Analysis, Design & Performance

Full Session with Abstracts

342793-2 - Design and Construction of Long-Span Composite Beam Specimens for Large Structural-Fire Tests

Saturday, April 21
10:00 AM - 11:30 AM
Location: 203BC

Long-span composite floor beams have been widely used in steel-framed office buildings because of providing cost effective large column free space. In current practice, the fire resistance of composite floor systems and beams are evaluated using standard fire tests. However, standard test methods often do not require evaluating the effects of steel connections and thermally-induced restraints that could be provided by the stiffness of surrounding assemblies. Due to the size limitations of custom-built furnaces, furthermore, most of the standard tests conducted in the past focused on measuring the fire ratings of composite beams with unrestrained concrete slabs. In real buildings, concrete slabs of composite floor beams are continuous over the support girders, and the steel reinforcements in concrete slabs resist the negative bending moments induced by gravity loading. Accurately analyzing the fire behavior of composite beams has been challenged since there are many influencing parameters, such as gravity connections, shear connectors, metal decking profiles, concrete slabs, and steel framing of adjacent bays.

The first test series at the newly commissioned National Fire Research Laboratory (NFRL) is being conducted on five 12.8-m long-span composite floor beams. Recommendations from leading structural engineering firms in the U.S. on the industry practice along with U.S. codes and standards were used to design the specimens. Each composite beam specimen consists of a W18x35 beam and a 1.83-m wide lightweight concrete slab cast over a steel decking attached on top of the W18x35 beam. Composite action is achieved using steel shear connectors. Slab is reinforced with welded wire reinforcement. No.4 reinforcing bars are used for crack control over the support girders. The steel beams are sprayed with fire resistive material that was designed in accordance with UL standards to provide a 2-hour fire rating. Two types of beam connections are considered: (a) bolted/welded double-angles and (b) single-plate shear connections. Slab continuity conditions are provided in two out of the five specimens by extending and anchoring the welded wire and crack control reinforcements over the support girder. Out of the five composite beams used in this research study, one will be tested at ambient temperature to measure its flexural moment capacity and the remaining four will be tested under combined structural and fire loads.

Mechanical load will be applied at six loading points, using hydraulic actuators, along the centerline of the specimen. For the specimen tested at fire conditions, a compartment constructed surrounding the specimens with a total ventilation area of 3 m2 is used to confine the fire below the composite beam specimen. The specimens tested under structural fire loads will initially be loaded to 50% of the bending moment capacity of the ambient temperature specimen and followed by thermal load using three 1 m x 1.5 m natural gas burners. A heat release rate of 1.33 MW will be maintained at each burner for 3.5 hours followed a controlled cooling for the next 30 minutes. The temperature, displacement, and load measurements will be used to evaluate the fire performance of long-span composite beams.

Selvarajah Ramesh

National Institute of Standards and Technology (NIST)

Research Associate at the NIST.

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