Standard versus natural fire resistance for partially restrained composite floor beams: 1-Testing

Two nominally identical one-way composite steel floor beams with SFRM-protected W-shaped sections were subjected to severe fire exposure in a large structural testing furnace. The first specimen was subjected to a “standard” fire (with indefinite heating until failure), and the second test utilized a “natural” fire (with heating followed by decay toward burnout). The prototype assembly was supported by shear tab connections to a test frame, which provided partial restraint to thermal expansion. Rotation of the longitudinal slab ends above each connection was also restrained to simulate realistic slab continuity in an actual building. Specimens were loaded in four-point bending to 65% of nominal flexural capacity (representative of a full-service load condition) prior to the start of fire exposure, and the load level was maintained throughout the test. For the standard fire test, the specimen was subjected to the ASTM E119 fire curve until flexural runaway was reached with no lateral or local instability, as well as no noticeable damage to the shear tab connections or support framing. The natural fire test maintained structural integrity through burnout and cooling, after which ∼40 mm of residual midspan deflection was observed. The results of testing provide a novel comparison between standard fire resistance and natural fire robustness for the same composite floor assembly. A relationship between structural behavior and the temperature of the steel beam under both fire exposures is discussed. In a companion paper (Part 2 of this study), data from these tests is used for model validation toward parametric analysis.