Answers to 19 questions about frame structures

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19 Answers to Questions about Frame Structures 1. How to choose the structure type? Explanation: (1) For multi-story buildings with a height of no more than 150 meters, a reinforced concrete structure is generally chosen ； (2) For high-rise projects with a height exceeding 150 meters, steel structures or concrete structures may be used ； (3) For residential houses or small-scale projects in backward and remote areas, masonry structures may be used. 2. How to choose a structural system? Explanation: For reinforced concrete structures, when the building height does not exceed 120 meters, the three conventional structural systems are generally used – frame structure, shear wall structure, and frame-shear wall structure. (1) For buildings that require large spaces, such as schools, office buildings, clubs, hospitals, and shopping malls, a frame structure is generally chosen when the building height does not exceed the values shown in the table below ； When the building height exceeds that specified in the table below, a frame-shear wall structure is generally chosen ； http://img.civilcn.com/d/file/zhishi/jggc/2019-07-12/221ad2326d3628db130831298113e1ca.jpg (2) For construction projects involving high-rise residences, apartments, hotels, and other buildings where the locations of partition walls are fixed and the available space is limited, shear wall structures are generally chosen. When the ground floor or several floors at the base of high-rise residential buildings, apartments, or hotel projects require large spaces for architectural functions such as lobbies or commercial areas, a partially braced shear wall structure is generally used. (3) For high-rise office buildings with a height exceeding 100 meters, a frame-core structure is generally adopted. 3. Is it reasonable to use a frame structural system for a 40-meter-high office building in the Guangzhou area? Explanation: Unreasonable. For frame structures in Zone 7, the economically viable height is 30 meters; when this height is exceeded by a significant amount, shear walls should be installed at appropriate locations (such as stairwells, elevators, and auxiliary rooms) to create a frame-shear wall structural system. In this way, the shear walls bear most of the horizontal forces, **reducing the stress on the frame section; as a result, the cross-sections and reinforcement of the frame columns and beams can be reduced, making the entire structure more economical and efficient. 4. What is the reasonable framework structure, column grid, and their dimensions? Explanation: (1) The column layout should be regular, usually with an orthogonal grid. (2) In multi-story frame structures with ordinary building functions, single-span frames should not be used except in certain areas; single-span frames shall not be employed in category B buildings such as schools and hospitals, nor in high-rise buildings. (3) From the perspective of structural economy alone, in low-seismicity areas (seismicity levels 6 and 7) where wind pressure is low (less than 0.4), a large column grid (around 9 meters) is advisable ； In high-seismicity areas (seismic intensity of 8 or above), a medium to small column grid (around 4–6 meters) is recommended. (4) Under normal circumstances, the column grid size should not exceed 12 meters ； Steel structures can be considered when the length exceeds 12 meters. 5. What is a reasonable selection of materials for frame structures? Explanation: (1) Concrete: For multi-story frame columns, the concrete strength grade can be C25 or C30; for high-rise frame columns, it can be C35 or C40. The concrete strength grade for the beams can be C25 or C30. (2) Reinforcing bars: Under normal circumstances, HRB400 is used for the reinforcing bars in beams, slabs, and columns, while HRB500 can be used for the longitudinal bars in beams. 6. How to choose the floor system for frame structures appropriately? Explanation: (1) The floor slabs of frame structures can adopt single-direction primary and secondary beams, grid beams, or cross beams. From the perspective of structural rationality, the layout of secondary beams should ensure that the span of one-way slabs is approximately 3.0 meters, while that of two-way slabs is around 4.0 meters. (2) From the perspective of building functions, generally speaking, schools and shopping malls tend to use grid beams and cross beams more often ； Office buildings, clubs, and hospitals generally use a large number of primary and secondary beams. 7. How to determine the appropriate cross-sectional dimensions for frame columns? Explanation: (1) The cross-section of frame structure columns is usually controlled by the axial compression ratio limit; generally, it is appropriate to use the column’s calculated axial compression ratio = specification limit for axial compression ratio – 0.1. (2) Unless Party A has special requirements regarding cost efficiency, under normal circumstances, the cross-sectional dimensions of multi-story frame columns shall not be changed more than 2 times ； The cross-sectional dimensions of the high-rise frame columns shall not change more than 3 times. (3) The cross-sectional shape of the columns is generally rectangular (with an aspect ratio generally not exceeding 1.5), and the longer side of the column cross-section is parallel to the shorter side of the structural plane. (4) When the number of layers is 10, the size of the square column is 700~1000 mm ； When the number of layers is 5, it ranges from 500 to 800; the higher value is used for large column grids, while the lower value is used for small column grids. 