《1    Engineering research hotspots and engineering research focus》

1    Engineering research hotspots and engineering research focus

《1.1    Development trends of engineering research hotspots》

1.1    Development trends of engineering research hotspots

The top 10 engineering research hotspots in the  field of civil, hydraulic engineering and architecture are sum- marized in Table 1.1.1, and they cover various disciplines including architecture, structural engineering, bridge en- gineering, engineering mechanics, construction materials, geotechnical and underground engineering, hydraulic engineering, infrastructural engineering, and survey en- gineering. Among these hotspots, “Material biological remodeling” and “Mixed-pixel decomposition and spa- tiotemporal location big data analysis” are the emerging frontiers, while in the other aforementioned hotspots, the traditional research is extended in the fields of civil, hy- draulic engineering and architecture. Table 1.1.2 presents the statistical data of publications on these hotspots from 2011 to 2016. From among the top 10 hotspots, the number of published papers is the highest in “High-performance civil, hydraulic, and building engineering structures” and “Structural defect analysis.”

(1)    High-performance civil, hydraulic, and building engineering structures

High-performance civil, hydraulic, and building struc- tures are structures in key civil and hydraulic engineering and multi-functional building structural systems with high-performance in safety, serviceability, construction- ability, environment-friendliness, durability, maintenance, and cost-efficiency throughout the life-cycle. This topic includes the following scientific and technological chal- lenges: ① development of novel design principles and implementation tools for consistent compatibility of archi- tecture (spatial arrangement), function, and structures; ② development of new concepts and methods of structural analysis and design that combine the rational optimiza- tion of the structural system and refined design; ③ de- velopment of new theories and techniques for an optimal design that takes into consideration the life-cycle and multi-performance tradeoffs while accounting for plan- ning, design, construction, service maintenance, and dem- olition. To address these issues, there is a requirement for a novel concept and the revolution of the implementation

《Table 1.1.1》

Table 1.1.1 Top 10 engineering research hotspots in civil, hydraulic engineering and architecture

《Table 1.1.2 》

Table 1.1.2 Annual number of core papers belonging to each of the top 10 engineering research hotspots in civil, hydraulic engineering and architecture

systems as well as the innovative investigations enabled by modern scientific development, which can be realized by combining physical and numerical modeling; macro-, meso- and micro-scale studies; and qualitative and quan- titative synthesis. The current hot subjects are as follows:① applications of novel materials; ② innovative structural systems; ③ refining of analysis and modeling of struc- tures; ④ structural performance monitoring and control; and ⑤ life-cycle reliability analysis and design. From 2011 to 2016, 377 core papers were published on this hotspot, and they received a total of 9948 citations and 26.39 cita- tions per paper.

(2)    Ultra-high performance and multi-functional cement-based composite materials

Owing to the rapid development of construction technology, traditional cement-based materials with low toughness and high brittleness could not meet the high-performance demands of several special applica- tions. Hence, ultra-high performance and multi-func- tional cement-based composites have become the focus of current research. Since their discovery, carbon nano- tubes have been widely studied in physical, chemical, and material areas owing to their excellent performance properties. Carbon nanotubes can greatly improve the strength or toughness of materials and their conductivity. Ultra-high performance civil-engineering materials can be obtained by adding carbon nanotubes and carbon fibers into cement-based materials. The following are some com- mon ultra-high performance civil-engineering materials: self-sensing carbon nanotube cement-based composites, multi-walled carbon-nanotubes-reinforced cement paste, and carbon-fiber-reinforced Portland cement mortar. Carbon nanomaterials in the cement matrix equip the composites with good physical and chemical properties, thus greatly enhancing the safety and reliability of con- crete structures, and they can be used for high-strength intelligent cement-based materials testing. Therefore, car- bon nanomaterials have broad applications in improving cement-based traditional building materials. From 2011 to 2016, 36 core papers have been published on this hotspot, and they received a total of 948 citations and 26.33 cita- tions per paper.

