Application of Unbonded Prestress in Building Construction
The most widely used unbonded prestressed concrete in housing construction is the floor structure, and the prestressed anchor technology is also after the emergence of unbonded prestressed tendons in the 1970s. It has been widely used in housing construction. The application of unbonded prestressed concrete in the floor structure reduces the height of the structure and increases the space of the building. In the general floor structure, it can be divided into the one-way slab and two-way slab. According to different structural functions, it can be designed into a two-way deck without beams and caps, two-way flat plates with caps or pallets, multi-ribbed plates, flat beams with wide beams, and two-way flat beams supported by peripheral beams. Slabs with wide flat beams have been widely used recently because of their good bidirectional seismic performance.
The structural height design of the unbonded prestressed concrete slab is based on the experience at home and abroad. For the unidirectional slab, the thickness should be 1/40-1/45 of the span; the thickness of the bidirectional slab of the slab-column system should be the long side of the column net. 1/40- 1/45; For two-way flat plates with flat support plates (measured by the extension of the column center in all directions, the extension length of the flat support plate should not be less than 1/6 of the span of the plate, and the thickness of the flat support plate should be greater than 1.5 times Plate thickness) The thickness should be 1/45-1/50 of the long-side dimension of the column net; for multi-ribbed plates, the rib height (including plate thickness) should be 1/30-1/35 of the long-side dimension of the column net.
Internal force analysis of unbonded prestressed concrete slabs According to the structural characteristics of the slab, elastic or elastoplastic analysis can be used. The force calculation is mainly divided into a one-way system and a two-way system. The rectangular column net unbonded prestressed concrete slab and multi-ribbed slab under vertical load can be calculated according to the equivalent frame method.
The equivalent frame method is to simplify the three-dimensional structure into two orthogonal equivalent frames passing through the axis of the column, as shown in Figure 4-22. When there is no lateral force member support, the effect of the lateral horizontal load should be considered, and the internal force generated by the horizontal load should be combined in the plate of the equivalent frame.
In the high-rise anchorage structure with shear walls or lateral force-resistant members, the horizontal load is mainly borne by the lateral-force members, and the floor slab is mainly subjected to the vertical load. At this time, the slab width of the non-drying prestressed concrete slabs and multi-ribbed slabs of the rectangular mesh can be taken as the sum of the half spans on both sides of the column.
The analysis method of the ACI specification decomposes the equivalent frame into three parts: 1) horizontal slabs; 2) columns or other supporting structures; 3) torsion plates on both sides of the columns that transmit bending moments between the columns of the slabs band. The bending moment transmission between the horizontal plate belt and the vertical members is related to the connection between them and the relative stiffness. In the three-way system, the bending moment of the column is smaller than the value obtained by the slab-column frame calculation model. The rotation capacity of the column must be considered. An equivalent column with an increased effective length can be used instead to reduce the bending of the column.