The Fire Resistance of Medium-Rise Timber Frame Buildings Summary Report
	© Chiltern International Fire
	Commercial in Confidence
	Report reference: TF2000 - Compartment Fire Test
Prepared by: Mostyn Bullock, Principal Engineer, Chiltern International Fire/TRADA Technology Ltd, UK Tom Lennon, Senior Engineer, Building Research Establishment (BRE), UK Vahik Enjily, Programme Manager Timber Engineering, Building Research Establishment (BRE), UK

Abstract

As part of the Timber Frame 2000 (TF2000) project a large-scale compartment fire test has been carried out in the full scale six storey timber frame building at BRE Cardington in the U.K. The test took place on the 15th September 1999. The programme of work is an important aspect of a unique collaborative project between Government, BRE, TRADA Technology Ltd and the timber industry, focused on enhancing the UK’s potential to become the world leader in the provision of medium-rise timber frame buildings. Changes to the Building Regulations in 1991 have made it possible to construct timber frame buildings of more than four storeys. The interest generated in the TF2000 project has shown that there is a demand for medium-rise residential timber frame buildings of five or more storeys. However, regulatory requirements between Scotland and the remainder of the UK are inconsistent. In particular the requirements regarding fire safety would benefit from harmonisation based on a rational approach to the problem.

As with other materials, fire resistance for elements of structure is based upon standard fire tests on individual elements. With new methods of construction and new demands on the industry there is a need to demonstrate the performance of medium-rise timber frame buildings subject to real fires. The construction of the six storey timber frame building at Cardington represented a unique opportunity to obtain valuable data on the performance of a complete building subject to a real fire.

The fire test compartment consisted of a single flat on level 3 of the building. The fire load was provided by timber cribs spread over the floor area of the flat. Although the test provided useful information on fire growth and development, the primary objective was to evaluate the performance of the structure subject to a fully developed post-flashover fire. Ignition took place in the living area. Flame spread was completely uninhibited and the ventilation to the compartment arranged to provide a worst case scenario for fire severity. One of the test objectives was to assess whether compartmentation of the building was effective in preventing fire spread from the flat of origin to adjoining flats through party walls, windows, floors or communal stair and lift shafts and in maintaining the integrity of the means of escape and structural stability.

In order to assess the performance of the structure in maintaining both the safety of the occupants and the integrity of the compartmentation for the required period of time the building was comprehensively instrumented. Instrumentation included thermocouples to measure the temperatures in the compartment and the heat rise in the cavities surrounding the compartment. Load cells were used to yield data on the rate of heat release in the compartment and automatic fire detection, gas analysis and heat flux meters were used to provide information on the tenability criteria. The purpose of this paper is to document the fire test programme for the timber frame building, to describe the performance of the structure and to present interim results and conclusions.

Introduction

The primary objective of the TF2000 compartment fire test was to evaluate the fire resistance of a medium rise six storey timber frame building subject to a severe natural fire exposure. The aspects of fire resistance being assessed were structural integrity and compartmentation. The test allowed for a realistic assessment of fire spread and provides a quantitative appraisal of the true performance of forms of construction tested to the Fire Resistance test. The test provided the opportunity to demonstrate that this form of construction can meet the functional requirements of the Building Regulations for England and Wales and the Building Standards for Scotland for such buildings.

Test Description

The fire test took place in the level 3 (2nd floor) flat in the South West corner of the building. The rooms of the test flat location are identified in plan in the top left hand corner of figure 1.

The fire was ignited in the living area of the flat and progressed to flashover after approximately 24 minutes. Initial burning was concentrated in the front of the living area closest to the ventilation opening. To accelerate the time to flashover the Fire Brigade was asked to intervene by breaking a single windowpane in the kitchen area. This took place 21 minutes and 30 seconds from ignition. Following flashover the Fireline boards over the windows to the floor above were subject to a heat flux of approximately 30kW/m² (peak plume temperature in excess of 500°C). The timber frame of the window would, if exposed, have ignited. Peak temperatures in the living area of the fire flat reached approximately 1000°C and remained at this level until the test was stopped at 64 minutes having reached one of the planned termination criteria.

Based on measurements taken of fuel mass loss, the peak rate of heat release has been calculated as approximately 6 MW.

The decision to stop the test was taken on the basis of predetermined termination criteria, one of which was related to duration of fire attack on exposed joists. The fire was extinguished by fire brigade intervention through the front door of the fire flat.