(3)    Methods for the analysis and simulation of complex structures

Owing to the rapid developments in computer science and technology in recent years, numerical simulations have played an increasingly important role in the study  of complex structures. The two main concerns in the de- velopment of modern computational techniques are the modeling of complex material behaviors and the model- ing of complex geometries. Engineering materials often sustain damage and fractures when they are subjected to external loading. The cracks could be considered as dis- continuities within continuum fields. Thus, the extended finite element method (XFEM) introduces discontinuous enrichments based on the concept of the partition of unity. Owing to its mathematical accuracy and convenience in application, XFEM has been well accepted by researchers and engineers. In recent years, several researches have fo- cused on the further development of XFEM. According to the continuum damage theory, cracks are often smeared within a certain domain and the effects of crack initiation and propagation are described by the continuum evolu- tion of the damage variables. Damage theory is suitable for the simulation of complex structures without changing the framework of structural finite element models. The recent developments in damage models have focused on nonlocal theory based on which the issue of mesh sensitiv- ities could be eliminated. Several new materials have been developed and designed in recent works, e.g., fiber-rein- forced composites and functional graded material. The numerical methods for the simulation of newly designed materials mainly belong to the class of multi-scale meth- ods. For structures with a rather complex geometry, the finite element meshing is quite complicated and time-con- suming. In order tackle this issue, the concept of isogeo- metric analysis is proposed, for which the basic functions used for the geometric modeling (e.g., NURBS) are also used for the interpolation of state variables. Isogeomet-  ric analysis has been developing very quickly, and it has recently found application in the problems of vibration, buckling, and fracture. From 2011 to 2016, 135 core papers have been published on this hotspot, and they received a total of 3699 citations and 27.4 citations per paper.

(4)    Built environment and occupant behavior

This hotspot involves the study of the relationship and interaction between the built environment and human comfort as well as behavior. It includes the study of the methods for meeting the psychological requirements of oc- cupants and improving their living environment through city planning, urban design, architectural design, and building equipment system design. In addition, it includes the study of the interaction and adjustment between the occupant and environment. The main topics of research include the requirements of human comfort with respect to temperature, humidity, air velocity, radiation, and oth- er environmental factors; the evaluation and prediction of thermal comfort based on thermal adaptation theory and dynamic thermal comfort theory; and the description and simulation of occupant adjustment to the built environ- ment. The study of the built environment and occupant behavior comprises the following engineering science branches: outdoor microclimate and thermal comfort, thermal adaptation to climate, and adjustment to the built environment. Building environment and occupant behav- ior is currently a research hotspot. The traditional thermal adaptation model and design theory of the indoor air-con- ditioning environment have been satisfactorily developed and thoroughly investigated. In recent years, research has been focused on outdoor environmental thermal comfort, dynamic thermal comfort and thermal adaptation, ther- mal comfort under special environments (such as high altitudes, public transport, and aerospace environments), and interaction between thermal comfort and occupants. These topics supplement the development of existing thermal comfort and occupant behavior theory. With the improvement in the quality of life and the popularization of the concept of green and sustainable development, this subject will continue to be a research hotspot in the field of built environment and will develop new connotations based on traditional theories. From 2011 to 2016, 34 core papers were published on this hotspot, and they received a total of 716 citations and 21.06 citations per paper.

(5)    Removal of heavy metals and other high-risk pollutants

Scientific and engineering methods were applied to restore water, air, or soil that had been contaminated by heavy metals and other high-risk pollutants. These meth- ods can be used to transform the pollutants into less toxic forms or maintain their concentrations within a safe range. Heavy-metal pollution occurs when the concentrations of mercury, cadmium, lead, chromium, arsenic, and their compounds exceed their safe value in water, air, soil, or other carriers. They can cause significant harm to the en- vironment and to lives. To prevent heavy-metal pollution, the control of the total amount of the pollutants should   be combined with concentration control. Furthermore, source prevention, process disruption, cleaner production, and terminal treatment should be realized while focusing on the key prevention and control zones, key prevention and control industries, and key sources of pollution. In addition, to control the emission of heavy metals from the source, the removal of discharged metals is particularly critical, and the main technologies for achieving this pres- ently include leaching, adsorption, ionic liquid extraction, oxidation-reduction, the electrochemical method, and photocatalytic method. Furthermore, increasing attention has been focused on the recycling field. Other high-risk pollutants in the environment include pharmaceuticals and personal care products, which can be eliminated mainly via adsorption and oxidation. Though these high- risk pollutants exist in low concentrations, long-term exposure to them is relatively risky. From 2011 to 2016,   29 core papers were published on this hotspot, and they received a total of 1243 citations and 42.86 citations per paper.

