Flame propagation at the curtain wall-floor slab joint

In the 1970s, the use of combustible materials in external, non-load-bearing walls in non-combustible constructions raised questions on how this would affect the horizontal and vertical fire propagation over these walls and through their combustible cores [1].  During this time, it was observed that fires in buildings with curtain walls spread through the windows, curtain walls, and floor slab intersections.

There are three (3) ways a fire originating in one compartment of a given floor spreads to other floors [2].


1. Through the gap between the curtain wall and the floor slab edge

This void is protected using perimeter fire barriers tested per the ASTM E 2307, Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using Intermediate-Scale, Multi-story Test Apparatus.


2. Through cavities within the curtain wall

The fire would spread along the exterior curtain wall or through cavities within the combustible core.  This is prevented by installing assemblies compliant with the NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components.


3. By leapfrogging

This is the process in which combustible materials behind a window on the floor above are ignited due to the radiant heat emanating from flames from the window below.  This (radiant heat emanating) behaviour is now addressed by the ASTM E 2874, Standard Test Method for Determining the Fire-Test Response Characteristics of a Building Spandrel-Panel Assembly Due to External Spread of Fire.


Case Studies

On May 4, 1988, a fire broke out in the 62-floored First Interstate Building in Los Angeles, starting on the 12th floor [2].  The fire proliferated due to the ignition of combustibles on this floor.  The glass panels on the exterior façade began breaking, thus providing an alternative path for the fire to leapfrog to the floors above.  The fire also spread through the gap between the curtain wall and the floor slab edge.  At that time, the building’s sprinkler system was being reedified and therefore was not operational.  This allowed for the easy spread of fires from floor to floor.  It took firefighters three and a half hours to contain it [2].


In Madrid, Spain, a fire broke out in the Windsor Tower (or Torre Windsor) on February 12, 2005.  The building comprised a reinforced concrete core and steel load-bearing mullions around its perimeter.  During the building construction, the building codes did not require steel mullions to be fire protected [3].  In 2005, due to code changes, the building was refurbished to bring it up to code by installing active and passive fire protection. During the refurbishment, a fire broke out on the 21st floor of the 32-floored building, spreading downwards to the 2nd floor and upwards to the topmost floor.  The fire burned for approximately 20 hours, engulfing the entire structure [2].  A review of this event revealed that the fire spread and failure of the vertical compartmentation of the building were a result of the lack of perimeter fire barriers along the edge of the floor slabs, failure of the unprotected mullions and the exterior cladding, and the radiant heat produced by the leapfrogging effect of the fire [3].


Perimeter Fire Barriers

There has been a significant evolution in the design and incorporation of fire protection systems in high-rise buildings.  This can be attributed to the lessons learnt from the significant fires recorded over the past decades.

In high-rise buildings, the Life Safety of the occupants is highly dependent on the building’s compartmentation.  Perimeter fire barriers are installed to fill in the void between the curtain wall and the floor slab edge.  The ASTM E 2307 defines a perimeter fire barrier as “the perimeter joint protection that provides fire resistance to prevent the passage of fire from floor to floor within the building at the opening between the exterior wall assembly and the floor assembly” [4].  The perimeter fire barrier is expected to maintain a tight seal between the curtain wall and floor assemblies during the deflection or movement of the exterior curtain wall in a fire scenario.  Mineral wool is a suitable material for this application because of its non-combustibility, high self-ignition and flash ignition temperatures, and its ability to maintain its integrity during fire scenarios.


Code Requirements

In the United Arab Emirates, the UAE Fire and Life Safety Code of Practice recommends the minimum requirements of Civil Defence for a building’s life, fire safety design, and installation [5].  It is a requirement that all perimeter barrier systems shall be listed and approved system assemblies.



The tragic history of fires that resulted from the lack of or improper installation of fire protection systems stresses the importance of selecting the right system, hiring qualified installers and inspectors, and maintaining the system throughout the building’s lifetime.  Listed and approved perimeter fire barrier systems that prevent the spread of fire, smoke and hot gases should be installed at the gap or joint between a rated/non-rated wall assembly and the rated floor assembly.


Works Cited

[1] National Fire Protection Association, "Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components," National Fire Protection Association, 2023.

[2] P. M. Tony Crimi, "Perimeter Fire Barrier Systems: Taking a team approach to fire-safe construction," The Construction Specifier, South Bend, 2017.

[3] Standing Committee on Structural Safety, "THE FIRE AT THE TORRE WINDSOR OFFICE BUILDING, MADRID 2005," Collaborative Reporting for Safer Structures UK (CROSS-UK), 2008.

[4] ASTM International, "Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using Intermediate-Scale, Multi-story Test Apparatus," ASTM International, 2020.

[5] Directorate General of Civil Defence, UAE Fire and Life Safety Code of Practice, 2018.