Introduction The predominant characteristics of high-altitude climates include low air pressure,low humidity,and large diurnal temperature fluctuations.In practical engineering scenarios in high-altitude regions,many ...Introduction The predominant characteristics of high-altitude climates include low air pressure,low humidity,and large diurnal temperature fluctuations.In practical engineering scenarios in high-altitude regions,many pier surface cracks only appear several months after erection,and cyclic thermal stress is identified as the main trigger for such cracking.The thermal stress in concrete structures has been investigated for decades but remains incompletely understood.Structural engineers typically regard concrete as an isotropic material and calculate the thermal stress using code-specified coefficients of thermal expansion(CTEs)along with temperature conditions and constraints.Because the CTE of hardened cement paste is more than twice that of many aggregates,reducing the CTE of coarse aggregates can further exacerbate the thermal deformation incompatibility between the coarse aggregate and mortar matrix.In this paper,a comprehensive thermal-elastic mechanics model for pier concrete was developed to analysis the temperature-induced surface stress.Methods A series of mechanical and thermophysical tests were conducted on the diorite aggregate,ITZ cement paste,and mortar,and concrete.A test pier was constructed on open ground near the Yarlung Zangbo River at an altitude of 3800 m.The pier had a diameter of 1.8 m and height of 2 m.Temperature sensors were embedded in the cross-section at a height of 1 m,positioned along the southnorth and east-west directions.The embedding depths(distances from the pier surface)were 0,1.5,3,4.5,6,7.5,9,12,15,20,25,30,35,40,50,60,70,80 cm,and 90 cm.A model of the bridge pier concrete for surface-level analysis was constructed.The model consists of a concrete unit formed as a sphere-shell-shell composite,including the aggregate,ITZ,and mortar layers,embedded in the surface layer of a bridge pier.Outside this unit,the pier concrete was treated as an isotropic,homogeneous elastic material.The real-time internal temperature fields of high-altitude concrete bridge piers,measured on-site,were incorporated into the model.By applying thermoelastic mechanics theory and finite element solutions for plane strain problems,the three-dimensional thermal stresses on the surface layer of high-altitude bridge piers were analyzed Results and discussion During the experimental period,the lowest and highest temperatures on the bridge pier in the high-altitude region were 9.6 ℃ and 42.6 ℃,respectively.These occurred before sunrise and sunset on sunny days,respectively,corresponding to the local maximum temperature gradients during the surface heating and cooling stages,as well as the maximum temperature difference between the surface and center during these stages.The thermal stress on the pier concrete surface was obtained by superimposing the stresses caused by the uneven distribution of the internal temperature field and those caused by the incompatible thermal deformation among the different components in the surface concrete Before the erection of the upper structures,the absolute values of the tangential and vertical stresses were the same;therefore,only one curve was observed.From 22:00 to 8:00,the pier concrete surface was in tension,whereas from 11:00 to 22:00,the pier concrete surface was in compression.The surface of the pier concrete was subjected to biaxial forces of equal magnitude with a maximum compressive stress of 12.52 MPa and maximum tensile stress of 2.15 MPa,respectively at 18:00 and 8:00.According to the fatigue equation,the concrete was predicted to crack after 21 d of temperature cycling.Moreover,if humidity-induced stress is added on top of this,the tensile stress may approach or even exceed the concrete's tensile strength,thereby posing a significant risk of cracking.After the erection of upper structures,the tangential and vertical stresses no longer coincide because the upper structures have been erected.The curve of the tangential stress is unchanged,whereas the curve of the vertical stress is translated downwards by 1.57 MPa due to the structural deadweight.Therefore,the maximum tangential compressive stress remained 12.52 MPa,whereas the maximum vertical compressive stress increased to 14.09 MPa.Additionally,the maximum tangential tensile stress was 2.15 MPa,and the maximum vertical tensile stress was 0.58 MPa.According to Appendix C of GB/T 50010 and the fatigue equation,stresses are unlikely to cause cracking of the pier concrete surface.Although a higher CTE of the coarse aggregate slightly increased the maximum compressive stress,the differences among the three groups of concrete were minimal and could be ignored.Specifically,the maximum compressive