Risk Assessment And Load Modification Factor Of High-Rise Buildings Subject To Earthquakes And Strong Winds

With the rapid development of the economy,the population expansion and the unprecedented urbanization in China,there brings a tremendous demand for high-performance structures,such as the high-rise buildings,for the overall social development.In the past decades,the earthquakes,strong winds and blasts occurred frequently in all over the world that has caused high economic losses and serious casualties.China lies between the Asia-Europe plate,Pacific plate and Indian Ocean plate as well as it has the long coastline,which causes our country where is prone to multiple hazards,e.g.,earthquakes and strong winds.The analysis of meteorological data shows that,before and after the earthquakes,the atmospheric pressure and temperature will change vastly in the epicenter area,which may induce the formation of winds.The high-rise buildings with long service period and huge investment are inevitable to be subjected to individual and concurrent hazards.However,the traditional design procedure commonly treated the multiple hazards(or referred to multihazard)separately,and focused on the worst-case load combination without considering the combined effects of both hazards acting simultaneously on structural responses as well as there is lack of the probabilistic model of earthquakes and strong winds.In this context,this study presents a probabilistic framework for risk assessment of high-rise buildings and proposes a reliability-based load modification approach under earthquakes and strong winds,which will be implemented in a 42-story steel frame-reinforced concrete core tube(SFRCT)building.The main research contents and conclusions are summarized as follows:(1)According to the instrumental earthquake data and earthquake catalog with Ms≥4 from 1970-2017 in Dali Prefecture provided by the China Strong Motion Network Center,the Frechet distribution is verified to be the optimal model to describe the distribution of PGA,and the seismic hazard curve is obtained by the probabilistic seismic hazard analysis,in which,the logic tree is applied to consider the uncertainties.Besides,the Weibull distribution is the best model to depict the distribution of wind speed in Dali region based on the daily maximum wind speed from 1971-2017 recorded in the height of 10m above the ground level,which was obtained from the China Meteorological Administration.(2)Based on the statistics and recorded data,this study presents a novel approach to model the joint probability distribution(JPD)of earthquakes and strong winds,which can be used to assess the risk of structures under multihazard.Firstly,this innovative copula-based approach is applied to describe the JPD of PGA and strong wind speed with v≥10 m/s based on the 76 pairs of simultaneously recording the earthquake and strong wind events.The statistical analysis results show that the Joe Archimedean copula can yield good joint distribution estimation comparing to the measured values.Then,the presented method is used to construct the joint model of wind speed and direction,and its effectiveness and accuracy are validated by comparing the analytical results with those obtained by the traditional approaches,i.e.,the methods based on the multiplication rule and AL model.(3)This study proposes a probabilistic approach for assessing risk of high-rise buildings under multiple hazards,which can be separated into three circumstances:individual earthquakes,individual strong winds and simultaneous earthquakes and strong winds(or bi-hazards for short).Firstly,this approach is used to evaluate the damage probability of a high-rise building subject to wind excitations by considering the effects of wind direction,structural orientation and parameter uncertainties.The results indicate that the wind-induced fragility of the high-rise building is sensitive to the attack angle,and the damage probability is strongly dependent on the structural orientation.Secondly,this presented approach is respectively employed for the risk assessment of this high-rise building against individual seismic loads and concurrent seismic and wind loads.In practice,the effects of uncertainties associated with random variables,input seismic and wind loads as well as various load conditions are considered.The results show that the damage probability is sensitive to the different loading condition,and decreases with the damage severity increasing.(4)According to the risk results under earthquakes and strong winds,the total annual damage probability is defined as the summation of all possible damage probability corresponding to these three hazard circumstances.The contribution results of each hazard circumstance to the total probability illustrate that the damage probability induced by the bi-hazards dominates the total probability under most damage states.In the available researches,the joint effects of earthquakes and strong winds on the risk of high-rise buildings are commonly ignored,and the bi-hazards is always assumed to be independent.The comprehensive application of this study highlights that the widely accepted assumptions are irrational,conversely,it is necessary to examine the responses of high-rise buildings subject to multihazard.(5)Finally,upon obtaining the structural damage probability under multiple hazards,this study proposes a reliability-based approach to calculate the seismic and wind load modification factors of high-rise buildings against the seismic and wind excitations,which can be utilized as a valid guideline for the calibration of current codes and extended to the combinations of other extreme hazards.For the given rational reliability indexes,the load modification factors and the corresponding combined factors of seismic and wind loads in the extreme event limit state are directly computed,which are larger than or equal to the code-specified values.Indeed,the load modifications may vary vastly for different high-rise buildings and combinations of various hazards,and there needs more experiments and finite element analysis before the load modifications can be suggested for practice.However,this study offers a practical and extensible approach to improve future calibration of load combination criteria for designing the high-performance structures against various hazards.Besides,this proposed reliability-based method can be well extended to determine the corresponding factors of other hazards involved in the combinations.