Guidelines to Improve Earthquake Resistance of Small Buildings

Earthquakes are natural phenomena characterized by unexpected and rapid shaking of the Earth’s surface, triggered by the movement of tectonic plates and rocks beneath the ground. This sudden movement releases energy that propagates as seismic waves, causing the ground to vibrate intensely in all directions. These vibrations induce inertia forces on structures, which can lead to significant property damage and loss of life. Interestingly, it’s not the earthquakes themselves that are directly responsible for casualties but the collapse of susceptible structures that fail to withstand these forces.

In regions prone to seismic activity, it becomes imperative to understand how to construct earthquake-resistant buildings. By implementing specific design principles and construction practices, especially in vulnerable zones, we can mitigate the impact of earthquakes. This article provides comprehensive guidelines to improve the earthquake resistance of small buildings, focusing on critical aspects such as site selection, structural planning, foundation design, masonry work, and more.

Guidelines to Improve Earthquake Resistance

1. Site Selection

The first step in constructing an earthquake-resistant building is choosing an appropriate site. The following locations should be avoided:

Near Unsafe Embankments: Areas adjacent to unstable slopes or embankments are prone to landslides during seismic events.
Sloping Ground with Uneven Column Heights: Building on slopes can lead to uneven stress distribution due to varying column heights, increasing the risk of collapse.
Flood-Prone Areas: Sites susceptible to flooding can suffer from soil liquefaction during earthquakes, undermining the building’s foundation.
Subsoils with Significant Discontinuities: Locations where the subsoil composition changes abruptly (e.g., transitioning from rock to soft soil) can result in differential ground movement.

Illustration: Constructing on sloping ground requires careful consideration. Uneven column heights can cause asymmetrical load distribution, making the structure more vulnerable during an earthquake.

2. Structural Planning

A well-planned building layout significantly enhances earthquake resistance.

Symmetric Layouts: Buildings with symmetric shapes, such as squares or rectangles, perform better under seismic loads. Symmetry ensures uniform distribution of mass and stiffness, reducing torsional effects.
Avoid Unsymmetric Shapes: L, E, H, and T-shaped buildings tend to have irregular stress concentrations, making them more susceptible to damage.
Proportional Dimensions: For rectangular layouts, the length should not exceed twice the width to maintain structural integrity.

3. Foundations

A strong foundation is crucial for any building, especially in seismic zones.

Minimum Width:
Single-Storey Buildings: At least 700 mm.
Multi-Storey Buildings: At least 950 mm.
Minimum Depth:
Soft Soil: Not less than 1.0 m to reach stable strata.
Rocky Ground: At least 0.45 m to ensure anchorage.
Preparation:
Remove all loose materials and standing water from the trench.
Compact the soil to achieve the desired bearing capacity.
After laying the foundation, backfill with appropriate material and compact thoroughly.

Illustration: Proper backfilling and compaction along foundation walls prevent settlement and provide lateral support to the foundation.

4. Masonry

The choice of masonry material and construction technique affects the building’s seismic performance.

4.1 Stone Masonry

Placement: Set each stone on its broadest face for stability.
Interlocking: Arrange stones so that they extend into the wall’s thickness, ensuring a strong connection between inner and outer faces.
Filling Voids: Use small stone chips with minimal mortar to fill gaps, enhancing solidity.
Stone Selection: Avoid rounded stones; angular stones provide better interlocking.
Through Stones: Incorporate stones that extend through the wall thickness at intervals of 600 to 750 mm to tie the wall together.

4.2 Brick Masonry

Quality Bricks: Use well-fired bricks with sharp edges for uniformity and strength.
Orientation: Lay bricks with their grooves facing upward to maximize mortar bond and load distribution.

4.3 Concrete Block Masonry

Surface Texture: Place blocks with rough faces on top and bottom to improve mortar adhesion.
Strength: Ensure blocks have adequate compressive strength to withstand seismic forces.
Preparation: Clean and moisten block surfaces before laying to enhance bonding.

General Recommendations:

Wall Thickness: Avoid walls thicker than 450 mm, as excessively thick walls can be rigid and brittle.
Wall Length: Limit wall lengths to 6 m to reduce vulnerability to bending and cracking.
Cross Walls: Incorporate cross walls or partition walls connected to main walls to increase overall structural stability.

5. Windows and Door Openings

Openings in walls are points of weakness during an earthquake.

Limited Openings: The total width of all openings should not exceed one-third of the wall’s length.
Alignment: Position windows at the same vertical level to maintain uniform stress distribution.
Door Placement: Avoid placing doors near wall ends; maintain a distance of at least 400 to 500 mm from intersecting walls.
Spacing Between Openings: Ensure a minimum clear width of 600 mm between adjacent openings to preserve wall integrity.

6. Roofing

The design of the roof plays a significant role in the building’s seismic performance.

Trusses vs. Rafters: For roofs with spans longer than 6 m, use trusses instead of rafters to distribute loads more effectively.
Roof Shape:
Four-Sided Sloping Roofs: These are stronger and more stable during seismic events compared to two-sided (gable) roofs.
Gable Walls: Two-sided roofs with gable walls are prone to collapse due to the weak connection between the wall and roof.

7. Chajjas (Projections)

Overhanging elements can be hazardous if not properly designed.

Projection Limit: Limit chajjas or terrace projections to 0.9 – 1 m.
Support for Larger Projections: For projections exceeding this limit, provide support using beams and columns to prevent detachment during an earthquake.

Illustration: Supporting large projections with columns and beams integrates them into the building’s structural system, enhancing safety.

8. Parapet Walls

Parapets are vulnerable during seismic activity.

Construction: Build parapet walls with bricks up to a thickness of 300 mm.
Reinforcement: Incorporate iron railings or reinforced concrete to strengthen parapets and prevent collapse.

