1 preface
Multi-storey concrete frame building structure is flexible and clear, especially this kind of building frame has strong integrity and good earthquake resistance. The characteristic of multi-story concrete frame building structure is that it is widely used in modern building structures. However, we should grasp it well in actual design and application, otherwise various design problems will appear. Based on the author's years of practical experience, the structural design of multi-story concrete frame building is analyzed and discussed below.
2 Multi-storey concrete frame foundation bearing capacity and independent foundation design analysis
In the design of multi-storey concrete frame buildings, certain principles are generally followed, especially when multi-storey concrete frame buildings are below six floors, most of them use independent foundations. This is carried out according to the requirements specified in the relevant codes for seismic design of buildings. If the main bearing layer of the foundation does not contain too much soft clay, and there are no eight floors of the building, and the height is generally not higher than 25m, the calculation of the seismic bearing capacity of the natural foundation and the related design calculation of the seismic design of the foundation can be omitted. These civil multi-story concrete frames or multi-story frame factories with similar building foundation loads can be exempted from these designs and related seismic bearing capacity calculations.
However, the influence of wind load must be considered in the design of the related foundations of these multi-story concrete structure frames. It can be said that it is very important to consider the influence of wind load in the overall calculation of the structural design of reinforced concrete multi-story frame buildings. We can't ignore or not calculate the design value just because the wind load of multi-story concrete frame buildings in earthquake area is not the main control function. In addition, when designing an independent foundation, it is necessary to analyze the external load at the top of the foundation, which is generally the internal force design value of the column foot. Not only some design values of axial force and bending moment, but also shear force should be considered. When considering these design values, we must comprehensively analyze the design to avoid the problem of small foundation design size. At the same time, we should also pay attention to ensuring the safety of the independent foundation itself and the construction.
3 Design of calculation diagram of multi-story concrete frame and analysis of examples
In view of the multi-story concrete frame building, if there is no basement, its independent foundation is generally buried deeply. Generally, the relevant foundation beam should be set at -0.05m, and the foundation beam should be input according to the first floor design. Specifically, taking a multi-story concrete frame building in a certain place as an example, in actual design, it is a three-story concrete frame structure, which belongs to Class C building, and the construction site in this case belongs to Class II building. The height of the frame is 3.3m, the buried depth of the foundation is 4.0m, the height is 0.8m, and the height difference between the inside and outside of the frame building is 0.45m.
According to the relevant seismic design standards, this case is generally located in the 7-degree earthquake zone, and the seismic grade of this multi-storey concrete frame building is Grade III. Relevant designers have calculated and designed the three-story concrete frame building. In the calculation, the height of the first floor is 3.35m, and the frame building is set to be embedded in the top surface of the -0.05m foundation tension beam, and then the cross section of the foundation tension beam is designed, and the reinforcement structure is analyzed and designed. Generally speaking, the first value is calculated according to the central compression. To be sure, in this way, the calculation result of the sketch is improper.
In actual design, designers must pay attention to the structural design of the beam, which can not balance the bending moment of the column foot, and also pay attention to the value of the height of the bottom column in the multi-story concrete frame structure, which is generally calculated from the top surface of the foundation to the top surface of the first floor. According to the practical experience of building engineering, the design and analysis should be carried out according to the four-story frame structure, that is to say, in the foundation beam layer, the input must be according to the first story, and if there is a load on the beam, the load on the beam should be calculated. Finally, it is concluded that the height of the first floor of the frame shear is 3. 15m, while the height of the second floor is 3.35m, which is 3.3m relative to the third and fourth floors. Then, according to the relevant regulations and standards of building seismic design, the bending moment of the column foot at the bottom of the frame column is calculated, which is usually multiplied by the coefficient 1. 15.
In the process of frame design, if there is a beam layer, the problem that needs attention is that the reinforcement of the bottom column needs to be analyzed to see which aspect to control, which is the control of the top section of the foundation or the equal section control of the top section of the beam. In addition, it is scientific to comprehensively analyze the foundation soil problems and then calculate the schematic diagram. In the actual computer calculation process, the number of floors in the basement can be set to 1, and then the calculation is repeated, and the reinforcement design is analyzed according to the results of these two calculations.
4 model analysis of foundation beam layer in multi-story concrete frame design
In the design process of multi-story concrete frame, attention should be paid to the calculation of foundation beam layer, which should meet the actual requirements of site construction. In general, there is no floor in the foundation beam layer. In the overall design and calculation process of the frame, the thickness of the floor is mostly set to 0, and the total stiffness analysis method is adopted. In another case, although the calculated thickness is set to 0 in the floor design calculation, the total stiffness analysis method is not adopted. In the process of program analysis and design, the analysis and design are carried out according to the rigid floor, which is inconsistent with the actual situation. Especially when there are some irregularities in the building plane of multi-story concrete frame, we should pay more attention to this problem. In the design process of foundation tension beam, it is necessary to scientifically reduce the relevant length of bottom column to avoid excessive bottom displacement. When setting the foundation beam, it should be designed according to the frame beam to ensure the rigor and scientificity of stirrup design. In order to strengthen the seismic performance, the short column design scheme should be adopted.
5. Parameters that should be paid attention to in frame structure design.
First of all, when determining the periodic reduction factor of the structure, the infilled wall should be considered to make the actual stiffness of the structure exceed the calculated value. The calculated period value should be longer than the actual period. Therefore, when the designed and calculated seismic shear force is relatively small and the structure is easily calculated as unsafe, the calculation period of building construction should be effectively shortened. It should be noted that when calculating the period, the period cannot be completely reduced, and the reduction factor cannot be too large or too small, which should reflect scientific and reasonable reduction. Generally, in frame structures, if masonry infilled walls are used, the period reduction coefficient can be 0.6 to 0.7, and there is not much filler compared with masonry.
Or using lightweight blocks, the coefficient of 0.7 to 0.8 can be used, and when there is no wall frame, the calculation period coefficient can be ignored. Secondly, referring to the analysis of the frame beam stiffness amplification factor, when the computer is used to analyze and calculate the whole frame reasonably, the frame beam stiffness amplification factor must be consistent with the floor structure. This problem is easily overlooked in frame structure engineering. If it is a cast-in-place floor slab or an integral floor slab, the stiffness amplification factor of the frame beam can be determined between 1.3 ~ 2.0, and the calculated amplification factor is reasonable.
To sum up, in the design process of multi-story concrete frame building structure, we must pay attention to the practical application problems, consider comprehensively in the design process, avoid some bad problems, ensure the reliability and safety of the building, and reach the corresponding quality standards. This makes the multi-story concrete frame building structure model more widely used in the construction field, giving play to its more advantages and promoting the development and progress of China's construction industry.
The structural design of the above-mentioned multi-story concrete frame building was collected and arranged by Zhong Da Consulting Company.
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