Discussion on the design of reinforced concrete multi-story frame structure?
According to the author's related problems and design experience in structural design and calculation for more than ten years, the author believes that the following problems in the structural design of reinforced concrete multi-storey frame buildings should attract the attention of designers.
1 Checking calculation of seismic bearing capacity of foundation and design load value of independent foundation
When the number of floors of reinforced concrete multi-storey frame buildings is low (generally less than six floors), independent foundations under columns are often used. According to article 4. 2. Code for Seismic Design of Buildings 1(GB 500 1 1) When there is no soft cohesive soil layer in the main bearing layer of foundation, it should not exceed eight layers. But the foundation design of these houses should consider the influence of wind load. Therefore, in the overall calculation and analysis of reinforced concrete multi-storey frame buildings, wind load must be input, and it cannot be ignored because the wind load of general buildings other than high-rise buildings in earthquake areas cannot be controlled. On the other hand, when designing an independent foundation, the external load acting on the top surface of the foundation (the internal force design value of the column foot) only takes the design value of axial force and bending moment, and does not take the shear force, or even only takes the design value of axial force. In both cases, the design size of the foundation is smaller and the reinforcement is less, which will affect the safety of the foundation itself and the superstructure.
2 frame calculation diagram should be reasonable
For reinforced concrete multi-story frame houses without basement, the buried depth of independent foundation is relatively large. When the foundation beam is installed at about -0. 05m, the foundation beam shall be input according to 1 floor. Taking a student dormitory as an example, the project is a three-story reinforced concrete frame structure, a class C building, and the construction site is a class II building. The floor height is 3. 3m, and the foundation depth is 4. 0m, and the foundation height is 0. 8m, indoor and outdoor height difference is 0. 45m。 According to article 6. 1. Code for Seismic Design of Buildings 2. The seismic grade of the frame structure of this project is Grade III in the 7-degree earthquake zone. According to the designer's calculation of a three-story frame house, the height of the first floor is 3.35m, which means that the frame house is embedded in the top surface of the foundation beam at -0. 05m According to the structure, design the section and reinforcement of the foundation tension beam; The foundation is calculated according to the central compression. Obviously, it is not appropriate to choose such a calculation chart. Because, firstly, the tension beam designed according to the structure can not balance the bending moment of the column foot; Secondly, according to article 7. 3. Code for Design of Concrete Structures (GB 50010-2002)11,the height of the bottom column of the frame structure should be the height from the top surface of the foundation to the top surface of the first floor.
According to the experience of engineering design, such a frame structure should be analyzed and calculated as a whole according to four layers, that is, the foundation beam layer should be input as 1 layer, and if there is a load on the beam, the load should be input together. In this way, the height of the first floor for calculating shear force should be h1= 4-0.8-0.05 = 3.15m, and the height of the second floor should be 3. 35m, the height of the third and fourth floors is 3. 3m。 According to article 6. 2. According to Article 3 of Code for Seismic Design of Buildings, the design value of bending moment at the foot of frame column should be multiplied by the increase factor 1. 15. When setting beam layer, in general, it is necessary to compare whether the reinforcement of bottom column is controlled by the top section of foundation or the top section of foundation beam. Considering the constraint of foundation soil, for such a calculation diagram, the number of floors in the basement can be filled as 1 in the total information input of the computer program, and the calculation is repeated once, and the reinforcement design of the bottom column of the frame structure is carried out according to the envelope diagram of the two calculation results.
3 The calculation model of foundation beam layer should conform to the actual situation.
There is no floor in the foundation beam layer. When using TAT or SATWE computer programs to calculate the whole frame, the floor thickness should be zero, and elastic joints should be defined, and the total stiffness analysis method should be used for analysis and calculation. Sometimes, although the floor thickness is zero, elastic nodes are defined, but the overall rigid analysis is not adopted, and the program analysis is automatically calculated according to the assumption of rigid floor, which is inconsistent with the actual situation. Pay special attention to this when the plane of the house is irregular.
4 foundation beam design should be appropriate
When the buried depth of multi-story frame building foundation is large, in order to reduce the calculated length of bottom column and the displacement of the bottom floor, the foundation tension beam can be set at an appropriate position below 0. 000, but it should not be set according to the structural requirements. Should according to the frame beam design, and in accordance with the provisions, set the stirrup encryption area. But as far as earthquake resistance is concerned, short column foundation scheme should be adopted. Generally speaking, when the independent foundation is not deeply buried, or the short column foundation is adopted although it is deeply buried, because the foundation is poor or the column load is quite different, or according to the seismic requirements, the structural foundation beam can be set along the two main shafts. The section width of the foundation beam can be 1/20 ~ 1/30 of the column center distance, and the height can be112 ~1/8 of the column center distance. The section of the structural foundation beam can be taken as the lower limit of the above-mentioned limit range, and the longitudinal reinforcement can be taken as 10% of the maximum axial force design value of the connecting column for tension or compression calculation. Structural reinforcement, besides meeting the minimum reinforcement ratio, should not be less than 2 φ 14, and stirrup should not be less than? 8@200。 When the load of the infilled wall or stair column acts on the pull beam, the cross section of the pull beam should be appropriately increased, and the calculated reinforcement should be superimposed with the above structural reinforcement. The height of the beam top of the foundation structure is usually the same as the height of the foundation or the elevation of the short column top. In this case, the foundation can be designed as eccentric compression foundation. When the height of the bottom layer of the frame is not large or the foundation is not deeply buried, sometimes the foundation beam should be designed firmly to balance the bending moment at the bottom of the column. At this point, the positive bending moment reinforcement of the pull beam should be pulled through the whole span, and the negative bending moment reinforcement should be pulled through at least 1 /2. The anchorage of positive and negative bending moment steel bars in frame columns, reinforcement of stirrups and seismic structure requirements are exactly the same as those of upper frame beams. At this time, the tension beam should be set at the top of the foundation, not above the top surface of the foundation, and the foundation can be designed according to the central compression.
