Dynamic Response Characteristic Of Buildings Subjected To Blasting Excavation In Under-Crossing Tunnels

With the continuous expansion of underground pipeline network and the further increase of density of urban buildings,the situation that the tunnel passes beneath buildings,which are rare now,will be common in the future.Compared with the tunnel pass alongside the long-distance building structure,the blasting construction of undercrossing tunnel has two characteristics: the structure is close to explosion source and the blasting dynamic effect is obvious,which will inevitably have a more serious impact on the safety and usability of the upper building structure.Research on the vibration response characteristics of structures under the tunnel blasting is a basic work for reasonably evaluating and controlling the harmful effects of blasting vibration.At the same time,it is also a frontier and hot topic of multidisciplinary intersections such as explosion mechanics,rock dynamics and structural dynamics.It has important theoretical significance and engineering application value.Based on the under-crossing tunnel blasting excavation of Zhifang station section at Wuhan metro line 7,the comprehensive research methods of "field test","numerical simulation" and "theoretical calculation and analysis" are used to study the dynamic response characteristics of structures under blasting vibration induced by under-crossing tunnel excavation.The analysis of structures blasting vibration tests,dynamic strain characteristic tests,time-frequency-instantaneous energy characteristics,motive stress centralization of underground structures,vibration and stress-strain characteristics of the whole structure and various components,etc.are carried out in this thesis.The main contents and achievements are as follows:(1)Analysis on the blasting vibration tests of structure.Depending on the site conditions,the blasting vibration test of the structure including basement during the tunnel blasting excavation process is carried out.According to the test results,the combination of wavelet transform and fast Fourier transform is used to extract the main vibration frequency of the velocity components in each direction.Based on this,the variation and distribution of vibration velocity and frequency spectrum along the height of structure are studied.The test results show that under the blasting vibration induced by under-crossing tunnel excavation,the horizontal radial and horizontal tangential peak particle velocity(PPV)increases first and then decreases with the increase of the floor;The vertical PPV is firstly decreased and then increased,and the vertical PPV is always higher than the horizontal radial and horizontal tangential PPV;At the 1st floor which has the same elevation with the ground surface,the difference between the vertical and horizontal structural response velocities is the smallest,and the difference between the vertical and horizontal PPV increases with the distance from the ground surface.The main frequency of the structural response is 11 Hz ~ 44 Hz,and the dominant frequency of the speed signal is vertical > horizontal;Under the same shot,The bandwidth of the dominant frequency band of the velocity response signal in the three directions of the structure has the trend that: the first floor < 2nd floor < basement;With the increase of floor,the main frequency of horizontal radial and horizontal tangential vibration gradually attenuates and tends to the natural frequency of the structure,while the main frequency of vertical signal increases with the increase of floor.(2)Analysis of dynamic strain characteristics tests of structure under blasting vibration.Based on technical conditions of tunnel blasting excavation and the characteristics of the building structure,structural dynamic strain test scheme is formulated and tested subjected to tunnel blasting excavation.According to the test results,comparative analysis of strain peak,duration,and number of oscillations in the structural basement,the constructional column on the 2nd floor,and the internal and external partition walls are conducted while blasting occurs directly below the tunnel excavation area.Analysis shows that for the structural basement,the peak vertical strain of the side column decreases with the increase of the number of oscillations,and with the height increases,the strain in the middle of the constructional column is significantly larger than the bottom of the column,and the number of strain oscillations is larger than the bottom of the column.For the top layer of the structure,the peak strain is presented: the inner partition wall <column at 1 m <column at 2 m <exterior wall.The duration is presented: column at 1 m <inside partition wall <column at 2 m < exterior wall.Meanwhile,the dynamic strain of the monitoring points in the outer wall of top floor and the monitoring points in the middle of the side column are different from the dynamic strain of other structural members due to the whiplash effect and weak lateral restraint,after undergoing strain oscillation for a period of time,the strain does not return to zero,but continues to oscillate continuously during the monitoring process subjected to blasting vibration.The weakest floor of structure is the basement,and its strain effect is much larger than the other floors.At the same time,the strain effect of the outer wall of top floor cannot be ignored.