Research On On The Opening Retrofitting Technology Of Prestressed Concrete Basement Roof

Currently, the prestressed concrete basement roof is widely used, since prestress can effectively overcome the load applied by cover earth, and thereby the crack resistance and durability of slabs are improved accordingly. However, there are increasing cases that structural opening need to be carried out for the existing prestressed concrete basement roof when the building function changes. It is worth noting that relaxation tendons and tensioned tendons as well as tendon cutting are inevitable due to the opening in prestressed concrete basement roof, which may result in significant change of internal force and displacement. Moreover, the structural opening construction is hard to be prepared and the structure may be put at high risk. This paper discusses the opening retrofitting technology of prestressed concrete basement roof, on account of Nanjing Beijige square retrofitting project, and the following conclusions can be drawn:(1) A clamped prestressed concrete slab is designed based on the method of equivalent loads to investigate the influence of the size, location and shape of opening on stress and deformation of unbonded prestressed concrete basement roof. The analysis results indicate that the size of opening is independent of negative reinforcement amount arranged in the support of slab; rectangle opening placed in the middle of slab has little effect on the displacement and moment of slab; when the hole is smaller or larger, it can ensure the security without support; when hole size in the middle, support is necessary to ensure the security(2) The design method of prestressed concrete frame is summarized and a corresponding design example, which includes a three layers prestressed concrete frame, is presented. Based on the different column spaces, namely 1×1?1×3 and 3×3, the effect of opening on mechanical properties of structural members including slab, beam and column is investigated. The analysis results indicate that opening has a significant effect on the members in the span close to opening, and yet the influence of opening can be neglected for the members in the span away from the opening; the displacement of members in the first floor of basement is changed due to opening, while the stress of members in the first floor of basement, as well as the stress and deformation of members in the second floor of basement can all be neglected; the axial force of column in projection region of opening reduces considerably, and structure in this region tends to move upward integrally; the length of opening in one direction has an effect on the mechanical properties of corresponding members in another direction.(3) Based on the prestressed concrete frame with column spaces 3×3, the effect of unbounded prestressed concrete secondary beam cutting on the security of structure is investigated. The analysis results indicate that the braces should be arranged under the secondary beam whose tendons are cutted. Comparison of the different layout scheme of brace system indicates that it is effective to arrange the braces in the mid-span of secondary beam and these braces have significant influence on the internal force and displacement of members in the first floor of basement. Particularly, to overcome the load applied by the braces due to opening, additional braces are supposed to be placed continuously in the same location of other floor, or alternative method, external prestress technology, can be also adopted.Lastly, Based on Nanjing Beijige square project, construction technology of opening retrofitting of prestressed concrete basement roof is presented, in terms of reinforcement of members, relaxation tendons, tensioned tendons and arrangement of brace system. Furthermore, the effectiveness of conclusion mentioned above is proved in this project.