Enhancing Earthquake Resistance in Small Buildings

Earthquakes are natural phenomena that can cause significant damage to structures, especially small buildings that may not be designed with seismic resistance in mind. Enhancing the earthquake resistance of such buildings is crucial to protect lives and property. This article explores various precautions and measures that can be implemented in site selection, building planning, and construction to improve the seismic performance of small buildings.

I. Site Selection for Small Buildings

Choosing the right site is the first step in constructing an earthquake-resistant building. The following areas should be avoided when selecting a site:

1. Near Unstable Embankments: Building close to steep slopes or embankments increases the risk of landslides during an earthquake. The ground movement can undermine the foundation, leading to structural failure.
2. On Sloping Ground with Varying Column Heights: Constructing on slopes where columns have different heights can cause uneven seismic forces distribution. This irregularity may result in torsion and twisting during an earthquake, increasing the likelihood of collapse.
3. Flood-Affected Regions: Areas prone to flooding can have weakened soil foundations. Saturated soils can amplify seismic waves, causing more intense shaking and potential liquefaction, where the ground behaves like a liquid.
4. Subsoils with Marked Discontinuities: Sites with varying soil conditions, such as part rock and part soil, can lead to differential settlement. During an earthquake, these inconsistencies can cause uneven ground movement, stressing the building’s structure.

Proper site investigation and soil testing are essential to ensure the chosen location is suitable for construction.

II. Building Planning

The architectural design of a building significantly influences its earthquake resistance.

• Symmetric Plans: Buildings with symmetric shapes (e.g., squares or rectangles) perform better during earthquakes due to uniform distribution of mass and stiffness. Symmetry helps in reducing torsional movements.
• Avoid Complex Shapes: Designs with L, E, H, or T shapes create stress concentration points and irregular seismic response. If complex shapes are necessary, seismic joints should be incorporated to separate the building into simpler geometric forms.
• Rectangular Plan Proportions: For rectangular buildings, the length should not exceed twice the width. Elongated structures are more susceptible to seismic forces along their longer axis.

Thoughtful planning can enhance the building’s ability to withstand seismic forces by promoting uniformity and simplicity in design.

III. Foundations

A strong foundation is crucial for the stability of any building.

• Width Requirements:
• Minimum of 750 mm for single-story buildings.
• Minimum of 900 mm for multi-story buildings.
• Depth Requirements:
• At least 1.0 m for soft soils to reach a stable stratum.
• At least 0.45 m for rocky grounds due to the inherent strength of rock.
• Construction Precautions:
• Site Preparation: Remove all loose material, debris, and water from the trench before laying the foundation. A clean and compacted base ensures better load distribution.
• Backfilling: After laying the foundation, backfill with suitable material and compact in layers to prevent settlement.

Proper foundation design distributes seismic forces effectively and prevents excessive movement or settlement.

IV. Masonry

The choice of masonry materials and construction techniques impacts the building’s seismic performance.

Stone Masonry

1. Placement of Stones: Lay each stone flat on its broadest face to provide stability and maximum contact area.
2. Interlocking Stones: Use length stones that extend through the wall thickness to tie the inner and outer faces together, enhancing wall integrity.
3. Filling Voids: Fill gaps with small stone chips and minimal mortar to avoid weak points that can lead to cracks.
4. Avoid Rounded Stones: Angular stones interlock better than rounded ones, which can slip during shaking.
5. Through-Stones: Install through-stones or bond stones every 600–750 mm vertically to tie the wall mass together.

Brick Masonry

1. Quality Bricks: Use well-burnt bricks with high compressive strength and low water absorption.
2. Proper Laying: Place bricks with the frog (groove) facing upwards to improve mortar adhesion and bonding between layers.

Concrete Blocks

1. Surface Texture: Use blocks with rough surfaces on top and bottom to enhance mortar grip.
2. Strength: Ensure blocks meet strength requirements and are free from defects.
3. Cleanliness: Brush off dust and debris from block surfaces before laying to improve bonding.

General Guidelines

• Wall Thickness: Avoid walls thicker than 450 mm to reduce mass and seismic forces.
• Wall Length: Limit the length of continuous walls to 6 m to prevent excessive flexing and cracking.
• Cross Walls and Partitions: Incorporate cross walls to divide larger wall spans and provide additional support, increasing overall rigidity.

Selecting appropriate masonry practices enhances the ductility and strength of walls, making them more resilient to seismic activity.

V. Doors and Window Openings

Openings in walls can weaken structural integrity if not properly designed.

1. Limit Openings: Too many openings close together can create weak zones. The total width of all openings should not exceed one-third of the wall length.
2. Uniform Window Levels: Align windows at the same vertical level to maintain consistent load paths and reduce stress concentrations.
3. Door Placement: Avoid placing doors at wall ends. Maintain at least 500 mm distance from cross walls to doors to provide sufficient wall area for structural support.
4. Spacing Between Openings: Ensure a minimum clear distance of 600 mm between adjacent openings to preserve wall strength.

