Fire Resistance

Stainless Steel Fire Testing

By Catherine Houska, Consultant to IMOA

 Fire resistance, containment, and prevention are important aspects of building and structure design. The ability of a loaded structural material to retain its strength can provide additional valuable time for building evacuation.  The metals used for architecture exhibit significant differences in strength retention at elevated temperatures after even brief exposure to high temperatures.

Copper and aluminum start to experience strength reductions at fairly low temperatures. Aluminum alloys begin to show a reduction in strength at temperatures above 212ºF (100ºC).⁽¹⁾  At 400ºF (204ºC), the tensile strength of copper has decreased about 25% and 6061-T6 aluminum has decreased by about 60%. (2)

Under continuous loading, carbon steel is usually limited to a maximum temperature of 700ºF (370ºC). ^{(3, 4)} By the time steel reaches 930ºF (500ºC), it has lost about 30% of its tensile strength. Unprotected weathering steel loses about half of its tensile strength above 1000ºF (538ºC).⁽⁵⁾ Both carbon and weathering steel are normally encased in insulating materials to limit temperature increase during fires. Types 316, 304, and 430 stainless steel can withstand short exposure times up to 1600ºF (870ºC).

Five industry associations and metals producers sponsored fire and radiation resistance testing of structural materials: galvanized carbon steel, fiberglass-reinforced plastic (FRP), aluminum, and molybdenum-containing Type 316 stainless steel.(6) The tests used commercially available 33-foot long (10-m) cable ladders uniformly loaded to simulate the weight of electrical cables. The FRP and aluminum ladders failed both tests within seconds. A copy of this report, Stainless Steel for Durability, Fire-Resistance, and Safety (10042), can be downloaded from the Nickel Institute website.

In this 5-minute test for fire resistance, the metals were exposed to direct flames at 1000-1050ºC (1832-1922ºF). The results are summarized in Table 1. The galvanized mild steel passed the five-minute exposure requirement and reached 642ºC (1188ºF). The Type 316 stainless steel cable ladder passed the 5-minute test. The test time was extended to 45 minutes and it reached 705ºC (1300ºF). The deflection of the galvanized steel after five minutes was 166.5 mm (6.6 inches). The testing for the Type 316 stainless steel ladder was extended for 45 minutes, because there was additional gas in the canisters. At the end of the extended test, the deflection of the Type 316 stainless steel was only 80.5 mm (3.2 inches).

Table 1: Five minute fire resistance testing

Metal Tested	Test Result	Comments
Stainless Steel	Passed	Passed 5-minute test. The test was extended for 45 minutes until the gas ran out. The deflection was only 80.5 mm (3.2 inches) after 45 minutes.
Galvanized Steel	Passed	Passed 5 minute test with 166.5 mm (6.6 inches) deflection, some molten zinc observed
Aluminum	26 sec.	Molten aluminum was visible as it collapsed
Fiberglass	0 sec.	Collapsed

The radiation testing simulated heating by radiation rather than with direct flame and the results are summarized in Table 2. The test was continued until the ladder temperature stabilized or structural failure occurred. The galvanized steel ladder reached a stable temperature of 552ºC (1026ºF) in two hours. The stainless steel reached temperature stability at 556ºC (1033ºF) in three hours. The deflection of the stainless steel after three hours was about one-third that of the galvanized steel ladder after two hours.

View a video clip of the fire resistance and fire radiation testing programs.

Video: Extracted from a Nickel Institute video

Table 2: Two-hour radiant heat testing

Metal	Result	Comments
Stainless steel	3 hrs.	Passed 2-hour test.  Testing extended for 3 hours. The deflection after 3 hours was 1/3 less than galvanized steel deflection after 2 hours
Galvanized steel	Passed	Passed 2 hour test some molten zinc observed
Aluminum	Failed in 12 min.	Total failure
Fiberglass	Failed in 6 min.	Total failure

Darchem Engineering also determined the length of time necessary for heat to be conducted through walls.  The experiment was designed to determine how much heat is transferred through insulated cavity walls when structural sections extend through them.  The results for stainless steel and aluminum cable ladders are shown in Table 3.  The hot side was exposed to flame temperatures of 1,000 - 1,050 C (1832 - 1922 F).  The aluminum ladder collapsed due to melting after only 1:08 minutes but the test was continued on the remaining end pieces until the cold side stabilized after 37 minutes.  The aluminum legs extending through the wall to the cold side were 134 and 152 C (273 and 306 F).  There was no deformation of the stainless steel ladder during this test, and after 90 miutes the cold side legs stabilized at 80 and 58 C (136 and 176 F).  This result is not unexpected given the metals' significant difference in thermal conductivity.

