Fire Resistance Testing

PO-Laboratories conducts fire resistance tests for developers of fire protective coatings. Intense hydrocarbon fire can rapidly heat steel construction elements like columns and beams to the critical temperature, where the steel loses its strength and bends, and as a result, the whole construction collapses.

PO-Laboratories can test relatively small but representative samples with the full ranges of standard fire conditions, including ISO 22899-1, GOST Р 53295 and UL 1709.

Two pictures above showing the test conducted in a tunnel furnace, made of fire-resistant bricks and equipped with high-pressure jet burner, carefully calibrated and operated horizontally. A sample was placed in the furnace with its outer angle facing the jet burner, so the sample evenly splits the horizontal stream of fire. The temperature of the sample was recorded using thermocouples attached to the “inner” side of the sample, opposite to sides, exposed to the fire. For this test, the “sample failure” considered a time point when the temperature of the steel reaches 500 °C at any one of the measurement points. The intumescent coating properties were tested under the influence of the most severe fire conditions that possibly may occur during a real fire resulting from high-pressure releases of flammable gas. The specimens were subjected to high temperature, oxidation and mechanical effect of high-pressure jet fire. The fire intensity exceeds the jet fire standard ISO 22899-1 and the UL 1709.

Summary of standards and methods.

Fire resistance ratings of structural steel elements and assessment of performance of fire resistant coatings.

Fires cause extreme damage to commercial, residential and industrial buildings, plants, and other constructions. There are a huge variety of construction materials with different fire-resistive properties, such as highly flammable wood and plastics, moderately flammable and non-flammable, such as concrete and steel. However, least flammable steel construction elements could be damaged by high-temperature flame and cause building collapse and destruction. There are many standards and methods to assess fire resistance properties and ratings of steel construction elements. Most standards describe tests and ratings in a severe fire condition, where the steel construction is exposed to a hydrocarbon rapid-temperature-rise pool fire, as may happen on a refinery plant or was the cause of the collapse of WWT towers. The most popular standards are: ASTM E 2924, UL 263, UL1709, ASTM E119, ASTM E 1529, API 2218 and GOST Р 53295.

Fire-resistance ratings.

All of the standards describe so-called “fire-resistance ratings”; those are the number of hours or minutes that the tested specimen could be exposed to the fire in a standardized condition without reaching a failure criterion.

The failure criteria are slightly different between different standards, some describe failure criterion as certain mechanical deformation under specified load, but most standards accept failure criterion as the steel specimen reaches the critical internal temperature. The critical temperature is the temperature of the specimen at which the steel loses its 50% mechanical strength. As per UL1709 and ASTM E119, the critical temperature is 1000°F (538 °C), and no thermocouple shall indicate a temperature greater than 1200°F (649°C). The Eastern European standard GOST Р 53295 sets as a maximum just 500 °C for any of thermocouples, which is more conservative.

Standard fire parameters.

Most tests describe the standard fire parameters the same as UL1709:

The ﬁre environment within the furnace is to develop a total heat ﬂux of 65,000 ±5000 Btu/h·ft2 (204 ±16 kW/m2) and an average temperature of 2000 ±100°F (1093 ±56°C) within 5 min from the start of the test. The ﬁre environment is to be controlled by reproducing the furnace temperatures recorded during the furnace calibration.

The fire's temperature must be uniformly distributed within the furnace near all parts of the test specimen. However, the UL1709 accepts the average temperature obtained from the readings of eight thermocouples symmetrically disposed and distributed within the test furnace; proper temperatures are to be 2000 ±400°F (1093 ±219°C) 5 min after the start of the test and until the end of the test.

Test Sample

Most standards, including ASTM E 2924, UL 263, UL1709, ASTM E119, ASTM E 1529 and GOST Р 53295, suggest that the test sample (specimen) should be representative of the design, materials, and quality for which Classiﬁcation is desired. The protection material shall be applied to the steel column following acceptable ﬁeld practice.

