This article explores why the disconnect exists. It examines how fire affects structural elements, why code-based design falls short, and what must change to close the gap between theoretical safety and real resilience.
Structural fire safety should be a pillar of building design. However, it is often the most neglected. Fires remain one of the most devastating threats to buildings, yet most structures are not designed to survive them. Codes provide guidance, but that guidance rarely reflects the complexity of real-world fire behavior.
Design teams focus intensely on vertical and lateral loads. They spend time analyzing wind pressures, seismic motions, and service loads. Fire design, on the other hand, is handled with less attention. It is commonly reduced to time-based resistance ratings that fail to account for real thermal effects or system interactions. Many engineers believe compliance equals safety. That belief has been proven false, again and again.
The current practice in structural fire safety does not match the reality of fire behavior. This article explores why the disconnect exists. It examines how fire affects structural elements, why code-based design falls short, and what must change to close the gap between theoretical safety and real resilience.
Why Fires Are Different from Other Design Scenarios
Fires are unique because they affect both the material and geometry of a structure. As heat builds up, the strength of most materials begins to degrade. Steel softens rapidly and begins to sag. Concrete starts to spall, breaking away in chunks. Timber chars and loses cross-sectional area. These effects do not happen all at once. They progress unevenly and interact in ways that are difficult to predict using static assumptions.
Unlike live or dead loads, fire introduces an evolving set of conditions. Temperatures rise and fall. Flames move through compartments. Hot gases collect under ceilings and spread heat far from the ignition point. Each of these variables changes the performance of structural members. Most importantly, fires cause redistribution of forces. A heated beam might lose strength and pass its load to adjacent members. If those are not fire-protected, they may fail too. What begins as a localized event can grow into global instability.
Real fires do not behave like furnace tests. The assumptions used in time-temperature testing do not capture the complexity of real buildings. Most fire tests apply uniform heating to an isolated member. They do not consider restraint from surrounding elements, asymmetric heating, or the compounding effects of material degradation and thermal expansion. These are the conditions that cause failure. Yet most designs never consider them.
Code Compliance Does Not Mean Real Fire Safety
Building codes provide fire resistance requirements. These requirements typically use time ratings: 30, 60, or 120 minutes of fire endurance. Engineers assign these ratings to structural members and assume they are safe. But the process used to arrive at these numbers is flawed. Fire resistance tests are standardized, but real fires are not.
The code assumes that a fire will occur in a single compartment, last for a defined period, and not exceed certain temperature curves. This assumption works on paper but not in the field. Fires can breach compartments. They can burn hotter and longer than expected. They can be influenced by fuel loads that far exceed design assumptions. Worse still, they can affect structural details that were never designed to handle thermal stress.
Most designs assume that fire protection—like cladding or intumescent paint—will remain in place. But fireproofing can fail. It can be damaged during construction or renovation. It can degrade over time. Also, it can be improperly installed. Once compromised, the protective layer becomes useless. If the underlying structure was not designed to handle heat, collapse becomes a real possibility.
Designs that follow code requirements are often brittle. They work well under assumptions but fail in uncertain conditions. Fire design must become more robust. It must consider performance, not just compliance.
The Role of Material Behavior in Structural Fire Safety
Every structural material responds differently to fire. Engineers must understand these responses in detail.
Steel loses strength quickly under heat. Its modulus of elasticity drops as temperature increases. By 500°C, steel retains only half of its room-temperature strength. At 600°C, it begins to sag significantly. Beams start to deflect, connections begin to distort, and columns buckle under load. These effects are not theoretical. They happen in real buildings, often before the design-rated time is reached.
Concrete performs better initially. It does not combust, and it has good thermal mass. But it is not immune to fire. Moisture inside the concrete can vaporize and cause spalling. When this occurs, the concrete breaks apart, exposing reinforcement to direct heat. Once exposed, the steel within the concrete behaves like any other steel—losing strength and stiffness. Reinforcement bars also expand faster than the surrounding concrete, causing internal stress and cracking.
Timber, especially engineered timber like glulam, burns in a more predictable way. It develops a char layer that slows further combustion. However, it loses structural section in the process. If connections are not well protected, fire can weaken them before the timber fails. The failure may be sudden and complete.
Composite systems are particularly vulnerable. They rely on the bond between materials—like steel and concrete slabs. Under fire, these bonds can break down. Differential expansion can introduce shear and separation forces that weaken the entire assembly. Yet most designs do not check composite behavior under thermal loading.
Understanding these behaviors is essential. It is not enough to rely on generic fire ratings. Engineers must consider how each material reacts, how the structural system interacts, and how failure might propagate under fire.
