Measurement of Structural Performance in Fire

Summary

This project encompasses real-scale experiments to provide the technical data necessary to advance the performance-based design of structures under realistic fire conditions, validate physics-based computational models to predict the performance of structures in fire, and evaluate new metrology techniques to assess structural responses during a fire.

Description

Compartment fire test of a two-story steel gravity frame with composite floors.

Credit: NIST

Objective - To support the development of performance-based building codes, validate computational tools for structural fire resistance, and apply new metrology techniques to characterize structural responses to fire through large-scale experiments.

What is the new technical idea? Innovation in design and materials for fire resistance of structures will be accelerated through a transformation from the current prescriptive approach to performance-based methods. The nearly century-old, prescriptive testing standard for the fire endurance rating of structural components stifles industry innovation in the design of structural systems and connections. The standard fire test method provides little insight into the performance of the structure as a system – including its connections and nonstructural elements – in real building fires that can be vastly different from the standard fire. The transition from prescriptive to performance-based design requires experimental evidence of the performance of structures as complete systems.

The National Fire Research Laboratory (NFRL) provides the unique capability to measure the performance of real-scale structural systems mechanically loaded to simulate service (or collapse) conditions and exposed to a real fire. Data resulting from experiments conducted in the NFRL will be used to investigate the behavior of structures in fire and validate computational models that predict the system-level fire performance of the structure. The new research capabilities of the NFRL also enable researchers to explore innovative measurement techniques to better characterize fire-structure interaction and develop an experimental database on the performance of large-scale structural connections and systems under realistic fire and mechanical loads. The National Institute of Standards and Technology (NIST) works closely with code-writing bodies such as the International Code Council (ICC), and standards-developing organizations such as the American Society of Civil Engineers (ASCE), Society of Fire Protection Engineers (SFPE), National Fire Protection Association (NFPA), American Society for Testing and Materials (ASTM), American Institute of Steel Construction (AISC), and the American Concrete Institute (ACI), as well as the International Standards Organization (ISO) to incorporate results into design methodologies.

There are currently two thrust areas in this project:

(1) Steel framed structures with composite floor systems
Composite floor system is commonly used in the design and construction of steel-framed buildings. Analyzing the fire performance of composite floor systems, however, is challenging since there are influencing factors, including gravity connections, concrete slabs, and shear connectors, that result in a complex fire-structure interaction modeling of the overall system. The NIST World Trade Center investigation [1] identified potential vulnerabilities of composite floor systems in uncontrolled fires. Further, a recent study of composite steel structures by Ove Arup and partners [2] described “lessons learned about the detailed response of [composite] structures under fire loading” that included issues related to structural layout, sources of thermal restraint, and connections. As noted in the NFPA report [3], “a focus on large scale experiments related to the many unanswered questions about composite floor system performance would have great practical import and a major impact on design methods.”
The primary goal of this project is to conduct a series of experiments on a two-story, multi-bay steel-framed structure with steel-concrete composite floors subjected to a real fire. The test structure will be designed and constructed as a typical mid-rise steel-framed office building following U.S. standards and practice. The work also includes a sub-project in which long-span (12.8 m), one-way spanning, isolated composite floor beams will be tested to failure under combined mechanical loads and a structurally significant fire. Test variables examined through this research area include:

• Floor plate geometry as it affects the development of a compression ring;
• Influence of beam-to-floor slab shear transfer via headed shear studs;
• Thermal restraints provided by connections and stiffness of surrounding structures; and
• Fire exposure.

(2) Cold-formed steel structural systems
Cold-formed steel construction represents roughly 25 percent of the multi-story nonresidential building market in the United States; enabling about $140 billion in construction in 2015 [4]. There is little data about the performance of lateral force-resisting systems (for wind and earthquake) for cold-formed steel framed structures under combined mechanical and fire loading. There is a need to characterize the behavior of these systems under multi-hazard actions to understand: Post-fire lateral capacity (the strength to withstand horizontal force after a fire); Post-earthquake fire behavior (the ability of the building to limit the spread of fire in the case of fire following the earthquake); and Post-earthquake, post-fire lateral capacity (the residual lateral strength after a fire following earthquake event). Such data will inform design decisions about fire compartmentation when significant lateral deformation of the building is anticipated, post-fire assessment of structures, as well as first responder decisions to enter a building when earthquake aftershocks are likely.

In the first two Phases of work in this thrust area, individual cold-formed steel shear walls common in the United States will be subjected to quasi-static, revised-cyclic mechanical load and exposure to real-fire. Variables addressed by this research include:

• Wall type: Sureboard™ shear panels; Oriented strand board (OSB) shear panels; Steel strap braced walls; and Mixed lateral and gravity walls system.
• Fire exposure: Standard fire (ASTM E119); Natural Severe Fire; Natural Mild Fire.
• Wall fire-resistance.

A third Phase to test a multi-story cold-formed steel structure is being considered.

