This article explores the assessment of structures damaged by fire as well as the measures available to strengthen them for reuse
Fire remains one of the most destructive forces acting on built structures. When it occurs, it does not only threaten human safety but also challenges the integrity of the structural system that supports a building. Understanding what happens to a structure after a fire, and how to assess and restore it, forms a crucial part of modern structural engineering practice.
Assessment of fire-damaged structures involves careful observation, testing, and interpretation of how heat has altered material strength and connections. Engineers must distinguish between superficial damage and deep structural compromise. Each decision, whether to repair, strengthen, or demolish, depends on the accuracy of this evaluation.
In recent decades, research and field experience have improved how engineers assess fire damage. Modern methods combine visual inspection, non-destructive testing, material sampling, and computational modelling. Strengthening approaches now include advanced materials such as fibre-reinforced polymers (FRP), high-performance concrete overlays, and steel jacketing systems. Together, these techniques allow safe restoration while reducing unnecessary demolition and waste.
Fire Effects on Structural Materials
Fire affects materials differently depending on their composition, exposure time, and temperature. The physical and chemical transformations during heating can alter strength, stiffness, and bond properties permanently.
Concrete Structures
Concrete performs better under fire than most materials due to its low thermal conductivity. However, prolonged exposure can lead to surface cracking, spalling, and internal micro-cracking. Above 300°C, dehydration of cement paste begins, reducing cohesive strength. Beyond 600°C, aggregates may expand or explode, causing delamination between layers. Reinforcement embedded in concrete loses strength if the cover is insufficient to limit heat transfer.
In severe cases, the outer concrete layer detaches, exposing steel bars directly to heat. This exposure can lead to loss of bond, reduction in yield strength, and permanent deformation once cooled. Engineers often examine colour changes as a rough indicator of maximum temperature: pink or red hues suggest heating between 300°C and 600°C, while grey or white tones indicate higher exposure.
Steel Structures
Steel reacts rapidly to temperature increase. Its yield strength drops by nearly 50% at 550°C and continues to reduce with further heating. Because steel conducts heat efficiently, even localized fires can cause large portions of a frame to weaken. The key issue is loss of stiffness and stability, leading to buckling or collapse under existing loads. After a fire, steel members may remain visually intact but internally warped or stress-relieved.
Residual stresses, distortions, and reduced ductility must be carefully assessed. Nondestructive tests such as hardness measurement, ultrasonic thickness gauging, and metallographic analysis help determine whether the steel can be reused or must be replaced. In mild cases, straightening and heat-treatment can restore performance.
Timber Structures
Timber responds differently. It chars on the surface while retaining a relatively cool core beneath. This charring acts as insulation, allowing timber elements to survive moderate fires better than unprotected steel. However, the rate of charring—often 0.6 to 1.0 mm per minute—reduces the effective cross-section. Engineers must determine remaining section size and verify that it can carry design loads.
Moisture loss during fire also alters strength. Once reheated timber cools, internal cracking or delamination in glued-laminated elements may reduce stiffness. Visual grading, mechanical testing, and resistograph drilling are common methods to assess remaining capacity.
Post-Fire Assessment Procedure
The assessment process aims to establish the residual capacity and safety of the structure. It typically proceeds in four main stages: site inspection, data collection, testing, and analytical evaluation.
Site Inspection and Preliminary Evaluation
The first step after a fire is to ensure safety and stability. Engineers inspect the site visually to identify areas of severe distortion, spalling, or collapse risk. They record the fire pattern, direction of heat flow, and cooling conditions—since rapid quenching with water can worsen damage in concrete and steel.
Photographic documentation and mapping of affected zones are vital. Structural drawings, fire reports, and eyewitness accounts help reconstruct the sequence and intensity of the event. From this information, engineers divide the structure into zones of high, moderate, or low damage for focused investigation.
Material Testing and Investigation
Laboratory and field tests confirm the extent of damage. For concrete, core samples are extracted for compressive strength testing, density measurement, and petrographic examination. Ultrasonic pulse velocity testing helps detect internal cracks. For steel, tensile tests and hardness measurements reveal residual strength. Metallurgical microscopy can identify grain coarsening or phase transformations caused by overheating.
In timber, mechanical probing, drilling resistance, and moisture tests indicate residual capacity. Char depth measurements help determine remaining effective cross-sectional area. Non-destructive testing (NDT) such as infrared thermography or ground-penetrating radar (GPR) may assist in identifying hidden voids and delamination.
