Integrating Seismic Safety into Development Control: A Screening Approach to Setbacks and Inter-Building Separation in Siliguri, India

In a city, the space between buildings is often treated as a mere detail of urban planning, a leftover margin for light, air, or fire safety. But in regions where the ground can shake violently, that empty space becomes a matter of life and death. When an earthquake strikes, buildings do not just stand or fall in isolation; they interact. If structures are too close, they can slam into one another as they sway, causing severe damage to walls and columns. Even if a building does not collapse entirely, heavy pieces of its exterior—like parapets, tiles, or cladding—can break off and tumble outward. If the street in front is too narrow, this falling debris can completely block the road, trapping residents inside and preventing emergency vehicles like ambulances and fire trucks from reaching those in need. This is the core problem of seismic safety in dense cities: it is not just about how strong a single building is, but about how much room it has to fail without crushing its neighbors or sealing off the street.

Researchers in India have developed a new way to look at this problem, moving beyond complex engineering simulations to a method that relies on simple, visible facts. Focusing on Siliguri, a bustling city in the Himalayan foothills known for its rapid growth and dense, narrow plots, a team of planners and engineers set out to see if they could predict danger just by walking the streets and measuring what they saw. They examined over 1,100 buildings, recording their height, the width of the gaps between them, and the distance from the building to the road. Their goal was to create a practical screening tool that local officials could use to identify which streets are most likely to be blocked by falling rubble or which buildings are at risk of crashing into each other during a quake. The approach does not require expensive computer models or deep structural analysis; instead, it uses basic geometry to estimate how far debris might travel and whether the space between buildings is wide enough to prevent a collision.

The study began by asking a straightforward question: if a building were to partially collapse, would the falling pieces reach the street? To answer this, the researchers used a simple concept of a collapse angle. Imagine a pile of debris falling from a height; the higher the building, the further the debris will travel outward. By measuring the height of each building and the width of the setback—the space between the building and the road—they could calculate whether the current gap was large enough to catch the falling material. They found that for nearly 69 percent of the buildings surveyed, the existing space was too small. If a building were to suffer a partial collapse, the debris would spill over the sidewalk and onto the road, likely blocking traffic. This risk was not evenly distributed; it grew sharply with height. For the tallest buildings in the sample, almost every single one had a setback that was too narrow to be safe. The problem was so widespread that the researchers concluded that in many parts of the city, the current rules for how far back a building must be set from the road are simply insufficient for the heights being constructed today.

The team then turned their attention to the spaces between buildings. In many older, organically grown neighborhoods, structures are built right up against their neighbors, leaving no gap at all. The researchers wanted to know if these narrow or non-existent gaps were enough to prevent buildings from hitting each other when they sway during an earthquake. They applied a simplified check based on how much a building might move side-to-side. Their analysis revealed that about 22 percent of the buildings had inadequate separation. However, the nature of this problem was different from the debris issue. While the debris risk grew with height, the separation problem was almost entirely about whether a gap existed at all. The researchers found that the buildings flagged as unsafe were the ones with literally zero space between them. For the buildings that did have a gap, even a small one, the space was generally large enough to handle the expected movement. This meant that the danger was not a matter of fine-tuning the size of a gap, but of enforcing the basic requirement that buildings must not be built touching each other.

To make sense of these findings, the researchers built a model to identify which factors made a building the most dangerous. They combined the data on debris risk and separation risk to create a single "high-risk" category. They discovered that the strongest predictors of danger were not the type of material a building was made of, but rather its physical location and dimensions. The most critical factors were the width of the gap between buildings, the height of the building, and the level of the building's foundation relative to the road. Surprisingly, the structural style of the building mattered less than these geometric features in this specific screening context. Nearly half of the buildings classified as high-risk were flagged not because their structure was weak, but because they failed the geometric checks: they were too tall for their setbacks, or they were built too close to their neighbors. This suggests that a screening system that only looks at structural strength would miss a huge portion of the most vulnerable buildings.

The implications for city planning are direct and urgent. The current rules in Siliguri, and likely in many similar cities, treat setbacks and gaps as generic planning standards, often fixed at a single number regardless of how tall a building gets. The study shows that this approach is flawed. A setback that is safe for a two-story house becomes dangerously small for a six-story apartment block. The researchers propose a new framework where the required distance from the road increases as the building gets taller, ensuring that falling debris has a chance to land safely before reaching the street. Similarly, they emphasize that the rule against zero-gap construction must be strictly enforced, as the presence of any gap, however small, is far better than none. For existing buildings where the geometry cannot be changed, the study suggests compensatory measures, such as reinforcing the walls or installing catch systems, to mitigate the risk.

This work represents a shift in how seismic safety is viewed in dense urban environments. It moves the focus from the internal strength of a single building to the relationship between the building and the street. By using simple, observable data, the researchers have created a tool that local officials can use immediately to spot dangerous frontages and prioritize interventions. The findings confirm that in rapidly growing cities with small plots, the geometry of the built environment is just as critical as the engineering of the structures themselves. If the space between buildings is not managed correctly, even a structurally sound building can become a hazard to the community, blocking escape routes and endangering neighbors. The study does not claim to solve the problem of earthquake safety, but it provides a clear, evidence-based path for cities to start fixing the most immediate and visible risks before the ground ever begins to shake.