On the design of adding floors and strengthening in the structural design of building engineering

1. Project Overview: The original main building of a high-rise building had 15 floors, with an underground floor area of 3980m2 and a main building area of 43200m2. The original main structure adopted a reinforced concrete frame shear wall structure system, with a seismic fortification intensity of 7 degrees, a shear wall seismic grade of level 2, and a frame seismic grade of level 3. The foundation adopts high-strength prestressed concrete pipe piles with a diameter of 500 and a wall thickness of 125mm, AB type. The bearing layer at the pile end is sandy soil and highly weathered granite. The pile length is about 25 meters, and the standard vertical ultimate bearing capacity of a single pile is 5000kN. Due to the owner's request for additional layers, the original 15 floors were increased to 21 floors, and the height of the building roof was increased from 50.6 meters to 79.7 meters, increasing the area by about 9800m2. This project is a continuation of the construction by adding layers during construction, and both designs adopt the same current specifications. Therefore, there are no related standard issues such as the design service life and the connection between new and old specifications in this project.

2. Structural layer design analysis

The key to expanding the under construction project from 15 floors to 21 floors is to solve the overall performance of the structural system, the bearing capacity of existing structural components, and reinforcement methods under the newly added vertical loads, seismic and wind loads. Firstly, conduct a comprehensive analysis and calculation of the structure to preliminarily determine the possibility of adding layers. In order to fully utilize the completed structural components and anti lateral force system, an additional 6 floors will be added for analysis and calculation according to the original structural layout plan. Using the assumption of rigid floor slabs and simulating construction loading methods, taking into account ± 5% accidental eccentricity and bidirectional seismic torsion effects, an elastic static analysis was conducted on the structure. The calculation results showed that the overall stiffness of the structure was slightly flexible and the period was relatively long. The ratio of the maximum displacement between floors to the floor height was 1/720, exceeding the limit value of 1/800 in the standard. The minimum seismic shear coefficient value of the floor was 1.51%, which was less than the limit value of 0.016 in the 7-degree zone. The seismic overturning moment borne by the bottom frame was greater than 50% of the total moment, and the axial compression ratio of some frame columns reached 1.15, severely exceeding the limit. There was an over reinforcement phenomenon in the frame beams and shear walls. Based on the problems of directly adding floors to the original structure, after multiple rounds of scheme trial design, it was determined to adopt the basic method of adding reinforced concrete shear walls, increasing the cross-sectional dimensions of shear walls and frame columns, which not only improves the structural stiffness but also enhances the bearing capacity of the structure. The upper continuation structure also adopts a frame shear wall structural system to maintain consistency and coordination with the original structure. According to the requirements of the building's functional use and the need for structural reinforcement effect, shear walls are added at the corners of the building during design, and the length of the shear walls is increased at the elevator of the original central core tube building. Considering the severe deficiency of the axial compression ratio of the original frame columns, the method of increasing the cross-section is adopted for reinforcement, while appropriately increasing the stiffness. The main results of the overall structural analysis are shown in Table 1. It can be seen that under wind and earthquake loads, the maximum inter story displacement angle of the structure meets the requirements of the current design specifications; The first natural period of the structure is the translational period, and the ratio of the first natural period dominated by torsion to the first natural period dominated by translational motion is less than 0.9; The shear to weight ratio of the structure, the overturning moment borne by the bottom frame, and the wall column axial compression ratio (maximum column axial compression ratio of 0.79) all meet the requirements of the specifications.