In prefab construction, quality assurance is inextricably linked to the accuracy of the assembly of structures. The quality of the design will be guaranteed while observing the errors of the processes of manufacturing elements and their installation, which are specified in the norms. The normalized random errors are called tolerances. Systematic errors are regulated by permissible deviations from the nominal. Geometric tolerances in construction are divided into functional and technological.
Functional tolerances regulate the accuracy of geometric parameters in the conjugations of structures and the accuracy of the relative position of structures. Functional tolerances are prescribed on the basis of strength, insulation or aesthetic requirements for structures.
Technological tolerances establish the accuracy of technological processes and operations for the manufacture and installation of elements, as well as the performance of the necessary alignment operations.
The purpose of the purpose of the tolerances is to ensure the accuracy of the assembly of structures, by which they imply the property of independently manufactured elements to guarantee the possibility of assembling the structures of buildings and structures from them with an accuracy of their geometrical parameters that meet the performance requirements of the structures. The quantitative characteristic is the level of collection, which evaluates the installation processes performed without additional operations on the selection, adjustment and regulation of the parameters of elements.
Collectability of structures depends on the accuracy of both the production of elements, and geodetic center works and the installation of elements. Technological tolerances are assigned to these processes.
To technological manufacturing tolerances are the tolerances of the linear dimensions of the elements, the shape and relative position of the surfaces. The tolerances of linear dimensions regulate the accuracy of their production in length, width, height, thickness, as well as the accuracy of landmarks applied to the elements. The accuracy of the shape of the surfaces is characterized by the straightness tolerances and flatness tolerances, and the relative position tolerances of the surfaces are characterized by the tolerances of the perpendicularity.
The accuracy of centering processes is characterized by the tolerances of the breakdown of axes (points) in the plan, the transmission of axes vertically, as well as the tolerances of breakdown and transmission of elevations.
The accuracy of the installation of elements of prefabricated structures is controlled by the tolerances of combining landmarks (points, lines and surfaces) and the tolerances of symmetry of the installation of elements.
Accuracy of installation of building elements with a free method of installation depends on the applied technological methods of performing processes, mounting devices and tools, as well as methods and means of accuracy control. Installed six classes of control accuracy of installation.
The first class of accuracy is ensured when the top of the element is installed in the design position by fine-tuning in several stages with the help of adjustable mounting devices (struts, end posts, jigs, jacks, etc.). In this case, the accuracy of the alignment of the installation risks is controlled by a theodolite.
The second and third classes of accuracy are achieved by controlling the accuracy of the installation of elements using a plumb, rack-plumb, rack-level and other simple measuring tools and fine-tuning them with adjustable mounting fixtures or a crowbar.
Fourth and fifth accuracy classes are provided when using a mounting crane for alignment. In this case, the control is performed using a plumb line. For the sixth class, the installation of the element in one step without finishing with a visual quality control is characteristic.
There are two methods of installation of prefabricated structures: free and restricted free. With the free method of installation, orientation and installation of structures are achieved by the joint actions of installers and the movement of the crane. The position of the structure is corrected by means of struts, clamps, braces, single conductors connecting the installed element with the previously mounted ones. The accuracy of the installation in this case depends on the qualifications of installers.
With a restricted-free method, the movement of the structure is limited to one or several directions. For such restrictions use stops, latches, group conductors. This method greatly simplifies the work of installers, contributes to improving the accuracy of installation and reducing the time spent by the crane and workers on the installation of the assembly element. The disadvantage of the method is the high consumption of metal for fixtures, the complexity of their installation and dismantling.
During the construction of large-panel buildings, deviations from the design position in the plan are allowed for walls within 5 mm, the height of the upper bearing surfaces should be aligned with errors of less than 10 mm, and the front surfaces of 5 mm. The offset of the axes of the panels and partitions in the lower section relative to the centering axes should not exceed 3 mm. The vertical axes of the panels of internal load-bearing walls located one above the other must coincide; misalignment of the axes of these panels is allowed no more than 10 mm. The offset in terms of floor slabs and coatings relative to their design position on the support surfaces should not exceed ± 20 mm.
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