Resilience-incorporated seismic risk assessment of precast concrete frames with“dry”connections

A resilience-incorporated risk assessment framework is proposed and demonstrated in this study to manifest the advantageous seismic resilience of precast concrete frame(PCF)structures with“dry”connections in terms of their low damage and rapid recovery.The framework integrates various uncertainties in the seismic hazard,fragility,capacity,demand,loss functions,and post-earthquake recovery.In this study,the PCF structures are distinguished from ordinary reinforced concrete frame(RCF)structures by characterizing multiple limit states for the PCF based on its unique damage mechanisms.Accordingly,probabilistic story-wise pushover analyses are performed to yield story-wise capacities for the predefined limit states.In the seismic resilience analysis,a step-wise recovery model is proposed to idealize the functionality recovery process,with separate considerations of the repair and non-repair events.The recovery model leverages the economic loss and downtime to delineate the stochastic post-earthquake recovery curves for the resilience loss estimation.As such,contingencies in the probabilistic post-earthquake repairs are incorporated and the empirical judgments on the recovery parameters are largely circumvented.The proposed framework is demonstrated through a comparative study between two“dry”connected PCFs and one RCF designed as alternative structural systems for a prototype building.The results from the risk quantification indicate that the PCFs show reduced loss hazards and lower expected losses relative to the RCF.Particularly,the PCF equipped with energy dissipation devices at the“dry”connections largely reduces the expected economic loss,downtime,and resilience loss by 29%,56%,and 60%,respectively,compared to the RCF.

Study on Evacuation Strategy of Commercial High-Rise Building under Fire Based on FDS and Pathfinder

With the development of economy and society and the growth of population,the high-rise and multi-function of commercial buildings have become an international trend.But it also poses huge fire hazards.Most of the existing studies’research objects are predominantly high-rise residential buildings,without considering the impact of different functional zones(Standard floor,entertainment zone,office zone,equipment room and so on)and personnel distribution of commercial buildings evacuation.And the influence of using elevators to carry evacuees on the refuge floor on personnel evacuation is rarely studied.In this work,the fire scenario of the Yangtze River InternationalConferenceCenter,a high-rise commercial building,is simulated with the Pyrosim programto get the necessary parameters under various fire scenarios and to calculate the available evacuation time TASET.At the same time,according to the complex functional zone of the commercial high-rise building and the distribution of people in different time periods,a reasonable evacuation strategy is developed and simulated by Pathfinder software.The results indicate that unorganized evacuation will lead individuals to take the erroneous evacuation route,resulting in a vast region of congestion;comprehensive consideration of the time staggering and the reasonable distribution of evacuation routes can significantly improve evacuation efficiency,and the TRSET of night and working hours is 36.6%–55.3%and 49.9%–79.6%of unorganized evacuation,respectively.For the night fire,60%of the people use elevator-refuge floor to evacuate is the optimal strategy;for the fire during working hours,half of the people on standard floors use the elevator to evacuate and people on multifunctional floors evacuate in four batches is the best plan.The results of this study can provide viable solutions and a foundation for analyzing the fire evacuation and safety of big commercial high-rise buildings.

A symmetric substructuring method for analyzing the natural frequencies of conical origami structures

Conical origami structures are characterized by their substantial out-of-plane stiffness and energy-absorptioncapacity.Previous investigations have commonly focused on the static characteristics of these lightweight struc-tures.However,the efficient analysis of the natural vibrations of these structures is pivotal for designing conicalorigami structures with programmable stiffness and mass.In this paper,we propose a novel method to analyzethe natural vibrations of such structures by combining a symmetric substructuring method(SSM)and a gener-alized eigenvalue analysis.SSM exploits the inherent symmetry of the structure to decompose it into a finiteset of repetitive substructures.In doing so,we reduce the dimensions of matrices and improve computationalefficiency by adopting the stiffness and mass matrices of the substructures in the generalized eigenvalue analysis.Finite element simulations of pin-jointed models are used to validate the computational results of the proposedapproach.Moreover,the parametric analysis of the structures demonstrates the influences of the number of seg-ments along the circumference and the radius of the cone on the structural mass and natural frequencies of thestructures.Furthermore,we present a comparison between six-fold and four-fold conical origami structures anddiscuss the influence of various geometric parameters on their natural frequencies.This study provides a strategyfor efficiently analyzing the natural vibration of symmetric origami structures and has the potential to contributeto the efficient design and customization of origami metastructures with programmable stiffness.