stresses on the pier concrete surface were 12.54,12.45 MPa,and 12.56 MPa when using diorite,limestone,and basalt,respectively.By contrast,a lower CTE of the coarse aggregate results in a greater maximum tensile stress on the pier concrete surface.For example,when using limestone,which has a low CTE,the maximum tensile stress on the pier concrete surface is 2.28 MPa,compared to 2.17 MPa when using diorite and 2.14 MPa when using basalt.The finite element simulation results indicated that the maximum compressive stress on the pier concrete surface was 11.72 MPa,whereas the maximum tensile stress was 2.10 MPa.These results are approximately consistent with the theoretical calculations.This consistency provides mutual verification.Conclusions Surface cracking in pier concrete occurs predominantly before the erection of upper structures.Under sunny conditions,the orthogonal decomposition of the superficial stress revealed that the maximum compressive stress during the day was approximately 12.52 MPa,whereas the maximum tensile stress was approximately 2.15 MPa.This tensile stress approached the tensile strength of the C35 concrete under biaxial tension.The risk of cracking increased significantly when humidity-induced stress was considered.After the erection of upper structures,the maximum tangential tensile stress on the pier surface remained at 2.15 MPa while the maximum vertical stress decreased to 0.58 MPa,both of which are well below the tensile strength of C35 concrete under biaxial tension.Although the use of coarse aggregates with a lower coefficient of thermal expansion reduced the tensile stress induced by temperature gradients,it increased the stress owing to material deformation incompatibility,leading to a slight increase in the maximum tensile stress on the pier concrete surface.展开更多
Many studies on land use change （LUC）, using different approaches and models, have yielded good results. Applications of these methods have revealed both advantages and limitations. However, LUC is a complex problem...Many studies on land use change （LUC）, using different approaches and models, have yielded good results. Applications of these methods have revealed both advantages and limitations. However, LUC is a complex problem due to influences of many factors, and variations in policy and natural conditions. Hence, the characteristics and regional suitability of different methods require further research, and comparison of typical approaches is required. Since the late 1980s, CA has been used to simulate urban growth, urban sprawl and land use evolution successfully. Nowadays it is very popular in resolving the LUC estimating problem. Casebased reasoning （CBR）, as an artificial intelligence technology, has also been employed to study LUC by some researchers since the 2000s. More and more researchers used the CBR method in the study of LUC. The CA approach is a mathematical system constructed from many typical simple components, which together are capable of simulating complex behavior, while CBR is a problem-oriented analysis method to solve geographic problems, particularly when the driving mechanisms of geographic processes are not yet understood fully. These two methods were completely different in the LUC research. Thus, in this paper, based on the enhanced CBR model, which is proposed in our previous research （Duet aL 2009）, a comparison between the CBR and CA approaches to assessing LUC is presented. LUC in Dongguan coastal region, China is investigated. Applications of the improved CBR and the cellular automata （CA） to the study area, produce results demonstrating a similarity estimation accuracy of 89% from the improved CBR, and 70.7% accuracy from the CA. From the results, we can see that the accuracies of the CA and CBR approaches are both 〉70%. Although CA method has the distinct advantage in predicting the urban type, CBR method has the obvious tendency in predicting non-urban type. Considering the entire analytical process, the preprocessing workload in CBR is less than that of the CA approach. As such, it could be concluded that the CBR approach is more flexible and practically useful than the CA approach for estimating land use change.展开更多
Traditional methods of extracting the ocean wave eddy information from remotely sensed imagery mainly use the edge detection technology such as Canny and Hough operators. However, due to the complexities of ocean eddi...Traditional methods of extracting the ocean wave eddy information from remotely sensed imagery mainly use the edge detection technology such as Canny and Hough operators. However, due to the complexities of ocean eddies and image itself, it is sometimes difficult to successfully detect ocean eddies using these methods. A mnltifractal filtering technology is proposed for extraction of ocean eddies and demonstrated using NASA MODIS, SeaWiFS and NOAA satellite data set in the typical area, such as ocean west boundary current. Results showed that the new method has a superior performance over the traditional methods.展开更多