(6)    Reliability of civil engineering structures

Reliability is the probability that civil structures, under normal service conditions and/or subject to disastrous conditions, satisfy the prescribed demand function and safety and durability conditions, and it also includes the theory for reliability-based structural design. The key scientific issues with respect to this reliability include the identification and modeling of uncertainty in the struc- tural properties and actions , the structural life-cycle per- formance and complete process behaviors under loading, and the reliability analysis and design of structures. Since the late 1940s, structural reliability theory gradually be- came the core of structural design theory, and the golden epoch was from the late 1960s to early 1980s, which was enriched by the concept of performance-based design that originated in the early 1990s. Since the beginning of the 21st century, owing to the increasingly frequent effects of natural and manmade disasters, increasing importance has been attached to engineering reliability. Meanwhile, with the decreasing cost of data collection, emerging big data techniques, rapid development of the fundamental theory of mechanics and high-performance computation, and with the emerging uncertainty quantification theory in various fields, structural reliability theory is embracing new ideas. Presently, the hot subjects in this field include the modeling of uncertainty, the reduced model for en- gineering structures and systems, and high-efficiency stochastic analysis methods. Simultaneously, the research subjects are extended from overground structures to geo- technical and underground structures, the mathematical basis is expanded from probability theory to uncertainty modeling including the imprecise probability and interval methods, and the physical-mechanical basis is extended from simplified semi-empirical models to modern multi- scale physical mechanics. The structural reliability the- ory is transitioned from approximate probability to full probability and risk based decision-making, and from more mathematics-emphasized to physical-based global reliability analysis and design. From 2011 to 2016, 42 core papers were published on this hotspot, and they received a total of 1138 citations and 27.10 citations per paper.

(7)    Material biological remodeling

Biological materials are formed with perfect structural morphology and unique superior performance during their long evolution process. Inspired by these materials, researchers have attempted to reveal the structural fea- tures and mechanisms of the biological system, which is then applied to the design and preparation of building materials. The research on structural biomimetic materials is an important branch of bionics. Based on the study of the structural characteristics and structure-activity rela- tionship of the biomaterials, the bionic simulations of ma- terials, structures, and systems are performed to improve the engineering structural efficiency. Biological remodel- ing of materials is one of the main subjects of biomimetic research. It focuses on the non-uniform expansion and mechanical influence of gels and biomaterials on healing and strengthening during the stage of hardening defor- mation and biological growth. The main research includes material constitutive relations and the mechanics analysis method, the modeling of the boundary growth mechanics problem, and the solutions of static and dynamic prob- lems. Owing to inspiration from biological materials, bionic materials will become more and more microcosmic and intelligent. Future research will focus on constructing various biomimetic micro-devices using functional mate- rials and by assembling bionic micro systems with various structures and functions. Such studies will be widely used in the military, industries, and building. From 2011 to 2016, 34 core papers were published on this hotspot, and they received a total of 1345 citations and 39.56 citations per paper.

