This paper aims to address the problem of geometric state control of large-segment steel box girders in offshore hoisting during the construction of large-span bridges. First, the geometric state control indexes of a ...This paper aims to address the problem of geometric state control of large-segment steel box girders in offshore hoisting during the construction of large-span bridges. First, the geometric state control indexes of a large-segment steel box girder are determined, such as the manufacturing parameters of the top and bottom slabs, the width of the annular joint, and the support position. Second, the geometric state equations and state transfer matrixes of large-segment steel box girders under different conditions are deduced by taking the mileage and elevation of control points as basic state variables. In application of the geometric state transfer method in the construction control of the Hong Kong-Zhuhai-Macao Bridge, the width of the annular joint and the position parameters for the support of the large-segment steel box girder are predicted precisely. Moreover, the manufacturing parameters of the top and bottom slabs of the steel box girders are calculated reliably. The measured values show that the width of the annular joint is basically the same with the difference of less than 2 mm, the eccentricity of bridge support is less than 20 mm, and the elevation error of the bridge deck is within-10 mm to +15 mm, which meets the construction accuracy. Using the geometric state transfer method, the rapid and accurate installation of the Hong Kong-Zhuhai-Macao Bridge has been realized, demonstrating that the precise control of the geometric state of a steel box girder with ectopic installation and multi-state transition can be realized by using the geometric state transfer method.展开更多
To accurately control the full-span erection of continuous steel box girder bridges with complex cross-sections and long cantilevers, both the augmented finite element method(A-FEM) and the degenerated plate elements ...To accurately control the full-span erection of continuous steel box girder bridges with complex cross-sections and long cantilevers, both the augmented finite element method(A-FEM) and the degenerated plate elements are adopted in this paper. The entire construction process is simulated by the A-FEM with the mesh-separation-based approximation technique, while the degenerated plate elements are constructed based on 3D isoparametric elements, making it suitable for analysis of a thin-walled structure. This method significantly improves computational efficiency by avoiding numerous degrees of freedom(DoFs) when analyzing complex structures. With characteristics of the full-span erection technology, the end-face angle of adjacent girder segments, the preset distance of girder segments from the design position, and the temperature difference are selected as control parameters, and they are calculated through the structural response of each construction stage. Engineering practice shows that the calculation accuracy of A-FEM is verified by field-measured results. It can be applied rapidly and effectively to evaluate the matching state of girder segments and the stress state of bearings as well as the thermal effect during full-span erection.展开更多
In this study, we examined the thermal effects throughout the process of the placement of span-scale girder segments on a 6×110-m continuous steel box girder in the Hong Kong-Zhuhai-Macao Bridge. Firstly, when a ...In this study, we examined the thermal effects throughout the process of the placement of span-scale girder segments on a 6×110-m continuous steel box girder in the Hong Kong-Zhuhai-Macao Bridge. Firstly, when a span-scale girder segment is temporarily stored in the open air, temperature gradients will significantly increase the maximum reaction force on temporary supports and cause local buckling at the bottom of the girder segment. Secondly, due to the temperature difference of the girder segments before and after girth-welding, some residual thermal deflections will appear on the girder segments because the boundary conditions of the structure are changed by the girth-welding. Thirdly, the thermal expansion and thermal bending of girder segments will cause movement and rotation of bearings, which must be considered in setting bearings. We propose control measures for these problems based on finite element method simulation with field-measured temperatures. The local buckling during open-air storage can be avoided by reasonably determining the appropriate positions of temporary supports using analysis of overall and local stresses. The residual thermal deflections can be overcome by performing girth-welding during a period when the vertical temperature difference of the girder is within 1 °C, such as after 22:00. Some formulas are proposed to determine the pre-set distances for bearings, in which the movement and rotation of the bearings due to dead loads and thermal loads are considered. Finally, the feasibility of these control measures in the placement of span-scale girder segments on a real continuous girder was verified: no local buckling was observed during open-air storage;the residual thermal deflections after girth-welding were controlled within 5 mm and the residual pre-set distances of bearings when the whole continuous girder reached its design state were controlled within 20 mm.展开更多
We aimed to assess the associations of large artery stenosis(LAS)and cerebral small vessel disease(CSVD)with the risk of ischemic stroke and to investigate their respective and combined contributions.In the prospectiv...We aimed to assess the associations of large artery stenosis(LAS)and cerebral small vessel disease(CSVD)with the risk of ischemic stroke and to investigate their respective and combined contributions.In the prospective population-based Shunyi Study,1,082 stroke-free participants aged 55.9±9.1 years were included.Participants were followed for incident stroke throughout the study period(2013-2019).Total small vessel disease score was used to measure CSVD burden.Cervico-cerebral large artery stenosis was evaluated via brain magnetic resonance angiography and carotid ultrasound.We estimated the risk of ischemic stroke in relation to LAS and CSVD with Cox regression models.During a mean follow-up of 4.2 years,34 participants(3.1%)experienced at least one ischemic stroke.Severe LAS(≥50% stenosis versus no stenosis:HR=3.27(95%CI:1.31-8.18))and high CSVD burden(total small vessel disease score 2-4 versus 0 point:HR=12.73(4.83-33.53))were associated with increased stroke risk independently.In multivariate models,CSVD burden(7.72%)explained a larger portion of the variation in stroke risk than severity of LAS(3.49%).Our findings identified that both LAS and CSVD were associated with future ischemic stroke in asymptomatic subjects,while those with high CSVD burden deserve more attention in primary prevention of stroke.展开更多
基金Project supported by the Zhejiang Provincial Natural Science Foundation of China(No.LZ16E080001)the National Natural Science Foundation of China(Nos.51578496 and 51878603)。
文摘This paper aims to address the problem of geometric state control of large-segment steel box girders in offshore hoisting during the construction of large-span bridges. First, the geometric state control indexes of a large-segment steel box girder are determined, such as the manufacturing parameters of the top and bottom slabs, the width of the annular joint, and the support position. Second, the geometric state equations and state transfer matrixes of large-segment steel box girders under different conditions are deduced by taking the mileage and elevation of control points as basic state variables. In application of the geometric state transfer method in the construction control of the Hong Kong-Zhuhai-Macao Bridge, the width of the annular joint and the position parameters for the support of the large-segment steel box girder are predicted precisely. Moreover, the manufacturing parameters of the top and bottom slabs of the steel box girders are calculated reliably. The measured values show that the width of the annular joint is basically the same with the difference of less than 2 mm, the eccentricity of bridge support is less than 20 mm, and the elevation error of the bridge deck is within-10 mm to +15 mm, which meets the construction accuracy. Using the geometric state transfer method, the rapid and accurate installation of the Hong Kong-Zhuhai-Macao Bridge has been realized, demonstrating that the precise control of the geometric state of a steel box girder with ectopic installation and multi-state transition can be realized by using the geometric state transfer method.
