Brick houses with earthquake-resistant walls with bottom frames are mainly used for upper-level residential projects of shops facing the street. Compared with multi-storey reinforced concrete frame houses, this structure has the advantages of lower cost and convenient construction, and is widely used in urban construction in China.
1 seismic design requirements of structures
Brick building with seismic wall with bottom frame has the characteristics of rigid top and soft bottom, heavy top and light bottom. The degree of earthquake damage of buildings is closely related to the plane layout of buildings, the relative position of upper and lower walls and the lateral stiffness ratio of upper and lower floors.
1. 1 principle of "strong column and weak beam"
One of the basic principles followed in the frame design of brick buildings with seismic walls with bottom frames is the principle of "strong columns and weak beams" and "strong joints and weak members". The purpose is to make the plastic hinge appear at the beam end and then at the column end of the frame structure under strong earthquake. If the plastic hinge appears at any column end of the frame first, it may lead to the plastic hinge appearing at other column ends of the same floor one after another, and the house will collapse. However, it is difficult to meet the requirements of "strong column and weak beam" in the calculation of bending strength because the bottom frame beam has to bear the large bending moment caused by vertical load. Therefore, the design principle of "strong column and weak beam" should be realized as far as possible in the structure, especially in the configuration of stirrups.
1.2 structural plane design emphasizes unity and integrity.
The architectural layout should be concise, regular and symmetrical, and the unit form of the upper brick house should be reduced as much as possible. The vertical and horizontal walls of the upper brick house are evenly and symmetrically arranged, and should be aligned along the plane, and the wall width between windows on the same axis should be consistent. Stairwells should not be located at the end and corner of the house, and flue and air duct should not weaken the wall. For the seismic wall structure of the lower frame, it is required to arrange column grids corresponding to the upper brick houses, so that as many walls as possible fall on the column grids. Seismic walls should be symmetrically and dispersedly connected with longitudinal and transverse seismic walls as far as possible, and the longitudinal seismic walls should be arranged on the outer longitudinal axis to enhance the overturning resistance, avoid low seismic walls (the height-width ratio is less than 1), make the stiffness center of the structure coincide with the centroid, and reduce the torsional effect of the structure under earthquake action.
1.3 Unity and continuity of structural facade
The notable feature of brick building structure with seismic wall with bottom frame is "the top is heavy and the bottom is light". In order to reduce the center of gravity of the structure as much as possible, the number of floors and the total height of the building should be strictly controlled. According to the Code for Seismic Design of Buildings (GN 50011-2001), the height of the bottom structure should not exceed 4.5m, the building functions of each floor of the upper brick house are consistent, and the vertical walls are symmetrical and continuous. For the stairwell outside the roof, due to the sudden change of stiffness between water tanks, it is easy to cause whiplash effect during the earthquake, so the height should be reduced as much as possible. Only when the architectural design is vertical and regular can we ensure the unity of vertical strength and stiffness, avoid the weak layer in the upper brick house and reduce stress concentration and deformation concentration.
2 seismic design of brick building with seismic wall
2. 1 seismic wall design of bottom frame
At present, there are three problems in the bottom design of brick houses with seismic walls with bottom frames:
When the ground floor is a large shopping mall, there are fewer seismic walls (generally brick walls) on the ground floor, and the lateral stiffness of the ground floor is much smaller than that of the second floor with more vertical and horizontal walls. Because the overturning moment of this structure is mainly borne by the reinforced concrete frame columns, the bearing capacity of the reinforced concrete frame columns at the bottom layer is greatly reduced, and the bottom layer becomes the weak layer. Under the action of strong earthquake, the ground floor becomes the floor with concentrated elastic-plastic deformation and damage, which endangers the safety of the whole house. To solve the above problems, first of all, when building layout, some walls should be arranged in appropriate parts. Secondly, using reinforced concrete seismic wall instead of brick seismic wall, the lateral stiffness of concrete wall with the same thickness, height and length is several times that of brick wall, which can not only reduce the number of walls but also ensure the lateral stiffness of the bottom floor.
When a building faces a street, there are generally no earthquake-resistant walls along the street. The number of earthquake-resistant walls is too small, and the layout of the bottom floor is asymmetric, which leads to torsional effect during the earthquake and aggravates the damage of houses. In order to solve this problem, a certain number of reinforced concrete seismic walls should be arranged on the outer longitudinal wall along the street, and brick seismic walls with equivalent stiffness should be arranged on the outer longitudinal wall on the other side, so that the stiffness center of the bottom floor basically coincides with the centroid.
The ground floor is divided into several large spaces along the longitudinal direction. In some design schemes, the partition wall is designed as a brick wall with structural columns and ring beams, which leads to the failure to form a complete frame seismic wall system horizontally and vertically. Under the earthquake, these partitions cracked first because of their large lateral stiffness, and were seriously damaged because their bearing capacity and deformation capacity were worse than those of reinforced permanent frames. They quit work prematurely, which led to the redistribution of elastic-plastic internal forces and serious damage to the seismic wall of the bottom frame. Therefore, when arranging the structure, the bottom floor must be arranged as a vertical and horizontal frame seismic wall system to avoid the above problems.