(8)    Mixed-pixel decomposition and spatiotemporal location big data analysis

The contents of mixed-pixel decomposition and spatio- temporal location big data analysis include mixed-pixel decomposition, real-time precise point positioning, and spatiotemporal big data analysis. The breakthrough sen- sor spatial resolution limit in mixed-pixel decomposition depicts the true nature of the mixed-pixel distribution  at sub-pixel accuracy, and high-spatial-resolution im- age classification results can be obtained from a coarse-spatial-resolution image by scaling down in order to im- prove the accuracy of extraction of the image information. The real-time precise single-point positioning method uses a precise satellite ephemeris and precise clock prod- uct to obtain higher-precision spatial location data with    a pseudorange and carrier-phase observations using the global navigation satellite system (GNSS) receiver. The spatiotemporal data acquisition method is aimed at ob- taining high spatial resolution and spatiotemporal posi- tion accuracy in order to achieve the acquisition, storage, and reconciliation of high-dimensional spatial-temporal big data. Spatiotemporal big data analysis techniques are used for automatically discovering and extracting the im- plicit, non-obvious patterns, rules, and knowledge from massive, multi-source spatiotemporal location big data and for excavating values from the volume, velocity, and variety big data. From 2011 to 2016, 95 core papers were published on this hotspot, and they received a total of 3887 citations and 40.92 citations per paper.

(9)    Structural defect analysis

Various imperfections including cracks, defects, and material degradations are created in civil materials and structures during their construction and service pro- cesses, owing to the comprehensive interior and exterior factors such as changes in the service environment, fire, earthquakes, and explosion shocks. These imperfections also cause various types of serious accidents and even civil engineering disasters. Therefore, there is an urgent requirement for developing fundamental theories of frac- ture mechanics for bearing capacity analysis and safety assessment of civil structures with defects and diseases in the fields of civil engineering, hydraulic engineering, and construction engineering. The relevant research topics are listed as follows: crack initiation criterions and crack prop- agation theories for civil materials and structures; fracture analysis theories of plates and shells for civil engineer- ing; fracture behaviors, ultimate load-bearing capacity analyses, safety assessments, plastic fatigue, and fracture failure analyses for civil components and structures with odd-shaped notches under complex loading conditions; classifications, distribution surveys, characteristics, and statistical analyses of various defects or imperfections in civil structures and their suitable simulation models; and development of various types of defective specimens for various research purposes with materials such as poly-methylmethacrylate, steels, and graphite, and research on the corresponding new theories and methods for physical tests and fracture analysis. Currently, the hot research direction in the field of structural defect analysis is the development of new theories and methods for the simu- lation of structural fractures and failure processes. Some examples are nonlocal damage models for cracked struc- tures that are free of challenging issues such as the effects of mesh sensitivity and size, new numerical methods such as XFEM and meshless methods that circumvent the dif- ficulties of mesh refinement and mesh movement for the simulation of the fracture propagation process, and multi- scale theories and methods that can be used for analyzing new engineering materials and structures with fiber-com- posite and functionally graded materials ranging from the micro to the macro scope. From 2011 to 2016, 30 core papers were published on this hotspot, and they received a total of 483 citations and 16.10 citations per paper.

(10)    Numerical simulation of fluid-structure interaction

Fluid-structure interaction is the interaction of struc- tures with the surrounding fluid flow, and it includes var- ious mechanical behaviors of a deformable solid structure under the action of the flow field and the influences of the position and shape of a deformable structure on the flow field. Fluid flow problems comprise a large number of nonlinear phenomena, and for a fluid-structure coupling system, these problems may further involve large non- linear geometrical deformation, nonlinear elastic-plastic materials, and nonlinear problems induced by uncertainty and coupling at the contact interfaces. Generally, flu- id-structure interaction systems are often complex non- linear systems, and they are mainly analyzed using nu- merical simulations. The important studies on this subject include water/air flow-structure interaction and the cor- responding modeling technique. The key to the successful numerical simulation of fluid-structure interaction lies in adopting a unified coordinate system and considering the coordination problem at two-phase interfaces. In the case of nonlinear coupling problems, it is necessary to find a method for obtaining the solutions in the entire fluid and solid domain, and it is also a challenge to combine the effective methods of solving the nonlinear flow equations and the structural equations. From 2011 to 2016, 89 core papers were published on this hotspot, and they received a total of 2694 citations and 30.27 citations per paper.

