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Why can civil air defense components be designed according to plasticity? What is the ductility ratio for? Which civil air defense components cannot be calculated based on plasticity? Under explosive dynamic loads, the deformation of structural components in plastic civil engineering usually increases to its maximum value over time, followed by attenuation.
Therefore, it is possible to consider utilizing the plastic deformation generated by components to absorb the energy of explosive dynamic loads, allowing structural components to enter an elastic-plastic working state under the action of explosive dynamic loads.
Under the action of explosive dynamic load, even if the structural components enter a plastic working state, as long as the deformation caused by the dynamic load does not exceed the maximum allowable deformation, after the instantaneous dynamic load disappears, due to the combined effect of damping force, their vibration deformation continuously attenuates and can still reach a certain static equilibrium state.
At this point, although there are some residual deformations in the structural components, they still have sufficient load-bearing capacity and protective sealing capacity.
Due to the fact that structural components can absorb more energy during the elastic-plastic working stage, the potential of materials can be fully utilized.
In the design of civil air defense engineering, reinforced concrete top plates, bottom plates, exterior walls, and suspended walls can be designed according to the elastic-plastic stage.
For protective structures that are particularly important or have high sealing requirements, such as door frame walls of reinforced concrete protective closed doors, and structures with high waterproof requirements, which are still limited to the elastic working stage, the internal force should be calculated using the elastic analysis method.
(1) Civil air defense load belongs to pulse load, with high peak pressure and short action time as its characteristics.
Especially under the dynamic load of conventional weapon explosions, the structural components have not yet reached their maximum deformation and the load has disappeared, so their dynamic coefficient is often less than 1.
(2) After the structural components enter the plastic state, due to the disappearance of the load, most of the deformation can be restored, and only some residual deformation cannot be restored.
The allowable residual deformation is reflected by the allowable ductility ratio, which is based on meeting the sealing requirements of the component.
For example, if the allowable ductility ratio of the top plate is 3, under the action of civil air defense loads, the component may locally crack, but permanent transparent cracks will not occur.
The original text of Article 4.10.1 of the Code for Design of Civil Air Defense Basements (GB50038-2005), which describes the redistribution of plastic internal forces, is as follows: “For statically indeterminate reinforced concrete structures, the internal forces can be calculated based on the redistribution of plastic internal forces generated by inelastic deformation
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