Full-Scale Numerical Simulation of the Local Scour Under Combined Current and Wave Conditions Based on Field Data

The monopile is the most common foundation to support offshore wind turbines.In the marine environment,local scour due to combined currents and waves is a significant issue that must be considered in the design of wind turbine foundations.In this paper,a full-scale numerical model was developed and validated based on field data from Rudong,China.The scour development around monopiles was investigated,and the effects of waves and the Reynolds number Re were analyzed.Several formulas for predicting the scour depth in the literature have been evaluated.It is found that waves can accelerate scour development even if the KC number is small(0.78<KC<1.57).The formula obtained from small-scale model tests may be unsafe or wasteful when it is applied in practical design due to the scale effect.A new equation for predicting the scour depth based on the average pile Reynolds number(Rea)is proposed and validated with field data.The equilibrium scour depth predicted using the proposed equation is evaluated and compared with those from nine equations in the literature.It is demonstrated that the values predicted from the proposed equation and from the S/M(Sheppard/Melville)equation are closer to the field data.

Pulses in ground motions identified through surface partial matching and their impact on seismic rocking consequence

In seismology and earthquake engineering,it is fundamental to identify and characterize the pulse-like features in pulse-type ground motions.To capture the pulses that dominate structural responses,this study establishes congruence and shift relationships between response spectrum surfaces.A similarity search between spectrum surfaces,supplemented with a similarity search in time series,has been applied to characterize the pulse-like features in pulse-type ground motions.The identified pulses are tested in predicting the rocking consequences of slender rectangular blocks under the original ground motions.Generally,the prediction is promising for the majority of the ground motions where the dominant pulse is correctly identified.

Numerical Simulation of Fretting Fatigue Damage Evolution of Cable Wires Considering Corrosion and Wear Effects

In this paper,a numerical model of fretting fatigue analysis of cablewire and the fretting fatigue damage constitutive model considering the multi-axis effect were established,and the user material subroutine UMAT was written.Then,the constitutive model of wear morphology evolution of cable wire and the constitutive model of pitting evolution considering the mechanical-electrochemical effect were established,respectively.The corresponding subroutines UMESHMOTION_Wear and UMESHMOTION_Wear_Corrosion were written,and the fretting fatigue lifewas further predicted.The results showthat the numerical simulation life obtained by the programin this paper has the same trend as the tested one;the error is only about 0.7%in the medium life area;When the normal contact force increases from 120 to 240 N,the fretting life of cable wire decreases by 25%;When the evolution of wear morphology and corrosion effect are considered simultaneously,the depth of the wear zone exceeds 0.08mm after 600,000 loads,which ismuch larger than 0.04 mmwhen only the evolution of wear morphology is considered.When the evolution of wear morphology and corrosion morphology is considered simultaneously,the damage covers the whole contact surface after 300,000 loads,and the penetrating damage zone forms after 450,000 loads,which is obviously faster than that when only the wearmorphology evolution is considered.Themethod proposed in this paper can provide a feasible numerical simulation scheme for the visualization of the damage process and accurate life prediction of cable-supported bridges.

Exploration of instantaneous frequency for local control assessment in real-time hybrid simulation

Local control parameters such as instantaneous delay and instantaneous amplitude play an essential role in evaluating the performance and maintaining the stability of real-time hybrid simulation(RTHS).However,existing methods have limitations in obtaining this local assessment in either the time domain or frequency domain.In this study,the instantaneous frequency is introduced to determine local control parameters for actuator tracking assessment in a real-time hybrid simulation.Instantaneous properties,including amplitude,delay,frequency and phase,are then calculated based on analytic signals translated from actuator tracking signals through the Hilbert transform.Potential issues are discussed and solutions are proposed for calculation of local control parameters.Numerical simulations are first conducted for sinusoidal and chirp signals with time varying amplitude error and delay to demonstrate the potential of the proposed method.Laboratory tests also are conducted for a predefined random signal as well as the RTHS of a single degree of freedom structure with a self-centering viscous damper to experimentally verify the effectiveness of the proposed use of the instantaneous frequency.Results from the ensuing analysis clearly demonstrate that the instantaneous frequency provides great potential for local control assessment,and the proposed method enables local tracking parameters with good accuracy.

