1. Introduction

Hollow-core concrete slabs (also called precast prestressed hollow-core slabs) are widely used in building construction for floor and roof systems due to their high structural efficiency, cost-effectiveness, and speed of installation. These slabs are characterized by continuous voids (or cores) running through their length, reducing self-weight while maintaining strength.

However, their behavior under fire conditions is complex and critically important for structural safety. Fire analysis of hollow-core slabs focuses on understanding how they perform under elevated temperatures, considering factors like thermal degradation, spalling, loss of prestress, and shear failure.

2. Importance of Fire Analysis

Structural elements must maintain load-bearing capacity during a fire long enough to allow for safe evacuation and firefighting. For hollow-core slabs, fire can significantly impact performance due to:

• Reduced cross-sectional area due to spalling or core collapse
• Prestressing steel relaxation at high temperatures
• Shear failure near supports due to reduced material strength
• Loss of bond between concrete and reinforcement

These risks necessitate a detailed analysis to ensure code compliance and life safety.

3. Behavior of Hollow-Core Slabs in Fire

a. Thermal Response

• Fire causes a temperature gradient through the slab depth.
• The hollow cores act as thermal insulators, slowing heat transfer upward, but this can also lead to high local temperatures around the voids.
• Core collapse may occur if the upper surface overheats and thermal expansion causes internal pressure.

b. Structural Response

• Concrete strength decreases significantly above 500°C.
• Prestressing steel loses strength and stiffness rapidly above 300°C.
• Loss of prestress can lead to deflection, cracking, and shear failure, particularly at supports.
• Spalling (explosive removal of concrete layers) can reduce the effective section and expose reinforcement.

4. Fire Resistance Design and Assessment Methods

Fire analysis of hollow-core slabs can be conducted using:

a. Prescriptive Methods (Code-Based)

• Building codes define minimum fire resistance ratings (FRR) based on slab thickness, cover, and loading conditions.
• Fire resistance is often validated through standard fire tests

b. Performance-Based Design

• Finite Element Modeling (FEM) is used to simulate slab behavior under real fire scenarios.
• Models incorporate thermal conductivity, specific heat, and mechanical degradation of materials with temperature.
• Software tool ABAQUS is used to predict temperature distribution, deflection, crack formation, and failure time.

In fire analysis, heat transfer within the slab occurs through:

• Conduction (within concrete and steel)

• Convection (to/from the exposed surface)

• Radiation (from fire to slab surface)

Fire analysis of hollow-core slabs is crucial to ensure the structural integrity of these slabs during a fire. Due to their unique geometry and reliance on prestressing, these slabs are vulnerable to rapid degradation under high temperatures. A combination of prescriptive and performance-based methods should be used to evaluate and improve fire performance, ensuring compliance with safety regulations and minimizing risk to occupants and emergency personnel.