Behaviour of Steel-Polyurethane Sandwich Elements in Fire (Long-Term Experience)

For many years, light steel construction has occupied a most important position in modern industrial and commercial construction. The reasons for this are complex; questions of timing and cost have been mainly decisive.

Apart from the single and double skin method of construction with steel trapezoidal sections, it has no longer been possible for over 30 years to imagine the market without steel faced-polyurethane cored panels either.

If the use of the elements was limited in the early years to non-bearing external walls, roof and door elements have also been increasingly manufactured and have found world-wide usage.

The outstanding characteristics of the elements are:
- A higher level of pre-finishing including the surface finish,
- A low installation time, resulting in economical construction largely independent of the weather,
- Outstanding thermal insulation, optimum contribution to the regulation on insulation with optimum use of space as a result,
- Reasonably priced building installation and operation costs,
- Extensive re-usability, i.e. flexibility in the case of structural changes.

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Steel sandwich panels with a polyurethane core are large compounds consisting of synthetic coated steel sheets contact galvanised on both sides with a supporting core made of polyurethane rigid foam. They are continuously manufactured on double conveyor belts and after the foam has hardened, they are separated into the required lengths and packed automatically.

Although the polyurethane rigid foam used in the panels is unsurpassed in its technical thermal insulation, it is basically combustable, like all organic substances.

For this reason, significant reservations with regard to the behaviour in the case of fires, which could not be excluded, were often expressed both by architects and clients and, in particular, by building supervisory authorities, fire services and property insurers, because of a lack of adequate experience, particularly in the early years.

This has been the case even though there have been clear regulations for the use of combustible building materials in construction contained in the building regulations of the federal states and in the associated guidelines and prescriptions.

Now, following many years of testing the panels throughout the world, there is extensive experience and findings concerning the behaviour of the panels in the case of fire. For this reason, the following observations are supported by the findings of actual fires and small and large-scale experimental fires.

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1 Typical Cases of Fire
1.1 1975, Manufacturing Shed for Bodywork Constructions

In the areas directly affected by the flames or the heat of a fire, the elements were damaged or destroyed by the fact that the foam core had burnt out.
There was no spreading of the fire within the panels in shielded areas (Figure 1).

1.2 1977, Storage Shed for Household Goods

The unshielded areas of the wall attacked by fire are burnt out and distorted in shape. The areas separated by supports that have kept their composite are clearly discernible. The straight separation inside the panels illustrates that the fire did not spread inside the panels. (Figure 2)

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Comment of the Fire Service: "As has already been established at similar fires in sheds, the roof has made a significant contribution to spreading the fire quickly. Burning bitumen and adhesive dripped down onto the large quantities of stored goods present. The external facades have behaved differently. In this case, the polyurethane foam layer only burned if it was directly attacked by fire".

1.3 1984, Production Sheds for Glued Wood Trusses

The beneficial effect of a separation on the sandwich composite is particularly clear at the transition from the outer wall to the roof. (Figure 3)

Even after the fire burned for several hours, zones of the foam core and of the sealing strip used are discernible.

In the case of this fire, it was also possible to gain insights about the behaviour of the sandwich panels used.

The upper trapezoidal section shell did not collapse during the fire, but remained as temporary protection from the weather. The fire was not spread within the roof panels and a separating wall of the building was not passed. (Figure 4)

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In all cases, it is clear that, in the first place, the behaviour of the load bearing structure is decisive to the length of time that the building will remain standing in the case of fire.

If the structure, e.g. unprotected steel (Figure 5)

or even reinforced concrete (Figure 6)

fails, the non-load bearing sandwich panels alone will not be able to prevent the collapse of the structure and associated complete destruction.

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2. Experimental Fires

It has already been possible to discern a relatively defensive contribution of the steel sandwich panels with a polyurethane core to the course of the fire from all the fire events that have been known up to date.
However, it was a common feature of all fires that they were only recognised after the event and, for this reason, the behaviour or the participation of certain components during the actual course of the fire was not apparent.
Also for this reason, a sequence of smaller experimental fires according to the test norms of the day and also large-scale experiments in accordance with special arrangements were instigated.
When the results are being applied, it is to be taken into account that both the test conditions and the construction of the test object can change.