2.2 Design of transition layer
The second floor of brick house with seismic wall is called transfer floor. This floor is responsible for transmitting the seismic shear force of the upper floor and the overturning moment of the seismic force of the upper floor, which leads to the increase of the displacement between the two floors at the corner of the floor, so the stress on this floor is complex and very important. For brick houses with seismic walls with bottom frames, when a certain number of seismic walls are set at the bottom according to the requirements of seismic code, the lateral stiffness and horizontal bearing capacity of the bottom of the house are greatly improved; At this time, if the lateral stiffness and horizontal bearing capacity of the transfer floor wall are ignored, the transfer floor of the house may become a weak layer; Because the deformation capacity of brick masonry with transfer floor is relatively poor, the seismic performance of such buildings will be reduced. In order to avoid the above situation, the seismic structural measures of the transfer floor wall should be strengthened. The reinforcement of the two-story constructional column is one level higher than that of the upper constructional column at the same position, the stirrup at the lower end of the two-story constructional column is properly encrypted, and the longitudinal reinforcement of the constructional column is anchored into the bottom frame column and beam for 40 days; In addition to setting constructional columns according to the seismic code, constructional columns should be appropriately added according to the number of building floors and fortification intensity, especially where there is seismic wall at the bottom floor, in order to improve the performance of horizontal force transmission of the whole structure; In addition, structural columns should be set at the junction of vertical and horizontal walls around the house to increase the connectivity and integrity of the upper masonry structure and the bottom reinforced concrete frame seismic wall structure, so as to avoid the incomplete natural frequency of the structure caused by different materials on the upper and lower parts of the house and the second floor disconnection caused by the weak connection between the upper and lower parts under the earthquake. 3 problems that should be paid attention to in seismic design of bottom frame structure
3. 1 Focus on conceptual design.
Choosing a building site that is beneficial to earthquake resistance, simplifying architectural modeling, paying attention to regular symmetry, uniform changes in mass and stiffness, and reasonable and clear seismic structure system are the basic design contents to ensure reasonable seismic design. At the same time, the seismic design should achieve the fortification goals of "small earthquake is not bad", "medium earthquake can be repaired" and "big earthquake cannot collapse". Article 7. 1.8 of Code for Seismic Design of Buildings (GBJ50011) stipulates that the frame-seismic wall system should be evenly and symmetrically arranged at the bottom along the vertical and horizontal directions, and emphasizes that the bottom seismic wall should be symmetrically arranged in both directions and connected with the vertical and horizontal seismic walls. Because the shear wall in the bottom frame wall structure is a low wall, its shear stiffness is relatively large. If the wall limb is long and the plane form is complex, the local stiffness is too large and the stress is too concentrated, and even a small number of shear walls are often arranged to meet the lateral stiffness ratio limit of the upper and lower floors. If left untreated, the eccentricity between the stiffness center and the center of mass of the building will be large, which will cause torsion to the structure under the action of earthquake force.
The column network of the bottom frame wall structure should not be too large, generally controlled at about 7.5m, and the number of wall hanging on the frame beam should not exceed 1 piece. First of all, from the use function, the bottom frame structure is mostly commercial and residential buildings, and the span can be divided into two bays corresponding to the upper part. No matter whether the upper part is a residential building or an office building, the bay size must meet the function that the masonry structure can achieve.
3.2 Strictly control the lateral stiffness ratio
The lateral stiffness ratio between the second story and the bottom story in the current seismic code will not only affect the elastic displacement between stories under earthquake, but also have a great influence on the distribution of ultimate shear coefficient between stories, the position of weak stories and the concentration of elastic-plastic deformation of weak stories. Therefore, the lateral stiffness ratio should be strictly limited and controlled in the design. This is because the ratio analysis results show that when it is greater than 2, under the action of strong earthquake, the weak elastic-plastic deformation of the bottom layer will be concentrated, the elastic displacement will increase and the failure of the bottom layer will accelerate; But when?
3.3 the structural system should be reasonable
Two-way frame system should be set at the bottom or two floors of the bottom frame brick house, because the seismic shear force at the bottom is distributed according to the stiffness of each lateral force resisting member. In these mixed structural systems, the lateral stiffness of brick walls is much greater than that of frames. Under the earthquake, the brick wall cracks first, and its deformation ability is much worse than that of the frame, which will lead to the brick wall members quitting the work first and aggravate the damage of the half-frame or part of the frame.
conclusion
There is a great difference in seismic performance between the upper part and the bottom of the brick building with seismic wall with bottom frame. Because of its special structural form, unreasonable design leads to serious damage during the earthquake. Designing a regular and symmetrical building plane, controlling the stiffness ratio between the ground floor and the transfer floor, and rationally arranging the structural system of the bottom-frame seismic wall brick house can make the bottom-frame seismic wall brick house have greater seismic capacity and good seismic performance.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd