Time Domain Simulation of Transient Responses of Very Large Floating Structures Under Unsteady External Loads

A time domain finite element method （FEM） for the analysis of transient elastic response of a very large floating structure （VLFS） subjected to arbitrary time-dependent external loads is presented. This method is developed directly in time domain and the hydrodynamic problem is formulated based on linear, inviscid and slightly compressible fluid theory and the structural response is analyzed on the thin plate assumption. The time domain finite element procedure herein is validated by comparing numerical results with available experimental data. Finally, the transient elastic response of a pontoon-type VLFS under the landing of an airplane is computed by the proposed time domain FEM. The time histories of the applied force and the position and velocity of an airplane during landing are modeled with data from a Boeing 747-400 jumbo jet.

Seismic response of skewed RC box-girder bridges

It is critical to ensure the functionality of highway bridges after earthquakes to provide access to important facilities. Since the 1971 San Fernando earthquake, there has been a better understanding of the seismic performance of bridges. Nonetheless, there are no detailed guidelines addressing the performance of skewed highway bridges. Several parameters affect the response of skewed highway bridges under both service and seismic loads which makes their behavior complex. Therefore, there is a need for more research to study the effect of skew angle and other related factors on the performance of highway bridges. This paper examines the seismic performance of a three-span continuous concrete box girder bridge with skew angles from 0 to 60 degrees, analytically. Finite element （FE） and simplified beam-stick （BS） models of the bridge were developed using SAP2000. Different types of analysis were considered on both models such as： nonlinear static pushover, and linear and nonlinear time history analyses. A comparison was conducted between FE and BS, different skew angles, abutment support conditions, and time history and pushover analysis. It is shown that the BS model has the capability to capture the coupling due to skew and the significant modes for moderate skew angles. Boundary conditions and pushover load profile are determined to have a major effect on pushover analysis. Pushover analysis may be used to predict the maximum deformation and hinge formation adequately.

Response Analysis of Tension-Based Tension Leg Platform Under Irregular Waves

Tension Leg Platform（TLP） is a hybrid structure used as oil drilling and production facility within water depths of 1200 m. The extension of this TLP concept to deeper waters is a challenge and warrants for some innovative design concepts. In this paper, a relatively new concept of TLP which is christened as Tension-Based Tension Leg Platform（TBTLP） and patented by Srinivasan（1998） has been chosen for study. Response analysis of TLP with one tension base under irregular waves for three different sea states has been performed using hydrodynamic tool ANSYS？ AQWA？. Results are reported in terms of RAOs, response spectrums for surge, heave and pitch degrees of freedom from which spectral statistics have been obtained. The statistics of TBTLP have been compared with TLPs（without tension base） for two different water depths to highlight the features of the new concept. The effect of viscous damping and loading effects on the RAOs are also investigated.

Detecting damage to offshore platform structures using the time-domain data

A new method that uses time-domain response data under random loading is proposed for detecting damage to the structural elements of offshore platforms. In our study, a time series model with a fitting order was first constructed using the time-domain of noise data. A sensitivity matrix consisting of the first differential of the autoregressive coefficients of the time series models with respect to the stiffness of structural elements was then obtained based on time-domain response data. Locations and severity of damage may then be estimated by solving the damage vector whose components express the degrees of damage to the structural elements. A unique aspect of this detection method is that it requires acceleration history data from only one or a few sensors. This makes it feasible for a limited array of sensors to obtain sufficient data. The efficiency and reliability of the proposed method was demonstrated by applying it to a simplified offshore platform with damage to one element. Numerical simulations show that the use of a few sensors’ acceleration history data, when compared with recorded levels of noise, is capable of detecting damage efficiently. An increase in the number of sensors helps improve the diagnosis success rate.

Study of Nonlinear Seismic Response and TMD Primary Control of the Cable-Stayed Bridge Section of the Third Macao-Taipa Bridge

The practical design of the cable-stayed bridge of the 3rd Macao-Taipa bridge is investigated by the finite element analysis program ANSYS, and 3-D elements BEAM188 and BEAM4 are adopted to create a dynamic calculation model. In order to analyze the material nonlinear seismic response of the cable-stayed bridge, the nonlinear behaviors of the ductile plastic hinges of the bridge towers are taken into account by employing the nonlinear rotational spring element COMBIN40. To simulate a major earthquake, three earthquake records were chosen using a wave-choosing program and input into the bridge structure along longitudinal and transversal directions. Comparisons of the linear and nonlinear seismic responses of the cable-stayed bridge are performed. In addition, a study of TMD primary control is carried out using element MASS21 and element COMBIN14, and it is indicated that the effects of mitigation monitoring are evident.