《1.2    Understanding of engineering research focus》

1.2    Understanding of engineering research focus

1.2.1    High-performance civil, hydraulic, and building engineering structures

Civil, hydraulic, and building structures are the major material carriers in human society. The development of modern society, economy, and technology has raised the performance demands of engineering structures and has resulted in the development of new high-performance engineering structures. In general, high-performance civil, hydraulic, and building structures exist in key civil and hydraulic engineering and multi-functional building struc- tural systems with high life-cycle performance in safety, serviceability, constructionability, environment-friendli- ness, durability, maintenance, and cost-efficiency. With the development of economy, society, and technology, the population is becoming increasingly concentrated, and the function and structure of civil infrastructures are becoming increasingly specific and complicated. How- ever, the losses incurred owing to natural and manmade disasters are increasing. Therefore, superior performance is required of engineering structures for protecting human lives and property. Meanwhile, rapid developments in modern science and technology (particularly in material science, fundamental physics and mechanics, information and data science, and high-performance computing tech- niques) have provided unprecedented opportunities for realizing high-performance structures.

Attempts to develop high-performance engineering structures raise scientific and technological challenges:
① development of novel design principles and implemen- tation tools for the consistent compatibility of architecture (spatial arrangement), function, and structures; ② devel- opment of new concepts and methods of structural anal- ysis and design that combine the rational optimization of structural systems and refined design; ③ development of new theories and techniques for optimal design involving life-cycle and multi-performance tradeoffs while account- ing for planning, design, construction, service-maintenance, and demolition. To resolve these issues, we require a novel concept and a revolution in the implementation systems as well as innovative investigations that are enabled by mod- ern scientific development by combining physical and nu- merical modeling, macro-, meso-, and micro-scale studies, and qualitative and quantitative syntheses. 

The following are presently the hot subjects in “High- performance civil, hydraulic, and building engineering structures.”

(1)    Applications of novel materials

The development of novel materials is one of the major forces advancing innovation in engineering structures. In the past decades, material science has transited from the traditional paradigm of material fabrication—mechan- ical property studies—engineering applications to the paradigm of macro-microscale unified material design— integrated material-structure design. Various demands for high-performance civil, hydraulic, and building en- gineering structures have given rise to the development and application of materials of unique performance or comprehensive high performance (e.g., intelligent materi- als, high-damping materials, high-modulus materials, and high-durability materials). However, affordability is a bot- tleneck of high-performance civil, hydraulic, and building structures as massive amounts of materials are required in this field. Reducing the cost and improving the compre- hensive or unique performance, in particular via material modification, is an important alternative.

(2)    Innovative structural system

An innovating structural system is one of the major approaches for achieving high structural performance. In recent years, appreciable advances have been made in the development of new composite structures, new disaster- resistant structures (e.g., resilient structures and intelligent structures), and auto-growing structures based on bionics and topological optimization.

(3)    Refined analysis and modeling of structures

The advancement of fundamental physics and mechan- ics (e.g., the constitutive law of solids and soft materi- als) and high-performance computational facilities and techniques (e.g., smoothed particle hydraulics, mesh-free method, extended finite element method, and isogeo- metric analysis, large deformation and strong nonlinear analysis methods) greatly promoted the refined analysis theory and methods for complex civil, hydraulic, and building structures. In this aspect, the multi-scale physi- cal-numerical hybrid simulation incorporating the mod- ern experimental techniques and numerical computation is a significant direction for future investigations.

(4)    Structural performance monitoring and control

Structural performance monitoring will  become  a routine task for maintaining important civil, hydraulic, and building structures. The new multi-parameter sensors and sensing networks with high accuracy, disturbance insensitivity, superior weather-resistance performance, long service life, low-energy-consuming and wireless or remote transfer performance; analysis and high-efficiency property extraction techniques for big data; warning, intelligent control, and resilience or even self-healing of engineering structures are the challenging frontiers.

(5)    Life-cycle reliability analysis and design

The hot subjects include uncertainty quantification, the- ory of reduced or surrogate model, big data techniques, and life-cycle management systems. In particular, un- certainty quantification and disaster-resistant  reliability of civil, hydraulic, and building structures received a great deal of attention, which resulted in various break- throughs. The life-cycle global reliability analysis and optimal design of engineering structures that incorporates monitoring and control is a promising research direction and is supported with refined physical-mechanical mod- els and big data.

