Issue 3: Improving the Technical Basis for Testing and Design of Fire-Resistant Structures
By Kathleen Almand, P.E., FSFPE
Background:
In the aftermath of the World Trade Center disaster, there has been a renewed focus on the behavior of structures in fire conditions and with it, the standard fire resistance test (ASTM E-119 or equivalent). In their recently released report of the investigation of the World Trade Center twin tower collapse, the National Institute of Standards and Technology (NIST) made several recommendations related to the test. For example, NIST recommended (#4 and #5) that the technical basis for the standard fire resistance test, resulting fire resistance ratings, and pertinent code requirements be improved. NIST also recommended (#8 and #9) that procedures and practices used in the fire-resistant design of structures be enhanced and based on performance objectives.
Although there has been much work done to develop calculation methods based on the standard fire resistance test and many compilations of the results of those tests exist, the engineering community does not have a nationally recognized comprehensive reference document on the subject of structural performance in fire at this writing. In addition, some aspects of the fire resistance test, particularly with regard to the uncertainty of the test results, remain unquantified.
Research Goals and Topics:
American Society for Testing and Materials (ASTM) Committee E-5 on Fire Tests is in the process of reviewing the NIST recommendations to determine those which are pertinent to the standards over which they have jurisdiction. Several research needs have been identified based on ASTM's review. On March 3, 2006, 25 representatives from academia, various construction product manufacturers, insurance and trade associations, and SFPE met under the auspices of the Fire Protection Research Foundation. They discussed possible research projects aimed at developing the technical basis for using the information gathered in a standard fire resistance test in order to predict the performance of a structural member or assembly under different fire exposure conditions. Additional goals for the research are increased acceptance for calculation methods and recommended enhancements to the standard fire resistance test to reduce the uncertainty of calculated structural fire performance.
Several research topics were identified, and have been divided into five following subject areas.
Furnace. Large-scale test furnaces are used to test structural assemblies to determine their fire resistance when subjected to the standard fire exposure. The following subjects were identified as benefiting from research.
- Repeatability and reproducibility of the standard fire resistance test;
- Furnace control and impact of proposed changes (different temperature probes, use of heat flux, etc.) on test results;
- Effect of furnace geometry and construction on test results; and
- Measurement and effect of furnace pressure.
Materials. The standard fire resistance test only returns general information about the test specimen. Participants identified the following areas of research to better understand component performance:
- Need for practical devices and a standard test protocol to measure effective high-temperature properties of insulation and structural materials;
- Primer paint compatibility with fireproofing; and
- Fasteners – determine the effects of fastener spacing, orientation, type.
Structural performance. The standard fire resistance test only identifies the performance of individual structural elements. The following topics regarding the performance of structures in actual fire were identified:
- Influence of span length;
- Effect of column eccentricity;
- Influence of connections;
- Restraint – quantification of restraint forces;
- Structural limit state quantification – failure, deflection limits;
- Load calculation and distribution;
- Design fires; and
- Extrapolation of single-element performance data to system performance.
Additional measurements. The following additional measurements from standard fire resistance tests were identified as being desirable to better understand performance and to better enable results of standard fire resistance tests to be used in engineering applications:
- Temperatures inside and/or on the surface of specimens;
- Variations in applied load with time;
- Incident heat flux to the specimen;
- Strain;
- Deflection profile;
- Structural boundary conditions;
- Photo and video documentation; and
- Residual post-test strength.
Design Issues. In addition to the topics listed above, the following topics relevant to design were stated as areas in which research is desired:
- Design fires – evaluate European parametric and natural fire studies; and
- Evaluation of single-element performance in structural frames.
There was general consensus that the issues associated with furnace characterization have the greatest priority. Project descriptions in each of the key areas will be developed and implementation mechanisms sought.
Benefits
The fire safety community is seeking the appropriate means to respond to the recommendations issued by NIST with respect to the accuracy, reproducibility, and extrapolation of current test methods for determining the fire resistance of structural members and systems. A collaborative research program anchored in the wealth of existing data from the standard fire resistance test and targeting the community's collective understanding of the best way to enhance its value will maximize the research investment.
Kathleen Almand is with the Fire Protection Research Foundation.
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