2.1 DIN 4102, Behaviour of Building Materials and Components in Fire

The panels were examined in the fire shaft with the usual joint in the middle of the test piece. The first evaluation was carried out on the upper shells (Figure 7) and subsequently on the core layer uncovered. (Figure 8)

The middle value of the remaining non-burnt length of the test section must be at least 15 cm and, in order to achieve the building materials class B1 - resistant to fire, no test may exhibit a 0 value. Additional test criteria are also to be fulfilled. Since production began in 1968, the building materials class B1 has been achieved. Since 1980 the foam must also have a building material class B2.

2.2 NFP 92-501 Epiradiateur Test

The plate test object with the groove in the outer metal sheet is subjected to effect of a surface emitter. (Figures 9 and 10)

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Four coefficients, whose size is decisive to the classification, are determined.

Additional test criteria have also to be fulfilled. A relevant test and evaluation resulted in Class M1 (France).

2.3 SBI - Experimental Test

The SBI Test (Single Burning Item Test) has been planned within the framework of European harmonisation. In order to evaluate the chances of the usual panels in this case, an experimental test was carried out in the burner area with a thick panel and an originally unplanned groove.

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The fire in the test panel was extinguished after the experimental time had expired (Figure 11).

The foam core could be seen as only slightly affected, after the scorched outer metal sheet was removed. (Figure 12)

2.4 2.4 TGL 10685/12 Technical Construction Fire Protection

A test was carried out in accordance with the above Norm, which is no longer applied today, in the interests of orientation. After the test period of 15 minutes had expired, the remaining fire load was removed and the flames in the test object went out after a short time (Figure 13).

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This test also demonstrated only slight damage to the foam core in the direct area of the flames (Figure 14).

2.5 FMRC - Standard UBC 17-5

Both the Room Fire Test (Figures 15 and 16) and the test of the upper surface flammable characteristics of the foam core in accordance with ASTM E84 - 79a and the test of the so-called Convective Flame Spread Parameters were passed and led to the acceptance of the panels into the FMRC Approval Guide.

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2.6 Large-Scale Experimental Fire

A large-scale fire experiment was carried out to simulate the phase of a fire originating both during installation and in the case of possible repair work. To aggravate the situation, the outer metal sheet on the side of the fire was removed up to a height of 1.5m. This ensured that the B3 foam core, which was still permissible at that time, was directly subjected to the flames.

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The experiment gave the result that, even in an extreme repair situation, only a limited associated combustion of the uncovered foam core and the immediately neighbouring area is to be expected (Figures 17 and 18).

2.7 Repair Phase

A round opening for an additional tube passage was incorrectly burnt into an outer metal sheet with a welding torch (Figure 19).

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The flames in the panel were extinguished after several seconds, when the torch had been removed.
Only a barely visible bulge was to be established on the turned away outer metal sheet. There was no discolouring to the lacquer. Only a slight stress to the foam core was established, following the removal of the outer metal sheet on the side subjected to flames (Figure 20).

2.8 Natural Fire Experiment

The behaviour of the sandwich panels was also observed within the framework of a natural fire experiment on a multi-storey office and laboratory building. They also provided proof of their good behaviour in fire in this case (Figure 21).

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The first experiment on the 2nd floor did not lead to the fire spreading to the floors above (Figure 22).

After the experiment in the 3rd floor had finished, the event also came to a standstill of its own accord after the fire load had gone out. Fire-fighting measures were not undertaken in either case.

2.9 BVD Total Fire Experiment

In order to establish the behaviour of the sandwich panels in total fire, a realistic experiment was undertaken (Figure 23).

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Here, it was also clear after the removal of the outer metal sheet subjected to flames that a transmission of the fire within the interior of the panels had not taken place (Figure 24).

2.10 Large-Scale Experimental Fire in a Sandwich Roof

In accordance with expectations, the insulating core was burnt out in the area of the experimental roof directly subjected to flames. However, the fire had not spread across the combustible insulating core at the cross transition above a separating wall to the observed area. Apart from a narrow charred foam zone, the panels outside the area of observation did not demonstrate any other effects of fire (Figure 25).