In the supervised classification process of remotely sensed imagery,the quantity of samples is one of the important factors affecting the accuracy of the image classification as well as the keys used to evaluate the i...In the supervised classification process of remotely sensed imagery,the quantity of samples is one of the important factors affecting the accuracy of the image classification as well as the keys used to evaluate the image classification.In general,the samples are acquired on the basis of prior knowledge,experience and higher resolution images.With the same size of samples and the same sampling model,several sets of training sample data can be obtained.In such sets,which set reflects perfect spectral characteristics and ensure the accuracy of the classification can be known only after the accuracy of the classification has been assessed.So,before classification,it would be a meaningful research to measure and assess the quality of samples for guiding and optimizing the consequent classification process.Then,based on the rough set,a new measuring index for the sample quality is proposed.The experiment data is the Landsat TM imagery of the Chinese Yellow River Delta on August 8th,1999.The experiment compares the Bhattacharrya distance matrices and purity index zl and△x based on rough set theory of 5 sample data and also analyzes its effect on sample quality.展开更多
The Changbai Mountains,located in the temperate monsoon climate zone of East Asia,is an ideal loca-tion for the research on timberline response to global changes.In this study,the topsoils were collected from differen...The Changbai Mountains,located in the temperate monsoon climate zone of East Asia,is an ideal loca-tion for the research on timberline response to global changes.In this study,the topsoils were collected from different vertical vegetation zones on the northern slope of the Changbai Mountains,Northeast China in August 2009,and phytoliths in the soil samples were extracted by using wet oxidation method and identified with Motic 2.0 microscope in laboratory.The results show that phytoliths are abundant in the topsoils of the study area.The herbal phytoliths are primarily composed of elongated,tooth-shaped,point-shaped and hat-shaped phytoliths,as well as a small amount of fan-shaped and square-shaped ones.The elongated,tooth-shaped,point-shaped and hat-shaped phytoliths are representative of cold climate,while fan-shaped and square-shaped ones are representative of warm and humid climate.In the conifer broadleaved mixed forest zone,coniferous forest zone and broadleaf forest zone,there are close correlations between vegetation and woody phytoliths in the topsoils,indicating that the woody plants of a region can be reconstructed from the woody phytolith assemblages in the topsoils.Meanwhile,the topsoil phytolith assemblages can also be used to reconstruct the understory herbs effectively.The phytolith assemblages in the topsoils of the forest community and herbal community differ significantly,which can help indicate the historical location of the timberline.展开更多
文摘Introduction The predominant characteristics of high-altitude climates include low air pressure,low humidity,and large diurnal temperature fluctuations.In practical engineering scenarios in high-altitude regions,many pier surface cracks only appear several months after erection,and cyclic thermal stress is identified as the main trigger for such cracking.The thermal stress in concrete structures has been investigated for decades but remains incompletely understood.Structural engineers typically regard concrete as an isotropic material and calculate the thermal stress using code-specified coefficients of thermal expansion(CTEs)along with temperature conditions and constraints.Because the CTE of hardened cement paste is more than twice that of many aggregates,reducing the CTE of coarse aggregates can further exacerbate the thermal deformation incompatibility between the coarse aggregate and mortar matrix.In this paper,a comprehensive thermal-elastic mechanics model for pier concrete was developed to analysis the temperature-induced surface stress.Methods A series of mechanical and thermophysical tests were conducted on the diorite aggregate,ITZ cement paste,and mortar,and concrete.A test pier was constructed on open ground near the Yarlung Zangbo River at an altitude of 3800 m.The pier had a diameter of 1.8 m and height of 2 m.Temperature sensors were embedded in the cross-section at a height of 1 m,positioned along the southnorth and east-west directions.The embedding depths(distances from the pier surface)were 0,1.5,3,4.5,6,7.5,9,12,15,20,25,30,35,40,50,60,70,80 cm,and 90 cm.A