1. Seismic requirements
If the basement is not properly designed, it will have a great impact on the overall seismic performance. Generally, the buried depth of semi-basement is greater than the height above the ground outside the basement, so that the number of floors can be counted and the total height can be calculated from the outdoor ground. The wall column in the basement should be coordinated with the wall column in the upper structure. When the elevation of the indoor and outdoor panels of the basement roof changes, staggered floors will be formed when the elevation changes beyond the range of beam height, and they shall not be used as embedded parts of the upper structure unless measures are taken. The code clearly stipulates that the top floor of the basement as the embedded part of the superstructure should adopt beam-slab structure, and the basement roof should not be used as the embedded part of the superstructure when it is flat floor. Structural calculation should be calculated to the basement floor or the floor meeting the requirements of buried end, but the number of floors in the reinforced area at the bottom of shear wall should be calculated from the ground up and should include the basement.
Common problems, such as insufficient buried depth of semi-basement, eight floors including semi-basement, and the number of floors and total height exceeding the requirements, violate Article 7. 1-20 10 of GB 50065438. The seismic grade of the basement is Grade III, and the superstructure is Grade II. According to article 6. 1-20 1.3 of GB 50065438, the first floor of the basement should also be Grade II.
2. Load values and combinations
When calculating the bending shear of basement exterior wall, the effect caused by earth pressure is permanent load effect, and when controlling the combination of variable load effects, the partial coefficient of earth pressure load is1.2; For the combination of permanent load effect control, the partial load coefficient is 1.35. For the live load on the ground, the lateral pressure coefficient should also be taken, which is not calculated correctly in many designs. If the water pressure is the highest, it is generally designed according to the dead load. Please refer to Code for Design of Underground Ponds for the value of partial coefficient. When calculating the strength of basement floor, according to Article 3.2.5 of Load Code for Building Structures (GB50009-20 12), the load subentry coefficient of deadweight of battens and covering soil is 1.0. When calculating the anti-buoyancy, the partial load coefficient of the self-weight of slab and covering soil should be 0.9[ for this article, please refer to the Load Code for New Building Structures]. The earth pressure of basement exterior wall should be static earth pressure, and different calculation methods are adopted according to different soil properties. Cohesive soil is cost-effective, and sandy soil is divided into water and soil.
If there is no house at the top of the basement and it is an open space, it should be considered whether the usual load of the fire truck is greater than the possible load of the fire truck, and the load that plays a controlling role should be used as the design basis in practice. Another example is the basement roof with the elevation of-1.55m. Only 4.5KN/㎡ is considered for the live load, excluding the soil covering load and the fire truck load. The live load value of underground garage is 6.0KN/㎡, which does not meet Article 4. 1 2 of GB 50009-201. Regardless of the load of the fire truck, or the truck load that may occur during construction and use is greater than the load of the fire truck. The construction surcharge of 10kN/㎡ should be considered.
3. Calculation model of external wall
Calculation of reinforcement of basement exterior wall: In the calculation of reinforcement of exterior wall of some projects, the reinforcement is calculated according to the two-way slab regardless of the size of piers, while the reinforcement of piers is based on the computer analysis results of the whole basement structure, and the reinforcement of piers is not checked according to the load transmitted by the two-way slab of exterior wall. According to the principle of deformation coordination between external wall and buttress, the vertical reinforcement of external wall is insufficient, the reinforcement of buttress is less, and the horizontal reinforcement of external wall is surplus. Suggestion: The reinforcement of other external walls should be calculated according to the vertical one-way slab, except the external wall slabs connected with the reinforced concrete internal partition wall perpendicular to the external wall or the external wall slabs with larger cross-section (such as between the external frame columns of high-rise buildings). The internal and external main reinforcement of external wall pier pile with small vertical load (axial force) should also be strengthened appropriately. The horizontal distribution reinforcement of the external wall should be strengthened according to the section size of the buttress column, and external short horizontal negative reinforcement can be added appropriately, and the corner of the external wall should also be strengthened appropriately.