Fatigue Crack Propagation Law of Corroded Steel Box Girders in Long Span Bridges

In order to investigate the fatigue performance of orthotropic anisotropic steel bridge decks,this study realizes the simulation of the welding process through elastic-plastic finite element theory,thermal-structural sequential coupling,and the birth-death element method.The simulated welding residual stresses are introduced into the multiscale finite element model of the bridge as the initial stress.Furthermore,the study explores the impact of residual stress on crack propagation in the fatigue-vulnerable components of the corroded steel box girder.The results indicate that fatigue cracks at the weld toe of the top deck,the weld root of the top deck,and the opening of the transverse diaphragm will not propagate under the action of a standard vehicle load.However,the inclusion of residual stress leads to the propagation of these cracks.When considering residual stress,the fatigue crack propagation paths at the weld toe of the transverse diaphragm and the U-rib weld toe align with those observed in actual bridges.In the absence of residual stress,the cracks at the toe of the transverse diaphragm with a 15%mass loss rate are categorized as type I cracks.Conversely,when residual stress is considered,these cracks become I-II composite cracks.Residual stress significantly alters the cumulative energy release rate of the three fracturemodes.Therefore,incorporating the influence of residual stress is essential when assessing the fatigue performance of corroded steel box girders in long-span bridges.

Analysis on flutter performance of flexible photovoltaic support based on full-order method

Taking a three-cable flexible photovoltaic(PV)support structure as the research subject,a finite element model was established.Utilizing a full-order flutter analysis method,the flutter critical wind speed and flutter frequency of the flexible PV support structure at a tilt angle of 0°were calculated.The results showed good agreement with wind tunnel test data.Further analysis examined the pretension effects in the load-bearing and stabilizing cables on the natural frequency and flutter critical wind speed of the flexible PV support structure.The research findings indicate increasing the pretension in the load-bearing cables significantly raises the natural frequencies of the first four modes.Specifically,as the pretension in the load-bearing cables increases from 22 to 102 kN,the flutter critical wind speed rises from 17.1 to 21.6 m/s.By contrast,the pretension in the stabilizing cable has a smaller effect on the natural frequency and flutter critical wind speed of the flexible PV support structure.When the pretension in the stabilizing cable increased from 22 to 102 kN,the flutter critical wind speed increased from 17.1 to 17.7 m/s.For wind-resistant design of flexible PV support structures,it is recommended to prioritize increasing the pretension in the load-bearing cables to enhance the structural flutter performance.

A new isolation device using shape memory alloy and its application for long-span structures

The key points to consider in determining the effectiveness of using structural isolation with shape memory alloys （SMA） are the constitutive model, the SMA isolation device and the analysis method. In this paper, a simplified constitutive model based on the classic theory of plasticity is proposed to simulate the behavior of the superelasticity of the SMA, in which the martensite volume fraction is considered as one of the state variables. Comparisons between simulation results and experimental results are made and indicate that the proposed constitutive model yields stress-strain curves that are in good agreement with the experimental ones. Thus, the proposed model can correctly simulate the yield mechanism and energy dissipation capacity of the SMA. Next, in order to make full use of the superelasticity of SMA, a new SMA isolator composed of pre-tensioned SMA bars is presented. Then, a finite element analytical model is established to simulate the behavior of the SMA isolator according to its configuration and simplified constitutive model. Finally, a simplified design method for long-span structures installed with SMA isolators is proposed, which is further used to investigate the isolation effects of a space grid structure. Results show that the SMA isolator can reduce the seismic responses of the structure effectively, which indicates the effectiveness of the proposed SMA isolation method.

Investigations on serviceability control of long-span structures under human-induced excitation

The increasing strength of new structural materials and the span of new structures, accompanied by aesthetic requirements for greater slenderness, are resulting in more applications of long-span structures. In this paper, serviceability control technology and its design theory are studied. First, a novel tuned mass damper （TMD） with controllable stiffness is developed. Second, methods for modeling human-induced loads are proposed, including standing up, walking, jumping and running, and an analysis method for long-span floor response is proposed based on a finite element model. Third, a design method for long-span floors installed with a multiple TMD （MTMD） system considering human comfort is introduced, largely based on a study of existing literature. Finally, a design, analysis and field test is conducted using several large scale buildings in China including the Beijing Olympic Park National Conference Center, Changsha New Railway Station and the Xi＇an Northern Railway Station. The analytical and field test results show that the MTMD system designed using the proposed method is capable of effectively mitigating the vertical vibration of long-span floor structures. The study presented in this paper provides an important reference for the analysis of vibration serviceability of similar long-span floors and design of control system for these structures.

