Attention! The "Seismic Design Standard for Underground Structures" GB/T51336-2018 has been released! It will be implemented from April 1st, 2019.
This standard stipulates that underground structural engineering in areas with seismic fortification intensity of 6 degrees or above must undergo seismic design. Compared to previous standards, this standard covers a wide range of underground structures, including single underground structures, composite underground structures, shield tunnel structures, underground tunnel structures, open cut tunnel structures, sunken retaining structures, and reconstructed underground structures. The reconstructed underground structure is the common structure of the main building with a basement. Below the embedded end, there is no need to calculate seismic force? Delusions!
5. Seismic calculation of underground structures
5.1 General Provisions
5.2 Underground Structure Seismic Response Displacement Method I
5.3 Underground Structure Seismic Response Displacement Method II
5.4 Underground Structure Seismic Response Displacement Method III
5.5 Time history analysis method for underground structures
5.6 Section seismic verification
5.7 Seismic deformation verification
5.8 Anti floating verification
There are even more explosive ones! When there is saturated sand or silt layer under the bottom plate of the underground structure, anti floating calculation with vertical seismic force should be carried out!
F=γSFS+γpFp+γEvFEv
FEv - Effect of Vertical Seismic Action Standard Values on Structures and Overcover Soil
There are also seismic deformation regulations for underground structures!
When conducting elastic deformation calculations for structures with performance requirements of I and II, the maximum elastic interlayer displacement angle should be used as the indicator for rectangular sections, and the following formula should be met:
Table 5.7.1 Limit values of elastic interlayer displacement angle
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structural type
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Reinforced concrete frame
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1/550
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Reinforced concrete frame seismic wall, slab column seismic wall
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1/800
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Reinforced concrete seismic wall
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1/1000
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Reinforced concrete seismic wall
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1/1000
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There is also good news that there is a standardized basis for calculating seismic force in sunken squares!
11.1.1 This chapter is applicable to the seismic design of sunken retaining structures.
11.1.2 Submerged retaining structures can be subjected to seismic calculations using the quasi-static method.
Special research and demonstration should be conducted on sunken retaining structures with a retaining wall height exceeding 15m and a seismic fortification intensity of 9 degrees.
If there is a steel structure above the basement, that would be disastrous! The vertical continuous arrangement of supports in the steel frame support structure should extend to the foundation; The steel frame columns should extend at least one floor of the underground structure, and their vertical loads should be directly transmitted to the foundation.
The embedding requirements have undergone significant changes! Greater stiffness and relaxation!
When the outer walls of the reconstructed underground structure are in close contact with the soil layer, the fixing position of the upper structure shall be determined according to the following regulations:
When the upper structure is a seismic wall structure and the underground structure is a single or multiple story structure, the top plate of the underground structure can be used as the embedded part of the upper structure.
When the upper structure is a frame, frame seismic wall, or frame core tube structure:
1) When the underground structure is a single layer and serves as a box foundation, the top plate of the underground structure can be used as an embedded part of the upper structure.
2) For single-layer or multi-storey underground structures with raft foundation and multi-storey underground structures with box foundation, when the lateral stiffness of the underground structure is greater than or equal to 1.5 times of the lateral stiffness of the bottom floor of the superstructure connected to it, the roof of the underground structure can be used as the embedded part of the superstructure.