Dynamic Response Analysis of Risers Subjected to Combined Wave and Current Loading

A dynamic response analysis in the frequency domain is presented for risers subjected to combined wave and current loading. Considering the effects of current, a modified wave spectrum is adopted to compute the linearized drag force. An additional drag force convolution term is added to the linearized drag force spectrum, therefore the error is reduced which arises from the truncation of higher order terms in the drag force auto-correlation function. An expression of linearized drag force spectrum is given taking the relative velocity into account. It is found that the additional term is a fold convolution integral. In this paper dynamic responses of risers are investigated, while the influence of floater motion on risers is considered. The results demonstrate that the accuracy of the present method reaches the degree required in time domain analysis.

Simulating the Seismic Response of Concentrically Braced Frames Using Physical Theory Brace Models

The aim of this paper is to assess the accuracy of brace models formulated in Drain 2DX and OpenSees by comparing the simulated results with those obtained from experimental tests. Both, Drain 2DX and OpenSees rely on the physical theory brace model. In this study, experimental tests conducted on the behaviour of structural hollow section braces subjected to symmetric and asymmetric quasi-static cyclic loading were selected for calibrating the numerical model. In addition, the predicted failure strain parameter resulted from a proposed empirical equation as a function of slenderness ratio, width-to-thickness ratio and steel properties was used to define the low-cycle fatigue material that was assigned to model braces in OpenSees. It is concluded that both Drain 2DX and OpenSees brace models give a good prediction in terms of maximum tensile and buckling force, as well as interstorey drift. However, in Drain 2DX, the brace model is not able to replicate the out-of-plan buckling and the braced frame model cannot provide an accurate response when the system experiences highly nonlinear demand. To emphasise the differences in performance between Drain 2DX and OpenSees, the behaviour of a 4-storey concentrically braced frame with zipper bracing configuration, located in Victoria, BC, was investigated.

Dynamic Response of 100m, 200m, 300m Lattice Domes with LRB Seismic Isolator

The objective of this study is to analyze the seismic response characteristics of 100 m, 200 m, 300 m spanned lattice domes under both horizontal and up-down ground motion of El Centro earthquake. For the analysis of earthquake response of lattice domes, the time history analysis is used for the estimation of the dynamic response. Horizontal and up-down earthquake ground motions cause a large asymmetric vertical deformation in the large spatial domes. This study is to investigate the seismic characteristics of lattice domes for eigenvalue modes, displacement and acceleration response. The earthquake response of lattice domes with LRB (lead rubber bearing) isolation device by the horizontal and up-down combined ground motion is significantly reduced for the asymmetric vertical deformation and accelerations of domes.

Influence of Structure Plane Size on Seismic Response of Soil-Structure Interaction

The influence of the change of structure plane size on seismic response was studied for a soil-structure interaction system.Based on the finite element method,a soil-structure interaction calculation model was established to analyze the seismic response by changing the structure plane size and choosing different earthquake waves for different soil fields.The results show that when the natural periods of vibration for different structure plane sizes are close,under the same earthquake wave,the total displacement on the top layer of the structure and the foundation rotation displacement decrease with the increase of structure plane size,and the proportion of superstructure elastic selfdeformation displacement to the total displacement increases with the increase of structure plane size.While for different types of sites and seismic waves,under the horizontal and vertical seismic waves,the seismic responses of different plane sizes have a similar change rule.

Elastic and Inelastic Response of Structural Systems in Seismic Pounding

The present paper addresses the comparative study of three adjacent single-degree-of freedom structures for elastic and inelastic system with and without pounding under seismic excitations. For the gap between three adjacent structures, the simulation is done by using linear spring element without damping. The entire numerical simulation is done in time domain by considering the inputs of four real ground motions. The results of the study show that the response of elastic system is much different to that of response of inelastic system in the absence and presence of pounding, especially in lighter or more flexible structures. Elastic structures show much severe pounding response than inelastic structures. Modeling of colliding structures behaving inelastically is really needed in order to obtain the accurate structural pounding involved response under seismic excitation.

