Keeping up the Façade

Bt Andy Kay
Association of Specialist Fire Protection

Unfortunately, fantasy all too often mimics real life and there is a long history of fatal fires in tall buildings dating back well over 60 years. In 1946, a fire in the Winecoff Hotel, Atlanta GA cost 119 lives, many as a result of jumping. Other notable disasters – such as the 1974 Joelma building fire in São Paulo, Brazil (179 deaths), the Hotel Dupont Plaza in San Juan, Puerto Rico (97 deaths) in 1986, and the 1996 Garley Building fire in Hong Kong (41 deaths) – have made those responsible for building codes and regulations around the world sit up and take note.
Mercifully, there have been relatively few lives lost as a result of fires in high rise construction in Europe. However, recent incidents – for example, the complete destruction of the Torre Windsor Tower in Madrid in 2005, where issues with the curtain wall cavity fire breaks contributed to the fire spread; and the 2012 Polat Tower fire in Istanbul, where the flames spread behind a rainscreen cladding system – highlight the need for continued vigilance.

Role of Cavity Fire Breaks
The cavity fire breaks required in cladding systems vary with the type of façade. In a curtain wall system, the cavity break sits behind the façade, between the edge of the building slab and the façade itself. It is therefore inside the building and its purpose is to act as a continuation of the floor slab right up to the façade, while allowing for dynamic movement that can be caused by a number of differing factors. The taller the building, the more movement will need to be accommodated.

The installation of a cavity barrier that cannot cope with the continual stresses imposed by positive and negative wind loads will eventually lead to a breakdown in the fire compartmentation measures designed to prevent the passage of flames and smoke from one floor to the next. In 1997, a fire broke out in the President Tower Building in Bangkok. The fire started due to a vapour explosion on the 7th floor of the 36-storey tower. Although the fire spread to only four other floors, it claimed the lives of three occupants of the building. The building’s owners wanted to understand why the fire had spread outside of the fire compartments and so employed Building Diagnostics Asia Pacific (BDAP) to carry out a forensic study on the causes and spread of the fire.

Peter Hartog of BDAP wrote a detailed report in which he stated that the fire had “… spread rapidly through openings in the floor, including the gap between the curtain wall and the concrete slab”. The cavity barriers had all been installed correctly and, had the building been in London, Paris or Rome, the fire-stopping measures at the slab edge would have complied with the building regulations at that time. In fact, they would still comply now. The report went on to find that “… the conventional fibre fire stops were no longer flexible. All such forms of attachments are likely to fail as curtain wall extrusions deform”.

In addition, BDAP considered that “… the aluminium support members moved significantly during the early stages of the fire and that this movement would have formed routes for fire spread.”

Realistic Fire Tests
So how is it that there are products available in the European market that have the potential to fail in a fire? The problem has been the lack of a requirement to test the products in a realistic test scenario. There has been no requirement to test for cyclic movement or for the test set-up to replicate a curtain wall façade.

In the UK, products have traditionally been tested to BS 476: Part 20: 1987: Fire tests on building materials and structures. Method for determination of the fire resistance of elements of construction (general principles). This test provides a means of quantifying the ability of elements to withstand exposure to high temperatures, by setting criteria by which the load-bearing capacity, the fire containment (integrity) and the thermal transmittance (insulation) functions can be judged. It is a static joint test whereby the cavity barrier is fitted between two substrates, normally concrete to concrete but there are products tested concrete to masonry or concrete to aerated blockwork.

None of these can replicate in any way the performance of a curtain wall. It has therefore become commonplace for companies to manufacture variants on a standard rock-fibre slab that can meet the integrity and insulation values required by the test. These products have a horizontal fibre orientation that, while good for stability, cannot accommodate the sort of movement that is imposed daily through wind loadings, let alone the distortion of the mullions and transoms as seen in the President Tower fire.

Fortunately, a new test standard EN1364-4: 2007: Fire resistance tests for non-loadbearing elements. Curtain walling is now in place and European Technical Approval 26 clearly states that this is the method to which cavity barriers must be tested if they are to be used in a curtain wall application. The test is conducted using two floor levels of curtain wall and replicates the kind of movement of both the floor slab and façade that can occur in a fire.

In order for products to pass this more stringent test they need to be engineered in a -different way to the standard rock-fibre slabs. The orientation of the fibre needs to be changed from horizontal to vertical in order to make the product compressible. This can be done on site, involving traditional rock-fibre being hand cut, re-oriented to the vertical and then compressed into the gap. The rock-fibre is then coated in either an acrylic spray or a pourable silicone. These products can be problematical in wet or cold conditions.

Alternatively, the product can be engineered in a factory environment where the orientation and compression is regulated and therefore consistent. A foil coating is then applied to the rock-fibre assisting its integrity, insulation and acoustic performance. At least one spray system and one dry system have been successfully tested to the new test standard and the products are available in the European markets.

Rainscreen Façades
The purpose of the cavity fire barrier in a rainscreen façade is to prevent the spread of fire in the cavity between the exterior insulation and the cladding panels. In the UK, the building regulations require 30 minutes’ integrity and 15 minutes’ insulation (although there is no insulation requirement in Scotland). However, in a rainscreen system, there is a need to maintain constant airflow for ventilation purposes. This serves not only to aid the spread of flames and smoke, but also to make traditional cavity barriers that close right across the gap unusable.

The industry has therefore developed products that, while maintaining the requisite air gap, have an intumescent component that will react to close the gap in the event of fire. The maximum air gap that is required is usually 25mm and it is important to close this gap as quickly as possible before the fire has a chance to bypass the fire break.

There is currently an Association for Specialist Fire Protection (ASFP) working group looking at a fire test method for rainscreen cladding cavity barriers and while there is discussion on recommending a maximum closure time in the region of five minutes, it is always preferable to keep this time as low as possible.

It is always in the interest of designers and contractors to get a copy of the relevant fire test reports (or assessments) from a manufacturer to satisfy themselves that the products are suitable for the intended application. The responsibility, and therefore liability, will always lie with the designer to ensure that he has specified the right product, and the installer to ensure that he has installed it in accordance with the manufacturers’ recommendations.

Andy Kay is Sales & Marketing Manager with ASFP member company, Siderise Group

For further information, go to www.asfp.org.uk

Polat Tower blaze. Pic courtesy fotostory/Shutterstock.com