This means that there may be tested thousands of different steel samples, representing different columns, profiles, beams rods, and so one. From the other approach, the fire-resistance rating could be easily calculated for almost any steel element using tables and equations that connect fire rating with the perimeter of the cross-section that could be exposed to fire. This allows calculation of fire ratings using structural components geometry and dimensions.

Full-scale or small scale tests.

Most of the standards recommend testing of fire rating of full-scale construction elements, such as partitions, floors/ceilings (Steel-framed, including steel bar joist framing, steel C-joist framing, and steel truss; wood-framed, including dimensional lumber, engineered joist and truss; and structural concrete.), roof/ceilings (Steel-framed, including steel bar joist framing, steel C-joist framing, steel truss and

Steel roof deck engineered joist and truss; and structural concrete.), Horizontal Membranes, Structural elements (Column, beam, through-penetration walls and floors, and joists) and exterior walls. Although the “real world” “full scale” tests could be necessary, they usually have a broad variability of results due to many factors that could be difficult to control, such as fire uniformity and stability. The full-scale tests are very pricy and take a lot of time.

Building code requirements for structural fire resistance are based on laboratory tests conducted following the ASTM E119 Standard Test Methods for Fire Tests of Building Construction and Materials. NFPA 251 and UL 263 are fire endurance tests that are virtually identical to ASTM E119. Since its inception in 1918, the ASTM E119 fire test has required that test specimens be representative of actual building construction. Achieving this requirement in actual practice has been difficult since most available laboratory facilities can only accommodate floor specimens on the order of 15 ft. x 18 ft. plan area and 9 ft. x 11 ft. for walls in a fire test furnace. Even for relatively simple structural systems, realistically simulating the thermal restraint, continuity and redundancy globally present in actual buildings is physically impossible to achieve in a test assembly within the ASTM E 119 fire test furnace.

In some of the standards as UL1709 and T Р 53295 proposed “low scale” tests, so as a relatively small steel sample (as 1 to 2 sq.ft) is being exposed to a standardized well-controlled fire. “Low-scale” tests are much cheaper than “full scale”; however, they are much more accurate and repeatable because of better control on all test parameters.

However, none of those standards allow determination of the steel structure's fire resistance, and in many cases, not even a single structural element in a real-world fire. The designer of a construction must perform different levels of fire-resistance analyzes, such as global structural analysis, member analysis and sub-structure analysis. To do such analyzes widely accepted three calculation principles:

Tabulated data calculations: directly give fire resistance time as a function of a limited set of simple parameters, such as the thickness of insulation, dimensions of the section, the load level, and so one.

Tabulated data exist only for simple elements, and it is established only for the standard fire curve. Such tabulated data, for example, could allow verification that a steel element with a defined thermal massivity and defined load level can survive a fire exposure, providing that the fire load does not exceed a certain value.
Simple calculation models: Based on equilibrium equations, simple enough to be applied for everyday practice without using sophisticated numerical software. On the contrary to the tabulated data applicable for any temperature-time fire curve, provided that adequate material properties are known. The main field of application is element analysis and simple substructures.
Advanced calculation models: Those are sophisticated computer models that aim at representing the situation as close as possible to the scenario that exists in a real structure. Applicable to any time-temperature fire curve provided that the appropriate material properties are known. They can be used to analyze the entire structure because they take indirect fire actions into account.

