CPD: Large-scale system testing
Manchester Northern Quarter Skyline Central has its cladding removed after Grenfell fire
Large-scale testing to predict performance in real fires is a vital source of information for those construction professionals specifying and installing cladding and facade systems, says Alan Scupham.
The Independent Review of Building Regulations and Fire Safety due to be published in spring 2018 is long overdue. However, until any changes are implemented, the situation leaves construction professionals uncertain about where to turn to for clear guidance, especially when it comes to specifying and installing cladding and facade systems.
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One way to gain a good understanding of how a specific cladding system will behave in a fire is to look at whether it has undergone a large-scale test. This CPD sets out to provide evidence of the validity of using large-scale testing to predict cladding system and building envelope performance in real fires.
This approach is cited in the current regulatory guidance as providing a route to compliance for facades on buildings with a storey over 18m.
The performance-based route is referenced in Approved Document B2 (England & Wales), Technical Handbook 2 (Scotland) and BCA Technical Guidance Note 18 – Issue 1 June 2015, and the assessment criteria are set out in BR 135: Fire performance of external thermal insulation for walls of multi-storey buildings. BR 135 sets the required performance criteria of the complete assembly comprising the external cladding system when tested to BS 8414.
BR 135 to BS 8414
The BS 8414 test is a large-scale assembly test measuring fire spread and flame propagation on a rig representing a typical high-rise building geometry and cladding installation. It replicates a fire starting inside a room, breaking out through a window and exposing the external facade to fire.
Figure 1: BR 135 to BS 8414
- 9m test rig
- Resembles two floors above the combustion chamber
- The fire will release approximately 3MW during its peak heat release
- Incorporates system-specific details, cavity barriers and fire stopping
- 30 minute flame exposure
Figure 1 (see below) shows the configuration of the rig. A wooden crib is located in the 2m x 2m combustion chamber. Once ignited, the crib can reach a peak heat release of 3MW, replicating a severe fire. The marks indicated in grey and green are thermocouples, located 2.5m and 5m above the combustion chamber, which are used to measure temperature during the test.
BR 135 sets three performance criteria: external fire spread, internal fire spread and mechanical performance. To comply with the Building Regulations, using the performance-based route to compliance, the thermocouples cannot exceed 600°C for a period of at least 30 seconds within 15 minutes of the start time.
Products which are successfully tested as part of complete cladding systems are deemed compliant in the specific configuration in which they were tested. In other words, if any part of the system is to be altered, it should be retested or assessed.
Why not just use ‘non-combustible’ materials?
Despite the importance of understanding how the whole system will behave in a fire, there currently exists another route to compliance for high-rise buildings that relies on the basic classification of products as “non-combustible” or “of limited combustibility” (in England and Wales).
These product or material classifications use small-scale laboratory tests, such as BS 476-4 and BS 476-11, that do not reflect real-life fire situations or establish how various products perform together when installed as a system.
It is important to remember that, in the case of insulation materials, even products that are classed overall as “non-combustible” can contain varying quantities of binders and other elements that will burn.
A rainscreen facade will also contain any number of combustible elements, regardless of the classification of the insulation and facade material. These include membranes, gaskets, sealants and fillers.
Other factors, such as cavity sizes, quality of installation and the overall design of the facade can have an impact on whether and how easily a fire can spread. This is why it is crucial to test the whole cladding system on a realistic scale, not simply consider the performance of individual components in isolation.
Large-scale insurer-backed testing
Insurance companies have long recognised the limitations of the small-scale testing required for many aspects of Building Regulations compliance, and have devised their own – generally far more rigorous – test and certification regimes to provide an assurance of asset protection, regardless of the height of the building.
At a time when insurance premiums are becoming more of an issue for contractors and building owners, it is helpful to understand what these tests are, and which systems to look for that have insurer-rated test approval.
UK insurance requirements
In the UK the loss prevention certification board (LPCB) has specific insurance requirements, and has developed the relevant test standards for the building envelope:
- LPS 1181-1 for exterior walls and roofs;
- LPS 1181-2 for interior walls and ceilings;
- LPS 1208 for external walls and internal walls and ceilings.
LPS 1208 focuses on various periods of fire resistance, whereas LPS 1181-1 and 2 focuses on reaction to fire tests and can also make provision for a period of fire resistance.
Each of the FM standards deals with different panel applications
LPCB LPS 1181-1: Large-scale external wall and roof test
This is designed to determine a number of key performance requirements:
- Internal surface flaming
- External surface flaming
- Concealed burning
- Burning brands
The configuration of the test is as a “garage” test consisting of a structure that is 10m by 4.5m wide and 3m high. The test is designed to measure fire spread and propagation.