基金Project supported by the National Natural Science Foundation of China(Nos.51578496 and 51878603)the Zhejiang Provincial Natural Science Foundation of China(No.LZ16E080001)。
文摘To accurately control the full-span erection of continuous steel box girder bridges with complex cross-sections and long cantilevers, both the augmented finite element method(A-FEM) and the degenerated plate elements are adopted in this paper. The entire construction process is simulated by the A-FEM with the mesh-separation-based approximation technique, while the degenerated plate elements are constructed based on 3D isoparametric elements, making it suitable for analysis of a thin-walled structure. This method significantly improves computational efficiency by avoiding numerous degrees of freedom(DoFs) when analyzing complex structures. With characteristics of the full-span erection technology, the end-face angle of adjacent girder segments, the preset distance of girder segments from the design position, and the temperature difference are selected as control parameters, and they are calculated through the structural response of each construction stage. Engineering practice shows that the calculation accuracy of A-FEM is verified by field-measured results. It can be applied rapidly and effectively to evaluate the matching state of girder segments and the stress state of bearings as well as the thermal effect during full-span erection.
基金Project supported by the National Natural Science Foundation of China(Nos.51578496 and 51878603)the Zhejiang Provincial Natural Science Foundation of China(No.LZ16E080001)。
文摘In this study, we examined the thermal effects throughout the process of the placement of span-scale girder segments on a 6×110-m continuous steel box girder in the Hong Kong-Zhuhai-Macao Bridge. Firstly, when a span-scale girder segment is temporarily stored in the open air, temperature gradients will significantly increase the maximum reaction force on temporary supports and cause local buckling at the bottom of the girder segment. Secondly, due to the temperature difference of the girder segments before and after girth-welding, some residual thermal deflections will appear on the girder segments because the boundary conditions of the structure are changed by the girth-welding. Thirdly, the thermal expansion and thermal bending of girder segments will cause movement and rotation of bearings, which must be considered in setting bearings. We propose control measures for these problems based on finite element method simulation with field-measured temperatures. The local buckling during open-air storage can be avoided by reasonably determining the appropriate positions of temporary supports using analysis of overall and local stresses. The residual thermal deflections can be overcome by performing girth-welding during a period when the vertical temperature difference of the girder is within 1 °C, such as after 22:00. Some formulas are proposed to determine the pre-set distances for bearings, in which the movement and rotation of the bearings due to dead loads and thermal loads are considered. Finally, the feasibility of these control measures in the placement of span-scale girder segments on a real continuous girder was verified: no local buckling was observed during open-air storage;the residual thermal deflections after girth-welding were controlled within 5 mm and the residual pre-set distances of bearings when the whole continuous girder reached its design state were controlled within 20 mm.
基金supported by the National Key Research and Development Program of China(2016YFB1001402)National Natural Science Foundation of China(81971138)+2 种基金Chinese Academy of Medical Sciences(CAMS)Innovation Fund for Medical Sciences(CIFMS)(2017-I2M-3-008)Strategic Priority Research Program(Pilot study)“Biological basis of aging and therapeutic strategies”of the Chinese Academy of Sciences(XDPB10)Research Foundation for Young Scholars of Peking Union Medical College Hospital(PUMCH201911275)。
文摘We aimed to assess the associations of large artery stenosis(LAS)and cerebral small vessel disease(CSVD)with the risk of ischemic stroke and to investigate their respective and combined contributions.In the prospective population-based Shunyi Study,1,082 stroke-free participants aged 55.9±9.1 years were included.Participants were followed for incident stroke throughout the study period(2013-2019).Total small vessel disease score was used to measure CSVD burden.Cervico-cerebral large artery stenosis was evaluated via brain magnetic resonance angiography and carotid ultrasound.We estimated the risk of ischemic stroke in relation to LAS and CSVD with Cox regression models.During a mean follow-up of 4.2 years,34 participants(3.1%)experienced at least one ischemic stroke.Severe LAS(≥50% stenosis versus no stenosis:HR=3.27(95%CI:1.31-8.18))and high CSVD burden(total small vessel disease score 2-4 versus 0 point:HR=12.73(4.83-33.53))were associated with increased stroke risk independently.In multivariate models,CSVD burden(7.72%)explained a larger portion of the variation in stroke risk than severity of LAS(3.49%).Our findings identified that both LAS and CSVD were associated with future ischemic stroke in asymptomatic subjects,while those with high CSVD burden deserve more attention in primary prevention of stroke.
