Material Model Modifications

It is apparent from the modelling effort that although the displacement behaviour of the floors across the range of experiments has been simulated well, the failure behaviour is less well predicted due to a number of factors. Of the major parameters controlling the failure behaviour, one of the most significant inputs is the tension strength of the floor. Focussing on this parameter alone, the literature cites a wide range of values from the spread of test data for tension strength at elevated temperatures. Eurocode guidance was followed, hence a starting point for the improvement of the model lies with changing the least known parameter.

As a modification to the original material model described above, the strength reduction factor for timber in tension is changed from the value prescribed by the literature (CEN, 2004) of 65%

at 100°C to 25%, the value used for compression strength at 100°C. The large scatter of experimental data available on the influence of elevated temperatures on the tension strength of timber emphasised the importance of varying this value from the one prescribed in the modelling. These test values are commonly derived from small scale tests considering clear specimens in pure tension. A major issue with regards to timber floors in fire is that throughout the floor section a combination of compression, shear, tension, and bending stresses act, and this is further complicated by the redistribution effects and a constantly changing stress profile due to mass loss. It is likely that the combination of stresses have a greater impact on the floor section integrity, and this tension strength reduction factor is a method for accounting for the extra unknown stresses which are not present in the small scale testing from which the data is derived from.

The results of this simple modification to the strength reduction factor are shown for the test floors and compared with the original modelling results in the following sections.

5.9.1 Test A

Applying the material model modifications to the structural model and re-simulating the case of Test A, the output for both the modified and original models are shown in Figure 5-18 compared with the experimental results. The original modelling effort is represented by a red dashed line, while the modified model is represented by a blue dashed dotted line.

It can be seen from the results that the revised failure time is approximately 35 minutes as opposed to 38 minutes, which compares closely with the experimental result.

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Figure 5-18: Modified structural modelling of Test A

The major physical processes that take place which are being approximated by the numerical model are mass loss primarily from the bottom of the section as the floor burns away in the fire.

As the top of the slab is protected from the fire this results in the neutral axis of the section rising over the duration of the fire exposure. As the bottom elements of the floor become more slender (due to charring), the tension zone becomes the critical region in which failure will occur, and tension and bending stresses are redistributed throughout the heated bottom portion of the floor.

Failure occurs when the bottom elements can no longer redistribute these stresses and the floor begins to displace at an accelerated rate, simulating runaway failure where the ultimate capacity of the timber has been reached in the remaining bottom chord. The physics of these processes implies that the critical region is the tension zone, as it is most highly impacted by the fire. This emphasises the influence of strength reduction factors, especially for tension strength under the three-sided exposure of timber floors, and their importance in determining failure in the

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The modified modelling effort for Test B is shown in Figure 5-19.

Figure 5-19: Modified structural modelling of Test B

From the figure it can be seen that the original numerical model slightly over-predicts the displacement response of the experimental floors, and failure is predicted at approximately 47 minutes. The modified model improves greatly on predicting the displacement behaviour of the test, resulting in a very close approximation for the entire duration of the test until failure at 41 minutes. The stiffness of the experimental results is very well predicted for the duration of the simulation; more so than for Test A for which a less complete dataset of displacement was recorded in the experiment.

On comparison with Test A the modification has a larger impact on the modelling difference, being an 8% reduction for the Test A and a 13% reduction for Test B. This is due to the presence of the bottom flange in the composite box floor. The heated properties are more prominent in determining failure in the composite box floor scenario, as more timber is lost from the tension zone in a more rapid manner in proportion to the entire residual section of the floor.

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The modified modelling effort for the larger joist floor, Test C, is shown in Figure 5-20.

Figure 5-20: Modified structural modelling of Test C

From the experimental observations it was estimated that the floors would fail at approximately 120 – 125 minutes. The numerical prediction of floor failure is approximately 133 minutes using the original model, and 127 minutes with the modified model. The modified model results are much more conservative than its counterpart, as was seen previously with the smaller floors, and are a better approximation to the experimental results.

Due to the larger section sizes (and hence smaller ratio of heated to ambient temperature timber throughout the section), the effect of the reduced tension strength at elevated temperatures is not as pronounced as with the smaller floors, and is not seen until the final stages of the simulation. Also, as the duration is longer for this experiment the adequacy of the thermal model is tested to a much greater extent, as any error in the overall approximation is amplified by the duration of the simulation, as was seen with the thermal modelling.

Similar to the smaller floors, the stiffness of the larger joist floor is also predicted well by the modified model throughout the duration of the fire, with the predictions becoming slightly more

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conservative with an increasing effect in the latter stages of the simulation. As previously iterated about the charred elements still carrying load, due to the larger sections involved with Tests C and D the impact of the strength reduction on the modelling output is reduced. This result is also exacerbated by the higher load levels causing the section to fail at a time where a smaller proportion of total cross-sectional floor area is heated compared with the smaller floors.

Hence the modified method produces a smaller difference in the overall predicted displacement response for the larger floors.

5.9.4 Test D

The modified modelling effort for the larger composite box floor, Test D, is shown in Figure 5-21.

Figure 5-21: Modified structural modelling of Test D

As with Test C, experimental observations suggested that the floors would fail at approximately 120 – 125 minutes. The numerical prediction of floor failure is approximately 135 minutes using the original model, and 132 minutes for the modified model. The modified model simulation results are only slightly more conservative than its counterpart, as was seen previously with the smaller floors to a greater extent, and are a better approximation to the experimental results.

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A different effect is seen in comparison between Tests C and D than was seen for the smaller floors. In this case the joist floor has a wider spread of modelling results, while the composite box floor had only a smaller range. This is again attributed to section geometry, as seen from the thermal profiles shown in Section 4.9, the two-dimensional heat interaction has developed to such an extent that no part of the residual section of the joist is at ambient. In these conditions, the heat penetration and thus the charring rate upwards through the joist greatly increases and collapse occurs quickly. This behaviour was observed in the tests conducted by O’Neill (2009) on timber-concrete composite floors made from similar LVL and tested in the BRANZ furnace to destruction. This behaviour is not seen to such an extent in Test A as the residual section of the more slender joist cannot support the applied loads at a much earlier stage of the test, hence the heat penetration has not developed to such an extent as in the longer experiments.

5.9.5 Other Modifications

Although the only modification made here was the tension strength reduction for elevated temperatures, many other factors have varying influences on the failure results obtained and may produce similar results when changed. Some of these factors are investigated in the following sensitivity analysis.

As testing timber in its multiple failure modes and achieving reproducible results is difficult, especially at elevated temperatures, many proposed reduction factors have been derived from simple laboratory tests. These tests usually are testing clear specimens in pure shear, tension or compression. A major issue with regards to modelling floors in fire is that throughout the floor section a combination of these stresses act, and this is further complicated by the redistribution effects and a constantly changing stress profile due to mass loss.

Considering floor slabs, it is likely that the combination of stresses above and the bending stresses induced from the loading have a greater impact on the floor section integrity; hence the results for increasing the strength reduction at elevated temperatures were shown to be a closer match to the experimental results. Therefore it is suggested that further research into the impact of elevated temperatures on timber properties is conducted, especially with regards to combined actions on members.

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