Masonry construction is inherently seismic resistant and has been used for centuries to build structures of all sizes and types. However, in recent years, the emphasis on seismic design has increased significantly due to the advances in the earthquake analysis and design techniques. Masonry structures have several inherent features that make them able to resist seismic loads. However, this comes into play if and only if some good principles are adopted in the scheme.
The successful design of masonry structure, which is seismic resistant, does not depend on the exactness of calculation of seismic forces. The layout or the configuration of the masonry building or the load bearing building can play a significant role during an earthquake.
Provision of ties and following essential good practices and reducing brittle failure is the success for achieving seismic resistance for such masonry structures.
Even in large framed buildings, masonry wall design is many times neglected. In Such a scenario, many won't be even willing to adopt shear walls and ductile detailing practices in smaller buildings. However, all these seismic demands can be reduced by following carefully planned schemes. Let us discuss key points that will help in seismic resistance of masonry buildings.
Ensure Box Action
Brick masonry is bulky and has a large mass. It attracts lateral forces during ground movement and cracks up. It is subjected to both tensile and compressive forces.
A masonry wall is weaker in its thickness direction. It is slender as the height is large and thickness is less. The best way to make the masonry building perform well is by integrating the entire building as a box which acts together thereby reducing local effects due to inertia induced during ground movement.
The below mentioned points are going to ensure we can achieve a box action in a Masonry building effectively.
Regular Configuration
Regular configuration is important for any building and it is more significant for masonry buildings that lack a frame action to resist lateral movement. You can read about how a regular configuration helps in this blog here
One needs to avoid irregularities like offsets in plan, large openings etc.
Also, the loads in the building shall be uniformly planned and arranged rather than having load concentration at some points.
Avoid Cantilevers
Avoiding cantilevers like projecting balconies are necessary to ensure better seismic resistance with less costs spent in design and construction of masonry structures. All stakeholders including Architects and clients should plan the scheme well to ensure there are no cantilevers in the building.
Horizontal Bands
To ensure a box action as mentioned above, horizontal bands shall be used. Generally, you will need a continuous band at plinth level, lintel level and at roof level for a normal flat pitch masonry structure with concrete slab. The roof level band will not be needed as the slab as such will provide the necessary box action. The rebars in the plinth level beams shall be properly connected to ensure necessary box action and integrity.
Ensure sufficient Cross walls
A wall is weaker in its thickness direction. So, when the ground moves in the direction of width of the wall, the walls will undergo undesirable bending and is susceptible to failure.
Cross walls ensure that the building is stable. The cross walls act as a tie restraining the weaker direction of the masonry wall. If it’s an open design, you may have to introduce frames locally to reduce any undesirable actions in the masonry structure.
L shaped Dowels
L Shaped dowels at the corners where 2 walls join will ensure better inter connectivity between the walls and thus provide integrity to the masonry building. This is very important for earthquake resistance of the building. The peripheral walls are major and these walls need this strapping carefully. This helps in achieving the desired box action for the entire building. The complete building act as a cantilever vertically, not letting individual components or portions act independently.
Avoid Large wall openings
Openings weaken walls, especially large window openings. It will lose its integrity and lateral resistance. It is important to provide ties above the openings throughout the building. It would be like a horizontal band on top of the opening at the lintel level. Also, we have to avoid wall openings close to wall corners because it weakens the cross connections of the walls.
Avoid rigidity
Any rigidity in the structure will create undesirable forces. An example is a RCC staircase in a masonry building. The inclinedly connected stair acts as a bracing between the floors and transfers huge lateral forces at upper and lower connections of the stair. This will create a lot of lateral thrust in the masonry walls and may result in failure of the building.
It will be better to separate it from the main structure which will ensure the structure is safe during seismic activity. However, the gap between 2 structures shall be carefully planned so that the 2 structures don’t pound during earthquake.
Vertical Ties
Though we do all we can do to ensure a box action, the presence of openings like doors and windows create issues. The masonry in the structure behave in 3 different groups. The masonry below sill, from foundation or from plinth to sill has no openings and is group 1. The masonry from sill to lintel will have openings and will behave as a group and the masonry on top of the lintel will act as another group as there are no openings there. These 3 bands of masonry get disjointed due to rocking when the ground moves and it will move back and forth to create diagonal cracking or crushing depending up on if the walls are massive or not.
In masonry buildings that are not reinforced vertically, the cross-section area of the wall masonry is less at the opening. During seismic shaking, the building will slide under the roof slab, under the lintel beams or at the sill level. The building may also slide at the plinth beam location. The exact sliding location depends on various reasons like weight, inertial force, opening area etc.
This can be controlled by providing vertical reinforcement in the edges of the wall piers/ around openings. The bars shall be anchored in to the foundation and also at the roof. If we do this, the rocking won’t happen and there will be bending instead. In wider walls also the rebars gives additional resistance to the horizontal forces.
Quality of Building materials
Other than the detailing measures, materials used for the masonry construction decides its performance.
Strong mortar for jointing, high strength masonry like engineering bricks will contribute to the lateral strength of a masonry structure. In India, at many places, burnt clay bricks are used for building construction. These clay bricks will absorb water as they are porous. The water will be absorbed from the mortar and this results in poor bond between the bricks and mortar.
Remedy is to soak the brick before construction to reduce water absorption or to use bricks with less porosity.
Also using mud mortar should be avoided as a dry mud mortar crushes easily during earthquakes. Cement-sand mortar with lime is seen to be the best as it allows workability during construction which is good for the workmanship. Also, the bond between brick and mortar is seen to be better. The mortar also stretches without as much crumbling as mud mortar.
Relative strength of brick and mortar is very important for a better seismic performance. The bricks must be stronger than mortar. So unnecessary thick mortar will do more harm than good. There are IS code specifications for these.
Summary
Codes do not specify any Earthquake Resistant Design of Masonry Structures. However, adopting good proven engineering practices is going to help keep masonry structures safe against earthquakes.
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