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Update on a large-scale CLT experimental campaign for commercial enclosures

By: Danny Hopkin, OFR Consultants, UK; Michael Spearpoint, OFR Consultants, UK; Carmen Gorska, Stora Enso, UK; Harald Krenn, KLH, Austria; Tim Sleik, Binderholz, Austria; Gordian Stapf, Henkel Engineered Wood, Switzerland and Wojciech Wegrznski, ITB, Poland.


When conceiving of a new office building in the UK, timber is increasingly considered as part of any potential framing solution. This is driven by a combination of embodied carbon, aesthetic and constructability considerations. Commercial premises, such as offices, often have specific user / client demands, with emphasis placed on high floor-to-ceiling heights, long spans between column members and large areas of glazing. For this reason, the UK market is converging upon hybrid construction solutions where timber, in the form of cross-laminated-timber (CLT), is used in concert with other materials, such as steel and concrete (Figure 1).

Figure 1. Illustration of a hybrid commercial structure, utilising CLT as floor members.

Recent guidance [1] has been published which has clarified what design evidence should be provided by engineers when demonstrating that an adequate level of safety will be achieved when adopting a combustible structural framing solution. In higher-consequence-of-failure buildings, e.g., medium- to high-rise offices, the structure must be designed in a manner whereby it has a reasonable likelihood of surviving the full duration of a fire [2]. This necessitates that if the structure becomes involved as a source of fuel, it must undergo auto-extinction and remain capable of supporting the applied loads both during and beyond a fire. Allied to this, it is often considered that preventing glue-line integrity failure (GLIF) when adopting CLT is a prerequisite for auto-extinction.

The configuration, scale and fire design of in-demand commercial buildings are, however, increasingly detached from research that has been conducted to date, which has tended to focus on experimentally investigating fire dynamics in combustible residential-type enclosures. An extensive review of current large-scale testing is provided in Ronquillo et al. [3]. This focus on residential enclosures has meant little knowledge has been generated for large enclosures, where the combustible elements typically only comprise a single surface, e.g., a CLT floor slab. Therefore, on the one hand, good guidance exists directing designers towards what evidence should be generated, but on the other, little focus has been given to generate knowledge for the types of building increasingly most in demand. To this end, as part of a larger Structural Timber Association (STA) Special Interest Group project on mass timber compartment fire behaviour [4], a series of experiments have been recently undertaken at ITB in Warsaw to support designers in the realisation of mass timber commercial buildings. The emphasis is on those cases where only a combustible ceiling is exposed. This article serves to provide a brief update on the research programme, inclusive of some qualitative and quantitative findings from the experimental campaign. Further publications are intended to follow which will fully and rigorously analyse and report on the studies.


The primary objective of the experimental campaign was to address, first and foremost, whether averting GLIF is a prerequisite for auto-extinction for specific configurations representative of current UK commercial premise demands. As a secondary series of objectives, in support of understanding large compartment fire behaviour, the experiments were designed to:

  1. Quantify how the ceiling jet characteristics change below a mass timber soffit compared to that of an inert soffit;
  2. Quantify to what extent heat fluxes in advance of the fire front (i.e., from the near field region and ceiling jet) altered because of adopting mass timber soffits over inert soffits; and
  3. Quantify to what extent external flaming was a realistic proposition for enclosures with mass timber ceilings and large ventilation openings, with emphasis on the heat flux in the spandrel zone.

The enclosure

The experimental enclosure had internal dimensions of c. 4.5 m x 9.5 m x 2.6 m. Three of four elevations were enclosed in blockwork (Figure 2). The ceiling varied between experiments, i.e.:

  • Experiment 1: CLT lined with 2 x 15 mm layers of Type F(fire-rated) plasterboard;
  • Experiment 2: Exposed CLT using a regular polyurethane (PUR) adhesive (Henkel Loctite HB S); and
  • Experiment 3: Exposed CLT using a heat resistant PUR adhesive (Henkel Loctite HB X).

The CLT in experiment 1 and 2 was non-edge-bonded, whilst in experiment 3 it was edge-bonded. The layup was 40-20-40-20-40 mm. The CLT was supported on a rolled-steel frame, suspended from 8 load-cells, and spanned 4.5 m.

The fire exposure

The ceiling was heated using propane gas burners, elevated to sit 1 m below the soffit. The burners were located off-centre (Figure 3), with the aim of inducing a ceiling jet extending to 50% of the ceiling length in experiment 1. The fire’s heat release rate (HRR) was controlled via mass flow switches, leading to a HRR that ramped to a maximum of 1,250 kW (achieved over an 8 min period in 250 kW steps). The duration of steady-heating (at 1,250 kW) was chosen to induce GLIF in experiment 2, resulting in a steady-phase duration of 80 min. After this phase, the burners ramped down in 250 kW increments every 5 min, before being turned off. An image during the growth phase of experiment 3 is shown in Figure 4.

Figure 2. Experiment 1 compartment.

Figure 3. Rig floor plan showing offset propane burners and ceiling arrangement of plate-thermometers.

Figure 4. Flames from burner impacting ceiling in experiment 3.