Research on wind-induced responses of a large-scale membrane structure

The wind-induced responses of a large-scale membrane structure, Expo Boulevard, are evaluated in this study. To obtain the wind pressure distribution on the roof surface, a wind tunnel test is performed. A brief analysis of wind pressure on the membrane roof is conducted first and then an analysis of the wind-induced responses of the structure is carried out using a numerical integral method in the time domain. In the process of calculation, the geometrical nonlinearity is taken into account. Results indicate that mean, RSM and peak values of the structure responses increase nonlinearly while the approaching flow velocity increases. Strong nonlinear characteristics are observed in the displacement responses, whereas the responses of nodal stress and cable axial force show minimal nonlinear properties when the membrane structure is subjected to wind loads. Different values of the damping ratio only have a minimal impact on the RSM response of the structure because the background component is a dominant part of the total dynamic response and the resonant component is too small. As the damping ratio increases from 0.02 to 0.05, the RMS responses of vertical displacement, nodal stress and cable axial force decrease by 8.1%, 6.7% and 17.9%, respectively. Since the mean component plays a significant role in the wind-induced response, the values of the gust response factor are not high for Expo Boulevard.

Dynamic interaction numerical models in the time domain based on the high performance scaled boundary finite element method

Consideration of structure-foundation-soil dynamic interaction is a basic requirement in the evaluation of the seismic safety of nuclear power facilities. An efficient and accurate dynamic interaction numerical model in the time domain has become an important topic of current research. In this study, the scaled boundary finite element method （SBFEM） is improved for use as an effective numerical approach with good application prospects. This method has several advantages, including dimensionality reduction, accuracy of the radial analytical solution, and unlike other boundary element methods, it does not require a fundamental solution. This study focuses on establishing a high performance scaled boundary finite element interaction analysis model in the time domain based on the acceleration unit-impulse response matrix, in which several new solution techniques, such as a dimensionless method to solve the interaction force, are applied to improve the numerical stability of the actual soil parameters and reduce the amount of calculation. Finally, the feasibility of the time domain methods are illustrated by the response of the nuclear power structure and the accuracy of the algorithms are dynamically verified by comparison with the refinement of a large-scale viscoelastic soil model.

An investigation of the seismic behavior of a deck-type reinforced concrete arch bridge

This paper attempts to explore potential benefits of form in a deck-type reinforced concrete（RC） arch bridge in connection with its overall seismic behavior and performance. Through a detailed three-dimensional finite element modeling and analysis of an actual existing deck-type RC arch bridge, some useful quantitative information have been derived that may serve for a better understanding of the seismic behavior of such arch bridges. A series of the nonlinear dynamic analyses has been carried out under the action of seven different time histories of ground motion scaled to the AASHTO 2012 response spectrum. The concept of demand to capacity ratios has been employed to provide an initial estimation of the seismic performance of the bridge members. As a consequence of the structural form, a particular type of irregularity is introduced due to variable heights of columns transferring the deck loads to the main arch. Hence, a particular attention has been paid to the internal force/moment distributions within the short, medium, and long columns as well as along the main arch. A study of the effects of the vertical component of ground motion has demonstrated the need for the inclusion of these effects in the analysis of such bridges.

Effect of Lead-Rubber Bearing Isolators in Reducing Seismic Damage for a High-Rise Building in Comparison with Normal Shear Wall System

Seismic earthquakes are a real danger for the construction evolution of high rise buildings.The rate of earthquakes around the world is noteworthy in a wide range of construction areas.In this study,we present the dynamic behavior of a high-rise RC building with dynamic isolators(lead-rubber-bearing),in comparison with a traditional shear wall system of the same building.Seismic isolation has been introduced in building construction to increase the structural stability and to protect the non-structural components against the damaging effects of an earthquake.In order to clarify the influence of incorporating lead rubber bearing isolators in the seismic response and in reducing seismic damages;a comparative study is performed between a fixed base system(shear wall system)and an isolated base system(Lead Rubber Bearing)on an irregular high rise reinforced concrete(RC)building located in Beirut consisting of 48 storeys almost asymmetric orthogonally.For this purpose,a non-linear analysis of a real earthquake acceleration record(EI Centro seismic signal)is conducted,so that the mode shapes,the damping ratio and the natural frequencies of the two models are obtained using ETABS software.The results prove a substantial elongation of the building period,as well as a reduction in the building displacement,the roof acceleration,the inter-storey drift ratio and the base shear force of isolated building relative to fixed-base building.This study proves that this technology is applicable to high rise buildings with acceptable results.