How Fire Causes Collapse
Many structural failures in fire begin at the connections. Members expand when heated. If restrained, this expansion causes thermal axial forces. Connections must absorb these forces without tearing, slipping, or cracking. In most conventional design, they are not detailed for this. As a result, connections become the weak link.
When one beam fails, its load shifts to others. If they are not protected or are already weakened, the problem spreads. This process is known as progressive collapse. Fire is a common trigger for this sequence. A localized ignition point can destabilize the entire system.
Failure can also begin from above. Roof structures often collapse first, especially if they include lightweight steel trusses. Once the roof fails, fire spreads quickly and attacks the columns. If the building lacks redundancy in its load paths, the collapse becomes inevitable.
Another cause of fire-related collapse is excessive deflection. Heated members bend under their own weight. Floors sag, walls crack, and structural alignment is lost. This can result in complete system instability. Floors that deflect too much can break away from walls. This separation causes other members to lose support and collapse.
Fire also attacks stability systems. Bracing members are usually slender and sensitive to heat. Once lost, the structure may buckle even if vertical strength remains. Most engineers do not model this behavior during design.
Why Engineers Must Think Beyond Fire Ratings
The industry must move away from a time-rating mindset. Time ratings give a false sense of security. They assume uniform conditions and predictable behavior. Real fires are messy, chaotic, and unforgiving.
Performance-based fire design provides a better path. This approach models how a structure will behave during fire. It includes thermal loading, material degradation, and redistribution of internal forces. It requires advanced tools, like thermal-structural finite element analysis. While more complex, it results in designs that are better suited to reality.
Performance-based design also helps with robustness. A fire-resistant design is not one that survives alone, but one that prevents disproportionate failure. If one member fails, the rest should adjust. This approach is essential in buildings with large open spaces or complex load paths. It is the same thinking used in progressive collapse prevention. Fire must be included in these checks.
Post-fire behavior also matters. Some buildings may not collapse but are left structurally unsafe. Engineers must assess not only survival during fire, but structural condition after fire. Will the building need full replacement? Or can it be safely reused? These questions cannot be answered without proper fire response analysis.
Integrating Fire Safety into Structural Design
Fire design must be part of early planning. It cannot wait until final documentation. Structural systems, materials, and details must be selected with fire in mind. Architects, engineers, and fire consultants must work as a team. Each must understand how design decisions affect fire behavior.
Engineers must detail members with clear fire-resilient paths. They must protect critical load-bearing elements. They must also ensure that fire protection systems—sprays, wraps, boards—are inspectable and maintainable.
Ongoing inspection is vital. Fire protection degrades over time. Buildings change use. Maintenance workers damage coatings. These changes can render a fire-resistant design useless. A column may look intact but have lost its rating due to damaged insulation. Without regular checks, this becomes a silent risk.
Education is also important. Many engineers receive limited training in fire behavior. They must learn how materials respond, how systems interact, and how to model thermal effects. Fire safety is a structural issue, not just a life safety concern.
Conclusion
Structural fire safety remains the weakest link in modern design. Codes offer minimum requirements, but they often ignore the complexity of real fires. Time ratings cannot predict how a system will behave under heat, stress, and redistribution. Connections fail. Members deflect. Load paths collapse. These are not hypothetical problems—they are real risks. Engineers must treat fire as a design action. They must model it, plan for it, and detail for it. They must think beyond compliance and work toward resilience. Performance-based design offers a better way forward. It requires effort, but the reward is greater safety and longer-lasting structures.
Also See: Fire Safety in Modern Timber Buildings
Sources & Citations
- Structural Behaviour in Fire: A Summary of Experimental Research, Law, A., Stern-Gottfried, J., et al., 2010 Fire Safety Journal, Vol. 45, No. 1, pp. 3–13 DOI: 10.1016/j.firesaf.2009.09.003
- Why Buildings Collapse in Fire – Lessons from Structural Failures, Usmani, A. S., 2005 Fire Safety Science, Proceedings of the 8th International Symposium, pp. 1113–1124 DOI: 10.3801/IAFSS.FSS.8-1113
- NIST NCSTAR 1: Final Report on the Collapse of the World Trade Center Towers, National Institute of Standards and Technology (NIST), 2005 https://www.nist.gov/el/final-reports-collapse-world-trade-center-towers
- Guidance on Structural Fire Engineering, The Institution of Structural Engineers (IStructE), 2015 ISBN: 9781906334896
- Structural Fire Engineering, Torero, J.L., and Lennon, T., 2003 Fire Protection Engineering Magazine, Society of Fire Protection Engineers