The focus of research varies in different countries owing to their different geographical environments, develop- ment phase, economic status, scientific and technological level, and history and social thought. In the past two de- cades, researches conducted in the USA are transitioning from massive construction to maintenance, management, and post-disaster recovery, and thus included leading research on performance-based seismic design, struc- tural health monitoring, and life-cycle cost evaluation. Recently, they evolved from the structure scale to the community scale. The focus of research in Europe is very different. Reliability analysis and design theory for engineering structures and systems has received a great deal of attention in Norway, Denmark, and Germany; the fundamental theory for solid mechanics analysis plays a leading role in research in France and Spain; and the experimental and theoretical investigations in di- saster-resistant performance of engineering systems has gained importance in Italy. Japan leads the world in the development and applications of new disaster-resistant structural systems and devices. In the past decade, with massive infrastructure construction, the investigations into the above subjects have improved greatly in China. China now plays an important role in the study of novel materials, innovative structural systems, refined analysis theory and methods, monitoring and control of structur- al performance, and reliability design of structures and systems; China is expected to play a world-leading role   in one or several of these subjects.

As presented in Table 1.1.1, a total of 377 core papers   in “High-performance civil, hydraulic, and building structures” have been published with an average of 26.39 citations per paper. As shown in Table 1.2.1, the top five countries or regions producing the most core papers are China (excluding Taiwan of China), Italy, Australia, the USA, and Algeria, and the core papers from China ac- count for 31.03% (the Chinese mainland 21.14%; and Hong Kong, Macau, and Taiwan 9.33%), indicating that China is an important contributor in this subject. According to the average citations, the top five countries or regions are Por- tugal, Algeria, Germany, China, and the USA. The num- ber of average citations from China is 24.11, which is high- er than the average value, and this indicates that Chinese researchers are receiving global attention. As indicated by the collaborative network of countries or regions shown  in Figure 1.2.1, it can be observed that close collaborations exist among all the top five countries or regions except Al- geria.

The top five institutions producing core papers are the Univ Djillali Liabes Sidi Bel Abbes in Algeria, University of Hong Kong, City University of Hong Kong, University of Adelaide, and Univ Roma La Sapienza. The collab- orative network of the major institutions (Figure 1.2.2) indicates frequent collaborations in the same country or region.

As can be observed from Tables 1.2.3 and 1.2.4, the top five countries or regions producing the most citations are China, Australia, the USA, Italy, and Portugal. The citations from China account for 24.30%, and three of the top five institutions producing the most citations are from China (i.e., University of Hong Kong, City University of Hong Kong, and Wuhan University). This indicates that Chinese researchers are at the frontiers of these investigations.

In summary, China is maintaining the same pace as oth- er countries or regions in frontier research in “High-per- formance civil, hydraulic, and building structures” and has the potential for playing a leading role. It is essential to allocate more funding to research on these topics in or- der to advance such studies and to play a leading role in the world.

《Table 1.2.1》

Table 1.2.1 Major producing countries or regions of core papers on the engineering research focus “High-performance civil, hydraulic,  and building engineering structures”

《Table 1.2.2》

Table 1.2.2 Major producing institutions of core papers on the engineering research focus “High-performance civil, hydraulic, and building engineering structures”

1.2.2    Built environment and occupant behavior

The study of “Built environment and occupant be- havior” has been developed based on traditional steady state thermal comfort for approximately the past century. Its main aim is to examine the  human body’s reactions in a steady state thermal environment, which can be represented using a thermal comfort model proposed by Professor P. O. Fanger of Danish Technical University. The model became the basis of the standards used by American Society of Heating, Refrigeration and Air- conditioning Engineers (ASHRAE) and International Standardization Organization (ISO) for developing indoor thermal environments. China's thermal environment design standards generally cite these standards.

Based on the clustering analysis of keywords in core papers, the mainstream of engineering science and the branch engineering sciences focused on in this engineering research include the following: outdoor microclimate and thermal comfort, thermal adaptation to climate, and adjustment to built environments.