An Assessment of the Seismic Performance of the Historic Tigris Bridge

Turkey is a country that is vulnerable to earthquakes and has experienced many major earthquakes that completely destroyed or caused significant damage to numerous historic structures. Today, using computer software, it is important to numerically model and analyze historic structures that need significant restoration and strengthening, to evaluate them from a perspective of seismic resistance, and to reinforce them without altering their originality. In this study, a finite element model of the historic Tigris Bridge on the Tigris River was created. First, the stresses and deformation caused by its own weight were determined. Subsequently, dynamic analyses were performed in the time domain using past earthquake ground motion records. Displacement and stress values obtained for each earthquake record in these time domain analysis were compared to each other to evaluate the seismic behavior of the bridge comparatively. The seismic performance of the bridge was determined on the basis of the “Guidelines on the Management of Earthquake Risks for Historic Structures” published by the Directorate General of Religious Foundations in Turkey.

Seismic Analysis for Rigid-Framed Prestressed Reinforced Concrete Bridge in Tianjin Light Railway

The seismic analysis of a rigid-framed prestressed concrete bridge in Tianjin Light Railway is performed. A 3-D dynamic finite element model of the bridge is established considering the weakening effect caused by the soft soil foundation. After the dynamic characteristics are calculated in terms of natural frequencies and modes, the seismic analysis is carried out using the modal response spectrum method and the time-history method, respectively. Based on the calculated results, the reasonable design values are finally suggested as the basis of the seismic design of the bridge, and meanwhile the problems encountered were also analyzed. Finally, some conclusions are drawn as: 1) Despite the superiority of rigid-framed prestressed concrete bridge, the upper and lower ends of the piers of the bridge are proved to be the crucial parts of the bridge, which are easily destroyed under designed earthquake excitations and should be carefully analyzed and designed; 2) The soft soil foundation can possibly result in rather weakening of the lateral rigidity of the rigid-framed bridge, and should be paid considerable attention; 3) The modal response spectrum method, combined with time-history method, is suggested for the seismic analysis in engineering design of the rigid-framed prestressed concrete bridge.

浅水浮式风电结构非线性波浪荷载及动力响应特性研究

浮式风电结构在浅水海域中承受的波浪荷载及运动响应表现出非线性特征,对浮式风电安全性提出很高要求。本文建立了风机-浮式基础-系泊系统耦合分析模型,对于不同水深、不同波浪荷载计算方法下的频域水动力系数和时域计算结果分别进行了对比,讨论了水深以及波浪荷载计算方法对浮式风电结构响应和系泊张力的影响。结果表明:浮式风电结构的水动力性能和动力响应随水深变化显著。水深较浅时,浮体响应和系泊张力的低频部分明显增大,二阶波浪荷载的影响更加显著。同时,Newman近似方法与全二次传递函数方法相比,在浅水条件下会明显低估二阶波浪荷载效应,进而影响平台响应和系泊张力的分析。

地震动持时和不同持时指标对结构地震响应的影响

选取70条具有不同持时的天然地震波,并采用5%~75%重要持时将谱匹配后的地震记录划分为长、短持时地震记录集,对10层RC框架结构进行IDA分析和弹塑性时程分析,研究地震动持时对结构地震响应的影响,进一步选取35条天然地震波,采用有效持续时间、5%~75%和5%~95%重要持时等不同强震持时指标分别截取相应的加速度时程段,构建不同的地震记录集,分析不同持时指标对RC框架结构峰值响应和滞回耗能的影响。研究结果表明:在相同的地震动强度下,长持时地震动会导致更大的结构层间变形和结构倒塌概率,且随着持时的增加,结构总滞回耗能大幅增加;在罕遇地震水准下,可选取有效持续时间和5%~95%重要持时指标用于结构层间变形和楼层峰值位移分析,而在极罕遇地震水准下,选用有效持续时间具有更高的可靠性;对于楼层峰值速度,不同地震集计算结果均值和变异系数未表现出明显差异;在相同地震动强度下,建议优先选用有效持续时间用于结构滞回耗能分析。