Fire Endurance Testing

Author: Abbas Nanji

As per the National Building Code of Canada (NBCC), the fire rating of structural steel members, structural floor and roof assemblies is to be determined based on fire tests conducted in accordance with CAN/ULC-S101, “Standard Method of Fire Endurance Tests of Building Construction and Materials.” Based on fire tests, certification and testing laboratories publish designs with unrestrained and restrained conditions.

For conventional steel construction comprising of steel beams and open-web steel joist (OWSJ) with metal deck and concrete topping, the fire test standard provides for both the restrained and unrestrained ratings to de determined from one fire test.

The steel members are protected by thermal insulating materials. The most common thermal insulating materials are sprayed applied gypsum and cementitious based materials, intumescent and mastic coatings, gypsum board and ceiling membranes.

The temperature of the test specimen is measured with thermocouples that are installed on the unexposed surface of the test specimen and in the steel members. The test specimen is constructed in a rigid steel frame which restrains the thermal expansion of the structural members; rotational restraint is not provided.

The fire test is conducted with the test specimen loaded as close as practicable to the factored resistance of the test specimen. The loads are calculated in accordance with the appropriate design standard published by the CSA Group.

The underside of the assembly is exposed to a fire controlled by a time temperature curve. The fire exposure curve increases monotonically during the entire test period, implying an inexhaustible supply of fuel. The temperature of the fire specified in the standard is based on cellulose fire.

Representative temperatures of the fire at during the test are shown in Table 1.

Table 1. Fire Test Time Temperature.

Time (hours: minutes)Temperature oC

The limiting criteria to determine the fire endurance period are as follows.

  1. Temperature rise limit on the unexposed surface of the test assembly.
  2. No hot spots or opening created on the unexposed surface of the test assembly.
  3. The ability of the test assembly to support the test loads.
  4. Temperature of the structural member(s). (This represents structural failure to support the test load based on loss strength at elevated temperatures).

The unrestrained endurance period is determined at the time when any one of the four criteria is exceeded. For restrained fire endurance period of one hour or less the requirements are the same as for the unrestrained endurance period. For restrained endurance period over one hour the restrained rating is based criteria 1 to 3 and the temperature criteria of the structural member is at half the endurance duration but not less than one hour and not more then twice the unrestrained endurance period.

In the fifth edition of the standard issued in 2014, the standard was revised to include test requirements to address loaded unrestrained beams using deflection and rate of deflection criteria as an alternate to the beam temperature criteria. This made it possible for the unrestrained test and restrained test to be conducted separately on duplicate test specimen. This change was driven by new types of construction like, Slim-Beams, Delta Beams where the steel beam is partially or fully encased in the concrete floor. In this type of construction unprotected and partially protected parts of the steel member exposed to the fire would exceed the temperature criteria very early during the fire endurance test while the test assembly is still capable to support the test load.

In this case the unrestrained test is conducted on the test specimen, and for the restrained endurance period a separate fire test is conducted on a duplicate test specimen that is restrained in a rigid frame. When two different tests are conducted on identical test specimens the limiting criteria numbered 1 to 3 stated above are also applicable. However, the temperature criteria of the structural member is replaced with a limiting deflection and rate of deflection criteria.

From the results of the fire endurance fire test, testing and certification agencies publish Fire Resistance Ratings to the closest smaller duration.

Appendix A in the fire test standard provides a guide to determine conditions of thermal restraint for floor and roof assemblies and individual beams in buildings.

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