When calculating the basement exterior wall, the bottom is a fixed support (that is, the bottom plate is the embedded end of the exterior wall), and the bending moment at the bottom of the side wall is the same as that of the adjacent bottom plate. The bending capacity of the bottom plate should not be less than that of the side wall, and its thickness and reinforcement amount should match. This problem is most typical in underground lanes. The side wall of the driveway is a cantilever member, and the bending capacity of the bottom plate should not be less than that of the side wall. Similar problems often occur at the elevation change of basement floor: there is only one beam at the elevation change, and the Liang Kuan is even smaller than the floor thickness, so it is difficult to meet the requirements only by transferring the load-bearing bending moment of the plate by stirrups on both sides of the beam. There is no floor support at the top of the external wall at the bottom opening position (such as stairwell), and the calculation model and reinforcement structure should conform to the reality. When the driveway is close to the basement exterior wall, the floor of the driveway is located in the middle of the exterior wall. It should be noted that the external wall bears the horizontal concentrated force from the driveway floor, which is often ignored.
4. Top, bottom and stairs
The common problems in design are: the thickness of basement roof 100mm does not conform to article 6.1.10 of GB 500 1-20 10 0; The reinforcement of the bottom plate is φ 14 @ 100, which does not conform to Article 12.2.4 of JGJ3; The thickness of the basement roof and the reinforcement of underground columns do not meet the provisions of Article 6. 1-20 14 of GB 50065438.
5. Groundwater and anti-floating
Groundwater level and its amplitude is an important basis for anti-floating design of basement. In the actual anti-floating design of basement, only the normal use limit state is considered, and insufficient attention is paid to the construction process and flood period, which will lead to insufficient anti-floating in the construction process and cause local damage. In addition, in fact, there are many high-rise and low-rise buildings in the same large-scale basement, which is large in area and irregular in shape. In addition, there is no building above this part, so this anti-floating problem is relatively difficult to deal with and needs specific analysis and treatment.
6. Cracks and control methods
The concrete of basement exterior wall is easy to shrink, which is constrained by the structure itself and the side wall of foundation pit, resulting in greater tensile stress until shrinkage cracks appear. The crack width of basement exterior wall is controlled within 0.2mm, and the reinforcement quantity is often controlled by checking the crack width.
In many engineering designs, the bending moment amplitude modulation of basement waterproof structural members is calculated, some are hinged at the lower end, some do not consider the load subentry coefficient, and multi-span continuous calculation is not considered when multi-storey, and the crack resistance checking calculation of basement exterior wall is omitted in the calculation (in violation of Article 4.10/.6 of GB50/08-2008), the connection structure between basement exterior wall and bottom plate is unreasonable, and the building is too long without joints or post-pouring belt. , resulting in violation of design specifications and leakage. The basement of a project is designed as a large chassis, and the foundation forms under the large chassis include natural foundation, pile foundation and rigid pile composite foundation (in violation of article 3.3.4 of GB 5001-2010). Even if the post-pouring belt is installed, such a foundation is only suitable for the construction stage.
The overall length of the basement is too long, so corresponding measures should be taken to prevent cracks from developing. The main measures taken are as follows: ① compensating shrinkage concrete, that is, infiltrating UEA, HEA and other micro-expansive agents into concrete. If the difference between the expansion value of concrete and the final shrinkage value of concrete is greater than or equal to the ultimate tensile force of concrete, cracks can be controlled. (2) Expansion zone, because the expansion deformation of expansive agent in concrete will not be completely compensated by the early shrinkage deformation of concrete, according to some engineering practices, in order to realize the continuous pouring and seamless construction of concrete, the expansion strengthening zone is generally set above 60m. (3) As a technical measure to release the binding force of concrete in the early short term, the post-pouring belt has been greatly improved and widely used in the long-term deformation joints. (4) In order to improve the tensile capacity of reinforced concrete, anti-deformation steel bars should be added to the concrete, and horizontal temperature steel bars should be added to the side walls to strengthen the concrete surface. The side wall is constrained by the bottom plate and the top plate, and the expansion and contraction of concrete are inconsistent. A horizontal concealed beam can be set in the middle of the wall to resist the tension.
7. Thickness of protective layer and cushion layer
Technical Specification for Waterproofing of Underground Engineering (GB50 108-2008) stipulates that the thickness of waterproof concrete structure should not be less than 250mm；; The crack width shall not be greater than 0.2 mm, and shall not penetrate; The thickness of the protective layer of the steel bar near the water should not be less than 50 mm, the strength grade of the concrete cushion of the bottom plate of waterproof concrete structure should not be less than C 15, the thickness should not be less than 100mm, and it should not be less than 150mm in soft soil. Engineering practice shows that if the thickness of the structure or the thickness of the protective layer of the steel bar in the waterfront is less than the limit value of the code, it is often the common cause of water leakage, so the limit value is raised accordingly after the revision of the code, which should be paid attention to.
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