Table 3:  Conduction through walls test

Metal	Result	Comments
Stainless steel	Stabilized 90 minutes	No ladder deformation during test.  Cold side legs stabilized at 80 and 58 C (136 and 176 F).
Aluminum	Stabilized 37 minutes	Total collapse by melting after 1:08 minutes.  Test continued on protruding sections.  Cold side legs stabilized at 134 and 152 C (273 and 306 F)

Testing has repeatedly shown that stainless steel retains its stiffness better than carbon steel at elevated temperatures. Furthermore, molybdenum-containing Type 316 stainless steel retains its stiffness better than Type 304 stainless steel.  Figure 1 shows the stiffness retention behavior of stainless and carbon steels at elevated temperatures.⁽⁷⁾ By 800ºC (1472ºF), carbon steel has a stiffness retention level of about 10 percent, while stainless steel retains approximately 60 percent. This higher level of retained stiffness can make it possible to avoid fire insulation. Although the densities of these metals are similar, there are thermal expansion differences that need to be considered during design (Table 4).

Continued research and innovations in stainless steel structural design will allow designers and engineers to create even more compelling structures that capitalize on the fire resistance of bare metal to express details as sculptural design elements.

Table 4: Physical Properties

Type	Density		Thermal Expansion 20 to 100 C (68 to 212 F)
	g/cm3	oz/in3	(10-6/ C)	(10-6/ F)
A 36/ A 992/ A 500	7.7	4.5	12	6.6
316	8.0	4.6	16.5	9.2
2205	7.8	4.5	13	7.2

Additional Resources:

Design Manual for Structural Stainless Steel, Third Edition (Building Series, Volume 11), Euro Inox

Fire Resistance Rating and Testing of Stainless Steels, British Steel Association

Stainless Steel in Fire: Summary Final Report, and associated work packages, Steel Construction Institute

Software for designing cold formed structural stainless steel sections (including design for fire resistance), Steel Construction Institute

Yrjölä, P. and Säynäjäkangas, J., New Design Tools for Structural Hollow Sections of Stainless Steel, Proceedings of the 6th European Stainless Steel Conference Science and Market, Helsinki, Finland, June 10-13, 2008, Euro Inox, Luxembourg (2009)

Baddoo, N.R. and Burgan, B.A., A Fire Engineer's Approach to the Design of Stainless Steel Structural Systems, Proceedings of the 6th European Stainless Steel Conference Science and Market, Helsinki, Finland, June 10-13, 2008, Euro Inox, Luxembourg (2009)

References:

1) Doran, D.K., Construction Materials Reference Book, Butterworth-Heineman Ltd., Oxford, 1992.

2) Brandes, E. A. and Brook, G. B., Smithells Metals Reference Book, Seventh Edition, Butterworth-Heinemann Ltd, Oxford, 1992

3) Elevated Temperature Properties of Constructional Steels, Metals Handbook Desk Edition, American Society for Metals, Section 4-70

4) Metals Handbook Vol. 1, 10th Edition, Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990

5) Rozen, Harold and Heineman, Tom, Architectural Metals for Construction, McGraw Hill, 1996, New York

6) Waller, G. and Cochrane, D.J., Stainless steel for durability, fire-resistance and safety, Nickel Development Institute, technical series 10042, Toronto, Ontario, Canada

7) Gardner, L. and Ng’s, K. T., “Temperature development in structural stainless steel sections exposed to fire,” Fire Safety Journal. Also see G. Waller and D.J. Cochrane’s “Stainless Steel for Durability, Fire-resistance and Safety”, Nickel Institute Technical Series.