Application analysis of seismic strengthening methods for building structures?

China is a country with many earthquakes, and seismic activities are widely distributed, so the harm caused by earthquakes can not be ignored. 5. 12 Wenchuan earthquake and 4. 14 Yushu earthquake in Qinghai caused heavy casualties and huge economic losses, and also sounded the alarm for the people. Seismic strengthening of building structures has become a hot topic at present. In order to better protect the safety of human life and property and promote the harmonious development of society, it is very important to strengthen the engineering structure before and after the earthquake.

I. Details of Revision of 20 10 Seismic Code

After the Wenchuan earthquake, the Ministry of Construction revised the original national code for seismic design. The relevant details will be analyzed below.

1. 1 revised from the original general provisions to new mandatory provisions. 4. In article1.8, the increase factor is changed from "determined according to the specific situation, but not exceeding 1.6" to "adopted within the range of 1. 1 ~ 1.6", and the scope has been changed.

The stairwell shall meet the following requirements: ① Two φ6 long steel bars shall be set at intervals of 500mm along the horizontal wall and external wall of the top stairwell; 7 ~ 9 o'clock, the stairwell wall of other floors shall be provided with a reinforced concrete belt or reinforced brick belt with a thickness of 60mm and longitudinal reinforcement of not less than 2φ 10 on the rest platform or half height of the floor. The reinforced brick belt shall be no less than 3 skins, and the reinforcement of each skin shall be no less than 2φ6. The mortar strength grade shall be lower than m7.5 and not lower than the wall of the same floor.

(2) The supporting length of the beam at the external corner of the inner wall of stairwell and foyer should not be less than 500mm, and it should be connected with the ring beam.

(3) The assembled stair section shall be reliably connected with the beam of the platform plate, and the assembled stair section shall not be used at 8 or 9 degrees; Cantilever steps in the wall or stairs with vertical ribs inserted into the wall should not be used, nor should unreinforced brick balustrades be used.

(4) For buildings and elevator rooms that extend out of the roof, the constructional column should extend to the top and be connected with the ring beam at the top. All walls are provided with two φ6 long steel bars and φ4 short steel bars at 500mm intervals along the wall height, which are composed of Rachel net or φ4 spot welding net.

1.2 Modification of material properties specified in Article 3.9.2:

(1) The strength grade of ordinary brick and perforated brick should not be lower than mu 10, and the strength grade of masonry mortar should not be lower than M5;

(2) The strength grade of concrete small hollow block should not be lower than mu7.5, and the strength grade of masonry mortar should not be lower than that of mb7.5 for frames and brace members (including ladders) with seismic grade I, II and III. When ordinary steel bars are used as longitudinal steel bars, the ratio of measured tensile strength to measured yield strength of steel bars should not be less than1.25; The ratio of the measured yield strength of steel bars to the standard yield strength should not be greater than 1.3, and the measured total elongation of steel bars under the maximum tension should not be less than 9%.

Article 3.9.3 defines the limit value of "meeting the seismic performance index" for the performance of ordinary steel bars, and abandons the use of hpb235 grade steel bars as stirrups.

Article 3.9.4 is changed from "meeting the requirements of normal service limit state and seismic structural measures" to "meeting the requirements of minimum reinforcement ratio", and the specification requirements are more specific.

3.9.6 This article is revised from the original general provisions to the new mandatory provisions.

Article 7.3.6 is revised from the original general clause to a new mandatory clause. The new regulation is more specific, and the new code gives more explicit restrictions on the application scope of independent brick columns, stipulating that "only 6 degrees can be used, and 7 ~ 9 degrees cannot be used", and adding strengthening measures for supporting members of beams with a span of not less than 6 meters.

Second, the method of seismic reinforcement

At present, there are three commonly used seismic strengthening methods: adding additional members, strengthening members and isolation and shock absorption. The first two are traditional seismic strengthening methods, and the latter is a new seismic strengthening method.

2. 1 Add member reinforcement method

At present, seismic strengthening of structures is one of the most basic and effective methods, which can effectively improve the seismic capacity, deformation capacity and overall performance of structures. The concrete methods include adding seismic wall reinforcement method, adding ring beam reinforced concrete column reinforcement method, adding support reinforcement method and adding rigid frame reinforcement method. In this method, a new type of anti-seismic wall is used to bear the main earthquake action to reduce the deformation of the structure. When using seismic wall to strengthen, it is necessary to deal with the connection between the new wall and the original components to ensure the reliability of the connection and avoid causing excessive damage to the original structural components.