Analytical and Numerical Assessment
Once data are available, we can perform analytical evaluation to estimate residual strength and stiffness. Advanced models simulate temperature distribution during the fire and compare computed stresses with material degradation models. Finite element analysis helps assess whether deflections or rotations exceeded allowable limits.
For concrete frames, modelling may focus on residual compressive and bond strength. For steel structures, stability analysis and member slenderness under reduced modulus are critical. For timber, section loss models estimate post-fire capacity based on char thickness and exposure duration.
Structural Safety Categorization
After analysis, each element is classified as either safe, repairable, or beyond recovery. Safe elements require only minor maintenance. Repairable members undergo strengthening or partial replacement. Severely damaged components are removed and redesigned. The decision depends on cost, safety margin, and the intended use of the structure after rehabilitation.
Techniques for Strengthening Fire-Damaged Structures
Once assessment confirms residual integrity, engineers proceed with strengthening. The goal is to restore full or partial capacity while minimizing demolition and downtime. Strengthening strategies differ by material and damage type.
Concrete Structures
For concrete, several rehabilitation techniques are available:
Concrete Jacketing: Adding a new layer of reinforced concrete around existing members increases load capacity and restores stiffness. Shear connectors ensure composite action between old and new concrete.
Fibre-Reinforced Polymer (FRP) Wrapping: Lightweight carbon or glass fibres bonded with epoxy provide additional confinement and tensile resistance. FRP is useful where access is limited or weight must be minimized.
Steel Plate Bonding: Steel plates attached with epoxy or mechanical anchors reinforce beams and columns. This technique restores both strength and ductility.
Recasting and Patching: In less damaged zones, replacing spalled cover concrete and re-anchoring reinforcement can be sufficient. Proper surface preparation and use of compatible repair mortar are essential.
Steel Structures
In steel structures, strengthening may involve straightening, welding, or adding new sections. Local heat treatment can restore mechanical properties if the microstructure is not severely altered. When steel has lost significant strength, external bracing or member replacement is preferred. Fire protection upgrades such as intumescent coatings or encasement in concrete are typically included during repair to prevent future incidents.
Timber Structures
For fire-affected timber, strengthening often focuses on retaining the surviving core. Damaged layers are removed, and steel plates, FRP laminates, or epoxy injection restore stiffness. In historic or exposed timber structures, aesthetic considerations guide the repair technique. Engineers may combine traditional carpentry with modern bonding systems to maintain authenticity while ensuring safety.
Numerical simulations also improve decision-making. Coupled thermal-structural finite element models predict temperature distribution, stress variation, and residual deformation. Engineers can virtually test repair strategies before implementation, reducing errors and cost.
Code Guidance and Engineering Responsibility
Building codes such as Eurocode, ACI, and ASCE provide guidance for post-fire assessment and repair. However, many decisions depend on engineering judgment and experience. Codes often outline temperature-dependent material properties, residual strength factors, and limits for reusability. It remains the engineer’s duty to interpret test results conservatively and ensure that repaired structures meet full safety criteria.
Documentation and traceability are crucial. Reports must clearly describe testing, assumptions, and design checks for approval. In many jurisdictions, independent verification of repair design is mandatory before re-occupation.
Conclusion
The assessment and strengthening of fire-damaged structures combine engineering science with judgment and practical skill. Fire can severely degrade materials, yet with modern testing and repair techniques, many buildings can be safely restored. Understanding how each material responds, how to measure residual strength, and how to design effective strengthening ensures structural resilience and sustainability.
Each repaired building tells a story—not just of survival, but of the precision and responsibility that define structural engineering today.
Also See: Fire Resistance of Structural Elements – Timber, Steel, and Concrete Compared
Sources & Citations
- BS EN 1992-1-2:2004 – Eurocode 2: Design of Concrete Structures – General Rules – Structural Fire Design.
- Buchanan, A. H., & Abu, A. K. (2017). Structural Design for Fire Safety (2nd ed.). Wiley.
- SFPE (2016). SFPE Handbook of Fire Protection Engineering (5th ed.). Society of Fire Protection Engineers.
- Drysdale, D. (2011). An Introduction to Fire Dynamics (3rd ed.). Wiley.
