The Ground Motion Inputs Considering The Effects Of Existing Buildings

When a new building is built on flat ground,the foundation excavation and the surrounding buildings will affect the seismic response of it and the adjacent soil.Similarly,when a new building is built on a slope site,the surrounding buildings will have a significant impact on the seismic response of the new building.At present,the influence of the above factors on the ground motion input and the seismic response of the structure is not considered in the seismic design of the structure.To ensure the structural safety,the influence of the above factors should be systematically studied and evaluated.For this reason,this paper analyzes the seismic response of soil-structure group system on flat site considering the number of surrounding buildings,the process of construction,scale of foundation excavation and type of site.The influence of each factor on the amplitude and spectrum characteristics of ground motions and its law is compared and analyzed.Then,the response of slope site with different number of surrounding buildings and slope angle is analyzed and the influence of each factor on the amplitude and spectrum characteristics of ground motion and its law is compared.Finally,combined with the analysis results,some suggestions for adjusting the ground motion input for different site conditions so as to consider the influence of the above factors in the seismic design of the structure are provided.The main research results and conclusions of this paper are list as following.(1)On a flat site,the peak ground acceleration(PGA)and response spectrum of ground motions are studied.With the increase of the number of buildings on the site,the PGAs of ground motion increase about 40% at most compared with the case of no building on the site.The maximum value of the acceleration response spectrum appears in the period range between 0.05 and 0.15 s.The peak value increases with the increase of the number of buildings around,and the maximum value increases by about 3 times compared with that of no building on the site.At different stage during the construction process,the change trend of PGAs are also different.When the foundation excavation is completed,the ground motion PGA is the largest,which is 68% larger than that before the foundation excavation with no buildings around.The peak value of the acceleration response spectrum appears in the period range from 0.05 to 0.5s.The peak value was the largest when the foundation excavation was completed,which is about 3 times of that before excavation.After construction,the peak value decreased to about 1.5 times of that before excavation.When the building is built,the value of the response spectrum in period range from 0.3 s to 0.7s will increase.With the increase of foundation excavation scale,the increase of ground motion PGA tends to be smaller.The softer the soil is,the greater the increase of ground motion PGA is.Under the condition that other variables are the same,the influence of soft soil is at most 61% larger than that of hard soil.For site with softer soil,the peak of the acceleration response spectrum appears in the period range between 0.05 and 1.0s,and the maximum increase is about 4.2 times that in the case of no building.(2)On a slope site,the PGA and ground motion response spectrum influenced by the number of surrounding buildings and the size of the slope are studied.With the increase of the number of buildings around,the amplification of ground motion PGAs increases as well,the PGAs are at most 43% larger than the case of without building on the slope,and 54% larger than the case of without building on the flat site.The peak value of the acceleration response spectrum appears in the period range between 0.05 and 0.2s,with the increase of the number of buildings around,the maximum ratio of the peak value relative to flat site without building increases from 2.2 to 3.2.With the increase of slope,the amplification of ground motion PGA and the peak value of acceleration response spectrum also tend to increase.(3)On a slope site,there is a difference in the ground motion between the downhill buildings(0.35 time of slope height from the bottom of the slope)and the uphill buildings(0.65 time of slope height from the bottom of the slope).The amplification ratio of PGA in the downhill building is at most 11% lower than that in the study site.The increase of PGA in the uphill building is about 10% larger than that in the research site at most,and the amplification ratio tends to increase linearly with the height.The amplification of the acceleration response spectrum in the downhill position reduces,compared with that in the middle position,in the period range of 0s～0.2s with a reduce rate of about 10%～13%.The amplification of the acceleration response spectrum in the uphill position increases,compared with that in the middle position,in the period range of 0s～1s with an increase rate of about 8%～11%.(4)Based on the research results of the influence of foundation excavation and existing buildings on the amplitude and frequency spectrum of ground motion,a simplified method is proposed to consider their influence which can be used for structure seismic design.In this paper,the effects of foundation excavation excavation and the existing building group on the site and the proposed structures on flat ground or slope site are comprehensively investigated,the most unfavorable case of ground motion input from the beginning of construction to the completion of the building is identified and indicated,and the adjustment method of ground motion input considering above factors in the design is given in combination with the analysis results,which is helpful to further improve the seismic safety of new buildings.