The original structural design was not found, then samples (300 mm x 60 mm) were took off from living room's column and beams, for metallographic and tensile tests. The objectives were to obtain steel's chemical composition and follow any tensile reduction. One sample, A1, is representative of cold regions (it was protected by walls in four sides, and the original paint was intact after wall demolishment) and served as a reference to the others (Figs. 6 and 7). Table I gives the sample's chemical composition, and Table II shows the measured tensile properties. It can be shown that it was used a high strength low alloy steel containing Copper and Chromium, that is, a weathering steel.

Fig. 4 - Apartment's plant with furniture, showing where the fire started.

Fig. 5 - Composite slab damage detail.

The micrographs reveal a perlitic-ferritic structure, very typical for a structural grade steel. It can be seen a very thin layer of decarburized steel, caused by the reducing atmosphere along the fire. For most hot rolled shape production, final rolling occurs when the steel is about 870 ^oC or higher, depending the mill procedures. Austenite is the metallographic structure, that is transformed into ferrite and pearlite along cooling to ambient temperature.

Fig. 6 - Samples positioning.

Fig. 7 - Central column .

The Iron-Carbon diagram can be used to foresee the steel phases changes. The diagram has three invariant points of witch only one is relevant to the present study. This is known as the eutectoid point and occurs at 0,8% carbon and 723^oC. At this stage austenite will begin to transform to a constituent known as pearlite which consists of alternate plates or lamellae of ferrite and cementite.

The temperatures at which transformations take place are known as the critical temperatures. Thus, the eutectoid temperature, A1, is 723 ^oC. If the steel temperature doesn't exceed, for some time, this temperature, we expect that steel mechanical properties will be acceptable.

See tables 1: Sample's chemical composition. and table 2: Tensile properties for the test specimens.

The residual mechanical properties, after cooling to ambient temperatures, will be the same as in the pre-fire condition. Smith et al⁽²⁾ and Kirby et al⁽³⁾ give experimental data for structural steels submitted to different heating - cooling cycles. Any temperature rise between 720 ^oC and 870 ^oC has a low impact on steel's mechanical properties after cooling to ambient temperatures. Any heating beyond these values will cause a permanent transformation, resulting in a grain growth, and, sometimes, in hardening that, with the cooling, will affect adversely the residual mechanical properties. Tests carried on seven samples didn't reveal any significant change on microstructure; the same is valid for the tensile properties. This permits us to affirm that structural components were not heated to temperatures up to 870 ^oC. It was clear the existence of an original adherent mill scale on the central column, a strong indication that temperatures were below 550 ^oC. Samples took from beams showed a much more rougher surface, indicating a temperature range from 650 ^oC up to ≈ 720^oC.

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