Comparative Safety Assessment of Prescriptive Means-of-Egress Configurations Using Fault Tree Analysis

Every day, millions of people move through the corridors and stairwells of apartment buildings, trusting that the rules governing these spaces will keep them safe if a fire starts. In Germany, and in many places around the world, building codes act as the rulebook for this safety. These regulations are prescriptive, meaning they list specific designs that builders must follow, such as requiring two separate paths to escape or ensuring that stairwells are protected from smoke. The underlying assumption has long been that if a building follows these rules, it is safe, and that different designs allowed by the code offer roughly the same level of protection. However, this assumption has rarely been tested with a rigorous, side-by-side comparison. While we know that fire safety depends on a complex mix of building layout, human behavior, and equipment reliability, the regulations do not explicitly measure whether one approved design is actually safer than another. This gap leaves a question unanswered: do all the legally permitted ways to escape a fire truly provide an equal margin of safety, or do some configurations hide significantly more risk than others?

To answer this, researchers at the Technical University of Munich turned to a method originally developed for aerospace and nuclear engineering called fault tree analysis. Imagine a map of every possible way a system could fail, starting with a single, worst-case outcome at the top and working backward to the small, individual failures that could cause it. In this study, the researchers applied this logic to residential fire safety. They defined a specific, critical event: the moment when a fire becomes so dangerous that the fire department must rescue people from outside their apartments before the fire is put out. This scenario focuses on the shared escape routes—the hallways and stairwells—that serve everyone in the building, rather than just the apartment where the fire started. By building a detailed model of how a fire could spread, how smoke could fill a corridor, how doors might fail, and how people might react, the team created a transparent system to compare different building designs.

The researchers did not rely on historical accident data, which is often too sparse to compare every possible building layout. Instead, they used the expert judgment of a seasoned fire safety engineer to assign likelihoods to every single step in the failure chain. They identified 93 distinct factors, ranging from the probability of a door failing to close to the chance that a person might panic and block an exit. These values were not treated as absolute facts but as consistent inputs to see how different building shapes would perform under the same set of assumptions. They then modeled 21 different residential configurations derived from the German Model Building Code. These included standard designs with internal stairwells, open-air galleries, external stairs, and specialized safety staircases designed for high-rise buildings.

The results revealed a striking reality: designs that are legally equivalent can have vastly different safety profiles. When the researchers calculated the "score" for each configuration—a number representing the likelihood of the top event occurring—they found a wide spread. The safest configuration, which featured a highly protected internal safety staircase with multiple barriers against smoke and fire, had a score of 0.274. In contrast, a standard design with a protected corridor and a single internal stairway, relying on fire service rescue as a backup, scored 0.809. While the ratio between the absolute best and worst of all 21 models was nearly three to one, the comparison among the 14 standard-code configurations commonly treated as regulatory alternatives showed a comparative score ratio of 1.70. This means that under the same expert assumptions, even within the group of standard designs considered interchangeable by regulators, some configurations carry a significantly higher implicit risk than others.

The study also identified exactly why these differences exist. Configurations that offered two distinct directions to escape, or that used open-air galleries where smoke could dissipate naturally, consistently scored lower, meaning they were safer. Conversely, designs that relied on a single direction of travel or that used enclosed corridors where smoke could accumulate were less safe. The model showed that the number of apartments connected to a single escape route mattered greatly; the more people dependent on one path, the higher the risk if that path became blocked. Furthermore, the presence of self-closing fire doors and smoke barriers played a crucial role in interrupting the chain of events that leads to a trapped occupant. The researchers found that the ranking of these designs remained stable even when they adjusted the expert estimates up or down, suggesting that the differences are structural and not just a result of specific guesses about human behavior.

This work does not claim that the current building codes are unsafe or that any specific compliant building is doomed to fail. Rather, it demonstrates that the concept of "equivalent safety" in prescriptive regulations is more complex than it appears. The study shows that prescriptively admissible configurations can contain materially different implicit safety margins, meaning that different designs allowed by the same rules do not necessarily provide the same level of protection. The researchers emphasize that their numbers are comparative scores, not absolute predictions of how often fires will happen. They are a tool for understanding relative risk, showing that a building with a simple corridor and one stairway is inherently more vulnerable than one with a dedicated, smoke-protected safety staircase. The findings suggest that regulators and designers should be aware that not all approved solutions are created equal. By using this transparent method, it becomes possible to screen proposed changes to building codes or specific architectural plans against a common standard, ensuring that the implicit safety margins of our buildings are understood and, where necessary, improved. The study concludes that while the current system works, a deeper look reveals that some paths to safety are far more secure than others, and recognizing this difference is the first step toward more informed and effective fire safety engineering.