Why Stone and Brick Buildings Are Vulnerable During Earthquakes

When an earthquake strikes, not all buildings are equally at risk. While modern structures incorporate advanced seismic designs, traditional stone and brick buildings remain highly vulnerable to earthquake damage. Understanding why these structures fail during seismic events is crucial for disaster preparedness and safer construction practices.

Common failures in stone and brick buildings during earthquakes 🔗

Stone and brick structures face distinct failure patterns when subjected to seismic forces. Houses built of unreinforced masonry are very likely to be damaged during earthquakes because the mortar holding the masonry together is generally not strong enough to resist earthquake forces.

Wall separation and bulging: One of the most dangerous failure modes occurs when thick masonry walls split into two vertical layers or wythes. In traditional stone construction, walls often have two exterior vertical layers of large stones with loose rubble and mortar filling the space between them. During an earthquake, these layers can separate and bulge outward, leading to catastrophic structural failure.

Corner and junction failures: Walls frequently separate at corners and T-junctions where perpendicular walls meet. This happens because traditional masonry construction lacks proper interconnection between adjacent walls. When seismic forces act on the building, these weak points become critical failure zones.

Roof collapse: Heavy roofs on masonry buildings pose a severe threat during earthquakes. When walls fail or separate from the roof structure, the entire roof system can collapse inward, trapping occupants beneath debris. The lack of proper anchorage between walls and roofs makes this failure mode particularly common in older construction.

Complete disintegration: In severe earthquakes, unreinforced masonry buildings can completely disintegrate. During the 1993 Killari earthquake in Maharashtra, over 8,000 people died, most buried under rubble from traditional stone masonry dwellings. Similarly, the 2001 Bhuj earthquake in Gujarat saw more than 13,800 deaths, largely attributed to the collapse of this type of construction.

Key reasons for earthquake vulnerability 🔗

Several fundamental weaknesses make stone and brick buildings particularly susceptible to earthquake damage.

Weak mortar and poor bonding 🔗

The mortar used in traditional masonry construction often lacks the strength needed to withstand seismic forces. Mud mortar, commonly used in rural construction, offers minimal structural integrity. Even when cement mortar is used, it may not be strong enough if the mix ratio is inadequate. This weak bonding between masonry units means that the seismic vulnerability is often governed by local failure modes, such as out-of-plane overturning of structural portions.

Lack of structural interconnection 🔗

Traditional masonry buildings frequently lack proper connections between critical structural elements. Walls are not adequately tied together at corners and intersections. There is often no effective anchorage between walls and floors or roofs. This absence of a “box action” means the building cannot work as a unified system to resist lateral earthquake forces. Instead, individual components act independently and fail separately.

Heavy roof systems 🔗

Many traditional masonry buildings support heavy roofs made of timber with thick mud overlay, clay tiles, or stone slabs. During an earthquake, these heavy roofs generate substantial inertial forces that the brittle masonry walls cannot adequately resist. The weight compounds the problem by increasing the stress on already weak wall-to-roof connections.

Construction material deficiencies 🔗

Stone masonry buildings often use round, unshapen stones from riverbeds rather than properly dressed stones. This creates irregular contact surfaces and reduces the overall structural integrity. Older stone and brick buildings are more susceptible to shaking damage from earthquakes because of these inherent material weaknesses combined with age-related deterioration.

Excessive wall thickness without proper bonding 🔗

Paradoxically, the thick walls often seen in traditional stone construction can be a liability. Walls ranging from 600mm to 1200mm thick may appear robust, but without through-stones or proper bonding, they can split into separate vertical layers during seismic shaking. This thick, poorly bonded construction is actually weaker than thinner, properly reinforced walls.

Recommendations for safer earthquake-resistant construction 🔗

Modern engineering has identified several key features that significantly improve the seismic performance of masonry buildings.

Incorporating horizontal bands 🔗

Horizontal bands are the most important earthquake-resistant feature in masonry buildings. These reinforced concrete or timber bands tie all walls together, similar to a belt around a cardboard box. Four types of bands should be provided: plinth band at foundation level, lintel band at window and door height, roof band below the roof structure, and gable band for sloped roofs. The lintel band is particularly critical and should be included in almost all masonry buildings.

