: Recently steel flat decks used for concrete slab moulding have been utilized at many construction sites. At the same time labour accidents caused by fractures of steel flat decks are being reported. Typical accidents mainly are caused by fracture of steel flat decks were investigated. As a result, it became clear that there was a problem with the strength-property of a flat deck which causes occupational accidents. We intended to give the risk information of flat decks to safety managers in the construction industries.
    The typical flat decks which are used in the building construction now were examined to clarify their strength property. In order to know the strength property of flat decks, actual experimental measurements were carried out for three types of steel flat decks. Three thicknesses of steel materials of flat decks and two lengthes were examined. Bending load test, load test for the end-closed of flat decks and compression test for rigs of flat decks were carried out. Two different types of fixation of the end-closed of flat decks, i.e., welded and non-welded, are examined. The following results are obtained:
  (1) Safety factor of an allowable stress of flat decks is designed to be 1.3 on the base of the voluntary guideline made by the Association of Flat Decks Manufacturing Industries.
  (2) According to results of bending tests in case of non-welded edges of flat decks, average safety factorsactual fracture bending loads to allowable bending loads are 1.35 in 0.8 mm thickness, 1.51 at 1.2 mm thickness, and 1.82 at 1.6 mm thickness. It implies that risk of fracture of flat decks increases, as the thickness of flat decks material becomes thinner.
  (3) In the case of welded edges of flat decks, an average safety factors for bending loads are 2.28 at 0.8mm in thickness, 2.48 at 1.2 mm thickness, and 2.78 at 1.6 mm thickness.
  (4) Comparing the strength of end-closed of flat decks at 50 mm of bearing width in case of non-welded edges of flat decks, the strength at 20 mm of bearing width is 57% of strength at 50mm of bearing width, the one at 35 mm of bearing width is 76%. As the bearing width become shorter from the standard width of 50 mm, the risk of fracture of flat decks increases inversely proportional to the bearing width.
  (5) The strength of a rib for shoring loads is greatly safer, because safety factors are 4.0 at 0.8 mm thickness, and 4.2 at 1.2 mm thickness.