Experimental Comparisons

The validation of numerical studies is vital to ensure that any simplifications and assumptions made are appropriate within the scope of the problem being solved. This section presents the results obtained in the heat transfer analysis of the experimental tests described in Chapter 3.

Comparisons between the simulations and the actual experimental results are made to ensure the adequacy of the thermal transfer model used as input into the structural analysis.

4.9.1 Test A

To take advantage of the symmetry of the floor geometry, only a quarter portion of a strip of the floor needs to be modelled as described in Section 4.6.3. To ensure the thermal model can be used as an input into the structural model in ABAQUS, all node definitions and positioning in the thermal model must correspond exactly to the structural model. This restricts the size of the mesh as the structural model geometry is large; hence a uniform density 5 mm square mesh is used in the plane of the cross-section of the floors, and a 300 mm long element length is used for the general modelling. A sensitivity study on this element length is given in Section 5.8.1.

To simplify the following thermal contour figures, the contours have been set to display the following thermal profiles which show all temperatures above 300°C as grey, assuming this timber is char. Ambient temperature timber at 20°C is represented as blue, and the temperature gradient is linear throughout the remaining colours between the ambient timber and the char.

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Test A was terminated at 30 minutes, and the fire was extinguished approximately 6 minutes after this. The thermal profile of Specimen A at 36 minutes is shown in Figure 4-12. The full description of the results of Test A is described in Section 3.6.

Figure 4-12: Thermal profile of Specimen A at 36 minutes

The thermal model of Specimen A at 36 minutes shows approximately 25 mm charring (readout of the 300°C isotherm) for the one-dimensional regions, and 32 mm for the two-dimensional zone at the bottom of the joist. These values agree well with the charred section measurements after testing, and the corresponding calculated charring rates as presented in Table 3-6, where the joist sides and slab also measured 25 mm char depth, and the measured bottom charring of the joist was 30 mm.

On comparison with Figure 3-32 the two-dimensional charring damage of the bottom section of the beam and the beam-slab connection correlate well with the remaining residual section of the floor specimen, reinforcing the adequacy of the two-dimensional thermal model for modelling this type of behaviour.

4.9.2 Test B

As the Specimen B was tested to destruction and no meaningful recovery of the floor section could be made, no viable char damage measurements have been recorded. Figure 4-13 shows

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the thermal profile modelled for Test B at 41 minutes when the floor collapsed. The full description of the results of Test B is provided in Section 3.7.

Figure 4-13: Thermal profile of Specimen B at 41 minutes

The figure shows the extent of the expected damage to the floor slab, with a uniform char layer of approximately 28 – 30 mm surrounding the majority of the floor specimen. From the observations during the furnace testing it was noted that at 38 minutes there was a large increase in the volume of smoke escaping through the top of the specimen enclosure, corresponding to a partial loss in the integrity of the top slab. The thermal modelling output for Specimen B at 41 minutes shows the top of the slab temperatures are in excess of 250°C. This corresponds well with the temperature results shown in Figure 3-38 where the slab was close to burn through at the time of collapse.

When considering the results of Test A the thermal modelling effort predicted the impact of a fire on charring for a 36 minute timeframe well, hence it is assumed to also be comparatively accurate for the Test B results at 41 minutes. Given this is the time of collapse, the residual section as shown in Figure 4-13 shows that for the given office loading level a composite box floor would exhibit structural collapse after approximately two thirds of its cross-sectional area has been burned away under three-sided fire exposure.

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The thermal modelling output for Specimen C is shown in Figure 4-14 at 113 minutes of fire exposure time, corresponding to the time at which the specimen was removed from the furnace at 105 minutes and doused with water 8 minutes later. The full description of the results of Test C is described in Section 3.8.

Figure 4-14: Thermal profile of Specimen C at 113 minutes

The thermal output produced a char layer depth of approximately 68 – 70 mm for the one-dimensional regions of the floor including the sides of the joists and the underside of the slab, and 98 – 100 mm for the two-dimensional zone at the bottom of the joists. On comparison with the experimental results the charring ranged from 63 – 75 mm on the sides of the joists, 90 mm on the bottom chords of the joists and 73 – 76 mm on the underside of the timber slab. This shows the thermal model still produces a good estimate of damage at 113 minutes, as most measurements correspond very well with the experimental results.

A higher level of charring was measured on the underside of the timber slab which was attributed to greater re-radiation on the inside portions of the floor specimen, as shown by the cupping which occurred on the joists and the different levels of charring from the inside to

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outside edges of the specimen in Figure 3-48. Despite this, the thermal model still produces an estimate which is mere millimetres from the recorded damage after 113 minutes, hence would be appropriate for modelling applications of up to 120 minutes.

Due to the complexity of the mesh numbering system in ABAQUS and issues with creating partitions which were compatible with a structural model counterpart, the heating surface film conditions were applied to the entire edge surface as seen on the left of the figure. The section itself is small and its absence from the thermal model results in a more conservative thermal profile and thus structural results, hence it is not considered a major issue in the overall scheme of the modelling effort.

4.9.4 Test D

Similarly to Test C, Test D was terminated at 105 minutes and it took approximately 8 minutes to unload the furnace and extinguish the fire on the specimen. The thermal modelling of Specimen D is shown in Figure 4-15. The full description of the results of Test D is described in Section 3.9.

Figure 4-15: Thermal profile of Specimen D at 113 minutes

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At 113 minutes the thermal modelling output gives the same results for charring as for Specimen C, with a one-dimensional char layer located at approximately 68 – 70 mm deep for the majority of the box beam and slab. This matches well with the recorded char depths of Specimen D which ranged from 68 – 75 mm on the sides of each beam, and 75 mm on the bottom chords. The char depth on the underside of the timber slab was once again found to be 73 – 76 mm as with Specimen C, and similarly the modelling output correlates well to the measured values from the experiment.

Cavity temperatures from the thermal modelling reach approximately 90°C at the location of the thermocouples, corresponding to a measured 40°C from the testing. This shows that taking this value to be the temperature of the cavity gases is a conservative assumption.

The above thermal studies reinforce the adequacy of the thermal model in solving this type of problem, and the inputs, boundary conditions, and other factors (such as the refinement of the mesh) are also appropriate for these purposes. The current thermal modelling setup is therefore adequate to conduct the sequential structural modelling described in Chapter 5.