8. How to determine the appropriate size of the beam cross-section? Explanation: (1) Under normal loading conditions, the section height of frame beams can be estimated as L/13, the section height of one-way secondary beams can be estimated as L/15, and the section height of two-way grid beams can be estimated as L/18. (2) The width of the beam cross-section can be taken as 1/3 to 1/2 of the beam height. (3) The final cross-sectional dimensions of the beams are determined based on the calculation results. Under normal circumstances, the reinforcement ratio at the majority of the beam supports should be between 1.2 and 1.6%, and it should not exceed 2.0%; the reinforcement ratio at the mid-span should be between 0.8 and 1.2%. (4) The height of frame beams is generally 600~800 mm, and the width is generally 250~350 mm ； The cross-sectional height of the secondary beams is 500~600 mm, with a width generally ranging from 200~250 mm. 9. How to determine the appropriate thickness of floor slabs? Explanation: (1) Within the range of normal loads and normal spans, the thickness of a one-way slab is approximately h=L/30, that of a two-way slab is approximately h=L/38, and that of a cantilever slab is approximately h=L/10; moreover, the calculated reinforcement amount should be close to the required structural reinforcement. (2) In actual projects, plate thicknesses of 100 mm, 120 mm, and 150 mm are commonly used. 10. What are the considerations for cantilever structure design? Explanation: Cantilever structures are statically determinate structures with relatively low safety margins; therefore, during design, an appropriate increase in the safety reserve is necessary (the actually used amount of reinforcement should be about 30% greater than the calculated requirement). The span of the cantilever beam should be kept within 3.5 meters, and the length of the cantilever plate should be kept within 1.2 meters. If it exceeds this range, special attention should be paid to the verification of deflection and cracks, or other structural forms should be adopted (such as installing diagonal bracing). 11. What is the approximate proportion of materials used for various components in a frame structure? Explanation: A frame structure is composed of beams, slabs, and columns. In a multi-story frame structure, the approximate proportions of materials used are as follows: Concrete – about 30% for beams, about 55% for slabs, and about 15% for columns ； Reinforcement amount: beams—about 50%, slabs—about 25%, columns—about 25%. Therefore, when designing frame structures, attention should be paid to the column grid size, slab thickness, and control of beam reinforcement ratios to ensure the economic rationality of the structure. 12. What is the general pattern of the average weight per unit area for frame structures? Explanation: For conventional frame structures using lightweight blocks, the average weight capacity is 12–13 KN/㎡ in zones 6 and 7, and 13–14 KN/㎡ in zone 8 ； A lower value is taken when there are few internal partitions, while a higher value is taken when there are many internal partitions. 13. What overall indicators need to be controlled for frame structures? Explanation: It is necessary to control the inter-story displacement angle, displacement ratio, lateral stiffness ratio, and floor shear capacity ratio; the period ratio does not need to be controlled. The shear weight ratio and stiffness weight ratio can easily meet the regulatory requirements. 14. What should be done if the lateral stiffness ratio of the frame structure and the shear capacity ratio of the floors do not meet the code requirements? Explanation: When the height of the lower floor is large, it is particularly easy for the lateral stiffness ratio of the frame structure and the shear capacity of the floors to fail to meet the regulatory requirements. At this point, it is generally possible to increase the cross-sectional dimensions of the frame columns and beams in the floors with larger floor heights; if necessary, the structural system should be modified to use a frame-shear wall structure. The method of installing shear walls or bracing members solely on the floors with larger floor heights at the bottom can resolve the aforementioned issues in terms of calculation results, but it results in a rather unusual structural layout: a frame-shear wall structure at the bottom and a frame structure at the top, which is not appropriate and constitutes an over-limit structure. 15. What to do if the axial compression ratio of frame columns exceeds the limit? Explanation: There are two methods: (1) increase the column cross-section ； (2) Increase the strength grade of the column concrete. 16. What to do if the calculated longitudinal reinforcement for frame columns is too large? Explanation: (1) Frame columns generally have reinforcement for structural purposes; in rare cases, some frame columns or those on the top floor may require reinforcement based on calculations (that is, the calculated longitudinal reinforcement exceeds the minimum reinforcement ratio). In such cases, the cross-sectional shape of the column can be adjusted – if there is more reinforcement in the X direction, the column’s length in the Y direction is increased, and vice versa. (2) If calculated reinforcement is required for many frame columns, it should be considered to install shear walls at appropriate locations to form a frame-shear wall structure, thereby reducing the stress on the frame portion. 17. What to do if a beam is over-reinforced in bending? Explanation: When the building permits it, prioritize increasing the beam height ； Increase beam width if construction does not permit it ； When the dimensions of the beam cross-section cannot be changed, the layout of the floor beams should be adjusted to alter the loading conditions of the beams. 18. How to address the shear resistance issues at frame beam-column joints? Explanation: In high-seismicity areas (zones 8 and above), frame structures often suffer from insufficient shear resistance at joints, especially in structures with irregular columns. There are two effective methods to address insufficient shear resistance at joints: (1) widening the frame beams or adding horizontal bracing to the frame beams at the joints ； (2) Install shear walls at appropriate locations to form a frame-shear wall structure, thereby reducing the internal forces in the frame portion. 19. Is it reasonable for the longitudinal reinforcement in the upper layer of frame columns to be larger than that in the lower layer? Explanation: Frame columns are members subjected to both compression and bending. In the upper part of the structure (especially on the top floor), frame columns generally experience relatively low axial compressive forces but high bending moments, which represents a condition of large eccentric compression. Under conditions of large eccentric compression, axial pressure is advantageous; that is, the greater the axial pressure, the less reinforcement is required, while the smaller the axial pressure, the more reinforcement is needed. Therefore, in areas with high seismic intensity or in cases of large column grids, the longitudinal reinforcement in the frame columns increases as one moves to higher floors.