(1)    Outdoor microclimate and thermal comfort

This includes the adjustment of outdoor micro-climate parameters, building of comfortable outdoor thermal en- vironments, and proposing an evaluation index of safety

《Figure 1.2.1 》

Figure 1.2.1 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “High-performance civil, hydraulic, and building engineering structures”¹ 

¹ In the figure, the nodes refer to the countries or regions, the size of the nodes refers to number of papers, the connecting line between nodes refers to papers published based on research cooperation, and the thickness of the connecting line indicates the number of papers based on research cooperation. These are the same in full text.

《Figure 1.2.2 》

Figure 1.2.2 Collaboration network of the major producing institutions of core papers on the engineering research focus “High-performance civil, hydraulic, and building engineering structures”

《Table 1.2.3 》

Table 1.2.3 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “High- performance civil, hydraulic, and building engineering structures”

and comfort for an outdoor thermal environment through methods involving building layout design, outdoor floor pavement, the application of shade parts, residential and urban greening, the use of accumulated rainwater, the use of windshields and fountains, and other technical measures. The key technologies to be solved includes: the influence of short-wave and long-wave radiation on outdoor thermal comfort; outdoor thermal comfort eval- uation index and outdoor microclimate evaluation; and outdoor microclimate control measures based on thermal comfort. The currently developing trends include the de- velopment of urban-level planning theory, which is shift- ing from outdoor microclimate adjustment; the develop- ment of a solution for the urban heat island effect, urban ventilation corridor, and other urban-scale climate issues; the development of an index of comfort shifting from in- dex of outdoor labor safety; efforts to meet the demand of thermal comfort in large-scale outdoor waiting areas and cultural and recreational activity areas. The key countries and regions conducting research on these subjects include

《Table 1.2.4 》

Table 1.2.4 Major producing institutions of core papers that are cited by core papers on the engineering research focus “High-performance civil, hydraulic, and building engineering structures”

Taiwan of China, Egypt, Malaysia, Singapore, Greece, the Chinese mainland, Australia, France, Argentina, Italy, and Hong Kong of China.

(2)    Thermal adaptation to climate

This includes the difference of the human comfort index between non-air-conditioned naturally ventilated build- ings as well as free-running buildings and the traditional air-conditioned environment; the mechanism of thermal adaptation, such as physiological acclimatization, psy- chological adaptation, and behavioral adjustment; and the thermal comfort prediction model reflecting thermal adaptation. In this branch of engineering science, the key issues to be resolved include the investigation of the phys- iological parameters reflecting the thermal adaptation; the integration of the thermal adaptation model and the traditional thermal comfort model; the investigation of the design parameters and the control measures of the indoor environment while taking into consideration the thermal adaptation. The currently developing trends include the establishment of a worldwide database of on-site surveys regarding thermal comfort and experimental results; the study of interdisciplinary relations between medical-phys- iological indications and thermal comfort research; the development of a thermal adaptation dynamic evaluation model and indoor environment design theory for the ther- mal response of occupants to various climatic zones and economic development conditions. The key countries and regions conducting research on this subject include Aus- tralia, the Chinese mainland, Hong Kong of China, Japan, India, and the USA.

(3)    Thermal adjustment to built environment

This includes the driving factors of occupant adjustment behavior in a built environment, the quantitative descrip- tion methods and software simulation tools of occupant behavior, and the relation between occupant behavior and environmental parameters as well as the investigation of the influence of building energy consumption for meet- ing the practical requirements of scientific research and engineering applications through test data acquisition, model establishment and validation, questionnaires, and case studies. The key issues to be resolved include the de- velopment of a standardized testing method of occupant behavior in a building environment, realization of param- eter descriptions and regression of the occupant behavior model, and the simulation and validation of the occupant behavior model. The currently developing trends include the development of standardized testing techniques and   a quantitative description of multi-person occupancy and action; the study of environmental driving factors of occu- pant behavior and the interaction of multiple persons; and the development of a coupled computing platform for the occupant behavior model, indoor environment parame- ters, and building energy consumption. The key countries and regions conducting research on this subject include the Chinese mainland, the USA, Germany, Britain, Aus- tralia, and Canada.

