick: It is a clay that is fired to a hard consistency.
- Hollow concrete block: Known as “cinder block.”
- Hollow clay tile: Concrete block shaped with hollow cells, but brick-color.
- Stone: Used in its natural shape, “dressed” or cut into rectangular blocks
- Adobe: Formed by pouring mud into the form of walls or made of sun-dried bricks.
Influence of Material Properties on Behavior of Masonry Structures during Earthquakes
The behavior of masonry structures during earthquake depend on the properties of its materials like mortar and masonry units. The properties of these materials vary due to variation in raw materials and construction methods, which in turn depends on the source of the resources.
Burnt clay bricks are most commonly used for construction of masonry building. These are naturally porous and they absorb water. Excessive porosity is harmful to good masonry behavior because bricks absorb water from the adjoining mortar. This results in a poor bonding between brick and mortar causing difficulty in positioning masonry units.
To avoid this problem, bricks with low porosity are to be used, and they must be soaked in water before. This would minimize the amount of water drawn away from the mortar.
Various mortars are used in building construction, e.g., mud, cement-sand, or cement-sand-lime. Among these mud, mortar is the weakest. Mud mortar crushes easily when dry, flows outward and have very low earthquake resistance.
Cement-sand mortar with lime is the most suitable. This mortar mix provides excellent workability for laying bricks, stretches without crumbling at low earthquake shaking, and bonds well with bricks.
The earthquake resistance of masonry walls depends on the relative strengths of brick and mortar. Bricks must be stronger than mortar. Excessive thickness of mortar is not desirable.
Behavior of Masonry Structures during Earthquakes
The ground motion or ground vibrations due to earthquakes results in higher amount of inertia forces at the floor or at the location of the mass of the whole building. A building will remain safe, if the forces emerged finds a path to transfer into the ground, without any obstruction which in turn minimizes the damage or collapse.
Among the elements that involve in transferring these forces i.e. roof, wall and foundation, it is seen that walls are the one found most vulnerable to the damage (by the horizontal forces emerged due to the earthquake forces).
We will assume two possibilities in direction of horizontal forces acting on a masonry wall. Let the initial condition be the force which is acting horizontally at the top, which is in a direction perpendicular to its plane, as shown in figure.1. below
Fig.1: The wall is Pushed perpendicular to the plane of the wall
This direction is considered as ‘weak’, as the wall undergo toppling or a form of overturning.
The second possibility is that the wall being pushed in the same plane, and the result is shown in figure.2. This is considered as the strong direction because it offers greater resistance when pushed along its length.
Fig.2: The wall is Pulled in the plane of the wall
It is not always the case that only a single possibility can occur. The ground can shake simultaneously in horizontal as well as vertical directions. Hence both the possibilities have a chance to occur.
Horizontal inertia forces evolved because of ground motion are the most damaging response of normal masonry buildings. The transfer of the forces can take place from the roofs then to the walls. This transfer of horizontal forces can take place either in weak or stronger direction.
Fig.3: Walls A is considered to be loaded in the strong direction and the Walls B (loaded in weak direction). The wall B undergoes toppling.
Remedy to Prevent Damage of Masonry Structures during Earthquakes
When a measure to tie up the walls together like a box is not undertaken, there arise chances of the toppling of walls that are loaded in the weaker direction. So, the remedy for this problem is to join the walls together which will ensure good seismic performance.
This procedure would help the walls loaded in a weaker direction to seek the lateral resistance that is offered by the walls that are loaded in the stronger direction. As each form separate built elements, a rigidity in totality must be bought to ensure resistance as a single unit. So, to enable this, walls to have to be connected to the roof and the foundation.
Fig.4: Wall B properly connected to Wall A
Wall units made up of masonry behave like slender units because of their small thickness compared to their height and length. The simplest way of making these masonry wall units to behave appreciably during earthquake motion is by letting them act together like a box as mentioned before, along with the roof at the top and with the foundation at the bottom.
Box Action of Masonry Building to Prevent Earthquake Damage
The formation of box action as put forward requires several construction aspects. This can be ensured by undergoing following features of ensuring good connections between the walls. This can be achieved by:
- Ensuring good interlocking of the masonry courses at the junctions
- Employing horizontal bands at various levels, particularly at the lintel level.
- Smaller the openings, larger is the resistance offered by the wall.
The tendency of a wall to topple when pushed in the weak direction can be reduced by limiting its length-to-thickness and height-thickness ratios as shown in fig.5. Hence it is recommended to keep the sizes of door and window openings small.
Fig.5: Vulnerability of slender Masonry Walls
Design codes specify limits for these ratios. A wall that is too tall or too long in comparison to its thickness, is particularly vulnerable to shaking in its weak direction.