Patent Number: 051195984
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of construction of the top slab of a nuclear containment building 2. Related Art In the procedure for constructing a nuclear containment building (hereinafter referred to as the building), there have been two traditional methods of constructing the top slab on top of a cylindrical wall (hereinafter referred to as the wall) structure. A method of construction will be explained in reference to FIG. 7. In an area bounded by the wall 1, a floor slab 3 and the shield wall 2 of the pressurized reaction vessel (not shown), a plurality of support columns 4 are built to support radially-arranged support beams 4a which, in turn, act as a support for a slab support 6. After installing reinforcing members, the top slab 7 is poured, as illustrated by a dotted line in FIG. 7. Another method of constructing a top slab in an area sectioned off by the wall 1, a floor slab 3 and the shield wall 2 of the pressurized reaction vessel is explained with reference to FIG. 8. In this method, support components 9 and 10 are first constructed on the top sections of the wall 1 and shield wall 2. A plurality of large beams 11 are spanned across radially said components 9 and 10 in between the upper and lower steel reinforcements (hereinafter referred to as upper and lower reinforcements) of the top slab 7. Said large beam 11 are supported by means of hanging bolts 12 and anchors 13a which are welded to the back side of the slab liner 6. The top slab 7 is formed by pouring concrete as indicated by dotted lines in FIG. 8. Practising of these procedures presents the problems of installation as described in the following. In the method of FIG. 7, the support columns 4 and 4a sustain the stresses imposed by the slab liner 6, the weight of the reinforcements and the concrete as well as the stresses accompanying installation activities. However, the floor spaces on said floor slab 3 are limited because of a large number of facilities, such as other equipment and piping, which are already installed inside the building. Consequently, it becomes difficult to place said columns in all of the optimum positions to support the loads imposed by the heavy section top slab and accompanying installation activities. Furthermore, integral fastening metal components at the bottom surfaces of the slab liner 6 mechanically interfere with the top surfaces of the support column 4 at the points of contact with the said supports. In addition, this technique requires a large number of columns and trusses to be temporarily erected on top of the floor slab 3, and upon completion of the installation of the top slab 7, all of the said columns and beams need to be removed from the interior of the building. However, as stated earlier, much of the interior spaces are already filled with various equipment, and it is extremely difficult to secure open paths to dismantle and transport such masses of residual materials. In the method of FIG. 8, on the other hand, a load supporting member is constructed by pouring concrete in the spaces available in beam 11. However, because of the limited distances between the upper and lower reinforcements in top slab 7, the substantial height of said beam 11 becomes restricted and approaches that of a full web beam, and therefore, the elasticity of the top slab 7 presents a problem. This technique has an additional problem of not being able to optimize positioning of the load-bearing components of the top slab, such as beam 11, hanging bolts 12, liner anchor 13a, on the one hand, and the reinforcements within the top slab 7, on the other. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved means for constructing the top slab of a nuclear containment building to enable effective utilization of the spaces above and below said slab during the construction phase. Another object of this invention concerns an improved means for constructing the top slab of a nuclear containment building, to enable optimum placements of load bearing components of said slab without being constrained by the presence of steel reinforcements within the said slab. One aspect of this invention concerns an improved method for installing a top slab on top of a section of a nuclear containment building bounded by the wall, a floor slab and the shield wall of the pressurized reaction vessel. A top-load carrying member is spanned across several support columns constructed on top edges of the wall and the shield wall. A bottom-load carrying member is constructed by attaching slab anchors to slab liner by means of hanging devices, followed by the installation of reinforcements within the said member. The resulting assembly is encased in concrete to cover the slab liner assembly. By following this construction procedure, it is possible to reduce the number of beam components within the top slab, as well as the number of support columns to be erected on the floor. The doubly beneficial results are achieved because the construction load is supported by both the upper- and the lower-load carrying members thereby improving the structural elements, and also because the improved slab requires less support columns thereby reducing the column placement restrictions imposed by the existing equipment. The construction efficiency can also be improved by placing the said load carrying member in parallel to the shielding wall to create less interference with the flow of construction activities than that experienced in the traditional method. Furthermore, the efficiency of clean up operation is improved because there are lesser number of columns to be removed from the construction site than those in the traditional method of construction. It is a further aspect of the present invention to provide an alternative improved method of constructing top slab. In this method, several temporary support columns are constructed on top edges of the cylindrical and shield walls, and a lower load-carrying-member is constructed by attaching slab anchors to slab liner by means of hanging devices, followed by the installation of reinforcements within the said member. Next, an upper-load carrying member is spanned across temporary support columns, constructed on top edges of the cylindrical and shield walls. The upper and lower load carrying members are mechanically connected by means of hanging components, and this hanging assembly and the reinforcements are encased in concrete to cover the slab liner assembly. By following this construction procedure, it is possible to eliminate slab liner support columns erected on the floor slab before the construction of the upper load carrying member, because of the presence of the temporary support columns on top edges of the cylindrical and shield walls. This procedure avoids interference with equipment and facilities already existing on the floor slab and eliminates the problems associated with traditional techniques.