According to the present invention, a plurality of pillars (10); A plurality of first girders (100) connecting the adjacent columns (10); A plurality of second girders (200) connecting adjacent first girders (100); And a slab (300) supported by the first girder (100) and the second girder (200), wherein the first girder (100) comprises a frame (110); And a cross-sectional enlargement 120 formed between the frame 110 and the slab 300. According to the present invention, according to the present invention, there is an effect that the cost of the member can be reduced and the construction cost can be lowered.
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