model of the bridge pier concrete for surface-level analysis was constructed.The model consists of a concrete unit formed as a sphere-shell-shell composite,including the aggregate,ITZ,and mortar layers,embedded in the surface layer of a bridge pier.Outside this unit,the pier concrete was treated as an isotropic,homogeneous elastic material.The real-time internal temperature fields of high-altitude concrete bridge piers,measured on-site,were incorporated into the model.By applying thermoelastic mechanics theory and finite element solutions for plane strain problems,the three-dimensional thermal stresses on the surface layer of high-altitude bridge piers were analyzed Results and discussion During the experimental period,the lowest and highest temperatures on the bridge pier in the high-altitude region were 9.6 ℃ and 42.6 ℃,respectively.These occurred before sunrise and sunset on sunny days,respectively,corresponding to the local maximum temperature gradients during the surface heating and cooling stages,as well as the maximum temperature difference between the surface and center during these stages.The thermal stress on the pier concrete surface was obtained by superimposing the stresses caused by the uneven distribution of the internal temperature field and those caused by the incompatible thermal deformation among the different components in the surface concrete Before the erection of the upper structures,the absolute values of the tangential and vertical stresses were the same;therefore,only one curve was observed.From 22:00 to 8:00,the pier concrete surface was in tension,whereas from 11:00 to 22:00,the pier concrete surface was in compression.The surface of the pier concrete was subjected to biaxial forces of equal magnitude with a maximum compressive stress of 12.52 MPa and maximum tensile stress of 2.15 MPa,respectively at 18:00 and 8:00.According to the fatigue equation,the concrete was predicted to crack after 21 d of temperature cycling.Moreover,if humidity-induced stress is added on top of this,the tensile stress may approach or even exceed the concrete's tensile strength,thereby posing a significant risk of cracking.After the erection of upper structures,the tangential and vertical stresses no longer coincide because the upper structures have been erected.The curve of the tangential stress is unchanged,whereas the curve of the vertical stress is translated downwards by 1.57 MPa due to the structural deadweight.Therefore,the maximum tangential compressive stress remained 12.52 MPa,whereas the maximum vertical compressive stress increased to 14.09 MPa.Additionally,the maximum tangential tensile stress was 2.15 MPa,and the maximum vertical tensile stress was 0.58 MPa.According to Appendix C of GB/T 50010 and the fatigue equation,stresses are unlikely to cause cracking of the pier concrete surface.Although a higher CTE of the coarse aggregate slightly increased the maximum compressive stress,the differences among the three groups of concrete were minimal and could be ignored.Specifically,the maximum compressive stresses on the pier concrete surface were 12.54,12.45 MPa,and 12.56 MPa when using diorite,limestone,and basalt,respectively.By contrast,a lower CTE of the coarse aggregate results in a greater maximum tensile stress on the pier concrete surface.For example,when using limestone,which has a low CTE,the maximum tensile stress on the pier concrete surface is 2.28 MPa,compared to 2.17 MPa when using diorite and 2.14 MPa when using basalt.The finite element simulation results indicated that the maximum compressive stress on the pier concrete surface was 11.72 MPa,whereas the maximum tensile stress was 2.10 MPa.These results are approximately consistent with the theoretical calculations.This consistency provides mutual verification.Conclusions Surface cracking in pier concrete occurs predominantly before the erection of upper structures.Under sunny conditions,the orthogonal decomposition of the superficial stress revealed that the maximum compressive stress during the day was approximately 12.52 MPa,whereas the maximum tensile stress was approximately 2.15 MPa.This tensile stress approached the tensile strength of the C35 concrete under biaxial tension.The risk of cracking increased significantly when humidity-induced stress was considered.After the erection of upper structures,the maximum tangential tensile stress on the pier surface remained at 2.15 MPa while the maximum vertical stress decreased to 0.58 MPa,both of which are well below the tensile strength of C35 concrete under biaxial tension.Although the use of coarse aggregates with a lower coefficient of thermal expansion reduced the tensile stress induced by temperature gradients,it increased the stress owing to material deformation incompatibility,leading to a slight increase in the maximum tensile stress on the pier concrete surface.