5 frame structure should pay attention to the design of small shaft with stairs and elevators.
Multi-storey frame structures should avoid setting reinforced concrete stairs and elevator shafts as much as possible. Because the existence of reinforced concrete shafts will absorb large earthquake shear force, and correspondingly reduce the earthquake shear force borne by the frame structure, and the foundation design under the shafts is also difficult, so in the design process, these shafts mostly use structural columns and masonry materials as filler walls to form partition walls. When the reinforced concrete shaft needs to be designed, the wall thickness of the shaft should be reduced, and the stiffness should be weakened by opening vertical joints and structural holes. Only a small number of single-row steel bars need to be configured for reinforcement to reduce the role of the wellbore. When designing and calculating, besides determining and calculating the seismic grade according to the frame, it should also be rechecked according to the frame with vertical shaft (coupling should be considered when the plane is irregular), and the reinforcement of the column connected with the vertical shaft wall should be strengthened.
6 Reasonable selection of several important parameters in structural calculation
In order to analyze and judge whether the computer calculation results are reasonable, it is also very important to fill in the seismic fortification intensity and site category correctly and select other parameters in the general information of the computer program, in addition to the reasonable structural scheme and the correct structural calculation diagram. Taking the space finite element analysis and design program SATWE as an example, combined with the problems found in the process of structural design and calculation, how to reasonably select relevant parameters is explained. (1) Determination of seismic grade of structures In engineering design, most buildings are classified as Class C buildings according to their seismic fortification, such as civil houses, office buildings and general industrial buildings. Its seismic grade can be determined according to the intensity, structural type and height of the building in Table 6. 1. Article 2 of Code for Seismic Design of Buildings. For telecommunications, transportation, energy, fire protection, medical buildings, large stadiums, large retail malls and other public buildings, we should first determine which buildings belong to Class B buildings (or Class A buildings, which are not within the scope of this article) according to the Classification Standard for Seismic Fortification of Buildings (GB50223-95). For Class B and Class C buildings, the earthquake action is calculated according to the seismic fortification intensity in this area. For Class B buildings, in general, when the seismic fortification intensity is 6 ~ 8 degrees, the seismic measures should meet the requirements of increasing the seismic fortification intensity 1 degree in this area. ⑵ Reasonably determine the number of combinations of seismic force modes. For multi-storey buildings, when torsional coupling calculation is not taken, at least 3; When the number of vibration modes is greater than 3, it should be a multiple of 3, but it should not be greater than the number of layers; When the number of floors in a building is less than or equal to 2, the number of vibration modes can be selected. For irregular structures, considering torsional coupling, for multi-storey buildings, the number of vibration modes should be ≥ 9; If there are more structural stories or structural stiffness changes suddenly, the number of vibration modes should be more. For example, the structure has a transfer floor, a small tower at the top, and a multi-tower structure. , the number of vibration modes should be ≥ 12 or more, but not more than 3 times of the number of buildings; Only when the elastic floor is defined, the total stiffness analysis is adopted, and more vibration modes can be taken when necessary. The Code for Seismic Design of Buildings points out that the appropriate number of vibration modes can generally be the number of vibration modes required for participating vibration modes to reach 90% of the total mass. SAT2WE and other computer programs have this function, which can easily output the ratio of participation quality. Some designers do not attach importance to the application of computer program manual and choose the number of vibration modes at will, which needs to be improved. In addition, the seismic shear force calculated by coupling is usually smaller than that calculated by uncoupling. Coupled calculation is only used when the structure has obvious torsion, and uncoupling calculation is often necessary. ⑶ Determination of structural period reduction factor For frame structures and frame-seismic walls, due to the existence of infilled walls, the actual stiffness of structures is greater than the calculated stiffness, and the calculation period is longer than the actual period. Therefore, the calculated seismic shear force is too small, which makes the structure unsafe. Therefore, it is necessary to reduce the calculation period of the structure, but it is not appropriate to reduce the calculation period of the frame structure or obtain an excessive reduction factor. For frame structure, when masonry infilled wall is used, the period reduction coefficient can be 0.6 ~ 0.7; When there are few masonry infilled walls or light blocks are used, 0.7 ~ 0.8 is desirable; When light wallboard is used completely, 0.9 is preferable. Only a pure frame without walls can not reduce the calculation period.
7. Conclusion
Reinforced concrete frame structure is one of the building structures in China. The column ends and joints of reinforced concrete frame structures are seriously damaged, and its seismic design must meet the ductility design principles and relevant regulations such as "strong column and weak beam", "strong shear and weak bending", "strong joint" and "strong bottom column". In the practice of seismic design of multi-storey and high-rise reinforced concrete buildings, there are many controversies in the design due to the designers' understanding of codes and their mastery of dimensions, as well as the differences between people in structural selection, layout and calculation methods, so the seismic design method is worth further study.
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