(3)Theoretical analysis of motive stress centralization of the building basement.Considering the specificity of the structural basement being bound by the surrounding soil and the direct effect of seismic waves,the theory of wave function expansion method and complex function are used to study and establish the mathematical model for the scattering of SH wave because of the adjacent structural basement,and the mathematical calculation model of displacement amplitude of ground surface,soil-basement joint and basement wall subjected to SH wave are proposed.Based on the engineering practice,an application example of the mathematical model is given by the study of the influence law of displacement amplitude in basement and soil medium when SH wave impinge at the basement with different dimensionless frequency and different incidence angle under the given parameters.(4)The analysis of time-frequency-energy characteristics of structures subjected to blasting vibrations.Introducing the Hilbert-Huang transform to the analysis of structural dynamic response,the complicated blasting vibration is turned into the combined action of multiple IMF components.Substituting the IMF component frequency and instantaneous energy for main frequency and total input energy respectively,the variation law and characteristics of time-instantaneous frequency-instantaneous energy is analyzed.Results shows that the horizontal radial and horizontal tangential instantaneous energy increase firstly,then decrease with the increase of floor under blasting vibration,whereas the vertical instantaneous energy variation has the contrary feature.The instant of maximal peak value of instantaneous energy does not accord with the peak vibration velocity.The frequency and energy contribution of structural response signal change with time and differ considerably from main vibration frequency.Therefore the main vibration frequency derived from PSD frequency spectrums cannot precisely represent the structural dynamic response under blasting seismic wave.In contrast with the energy center of structural response signal in vertical direction,the energy center in horizontal radial and tangential directions moves from middle-low to lower frequency region with the increase of floors.The signal components of blasting seismic wave in different bench excavation are obviously different,and the blasting excavation of upper bench is more detrimental for the safety control of the structures.The horizontal radial response energy is stable in two frequency bands of 3~7 Hz and 17~19 Hz;The energy of the horizontal tangential response is stable in the two frequency bands of 5~9 Hz and 17~22 Hz;The vertical response energy is stable in three frequency bands of 10 Hz,28 Hz,and 39 Hz.(5)Numerical simulation analysis of dynamic response characteristics of multi-story building structures.Based on the actual blasting site,a three-dimensional calculation model of tunnel-rock-soil-structure was established,and the dynamic response characteristics,such as vibration velocity,stress and strain of the whole structure and structural members(columns,beams,plates and walls),were calculated and analyzed under the blasting excavation of the under-crossing tunnel.The results show that along the short axis of the structure,the peak vibration velocities in three directions of the structure are symmetrically distributed along the longitudinal axis of the structure under the influence of the blasting excavation of the under-crossing tunnel.And the farther the particle away from the central axis of the structure,the greater the value of the vibration velocity is.Along the height direction of the structure,the blasting vibration velocities in horizontal radial,horizontal tangential and vertical directions of the structure all show a trend of decrease firstly and then increase and the vertical vibration velocity is always greater than that of horizontal direction.Along the long axis of the structure,the peak blasting vibration velocity of the structure shows two completely different changes in the front and back of the tunnel face,and obvious vibration velocity amplification occurs behind the tunnel face.The peak particle vibration velocity on the side column shows that vertical velocity > tangential velocity > radial velocity in each floor.Stress on the side column shows that effective stress > shear stress > tensile stress.Strain on the side column basically shows that vertical strain > radial strain > tangential strain.As the height increases,the inflection point of the stress and strain on the column will move from the lower end of the column to the upper middle part of the column.In each floor,the maximum radial and vertical vibration velocity of the main beam all appear in the middle,but the maximum tangential vibration velocity appears in the far end of the main beam.The effective stress,shear stress and tensile stress on the beam are distributed as a trend of “U” shape along the length of the beam,and effective stress > shear stress > tensile stress.Radial strain and tangential strain basically show a trend of decrease firstly,then increase,and decrease finally in the length direction of the beam.With the increase of floor,the stress and strain on the beam reduce.The variation laws of particle stress,strain and vibration velocity on different floors are different in the length and width directions.The maximum vertical vibration velocity of the exterior wall appears at 1/2 of the length of the wall in each floor.The maximum tangential vibration velocity all appears at the position which is 1/2 of the wall height and 3/4 of the length.The variation rule of particle effective stress on the exterior wall is completely consistent with that of shear stress.The horizontal radial strain is substantially "W" shape along the length of the wall.The maximum value of effective stress and shear stress gradually decreases as the floor increases.The tangential and radial strains of the external wall increase continuously,but the vertical strain decreases first and then increases.