Test C and D Displacements

In contrast to the smaller 30 minute floors, the larger 90 minute floors were designed to be

- 99 -

This resulted in very similar levels of fire endurance and displacement behaviour, as seen in Figure 3-60 where the average displacements for each floor are plotted.

Figure 3-60: Comparison of averaged Test C and D mid-span deflections

The displacement response of both floors followed a distinct bi-linear trend, with subtle differences between the results. Figure 3-60 clearly shows the reduction in stiffness and strength of the floors over time due to the impact of the fire. The change in rate of deflection occurs at such a time when approximately two thirds of the original sections have charred away (about 60 mm of each exposed face), and the residual section has become so small that the stresses become concentrated in the remaining timber. This is because as the stresses are redistributed through the section during heating and charring, a limit is reached where the section is no longer of an adequate size to properly resist the applied loads and stress concentrations occur.

For the floors tested, as the fire damage is more localised around the bottom areas of the floor sections to the three sided exposure, the tension zone is the most affected region and structural failure occurs in this region. As the timber burns away and the neutral axis of the floor specimen begins to rise upwards, the thinning beam section has localised increases in stresses in the

- 100 -

tension zone and a brittle failure will occur once the stresses are too great for the section to resist. This behaviour was seen in Test B, where the floor specimen failed at the point of maximum moment in the span (between the loading plates) and the bottom chord of the beam failed in combined tension/bending, as seen in Figure 3-39.

The composite joist floor had higher deflections initially, and then followed a slightly lower rate of deflection during the latter stages of testing when compared to the composite box floor.

However the actual measured differences in displacement were so low for much of the test that no real conclusions can be drawn from differences in the results, as they will be within the experimental error of the testing apparatus, and could also be attributed to the difference in actual fire curve followed in each test. Both floor types were shown to withstand a minimum of 105 minutes of standard fire exposure, with an estimated failure time for both floors to be approximately 120 – 125 minutes.

3.10.3 Slab and Cavity Temperatures

The maximum point and average surface temperatures are presented in Table 3-5, along with the maximum cavity temperatures reached for the composite box floor tests. These are reported for the time at which the furnace was shut off.

Table 3-5: Temperature comparisons for Tests A – D

Specimen Furnace Time

On comparison of the slab insulation failures for Tests A and B, the 36 mm thick panel failed to provide an insulating function at approximately 37 minutes. The maximum temperatures recorded over the 108 mm slabs for Tests C and D were very low, indicating that an insulation failure would not occur before the structural failure of the floors.

- 101 -

Comparing cavity temperatures, both box floors showed that by the end of testing the maximum cavity temperature was approximately 110°C. With regards to a steel tendon or other structural element, this increase may not be problematic in causing major losses in strength or stiffness.

However for any temperature sensitive services and installations further protection measures may be warranted for installation in the cavities (such as a thermal blanket wrap or internal partitioning).

3.10.4 Char Damage

The average charring rates of each major surface of the floor specimens have been calculated in Table 3-6 with regards to the total burning duration of each specimen.

Table 3-6: Calculated charring rates for Tests A – D

Specimen

Burning Duration (min)

Charring Rate (mm/min)

Beam Sides Beam Bottom Slab Average

A 36 0.69 0.83 0.69 0.74

B 43 - - - -

C 113 0.66 0.86 0.84 0.79

D 113 0.68 0.71 0.84 0.74

At much longer durations the charring rates recorded become less variable, thus can be more reliably predicted. This is seen from the temperature results recorded through the slabs in Section 3.10.3, and from the table above where the charring rates for the 90 minute floors closely match. As Test B was conducted until destruction, no viable charring results could be obtained. The average values plotted in the fair right column are the averages of the three rates shown for beam sides, bottom and the slabs. This is presented to give an approximate estimate of the overall charring rate of each respective floor, and the charring rates for the major surface groups of the floors ranged from 0.66 – 0.86 mm/min across all specimens.

Charring rates for the vertical faces of the floors (sides of the beams) were all similar, ranging between 0.66 – 0.69 mm/min. For the bottom face of the box beams in Test D, the charring rate was similar being 0.71 mm/min.

- 102 -

Tests A and C had much higher charring rates on the bottom of the joists due to the corner rounding effect where in the latter stages of burning the radii of charring on each corner of the beam intersect, compounding the vertical rate of burning up the beam. This was observed in previous research on timber-concrete composite floors tested at BRANZ (O’Neill, 2009).

The charring rate of the smaller 36 mm thick timber slab was 0.69 mm/min in Test A, while the larger 108 mm thick slab charred at 0.84 mm/min for Tests C and D. This higher rate can be attributed to the longer duration of burning where the furnace temperatures are comparably higher at this time, being about 850°C at 30 minutes to reaching temperatures over 1000°C after 80 minutes duration of the standard fire.

3.11 Conclusions

Four furnace tests were conducted on unprotected timber floor systems in the full-scale furnace at the BRANZ facilities in New Zealand. The one-way strip floors had pinned support conditions and were exposed to the ISO 834 standard fire for varying durations of 30 – 105 minutes. The floors were loaded under standard office loading conditions of 3.0kPa live and 1.0kPa superimposed dead loading. From the four furnace tests it can be concluded that:

 The smaller floor designs based on 7 metre spans resisted 30 minutes of standard fire exposure under constant load without collapse.

 The larger floor designs based on 8 metre spans resisted 105 minutes of standard fire exposure under constant load without collapse.

 The insulation criterion for the 36 mm thick LVL slabs was exceeded at approximately 37 minutes of exposure, while the larger 108 mm slabs did not suffer any insulation failures up to 105 minutes of fire exposure.

 The charring rates of the timber members were found to range from 0.66 – 0.86 mm/min across all specimens.

 When designed to resist a similar load level both the composite joist and composite box floor types had a similar response to the fire loads, however the joist floors exhibited increased upward burning through the members in the latter stages of testing which may contribute to an earlier failure for smaller floor geometries.

- 103 -