Analysis of actuator delay and its effect on uncertainty quantification for real-time hybrid simulation

Uncertainties in structure properties can result in different responses in hybrid simulations. Quantification of the effect of these tmcertainties would enable researchers to estimate the variances of structural responses observed from experiments. This poses challenges for real-time hybrid simulation （RTHS） due to the existence of actuator delay. Polynomial chaos expansion （PCE） projects the model outputs on a basis of orthogonal stochastic polynomials to account for influences of model uncertainties. In this paper, PCE is utilized to evaluate effect of actuator delay on the maximum displacement from real-time hybrid simulation of a single degree of freedom （SDOF） structure when accounting for uncertainties in structural properties. The PCE is first applied for RTHS without delay to determine the order of PCE, the number of sample points as well as the method for coefficients calculation. The PCE is then applied to RTHS with actuator delay. The mean, variance and Sobol indices are compared and discussed to evaluate the effects of actuator delay on uncertainty quantification for RTHS. Results show that the mean and the variance of the maximum displacement increase linearly and exponentially with respect to actuator delay, respectively. Sensitivity analysis through Sobol indices also indicates the influence of the single random variable decreases while the coupling effect increases with the increase of actuator delay.

Cyclic Lateral Responses of Monopiles Considering the Influence of Pile−Soil Relative Stiffness in Sand

The existing studies have primarily focused on the effect of cyclic load characteristics(namely,cyclic load ratio and amplitude ratio)on cyclic lateral response of monopiles in sand,with little attention paid to the effect of pile−soil relative stiffness(K_(R)).This paper presents a series of 1-g cyclic tests aimed at improving understanding of the cyclic lateral responses of monopiles under different pile−soil systems.These systems are arranged by two model piles with different stiffness,including four different slenderness ratios(pile embedded length,L,normalized by diameter,D)under medium dense sand.The K_(R)-values are calculated by a previously proposed method considering the real soil stress level.The test results show that the lateral accumulation displacement increases significantly with the increment of the K_(R)-value,while the cyclic secant stiffness performs inversely.The maximum pile bending moment increases with the cycle number for the rigid pile−soil system,but shows a decreasing trend in the flexible system.For an uppermost concern,an empirical model is proposed to predict the accumulated displacement of arbitrary pile−soil systems by combining the results from this study with those from previous experimental investigations.The validity of the proposed model is demonstrated by 1-g and centrifuge tests.

Preparation of Novel Core-shell Non-sintered Lightweight Aggregate and Its Application in Wallboard for Better Properties

Due to the relatively high density of conventional non-sintered lightweight aggregate(NLA),a low-density core-shell NLA(CNLA) was developed.Moreover,two types of porous lightweight aggregate concrete (PLAC) for wallboard were designed,using both foam and lightweight aggregates.The effects of LA on lightweight concrete workability,compressive strength,dry shrinkage,and thermal conductivity were studied and compared.The bulk density of CNLA can be lowered to 500 kg/m^(3),and its cylinder crushing strength is 1.6 MPa.PLACs also have compressive strengths ranging from 7.8 to 11.8 MPa,as well as thermal conductivity coefficients ranging from 0.193 to 0.219 W/(m·K^(-1)).The CNLA bonds better to the paste matrix at the interface transition zone,and CNLA concrete has a superior pore structure than SLA concrete,resulting in a 20% improvement in fluidity,a 10% increase in strength,a 6% reduction in heat conductivity,and an 11% decrease in drying shrinkage.

Seismic behavior and mechanism analysis of innovative precast shear wall involving vertical joints

To study the seismic performance and load-transferring mechanism of an innovative precast shear wall(IPSW) involving vertical joints, an experimental investigation and theoretical analysis were successively conducted on two test walls. The test results confirm the feasibility of the novel joints as well as the favorable seismic performance of the walls, even though certain optimization measures should be taken to improve the ductility. The load-transferring mechanism subsequently is theoretically investigated based on the experimental study. The theoretical results show the load-transferring route of the novel joints is concise and definite. During the elastic stage, the vertical shear stress in the connecting steel frame(CSF) distributes uniformly; and each high-strength bolt(HSB)primarily delivers vertical shear force. However, the stress in the CSF redistributes when the walls develop into the elastic-plastic stage. At the ultimate state, the vertical shear stress and horizontal normal stress in the CSF distribute linearly; and the HSBs at both ends of the CSF transfer the maximum shear forces.

