Experimental Investigations of Fire-Structure Interaction: Advantages and Limitations

Transcription

1 Experimental Investigations of Fire-Structure Interaction: Advantages and Limitations Amit H. Varma, Sangdo Hong Purdue University NSF-NIST Fire Research Workshop June 11, 2007

2 Motivation for Experimental Research NIST BFRL researchers have conducted an exhaustive investigation of the 9/11 WTC collapse event. They have developed twenty-nine major recommendations for future work. Three of these recommendations R5, R9.1 and R9.1 are extremely important for structural engineers. R5 - The technical basis for the ASTM E119 standard fire test should be improved. R9.1 Develop and validate analytical tools, guidelines, and test methods necessary to evaluate the fire performance of the structure as a whole system. R9.2 Develop performance-based standards and code provisions, as an alternative to current prescriptive design methods, to enable the design and retrofit of structures to resist real building fire conditions.

3 Experimental Investigations of Fire Behavior There are three major types of experimental investigations that can be conducted to determine fire behavior. 1. Standard fire tests of structural components and assemblies 2. Non-standard or representative fire tests of structural components and assemblies 3. Realistic fire tests of large-scale structural systems The objectives and results of each of these types of fire tests are different. Their advantages, limitations, and uses are also different.

4 Common Issues For Fire Experiments Experimental investigations of structural fire behavior involve some common issues: a) Structural specimen, loading, and boundary conditions. These have to be representative of real structures. b) Heating following standard, estimated, or specified T-t curves. There is some possibility of applying heat flux-time curves. c) Measuring structural deformations, i.e., displacements, rotations, strains etc. at elevated temperatures, as the specimens are subjected to combined loading and heating. d) The structural loading and boundary conditions should be maintained as the heating is applied, which can be difficult. These issues have to be addressed adequately to conduct successful experimental investigations of structural fire behavior.

5 Standard Fire Tests Standard fire tests of structural components and assemblies are conducted to determine their design fire resistance rating (FRR) values. The design FRR values are central to the prescriptive design approach available in most building codes These tests are conducted using specially designed and built gas furnaces that are controlled to subject the structural elements to standard fire T-t curves This test method has been developed over several years of research and practice. It works well for its purposes. The major advantage is that the test method has been standardized and conducted all over the world. There are several limitations that have been identified over the years.

6 Standard Fire Test Advantages Standard fire tests can be conducted on structural columns, floor sub-assemblies, wall assemblies, etc. The specimens are subjected to standardized heating timetemperature curves. ASTM E119, ISO-834, and NFPA 251 T-t curves are very similar. Failure criteria include thermal and structural performance limit states. Limiting temperatures for steels, and limiting heat or temperature transmission for composite floor assemblies. Thermal performance criteria are very effective in establishing design limit states based on temperature. Excellent data on thermal behavior. Structural behavior requires interpretation Experimental data that can be used to develop and validate numerical models for predicting the thermal and structural behavior of structural components.

7 Standard Fire Test Limitations The standard temperature-time (T-t) curve keeps increasing. It does not account for the exhaustion of the fuel supply or changes in thermal boundary conditions etc. The required FRR values in the prescriptive design approach were developed using Ingberg s fire load concept with data from full scale fire tests conducted almost 100 years ago. The construction materials and contents of modern buildings have changed significantly, and the development of the required FRR values needs to be considered again. This is a limitation for the prescriptive design approach as well.

8 Standard Fire Test Limitations Gas furnaces are used to conduct the standard fire tests. The furnace temperatures can be difficult to control depending on the construction of the compartment walls. This can also lead to differences between the heating applied by two different furnaces. The heat flux incident on the specimens can vary depending on the radiation from the walls. The structural elements being tested are loaded to the maximum allowable stress, which is a combination of the nominal dead and live loads. The actual loads during a realistic fire event can be different from those used in the standard fire test. The effects of structural loading magnitude are not investigated

9 Standard Fire Test Limitations Standard tests require that the structural members be restrained at the ends (or sides) in a manner that is similar to the actual service condition. This requirement is vague, and the end restraints achieved at some of the laboratories are not quantified adequately. The end restraints can also vary from one furnace to the other and also during testing itself. There are no easy ways to account for the effects of changes in end restraints or conditions on the fire resistant rating (FRR) The measured FRR values are not normalized with respect to the material properties of the test specimens, which can vary between the specimen and actual construction.

10 Realistic Fire Tests Realistic fire tests of large-scale structural systems are very expensive and rare. They are conducted as demonstration projects to: Evaluate the performance of complete structural systems, or To develop knowledge and experimental data that can be used to verify numerical models and approaches for predicting thermal and structural behavior of systems while accounting for interaction between components etc. In some cases, real fire events provide incidental data or evidence that can be used to infer the performance of complete structural systems. The major advantage is that it provides knowledge of the realistic effects of fire loading and structural behavior The major limitation is that the experiment can be too expensive, detailed, and difficult to conduct successfully.