Preliminary Results

The atmosphere temperatures inside the fire flat are shown in figure 2 which includes a comparison with the standard ISO 834 fire exposure for correlation. The measured values of air temperature in the lobby, the adjacent flat, the flat above and the flat below are shown in figure 3.

The test was stopped once the fixity of the ceiling plasterboards in the living room had been lost and the joists were directly exposed to fire for a period of approximately 8 minutes. Figure 4 shows the temperature in the structural bays of the ceiling void in the area most directly affected by flame impingement while figure 5 shows the measured temperature profile on the floor joist in the centre of the living area.

Observations

At the time when the test was stopped both the compartmentation and structural integrity of the building were maintained. Maximum temperatures in the structural voids forming the boundaries of the compartment generally remained below 100°C with the exception of the localised areas where the timber members were exposed to fire for a period of time. The fire load in the bedrooms did not ignite although there was charring of the timber members of the walls enclosing the corridor in the hot smoke layer. Higher temperatures were recorded in the bathroom and had there been any fire load in the bathroom this may have ignited.

Figure 6 illustrates the fire dynamics of the test with the times and effects of key events highlighted. The temperatures shown were recorded at two different locations below the ceiling of the living area.

Compartmentation

Despite average atmosphere temperatures in excess of 900°C for 30 minutes there was no evidence of fire spread outside the compartment of origin during the test. At no time prior to the intervention of the Fire Brigade did air temperatures exceed 50°C in the lobby, the adjacent flat, the flat above or the flat below. With the exception of localised breakdown of the plasterboard linings between the kitchen area and the adjacent flat and the loss of plasterboards to the ceiling which led to the decision to terminate the test, temperatures in the cavities both internally and externally did not exceed 100°C and generally remained below 50°C. The cavity barriers remained effective for the duration of the test.

Tenability conditions in the Protected Lobby

Results from the test illustrate a gradual reduction in visibility to approximately 30m just prior to the Fire Brigade opening the entrance door to the flat at 59 minutes and 12 seconds from ignition. During firefighting operations the visibility dropped to a minimum of approximately 6 metres.

The smoke detector in the lobby was activated immediately following ignition as the flat entrance door remained open for some time. The CO detector in the lobby registered an alarm condition (300ppm) at 28 minutes and 30 seconds from ignition. The CO level at nominal respiratory height (1.58m above floor level) close to the stair access door registered only a few parts per million (ppm) above the ambient level up to the point where the Fire Brigade gained access to the fire flat.

Pressure measurements indicate that the fire rapidly established a dynamic positive pressure within the fire flat reaching a peak at the top of the entrance door of approximately 5.5 Pa post-flashover. This would have had the effect of forcing smoke and hot gases around door edges but video evidence supported by visibility, carbon monoxide and temperature measurements indicate that the smoke and intumescent seals around the door mitigated this.

The data collected in the protected lobby indicate that conditions favourable for means of escape would have existed for the duration of the test. This was borne out by observations from the Fire Brigade following the test.

Time Equivalence

The concept of time equivalence relates the severity of a natural fire exposure to an equivalent period in a standard ISO 834 fire resistance test. The fire test allowed an empirical estimate of equivalent duration to be calculated using data from the compartment test. Temperatures recorded at the back of the plasterboard in the ceiling void of the living room area in the test were compared to similar points in a section of replica ceiling tested in a furnace and exposed to standard temperature and pressure conditions. Figure 7 shows the average results from the comparison. The temperature in the TF2000 test exhibits a slower initial rate of heat rise. This was expected as flashover did not occur until 24 minutes into the test. At this point there was a severe heat flux which removed the chemically bonded water from the gypsum board at a faster rate than in a fire resistance test. The graph indicates that the TF2000 test was approximately 10% more severe than a 60 minute fire resistance test exposure. Inspection of the remains of the furnace specimen show very little charring of the timber joists. The joists in the TF2000 test were charred to a significantly greater extent in areas of the ceiling close to the window.

A number of softwood and hardwood cubes were positioned in the fire flat for the test and replicas of these cubes were subsequently exposed to a standard furnace exposure for exactly 60 minutes to provide a comparison of the extent of charring. The results of the charring analysis are summarised in table 1.

 

Conclusions

The compartment fire test met the stated objectives of the programme. The following conclusions may be drawn from an analysis of the data and from observations during and after the test.

In meeting the requirements of the regulations and the objectives of the research programme a number of issues have arisen.