A large wooden crib is located on a steel table inside the garage test. At 1.5m from the centre of the crib is a vertical red line on both adjoining walls and the ceiling, indicating a failure point for horizontal flame spread – flames must not spread beyond 1.5m from the centre of the wooden crib on either the walls or the ceiling.
After the test has been completed, the independent testing lab will remove the insulated panels from the test rig. These are then examined for evidence of concealed burning and damage. On the exposed side of insulated panels with the internal steel sheet removed evidence may be seen of the protective black char which typically forms upon exposure of high-performing PIR insulation to fire, and which self-extinguishes once the fire source has been removed.
FM 4880, FM 4881, FM 4471
While LPCB is known and accepted in the UK, it is far less well known on the world stage. FM Global, on the other hand, is a worldwide insurance and risk management enterprise.
FM Approvals,a member of the FM Global Group, is an international third-party testing and certification service. It tests property loss prevention products and services for use in commercial and industrial facilities, to verify that they meet rigorous loss prevention standards of quality, technical integrity and performance. Its FM Approved mark is recognised and respected worldwide.
Red line indicates failure point for fire spread
The FM suite of testing can include elements such as:
- Factory auditing;
- Combustibility from inside and outside;
- Wind uplift;
- Foot traffic;
- Hail resistance.
The most relevant standards for assessing the fire performance of insulated panel systems are: FM 4880, FM 4881 and FM 4471. Each of these deals with different panel applications (see image top left).
- FM 4880 is for Class 1 fire rating of building panels or interior finish materials. It deals with internal applications only;
- FM 4881 is for Class 1 exterior wall systems;
- FM 4471 is for Class 1 panel roofs, relating to the fire and spanning performance of exterior roof panels only.
An important point to consider is that for the external envelope to comply with FM requirements it needs to meet both FM 4880 and FM 4881.
A range of fire tests are performed in order to achieve FM Approval. These include tests on the insulation core and large-scale system tests which can include the FM 50ft Corner Test. In this test, the two wings/walls are 15.2m (50ft) high, and 6m (20ft) in width. The fire ignition source is 345kg of dry oak.
Based on the performance of the insulated panel system you can get the following classifications:
- Class 1 with no height restriction;
- Class 1 to a 50ft (15.2m) max height;
- Class 1 to a 30ft (9.1m) max height.
Another test in the FM suite which is applicable for FM 4880 and FM 4881 is the UBC 26-3 room fire test. It is essentially a room test with a wooden crib in the corner of the room, measuring flame spread and fire propagation.
A series of real fire case studies has given similar conclusions demonstrating that PIR cores char, fire is not propagated in the core, and there is no evidence to demonstrate that insurance industry approved panels increased risk of fire spread. See www.epic.uk.com for details.
Contractors and specifiers are coming under increasing pressure to specify roof, wall and facade systems that have been rigorously tested, irrespective of the amount of combustible or non-combustible products used.
We know from research, testing and observation of real fire case studies that it is possible for systems incorporating “combustible” insulation (for example, products that achieve a Euro Classification of B to EN 13501-1) to perform extremely well, and in some cases to outperform equivalent systems with non-combustible (for example, products that achieve Euro Classifications A1) insulation.
Design considerations should be based on large-scale system testing rather than individual product classifications, as it provides high-quality evidence of performance, offering peace of mind for building owners and assurance for insurers, helping to keep clients happy, tenants safe and premiums low.
Alan Scupham is design manager at Steadmans and EPIC Management Committee representative
This CPD has been sponsored by Epic
Real fire case study: Wharfedale Hospital
Large-scale testing provides a very good indication of how products are likely to perform, but the real test is how closely insulated panel systems live up to their results when exposed to a real fire.
A fire broke out at Wharfedale Hospital in Leeds in 2003. The building, which was steel framed with concrete floors, was under construction and was partially clad.
The first and second floors had been covered with PIR core insulated panels approved by LPCB to EXT-B of LPS 1181 Part 1. However, the ground floor glazing had not yet been installed and this level had been left open-sided.
It is thought the fire was started deliberately, by igniting adhesive stored on the ground floor. Security alerted the fire service, bringing it under control within 40 minutes.
The heat was significant, with 10m high flames impinging directly on the PIR core panels. The concrete floor cracked and the steel beams – protected with a fire-resistant intumescent coating – distorted.
Firefighters cut into the insulated panels, exposing the steel columns for inspection (pictured above). The PIR core revealed appears unaffected by fire. The fire service found light smoke but no fire spread on upper floors.
An independent report concluded that the PIR core of the insulated wall panels did not ignite, and did not promote fire spread within the core or to the eaves.
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