11 Representative Fire Tests Non-standard or representative fire tests are conducted to determine the fundamental behavior of structural components and sub-systems subjected to realistic fire loading effects The objective is to establish knowledge in form of experimental data that can be used to verify numerical models and approaches. The focus of these experiments is on the fundamental forcedeformation-temperature behavior and the effects of various parameters (loading, fire protection, etc.) Examples include non-standard fire tests conducted by researchers in U.K. (several), Hong Kong, China, etc. The major advantage is that it can have a significant impact on fire-structure interaction research. The major limitation is that there are not enough non-standard or representative fire tests being conducted. Require further developments and research.

12 Representative Fire Tests There is a growing research interest in conducting non-standard or representative fire tests, where the focus is on measuring fundamental behavior and conducting parametric studies. Specimens with more representative loading and boundary conditions, which are varied as parameters in the research The heating is applied to follow estimated or realistic T-t curves. In some cases non-standard heating focusing on the maximum temperature values is used. The effects of structural restraints and boundary conditions are quantified carefully and included as parameters. The structural deformations are measured using unconventional or innovative techniques. Research conducted in the U.K., Portugal, Singapore, etc. using such methods.

13 Representative Fire Tests Effects of axial and rotational restraint on the inelastic buckling behavior of columns subjected to different heating curves (U.K., Portugal, China) Effects of end restraints on the behavior of composite beams subjected to different heating T-t curves Component models of connection behavior at elevated temperatures Non-standard tests to quantify the behavior of different parts of the connection and force transfer: (a) compression, (b) tension, and (c) shear. These tests are very innovative and have been conducted in the U.K. and Singapore to develop fundamental knowledge of moment connection behavior at elevated temperatures Complete frame tests - with and without fire protection have been conducted recently in China (NIST collaborators).

14 Motivation for Experimental Research NIST BFRL researchers have conducted an exhaustive investigation of the 9/11 WTC collapse event. They have developed twenty-nine major recommendations for future work. Three of these recommendations R5, R9.1 and R9.1 are extremely important for structural engineers. R5 - The technical basis for the ASTM E119 standard fire test should be improved. R9.1 Develop and validate analytical tools, guidelines, and test methods necessary to evaluate the fire performance of the structure as a whole system. R9.2 Develop performance-based standards and code provisions, as an alternative to current prescriptive design methods, to enable the design and retrofit of structures to resist real building fire conditions.

15 Example - Representative Fire Tests Example from our research at Purdue University (NSF - sponsor) Research project focusing on the fundamental behavior and inelastic stability of columns under fire loading effects. Focus on fundamental F-δ-T behavior Section Moment - Curvature - Temperature (M-φ-T) behavior for different axial load levels and elevated temperature distributions Modify existing test methods and setups that measure the ambient M-φ behavior of members. Introduce heating in the critical segment, and measure deformations (curvature etc.) at elevated temperatures.

16 Structural Test Setup Axial Loading Beam P Hydraulic Cylinder Axial Tension Rod H CFT Specimen Ceramic Heaters Heated region Clevis and Pin Concrete Block

17 Heating Technology Ceramic Fiber radiant heaters were used to apply the heating The heaters were 12 x 12 in. and placed less than 1 in. away from the surfaces to be heated Each heater could provide heat flux density = 15 W/in 2 and surface temperatures of up to 1200 o C Four heaters were placed to surround the base segment of the CFT beam-column specimens PID controllers were used to control each heater individually to follow the specified heating T-t curves. Power controllers Thermocouple Heater PID Controllers HEATERS Power controller Temperature (PID) controller

18 Measurement Technology Deformations of the heated segment were measured using close-range photogrammetry and digital image processing 8 digital cameras were used to track and measure the deformations of the heated segment CFT specimens New Position Camera sensors Ceramic Heaters Initial Position Y, v X, u

19 Experimental Behavior End of test. Lateral displacement = 8 in. Local buckling failure

20 Applications The knowledge and fundamental behavior data was used in many creative ways: Was used to develop design equations for calculating the stiffness and strength of beam-columns subjected to fire loading effects. Was used to develop and validate numerical (fiber based finite element) models for predicting the fundamental section behavior, and the overall member behavior subjected to standard or realistic fire loading. Was used to develop and verify numerical approaches for predicting inelastic column buckling at elevated temperatures by integrating the fundamental section behavior along the length Currently being used to develop a new macro finite element for modeling the behavior of composite members under fire loading.

21 Even More Applications It can be used by other researchers to verify their numerical models and tools at a very fundamental level. It can also be used to develop experimental approaches for other types of structural members The heating approach is very versatile. Being used to develop a modular heating system using twentyfive radiant heating panels Each panel is 16 x 40 in. (4 sq. ft. area). It can provide heat flux up to 10 kw with a flux density of 17W/in 2 Each panel will be controlled with an individual power and temperature controller. Individual T-t or heat flux-time curves can be provided and controlled for each panel Driven by a 500 kw electrical generator System is completely modular and can be used to heat surface areas up to 100 ft 2 modularly

22 Acknowledgments NSF (Dr. Foutch is the program manager) NIST (Dr. Grosshandler is the program manager) Anil Agarwal, Ph.D. student Dr. Jay Gore, Purdue University, co-pi Peter Booth, M.S. student Caterpillar Inc. Bull Mosse Tube Thermatech Systems Inc.