Study on Unloading Creep Characteristics of the Soil and Application of the Stress-Dependent Creep Model in Suction Caisson Foundation

As the anchoring foundation of the tension leg platform(TLP),suction caisson foundation is subjected to the long-term vertical pullout loads.But there are few studies on the mechanism of the unloading creep of soft clay and long-term uplift bearing capacity of suction caisson foundations.To address this problem,unloading creep tests of soft clay were carried out to analyze the strain development with time under different confining pressures.The test results show that the creep curve rapidly develops in the early stage and tends to stabilize in the later stage.The unloading deviator stress is higher,the unloading creep deformation is greater and the soft clay has typical nonlinear creep characteristics.Therefore,by introducing the creep model and considering the influence of the deviator stress,the stress-dependent Merchant model is proposed to describe the unloading creep of soft clay.Then,the stress-dependent Merchant model is extended to a three-dimension constitutive model,and a finite element subroutine is developed to establish a finite element analysis method for analyzing the long-term uplift capacity of suction caisson foundations and validated with the long-term uplift bearing capacity results of caisson model.

Modeling and control of MR damper considering trapped air effect

Due to the high viscosity of magnetorheological(MR)fluid,eliminating air pockets dissolved in the fluid is very difficult,which results in a force lag phenomenon.In order to evaluate the performance of a semi-active control system based on the MR damper considering the trapped air effect,a performance test on a MR damper is carried out under different loading cases,and the influence of the input current,excitation amplitude and frequency on the force lag phenomenon is analyzed.A concise and efficient parametric model,combining the simple Bouc-Wen model and a spring with small stiffness,is proposed to portray the experimental characteristics of the MR damper with force lag,and then the response analysis of the semi-active controlled single-degree-of-freedom(SDOF)structure is performed using the classic clipped-optimal control strategy based on acceleration feedback.Numerical results show that the trapped air in the MR fluid can weaken the control effect of the MR damper,and the performance of the semi-active control system will be reduced more obviously and become close to the passive-off control with the increasing content of air trapped in the MR fluid.

Interface Mechanical Behavior of Flexible Piles Under Lateral Loads in OWT Systems

This paper investigates the interface mechanical behavior of flexible piles with L_p/D>10 under lateral load and an overturning moment in monotonic loading conditions.To modify the beam-on-Winkler-foundation model of piles in offshore wind farms,the energy-based variational method is used.The soil is treated as a multi-layered elastic continuum with the assumption of three-dimensional displacements,the pile modeled as an Euler-Bernoulli beam.A series of cases using MATLAB programming was conducted to investigate the simplified equations of initial stiffness.The results indicated that the interaction between soil layers and the applied force position should be taken into account in calculating the horizontal soil resistance.Additionally,the distributed moment had a limiting effect on the lateral capacity of a flexible pile.Moreover,to account for the more realistic conditions of OWT systems,field data from the Donghai Bridge offshore wind farm were used.

Flexural and longitudinal shear performance of precast lightweight steel-ultra-high performance concrete composite beam

In this study,the flexural and longitudinal shear performances of two types of precast lightweight steel–ultra-high performance concrete(UHPC)composite beams are investigated,where a cluster UHPC slab(CUS)and a normal UHPC slab(NUS)are connected to a steel beam using headed studs through discontinuous shear pockets and full-length shear pockets,respectively.Results show that the longitudinal shear force of the CUS is greater than that of the NUS,whereas the interfacial slip of the former is smaller.Owing to its better integrity,the CUS exhibits greater flexural stiffness and a higher ultimate bearing capacity than the NUS.To further optimize the design parameters of the CUS,a parametric study is conducted to investigate their effects on the flexural and longitudinal shear performances.The square shear pocket is shown to be more applicable for the CUS,as the optimal spacing between two shear pockets is 650 mm.Moreover,a design method for transverse reinforcement is proposed;the transverse reinforcement is used to withstand the splitting force caused by studs in the shear pocket and prevent the UHPC slab from cracking.According to calculation results,the transverse reinforcement can be canceled when the compressive strength of UHPC is 150 MPa and the volume fraction of steel fiber exceeds 2.0%.

A Simplified Method for Estimating the Initial Stiffness of Monopile-Soil Interaction Under Lateral Loads in Offshore Wind Turbine Systems

The interface mechanical behavior of a monopile is an important component of the overall offshore wind turbine structure design.Understanding the soil-structure interaction,particularly the initial soil-structure stiffness,has a significant impact on the study of natural frequency and dynamic response of the monopile.In this paper,a simplified method for estimating the interface mechanical behavior of monopiles under initial lateral loads is proposed.Depending on the principle of minimum potential energy and virtual work theory,the functions of soil reaction components at the interface of monopiles are derived;MATLAB programming has been used to simplify the functions of the initial stiffness by fitting a large number of examples;then the functions are validated against the field test data and FDM results.This method can modify the modulus of the subgrade reaction in the p-y curve method for the monopile-supported offshore wind turbine system.