3.2 Reinforcement methods of reinforcement members

When the new member cannot be used for reinforcement, the original structural member can be strengthened separately to improve the bearing capacity and ductility of the original member, thus achieving the purpose of seismic reinforcement. The main methods are:

1. externally bonded steel reinforcement method: it is suitable for the reinforcement of reinforced concrete beams, columns and other components that need to greatly improve the cross-sectional bearing capacity and seismic capacity. In this method, steel is wrapped around or at two corners of the member by grouting or welding, so that it can bear the load together with the original member, and its bearing capacity and deformation ability can be improved by restraining the original member. This method is suitable for large structures and long-span structures. Its advantages are simple construction, less on-site workload, small change of component section size, less increase of structural self-weight, obvious improvement of bearing capacity and improvement of stiffness and ductility of component section.

2. Bonding steel reinforcement method: reinforcement of reinforced concrete members in bending, eccentric compression and tension. This method is a reinforcement method to improve the bearing capacity of the structure by sticking steel plates on the surface of concrete members with building structural adhesive, so that they bear the same force. Compared with the traditional reinforcement method, the steel plate bonding reinforcement method is simpler and faster, and has less influence on the shape and clearance of the structure and production and life. The disadvantage is that the bearing capacity of reinforced members can not exceed 40% of the original structure, and the fire resistance is poor.

3. Fiber-reinforced composite reinforcement method: a method of seismic reinforcement of members by using composite materials containing fibers or their products as reinforcement materials. This method is suitable for reinforcement of reinforced concrete members under bending, axial compression, large eccentric compression and tension. The strengthening method of externally bonded fiber composite material is a new technology to repair and strengthen concrete members with frp sheets, which has high chemical corrosion resistance and protection ability to the strengthened structure and improves the durability of the structure. Suitable for reinforcement of irregular members; High material strength and small external reinforcement (small material thickness); The load increases very little and hardly changes the shape and size of the original structure; Short construction period and simple operation; The noise during steel bar construction is small, which has little influence on the use environment of the structure. The disadvantage is that the shear capacity is poor, so it is not suitable for shear reinforcement of components. The bearing capacity of reinforced members should not exceed 40% of the original structure.

4. External prestress is one of the important branches of post-tensioned prestress system, which refers to the prestressed structural system formed by arranging prestressed tendons outside the main body of load-bearing structure and applying prestress. External prestress reinforcement technology is an active reinforcement technology, which applies prestress to the tensile area of bridge structure through prestressed materials, eliminates the adverse internal force caused by local load and provides the bearing capacity of the structure.

5. The method of adding steel beams and columns to form composite structure: it is suitable for concrete structures with greatly improved bearing capacity, which is generally increased by ten times or even dozens of times. At present, this reinforcement method is widely used in the reinforcement of dense cabinets, filing cabinets, battery rooms and so on, and has good seismic performance. The disadvantage is that the fire resistance is poor, the clearance becomes smaller after reinforcement and the ground is raised, which is not conducive to decoration and spatial layout.

6. Method of increasing the section of members: reinforcement measures for beams, plates and columns with insufficient bending, shearing and seismic capacity. This method can greatly improve the bearing capacity of the original structure, has good corrosion resistance and fire resistance, and can strengthen the integrity and stiffness of the building. The disadvantage is that the construction period is long, which consumes a lot of manpower and material resources and increases the load on the basis of the original structure. Planting steel bars and drilling holes have great damage to the original structure and have adverse effects on earthquake resistance. This method is suitable for strengthening foundation and substructure.

3.3 isolation, energy dissipation and seismic reinforcement methods

1. A new method of applying isolation technology in the field of seismic strengthening, by setting isolation layer, the seismic deformation is concentrated on the isolation layer, thus reducing the seismic response of the original structure. The advantages of this method are good reinforcement effect and high seismic safety, and it is not necessary to strengthen non-structural members. The disadvantage is that after the isolation layer is set, the equipment wiring and pipelines passing through the isolation layer need to be connected flexibly, which is time-consuming and laborious.

2. Energy dissipation seismic strengthening method is a new seismic strengthening method, which reduces the seismic response by adding energy dissipation devices in some parts of the structure. This method mainly depends on increasing the energy consumption capacity of the structure. Shaking table tests of energy dissipation structures at home and abroad show that the seismic response of energy dissipation structures is reduced by 40%~60% compared with traditional seismic structures. According to foreign engineering data, compared with the traditional seismic structure system, the energy dissipation structure system can save 5%~ 10% of the structural cost. If it is used for the reinforcement and reconstruction of existing structures, the cost savings will be more considerable, and some reinforcement and reconstruction projects can save 60% of the cost.

To sum up, the two strong earthquakes in Wenchuan and Yushu have brought new problems and challenges to the traditional seismic resistance of structures. In the work, we should also sum up the experience of earthquake damage, study the failure mechanism of building structures under earthquake action, and improve the design theory.

Application analysis of seismic strengthening methods for building structures: collected and sorted by Zhong Da Consulting Company.

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