9. Concrete and Mortar

Quality materials are essential for constructing durable, earthquake-resistant buildings.

Sand Selection: Use river sand with pebble content less than 1%. Pebbles can disrupt the bonding in mortar and concrete.
Silt Content: Remove silt completely, as it weakens the mortar and concrete mix.
Aggregate Size: Limit the size of coarse aggregates to 30 mm for better compaction and strength.
Aggregate Shape: Use angular and well-graded aggregates to improve interlocking and load distribution.
Dry Aggregates: Ensure aggregates are dry before mixing to maintain the correct water-cement ratio.

10. Bands

Reinforced concrete bands are crucial for tying the building together and enhancing its ductility.

Types of Bands:
Plinth Band: Placed at the foundation level to distribute loads.
Lintel Band: Above openings to prevent cracks due to stress concentrations.
Roof Band: At the top of walls to integrate the roof structure.
Gable Band: Along gable walls to prevent separation from the main structure.
Specifications:
Thickness: Minimum of 75 mm.
Reinforcement: At least two 8 mm diameter steel bars.
Stirrups: 6 mm diameter stirrups spaced at 150 mm centers.

Additional Bands:

Vertical Bands: May be provided at wall junctions and corners to enhance stability.
Diagonal Bands: Useful in large walls to distribute seismic forces more evenly.

11. Retrofitting

Retrofitting involves modifying existing structures to make them more resistant to seismic forces.

Roof Truss Anchoring: Secure roof trusses to walls using metal brackets to prevent separation.
Bracing: Connect purlins and bottom chord members of trusses with bracings for added stiffness.
Gable Wall Strengthening: Install sloping belts on gable walls to reinforce them.
Seismic Belts: Strengthen building corners with seismic belts to absorb and distribute seismic energy.
Floor Joist Anchoring: Use brackets to anchor floor joists to walls, enhancing floor-to-wall connectivity.
Vertical Reinforcement: Install vertical steel bars at critical points to improve the building’s capacity to handle tensile stresses.
Corner Reinforcement: Provide reinforcement at all internal and external corners to reduce tensile stress from vertical bending.
Reinforced Openings: Encase wall openings with reinforcement to prevent cracking and collapse.

Indian Standards for Earthquake-Resistant Structures

India, being a seismically active country, has established comprehensive codes and guidelines to ensure the construction of earthquake-resistant buildings. The Bureau of Indian Standards (BIS) has categorized the country into seismic zones based on the severity and frequency of earthquakes.

The following Indian Standard (IS) codes are essential references for engineers and builders:

IS 1893–2002: Criteria for Earthquake Resistant Design of Structures

Provides guidelines for designing structures to withstand seismic forces, including calculations for base shear and load distribution.

IS 4928–1993: Code of Practice for Earthquake Resistant Design and Construction of Buildings

Offers practical instructions for constructing buildings that can resist earthquake effects, focusing on materials, construction methods, and detailing.

IS 13827–1992: Improving Earthquake Resistance of Low-Strength Masonry Structures

Recommends techniques to enhance the seismic performance of buildings made with low-strength masonry, common in rural areas.

IS 13920–1997: Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces

Emphasizes the importance of ductility in reinforced concrete elements and provides detailing requirements to prevent brittle failures.

IS 13935–1993: Repair and Seismic Strengthening of Buildings

Guides the assessment, repair, and retrofitting of existing buildings to improve their earthquake resistance.

Conclusion

Improving the earthquake resistance of small buildings is a multifaceted process that involves careful consideration from site selection to construction details. By adhering to the guidelines outlined above, builders and homeowners can significantly reduce the risks associated with seismic events. Implementing proper planning, using quality materials, and following established construction practices not only safeguard property but, more importantly, save lives.

Earthquakes are inevitable natural occurrences, but the extent of their impact on human society can be controlled. It is our responsibility to build structures that are resilient and capable of withstanding these forces. Continued education, adherence to building codes, and proactive retrofitting of existing structures are essential steps toward a safer built environment in earthquake-prone regions.

Frequently Asked Questions

1. Why is symmetry important in building layouts for earthquake resistance?

Symmetric building layouts ensure that the mass and stiffness are uniformly distributed, which reduces torsional forces during an earthquake. This uniformity helps the structure respond more predictably to seismic waves, minimizing stress concentrations that could lead to failure.

2. How do reinforced concrete bands improve a building’s seismic performance?

Reinforced concrete bands tie the walls together at various levels, distributing seismic forces throughout the structure. They enhance the building’s ductility, allowing it to deform without collapsing, and help prevent cracks and separations at critical junctions.

3. What role does proper foundation design play in earthquake resistance?

A well-designed foundation ensures that the building’s weight is adequately supported and that seismic forces are transferred to the ground effectively. Proper depth and width, along with soil preparation and compaction, prevent differential settlement and foundation failure during an earthquake.

4. How can existing buildings be retrofitted for better earthquake resistance?

Retrofitting methods include anchoring roof trusses and floor joists to walls, adding vertical and diagonal reinforcements, strengthening corners with seismic belts, and reinforcing wall openings. These modifications improve the building’s ability to withstand seismic forces without undergoing major reconstruction.

5. What is soil liquefaction, and why is it a concern during earthquakes?

Soil liquefaction occurs when saturated, unconsolidated soil loses strength and stiffness in response to seismic shaking, behaving like a liquid. This phenomenon can cause significant ground deformation, leading to the sinking or tilting of buildings and structures built on such soils.

By understanding and applying these guidelines, communities can build safer structures that stand resilient against the unpredictable forces of nature. Earthquake-resistant construction is not just a technical requirement but a vital component of sustainable and responsible development.