The most widely used standard for calculating structural steel fire resistance in North America is ASCE/SEI/SFPE Standard 29. The empirical equations there were derived based on the results of standard fire resistance tests carried out on steel structural elements and assemblies under standard fire exposure. The empirical equations often utilize factors such as W/D ratios, where W is the weight per unit length of the steel element (column or beam), and D is the heated perimeter. So the rate of the temperature rise in a structural steel member depends on its weight, surface area, exposed to fire and the thermal conductivity of protective coating if it’s used. In other words, this means that the surface of a steel structure (cylindrical column, for example) exposed to fire will adsorb a certain amount of energy per unit of time. That adsorbed energy is being distributed inside of steel, and according to the heat capacity will result in a certain temperature rise. So with standard flame, the heat ﬂux trough bare steel will be about 65,000 ±5000 Btu/h·ft2 (204 ±16 kW/m2). The rise of temperature per time (which is the fire rating) will depend only on the heat capacity of the steel sample, and as soon as the only heat adsorbing material is steel, this means that the fire rating will depend on the steel mass per surface unit or (which is the same) – wall thickness. In other words, this means that the steel element with a bigger thickness will have a higher fire rating than steel element of the same surface, exposed to the fire, but with a smaller thickness:

Fire resistance of protected steel elements.

Fire protective coatings for structural steel elements designed to reduce heat transfer from the fire to the surface of the steel element, so this could essentially lower the rate of the temperature rise inside of the steel element:

Again the fire rating depends on the steel massivity (mass or thickness per fire exposed surface) and the heat isolation properties of the coating, or (inversely) its thermal conductivity. Also, there is a specific dependence of fire rating on the shape of the steel element, which is accounted for by all three fire resistance analysis methods.

Testing and comparison of fire protection passive and intumescent coatings.

The primary function of steel fire protection is the reduction of heat transfer to the steel surface. This testing must be done using standard fire conditions because most of the coatings exhibit different transformations during heating, changing their heat resistivity at different temperatures. Also, the heat could cause altering or complete degradation of protective coating, loss of adhesion, and other effects.

Intumescent coatings require relatively high temperatures to be activated and then protect steel from further heating. The most desired orientation of the tested coating is horizontal, the coating side facing down, and the fire coming from below. This coating position is the most conservative; it allows the coating to fell off the steel surface in case of coating degradation and loss of adhesion with the steel surface.

Comparison of the different fire protection coatings, passive, active or intumescent possible by using the same steel elements of any size and any shape, providing that the standard 1000°F fire used and the zero rate reading obtained with not protected (bare) steel element of the same size and shape.

As suggested by UL 1709, small scale testing for assessment of fire protective coatings possible by using steel samples with the thickness of 3/16-in, or 4.8mm. The sample geometry is the square tube 6in X 6in heated from all four sides, which is equal to the 3/16-in X 12-in X 12-in flat steel sample heated from the coating side, providing that the opposite side is entirely thermally isolated.

From this point, the requirements of the standard UL 1709 is absolutely the same as per ASTM E 2924, UL 263, ASTM E119, ASTM E 1529, and API 2218. The only (slightly) different standard is GOST Р 53295, which suggests using the 5mm steel instead of 4.8mm as per UL 1709.

References:

1. W/D, A/P and M/D TABLES FOR STRUCTURAL STEEL SECTIONS : http://www.adfire.com/images/MD%20WD%20TABLE%20June%207%202007.pdf

2. Designing Steel Structures for Fire Safety, edited by Jean Marc Franssen, Venkatesh Kodur, Raul Zaharia, May 6 2009 CRC Press. ASIN: B00BPYPS7K

3. Standard calculation methods for structural fire protection: Standard ASCE/SEI/SFPE 29-05.

4. American Petroleum Institute Standard API 2218;

5. Fire Resistance Ratings - ANSI/UL 263;

6. Steel Design Guide, Fire Resistance of Structural Steel Framing #19 – American Institute of Steel Cconstruction Inc. AISC

7. Rapid Rise Fire Tests of Protection Materials for Structural Steel UL 1709 – Underwriters Laboratories Inc.

8. ASTM E 2924;

9. ASTM E119;

10. ASTM E 1529;

11. GOST Р 53295

12. Structural Engineer’s Guide to Fire Protection. CASE Fire Protection Committee, 2008.

If you have any questions please contact the author Dr. Oleg Nepotchatykh.