Properly designed openings help maintain wall integrity and distribute seismic forces evenly.

VI. Roof

The roof system plays a critical role in holding the structure together during an earthquake.

• Sloping Roofs:
• Trusses vs. Rafters: For spans greater than 6 m, use trusses instead of rafters. Trusses provide better support and distribute loads more evenly.
• Roof Shapes:
• Four-Sided Sloping Roofs: Preferred over two-sided (gable) roofs, as they eliminate gable walls, which are prone to collapse under seismic forces.
• Roof Connections: Securely anchor the roof structure to the walls to prevent separation during shaking.

A well-designed roof system contributes to the overall stability and integrity of the building.

VII. Chejjas (Projections)

Projections such as balconies and overhangs need careful consideration.

• Projection Limits: Limit the length of projections like chejjas to 0.9 m to reduce leverage effects during an earthquake.
• Support for Larger Projections: For projections exceeding 0.9 m, incorporate beams and columns to provide necessary support and prevent failure.

Controlled projection sizes minimize the risk of collapse and reduce additional stress on the main structure.

VIII. Parapet Walls

Parapet walls are vulnerable during seismic events.

• Masonry Parapets: Tall masonry parapets can topple easily due to their slenderness and lack of support.
• Alternative Design: Construct parapets with a maximum of 300 mm high brickwork, topped with iron railings. This design reduces mass and potential falling debris.

Redesigning parapet walls enhances safety by reducing the risk of collapse onto occupants or pedestrians.

IX. Concrete and Mortar

The quality of concrete and mortar affects the building’s strength.

• Sand Selection:
• Use clean river sand, free from organic impurities.
• Sieving: Remove pebbles and larger particles to achieve a uniform grain size.
• Removing Silt: Wash or air-clean sand to eliminate fine silt, which can weaken the mortar.
• Aggregates:
• Size: Use aggregates smaller than 30 mm to ensure better compaction and bonding.
• Grading: Well-graded aggregates fill voids more effectively, resulting in denser concrete.
• Shape: Angular aggregates interlock better than rounded ones, providing higher strength.
• Mixing:
• Dry Mixing: Thoroughly mix cement and aggregates in their dry state to ensure uniform distribution.
• Water Addition: Add water gradually to achieve the desired consistency without making the mix too wet, which can reduce strength.

High-quality concrete and mortar are essential for constructing durable and resilient structures.

X. Bands

Reinforced concrete (R.C.) bands are horizontal elements that tie the building together.

Types of R.C. Bands

1. Plinth Band: Placed at the foundation level to distribute seismic forces and prevent differential settlement.
2. Lintel Band: Located above door and window openings to tie walls together and support openings.
3. Roof Band: Installed at the top of the walls to connect them and support the roof structure.
4. Gable Band: Used in buildings with gable roofs to reinforce the gable end walls.

Specifications

• Thickness: Minimum of 75 mm to provide sufficient strength.
• Reinforcement:
• Use at least two steel bars of 8 mm diameter.
• Tie the bars with steel stirrups or links of 6 mm diameter at 150 mm spacing to form a closed loop.

Additional Reinforcements

• Diagonal Bands: Placed at corners and around openings to resist shear forces.
• Vertical Bands: Installed at wall junctions and corners to enhance vertical reinforcement.

R.C. bands act as ring beams, holding the building together and distributing seismic forces uniformly.

XI. Retrofitting

Retrofitting involves strengthening existing buildings to improve their seismic performance.

Methods of Retrofitting

1. Anchoring Roof Trusses: Secure roof trusses to walls using metal brackets to prevent separation during shaking.
2. Adding Bracings: Install diagonal bracings at the level of purlins and bottom chords of trusses to stiffen the roof structure.
3. Strengthening Gable Walls: Insert sloping belts or reinforce gable walls to prevent them from toppling.
4. Seismic Belts at Corners: Apply seismic belts or straps at building corners to hold walls together.
5. Anchoring Floor Joists: Use brackets to fix floor joists to walls, enhancing the connection between floors and walls.
6. Vertical Reinforcement: Introduce vertical steel bars at corners and junctions to improve the building’s ability to resist bending and shear forces.
7. Reinforcing Wall Openings: Encase doors and windows with reinforced concrete to prevent cracking and collapse around openings.
8. Improving Storey Connections: Strengthen connections between different floors to ensure the building acts as a single unit during an earthquake.

Retrofitting is often a cost-effective way to upgrade existing buildings, enhancing safety without the need for complete reconstruction.

Conclusion

Improving the earthquake resistance of small buildings requires a comprehensive approach that includes careful site selection, thoughtful architectural design, proper construction practices, and, where necessary, retrofitting existing structures. By implementing the measures outlined in this article, builders and homeowners can significantly reduce the risk of damage during seismic events, safeguarding lives and property.

Investing in earthquake-resistant construction is not just a structural decision but a commitment to safety and resilience in the face of natural disasters.