Using proper construction materials and techniques 🔗

Quality materials make a substantial difference in earthquake performance. Cement-sand mortar in a 1:6 ratio or richer should replace mud mortar, especially in higher seismic zones. Stones must be shaped using chisels and hammers rather than using round boulders. Wall thickness should not exceed 450mm for stone masonry to prevent splitting into separate wythes.

Ensuring proper bonding and through-stones 🔗

Through-stones extending over the full thickness of the wall or pairs of overlapping bond-stones must be placed at regular intervals. These should be positioned at every 600mm along the wall height and at maximum spacing of 1.2m along the length. This crucial detail prevents the wall from separating into distinct vertical layers during seismic shaking.

Controlling building dimensions 🔗

Structural proportions matter significantly for earthquake resistance. The unsupported length of walls between cross-walls should be limited to 5 meters. For longer walls, buttresses must be provided at spacing not exceeding 4 meters. Story height should not exceed 3 meters, and stone masonry buildings should generally not be taller than two stories when built with cement mortar or one story with lime or mud mortar.

Providing proper wall-to-floor and wall-to-roof connections 🔗

Effective anchorage between walls and horizontal elements is essential. L-shaped dowels at wall corners ensure better interconnectivity and provide structural integrity. These connections allow the building to act as a unified system rather than as isolated components during seismic events.

Retrofitting existing stone and brick structures 🔗

For the millions of existing vulnerable masonry buildings, retrofitting offers a practical path to improved seismic safety.

Adding steel frames and anchoring systems 🔗

One effective retrofitting solution involves installing steel frames inside unreinforced masonry walls and bolting the walls to these frames. Stainless-steel helical anchors can reattach existing masonry facades to backup structures, stabilizing veneers of brick, stone, or concrete without requiring complete reconstruction.

Strengthening wall-to-diaphragm connections 🔗

Retrofitting can include adding blocking between rafters, installing plate connectors to anchor rafters to walls, and using bolt anchors with metal plates to connect brick walls to floor joists. These modifications create proper load paths for seismic forces.

Improving diaphragm strength 🔗

Floors and roofs act as diaphragms that distribute seismic forces to vertical load-bearing walls. Strengthening these elements by adding structural sheathing to ceilings and subfloors significantly improves overall building performance. A continuous layer of plywood or oriented strand board on floor systems creates a rigid diaphragm that ties the structure together.

Post-tensioning and composite materials 🔗

Advanced retrofitting techniques include the application of fiber-reinforced polymers or textile-reinforced mortars to masonry surfaces. These materials add tensile strength that helps walls resist out-of-plane bending. Post-tensioning systems can also be installed to compress masonry elements and improve their cyclic behavior under seismic loading.

Adding reinforced concrete bands to existing structures 🔗

Where horizontal bands were not originally included, they can be retrofitted by carefully installing reinforced concrete bands at critical levels. These bands dramatically improve the building’s ability to act as a unified box during earthquakes, preventing the catastrophic separation of walls and roof elements.

What do you think? Given the vulnerability of stone and brick buildings, what steps should communities in earthquake-prone regions prioritize-retrofitting existing structures or enforcing stricter building codes for new construction? How can we balance the preservation of traditional architecture with the imperative of seismic safety?

References

1. https://www.earthquakecountry.org/step4/urmwalls/
2. https://www.iitk.ac.in/nicee/EQTips/EQTip16.pdf
3. https://link.springer.com/article/10.1007/s10518-014-9659-0
4. https://www.usgs.gov/programs/earthquake-hazards/science/washington-dc-stone-and-brick-buildings-vulnerable-distant
5. https://www.iitk.ac.in/nicee/EQTips/EQTip14.pdf
6. https://www.civilengineeringweb.com/2020/05/aspects-of-earthquake-proof-masonry-construction.html
7. https://prosoco.com/cost-effective-seismic-masonry-retrofits-can-offset-earthquake-damage/
8. https://basc.pnnl.gov/resource-guides/seismic-and-insulation-retrofits-solid-masonry-walls