基金Foundation： National 863 High Technology Programs of China, No.2011BAH23B04 The State Key Laboratory of Resource and Environment Information System, No.088RA500KA National Natural Science Foundation of China, No.41071250
文摘Many studies on land use change （LUC）, using different approaches and models, have yielded good results. Applications of these methods have revealed both advantages and limitations. However, LUC is a complex problem due to influences of many factors, and variations in policy and natural conditions. Hence, the characteristics and regional suitability of different methods require further research, and comparison of typical approaches is required. Since the late 1980s, CA has been used to simulate urban growth, urban sprawl and land use evolution successfully. Nowadays it is very popular in resolving the LUC estimating problem. Casebased reasoning （CBR）, as an artificial intelligence technology, has also been employed to study LUC by some researchers since the 2000s. More and more researchers used the CBR method in the study of LUC. The CA approach is a mathematical system constructed from many typical simple components, which together are capable of simulating complex behavior, while CBR is a problem-oriented analysis method to solve geographic problems, particularly when the driving mechanisms of geographic processes are not yet understood fully. These two methods were completely different in the LUC research. Thus, in this paper, based on the enhanced CBR model, which is proposed in our previous research （Duet aL 2009）, a comparison between the CBR and CA approaches to assessing LUC is presented. LUC in Dongguan coastal region, China is investigated. Applications of the improved CBR and the cellular automata （CA） to the study area, produce results demonstrating a similarity estimation accuracy of 89% from the improved CBR, and 70.7% accuracy from the CA. From the results, we can see that the accuracies of the CA and CBR approaches are both 〉70%. Although CA method has the distinct advantage in predicting the urban type, CBR method has the obvious tendency in predicting non-urban type. Considering the entire analytical process, the preprocessing workload in CBR is less than that of the CA approach. As such, it could be concluded that the CBR approach is more flexible and practically useful than the CA approach for estimating land use change.
文摘Traditional methods of extracting the ocean wave eddy information from remotely sensed imagery mainly use the edge detection technology such as Canny and Hough operators. However, due to the complexities of ocean eddies and image itself, it is sometimes difficult to successfully detect ocean eddies using these methods. A mnltifractal filtering technology is proposed for extraction of ocean eddies and demonstrated using NASA MODIS, SeaWiFS and NOAA satellite data set in the typical area, such as ocean west boundary current. Results showed that the new method has a superior performance over the traditional methods.
基金Supported in part by the National Natural Science Foundation of China(No.40671136)Open Research Fund from State Key Laboratory of Remote Sensing Science(No.LRSS0610)the National 863 Program of China(No.2006AA12Z215).
文摘In the supervised classification process of remotely sensed imagery,the quantity of samples is one of the important factors affecting the accuracy of the image classification as well as the keys used to evaluate the image classification.In general,the samples are acquired on the basis of prior knowledge,experience and higher resolution images.With the same size of samples and the same sampling model,several sets of training sample data can be obtained.In such sets,which set reflects perfect spectral characteristics and ensure the accuracy of the classification can be known only after the accuracy of the classification has been assessed.So,before classification,it would be a meaningful research to measure and assess the quality of samples for guiding and optimizing the consequent classification process.Then,based on the rough set,a new measuring index for the sample quality is proposed.The experiment data is the Landsat TM imagery of the Chinese Yellow River Delta on August 8th,1999.The experiment compares the Bhattacharrya distance matrices and purity index zl and△x based on rough set theory of 5 sample data and also analyzes its effect on sample quality.
基金Under the auspices of National Natural Science Foundation of China (No 40971116)Major State Basic Research Development Program of China (No 2009CB426305)Technology Innovation Project of Northeast Normal University in Eleventh Five-Year Plan Period (No NENU-Stb07002)
文摘The Changbai Mountains,located in the temperate monsoon climate zone of East Asia,is an ideal loca-tion for the research on timberline response to global changes.In this study,the topsoils were collected from different vertical vegetation zones on the northern slope of the Changbai Mountains,Northeast China in August 2009,and phytoliths in the soil samples were extracted by using wet oxidation method and identified with Motic 2.0 microscope in laboratory.The results show that phytoliths are abundant in the topsoils of the study area.The herbal phytoliths are primarily composed of elongated,tooth-shaped,point-shaped and hat-shaped phytoliths,as well as a small amount of fan-shaped and square-shaped ones.The elongated,tooth-shaped,point-shaped and hat-shaped phytoliths are representative of cold climate,while fan-shaped and square-shaped ones are representative of warm and humid climate.In the conifer broadleaved mixed forest zone,coniferous forest zone and broadleaf forest zone,there are close correlations between vegetation and woody phytoliths in the topsoils,indicating that the woody plants of a region can be reconstructed from the woody phytolith assemblages in the topsoils.Meanwhile,the topsoil phytolith assemblages can also be used to reconstruct the understory herbs effectively.The phytolith assemblages in the topsoils of the forest community and herbal community differ significantly,which can help indicate the historical location of the timberline.