The experiments were extensively instrumented to collect data on:

  • The HRR from the burner;
  • The HRR from the CLT slabs via load-cells;
  • Incident heat flux to the (internal) floor, ceiling and walls;
  • Incident heat flux to targets outside of the enclosure;
  • Incident heat flux to the façade extension above the openings;
  • Gas-phase temperatures throughout the enclosure;
  • Solid-phase temperatures within the CLT; and
  • Slab deflection at mid-span.

Preliminary observations

Analysis of the data from the experiments remains a work in progress. However, to date, the following observations have been made.

Impact of GLIF on auto-extinction

In both experiment 2 and 3, the exposed CLT underwent auto-extinction, with flaming combustion gradually ceasing from right to left, in the orientation given in Figure 2. In experiment 2, extensive GLIF was observed, with large pieces of lamella falling from the ceiling and continuing to burn on the floor (Figure 5). In experiment 3, GLIF was substantially reduced (Figure 6). Logically, for the specific and simple configuration of an exposed ceiling in an otherwise inert enclosure, avoiding GLIF was not observed to be a prerequisite for auto-extinction. It is important to stress that this is not a general finding, but an artefact of the specific configuration / geometry evaluated.

Figure 5. Extensive GLIF in vicinity of burner in experiment 2.

Figure 6. Limited GLIF above burner in experiment 3.


Combustible ceilings and the ceiling jet

In advance on the experiments, it would logically be postulated that the presence of a combustible soffit would alter the ceiling jet characteristics, both in terms of flame length and heat flux to the ceiling. This is confirmed in Figure 7, where the radiative heat flux to the ceiling is estimated from plate thermometers at different offsets from the propane burner. Results are shown for the growth and onset of steady phases. Near the burner, radiative heat fluxes converge upon 120 to 140 kW/m2. Away from the burner, the experiments with combustible ceilings (2 and 3) consistently show heat fluxes of c. 10 to 20 kW/m2 greater than the non-combustible counterpart. This difference broadly coincides with that attributable to flames at the surface of wood, as reported by other authors [5], [6].

Figure 7. Radiative heat flux to the ceiling at different offsets from the burner for experiments 1-3 (burner heat release rates annotated in -500 mm offset).

Combustible ceilings and heat flux to the floor

The presence of extended flames at the ceiling is shown to correspond with increased radiative heat flux to the floor (relative to experiment 1), as shown in Figure 8. This is more pronounced at larger burner offsets and would, in practice, likely translate to more rapid fire spread through a large compartment.

Whilst there was no discernible difference between regular (HB S) and heat-resistant (HB X) PUR adhesives in respect of radiative heat flux to the ceiling (Figure 7), there is a difference for radiative heat flux to the floor. Away from the burner, radiative heat fluxes to the floor were substantially higher where HB S adhesive was adopted over HB X. As plots are limited to the growth and onset of steady phases, such a difference is not attributable to GLIF and warrants further investigation. Other outcomes from the STA project have identified that HB X may impact fire performance more complexly than simply mitigating GLIF [7]. The difference in edge-bonding condition between experiment 2 and 3 is also noteworthy.

Figure 8. Radiative heat flux to the floor at different offsets from the burner for experiments 1-3 (burner heat release rates annotated in -500 mm offset).


This article has served to provide a brief update on the progress of the STA Special Interest Group project on mass timber compartments, summarising the motivations for and delivery of three large scale fire experiments, focussing on commercial type enclosures. Noting the primary objective of establishing if, for the specific case of exposed CLT ceilings (in commercial buildings), averting GLIF is a prerequisite for auto-extinction, it has been shown that extensive char fall off can occur in tandem with the cessation of flaming. Further analysis is required to better understand to what extent such a finding from the experiments translates to commercial buildings in practice. This will require a more thorough analysis of the data collected, with further publications expected in due course.


[1]        Structural Timber Association, Structural timber buildings fire safety in use guidance. Volume 6 - Mass timber structures; Building Regulation compliance B3(1), V1.1. Structural Timber Association, 2020.

[2]        D. Hopkin, M. Spearpoint, C. Gorksa, H. Krenn, T. Sleik, and M. Milner, ‘Compliance road-map for the structural fire safety design of mass timber buildings in England’, SFPE Europe, vol. Q4, no. 20, 2020.

[3]        G. Ronquillo, D. Hopkin, and M. Spearpoint, ‘Review of large-scale fire tests on cross-laminated timber’, J. Fire Sci., p. 073490412110344, Aug. 2021, doi: 10.1177/07349041211034460.

[4]        Structural Timber Association, ‘CLT Special Interest Group’, 2021. (accessed May 04, 2021).

[5]        M. J. Spearpoint and J. G. Quintiere, ‘Predicting the burning of wood using an integral model’, Combust. Flame, vol. 123, no. 3, pp. 308–325, Nov. 2000, doi: 10.1016/S0010-2180(00)00162-0.

[6]        D. Morrisset, R. M. Hadden, A. I. Bartlett, A. Law, and R. Emberley, ‘Time dependent contribution of char oxidation and flame heat feedback on the mass loss rate of timber’, Fire Saf. J., p. 103058, May 2020, doi: 10.1016/j.firesaf.2020.103058.

[7]        D. Hopkin et al., ‘Experimental characterisation of the fire behaviour of CLT ceiling elements from different leading suppliers’, presented at the Applications of Structural Fire Engineering, Ljubljana, Slovenia, 2021.