What is the research plan? The composite floor systems thrust area has five major tasks:
Task 1-1 – Design of Experiments: This task will provide the design of structural fire experiments in the NFRL. Two series of tests will be conducted: isolated member tests, and three-dimensional frame tests. The specifics of the test program have been determined through a series of stakeholder meetings with experts from national and international structural and fire protection engineering industry and academia.

• The isolated member tests consist of 12.8-m (42-ft) span composite floor beams loaded hydraulically to simulate the gravity service loads and simultaneously exposed to a post-flashover fire. Test variables include: (1) types of shear connection and, (2) floor slab continuity.
• The three-dimensional test frame is a two-story, two-by-three bay gravity frame with story heights of 3.3 m (11 ft). The test bay measures 6.1 m (20 ft) by 9.1 m (30 ft) in plan. The test bay will be loaded hydraulically to simulate the gravity service load condition. For this series of tests, the columns will not play a role in floor failure; rather the columns will be protected so that they provide a reliable load path. Test variables include: (1) the concrete slab reinforcement, (2) the fire protection scheme, and (3) restraints provided by adjacent bays and connections. The fire designed for the series of experiments will be confined to the test bay, and the thermal load will simulate a fire in an office setting, up to and beyond a flashover condition.

Task 1-2 – Design of Test Fires: This task will characterize and measure repeatable structurally significant fires for use in structural fire experiments in the NFRL. To conduct high-quality and repeatable fire experiments requires the design of the fuel delivery system (e.g., natural gas burners) to produce the desired heat release within an acceptable level of uncertainty.

Task 1-3 – Execution of Experiments: This task involves the execution of experiments under the 20 MW exhaust hood. The task will include fabrication and erection of test setup in accordance with standard construction practice as specified in (a) ANSI/AISC 303 Code of Standard Practice for Steel Buildings and Bridges and (b) ACI 117 Specifications for Tolerances for Concrete Construction and Materials and Commentary. Experimental data will be recorded along with video capture of critical regions. The measurement includes (1) the characteristics of test fire(s) using the heat release rate, gas temperatures, and radiation, (2) thermal response (temperatures and heat fluxes) of composite floor systems and fire barrier walls, and (3) structural responses (displacements, forces, and strains). Fire, thermal, and structural measurements noted above and any noteworthy observations critical to understanding the performance of the test structure under mechanical loads and fire(s) will be documented and disseminated.

Task 1-4 – Development of Standard Test Protocols: This task will develop standard test protocols for structural fire experiments, including characterization of fire and structural loads, and thermal and structural response measurements. The standard test protocols will ensure that results from a variety of tests can be compared and evaluated on a consistent basis.

Task 1-5 - Quantification of Uncertainty in Test Data: This task will implement statistical methods for uncertainty analysis to systematically measure and reduce the uncertainty in the fire, thermal and structural measurements carried out in this project.

The cold-formed steel structural systems thrust area has the following major tasks:

Task 2-1 – Market Analysis: This task reviews available market data on cold-formed steel construction in the United States to establish economic significance for the U.S. construction industry and the prevalence of various structural system types.

Task 2-2 – Design of Experiments: This task includes (i) development of test programs, (ii) design of the test setups, specimens, and fire loading, (iii) development of instrumentation plans, and (iv) preparation and approval of hazard reviews. Three Phases (series) of tests are anticipated: isolated wall tests linked to a 6-story building test conducted at the University of California, San Diego [5], isolated wall tests with various wall systems and fire exposures, and a multi-story building test.

• Phase I A series of 14 tests will be conducted on six 2.7 m × 3.7 m shear wall specimens consisting of cold-formed steel framing sheathed on one side with sheet steel adhered to gypsum board and on the opposite side with plain gypsum board (Sureboard™). The specimens will be subjected to various sequences of simulated seismic shear deformation and “mild natural fire” to study the influence of multi-hazard interactions on the lateral load resistance of the walls. The test program is designed to complement a parallel effort at the University of California, San Diego to investigate a six-story building subjected to earthquakes and fires.
• Phase II This phase extends the results of Phase I to two additional types of cold-formed steel shear walls common in the United States (Oriented strand board (OSB) shear panels; Steel strap braced walls) and two additional levels of fire exposure (Standard fire, Severe natural fire). Additionally, combined lateral and gravity walls may be investigated. About 50 tests of 2.7 m × 3.7 m shear wall specimens are anticipated. This phase of work also includes Monte Carlo simulations to establish the “natural fire” curves and deployment of new measurement techniques developed in the NIST Operations and Metrology project.
• Phase III This phase would study the structural collapse behavior of a multi-story cold-formed steel framed building under combined mechanical and real-fire actions. The test would afford opportunities to study fire dynamics in large open building spaces and floor-to-floor spread of fire on the exterior of buildings, in addition to investigating structure-fire interaction.

Task 2-3 – Execution of Experiments: Experimental data including heat release rate, temperature, heat flux, force, deformation and strain will be recorded and archived.

Task 2-4 – Documentation of results: This task will document and disseminate the test results.