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This also confirmed the behaviour of the sandwich composite already familiar from actual cases of fire.

3. 3. Calculating the Fire Load In Figures

An additional, not insignificant, aspect of the evaluation of the panels and, in particular, of evaluating risks of the application planned is the calculation of the fire load in figures (qR) in accordance with DIN 18230, 'Baulicher Brandschutz im Industriebau' (Building fire protection in industrial structures).
It can be established as the result of such a calculation that the calculated fire load will only rise very slightly by approx. 3 - 6 kWh/m² because of the use of rigid expanded polyurethane with an apparent density of 40 - 45 kg/m³, according to the thickness of the panels. At the same time, the combustion factor m was set at 0.2, taking into account the defensive behaviour of the panels, both in the case of a series of experimental fires and the resulting allocation in the building materials class B1 - resistant to fire, and because of the behaviour during various fire events.
Thus the rise in the existing fire load caused by the sandwich panels generally lies at the lower limit of the safety supplement usually fixed.

Consequently, the effects on the necessary design of the load bearing construction and on the whole thermal budget of a fire are correspondingly low.

4. The Opinion of Property Insurers, Departments of Planning and Building Inspection and Fire Services

Property insurers make higher demands on the building design with regard to the building regulations generally from the point of view of protection to property, i.e. the possible preservation of the substance of the building in the case of fire. For this reason, an extravagant technical fire protection - and thus usually significantly more expensive - construction was definitively compensated above the premium level for fire insurance in the past, according to applicable definitions of the construction class. In this context, a general change in the direction of 'made-to-measure fire protection' has taken place in the course of the years.

Today, the significance of the mode of construction is documented within the framework of the total concept of technical fire protection in the load bearing oriented evaluation of the building on the one hand and in the adjusted supplements or reductions to the basic premium rate of the Neutral Class N on the other.

The proved behaviour of the steel sandwich panels with polyurethane core has led to the fact that building structures of this type are allocated to the Neutral Class N, to the extent that the supporting frameworks are at least fire-retardant.

The departments of planning and building inspection and fire services responsible have also revised their initially justified reserve or rejection in relation to the use of these panels after giving their attention to various cases of fire and fire experiments and, as a rule, they do not raise any special reservations against their use.

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5. Summary

The following findings can be brought together as the result of the different small and large-scale experiments and many real cases of fire:

- The findings gained from standardised small experiments to determine combustibility are confirmed completely by the behaviour of the panels in practice.
- The panels do not contribute to keeping the fire going. They are only damaged in the immediate area of the effect of an external fire load and distinguish after the fire load has gone.
- The panels do not contribute to spreading the fire; the non-combustible outer steel plates prevent this on the upper surface. Because there are no internal layers of air, a chimney effect is excluded, as is the transportation of decomposing products.
- The durable, synthetic, rigid foam polyurethane will not melt or drip off. Thus, the danger of igniting secondary fires will not arise. The panels exhibit extremely favourable characteristic strengths associated with fire load. For this reason, the contribution to the whole thermal budget of a fire is extremely low.
- The opening of the connection grooves in the area of the fire causes an obvious thermal relief of the load bearing structure. This has a positive effect on its stability and, apart from this, it allows the targeted use of jet spouts, which is welcomed by the fire services.
- The toxicity of the effluent gases was evaluated as clearly lower in comparison to the usual materials. For this reason, there is no acute danger to people.
- The effluent gases will be drawn outwards together with those of the burning contents of the building.

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6. Conclusion

The behaviour of the steel sandwich panels with a polyurethane core in fire is such as can be expected from a fire-resistant building material. When they are used in accordance with the regulations, there will be no immediate increased risk.

Hans Fritz Karst
Academically trained engineer Hans Fritz Karst, born 1941, worked in the Research and Development Department of Hoesch-Siegerlandwerke GmbH for many years - his activity centred on protection against fire and corrosion. He has been a member of several national and international working committees.