“Built environment and occupant behavior” is a re- search hotspot. The traditional evaluation model of ther- mal comfort and the design theory of an indoor air-con- ditioned environment have been satisfactorily developed and thoroughly investigated. In recent years, research has been focused on thermal comfort in outdoor environ- ments, dynamic thermal comfort and heat adaptation, thermal comfort in special environments (such as high altitudes, public transport, and aerospace environments), and the relation between thermal comfort and occupant behavior. These hotspots are supplementary to the de- velopment of existing thermal comfort and occupant be- havior theory. With the improvement in the quality of life and the popularization of the concept of green and sus- tainable development, this subject will continue to main- tain its status as a research hotspot in the field of built environments and will develop new connotations based on traditional theories. There were 34 core papers on the field of “Built environment and occupant behavior,” with 21.06 citations per paper (Table 1.1.1). The top five coun- tries or regions with the greatest number of published core papers were Italy, the USA, India, China (excluding Taiwan of China), Japan, and Australia. Papers published by Chinese researchers accounted for 14.71% of the total core papers (8.83% by the Chinese mainland and 5.88%  by Hong Kong and Macao; Table 1.2.5), marking China as one of the main countries or regions conducting research in this field. The top five countries or regions with the highest average citations are England, Taiwan of China, Germany, Japan, and India. Chinese researchers’ papers were cited 16.2 times per paper on average, up to 87% of average level (Table 1.2.5), which indicates there exists an increasing possibility for Chinese researchers in this field. In the collaborative network of countries and regions (Figure 1.2.3), the top five countries or regions with the greatest number of core papers have a close collaborative relation.

In terms of the core-paper publishing institutions (Ta- ble 1.2.6), the top three are the University of Rome La Sapienza, Roma TRE University, and Tokyo City Univer- sity. However, according to the collaboration network of the top 10 publishing institutions (Figure 1.2.4), the in- ter-agency cooperation is not very close.

As can be observed from Table 1.2.7 and Table 1.2.8, the top five countries or regions by number of published core papers also have a greater number of cited papers. China has the second highest number of citations (along with India and Italy), which indicates that Chinese scholars pay close attention to and track the trends of research in this field.

Based on the above statistical analysis, in “Building en- vironment and occupant behavior” engineering research, China is currently on par with similar researches being conducted abroad. It is recommended that China continue to increase investment in such research in order to pro- mote the relevant research to the world-leading level.

1.2.3    Mixed-pixel decomposition and spatiotemporal location big data analysis

The Nature special issue on “Big Data” in 2008 and the Science special issue on “Dealing with Data” in 2011 pointed out the arrival of the big data era. The interdisci-

《Table 1.2.5 》

Table 1.2.5 Major producing countries or regions of core papers on the engineering research focus “Built environment and occupant behavior”

《Table 1.2.6》

Table 1.2.6 Major producing institutions of core papers on the engineering research focus “Built environment and occupant behavior”

《Figure 1.2.3》

Figure 1.2.3 Collaboration network of the major producing coun- tries or regions of core papers on the engineering research focus “Built environment and occupant behavior”

《Figure 1.2.4》

Figure 1.2.4 Collaboration network of the major producing institutions of core papers on the engineering research focus “Built environment and occupant behavior”

《Table 1.2.7 》

Table 1.2.7 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Built environment and occupant behavior”

《Table 1.2.8》

Table 1.2.8 Major producing institutions of core papers that are cited by core papers on the engineering research focus “Built environment and occupant behavior”

plinary fusion of surveying and mapping and information technology, is pushing the development of earth observa- tion, perception, and cognition to multiplatform, multi- scale, multi-resolution, multi-temporal, and space-air- ground integration. The integration of various means and methods of achieving spatial data acquisition, information extraction, network management, knowledge discovery, space perception, and intelligent service location on the earth, the solid object and human activities, to provide spatial information framework, basic mathematics and in- formation processing technique for earth science research problems, to offer reliable massive spatiotemporal infor- mation for people to fully accurate judgment and decision making.

The focus of engineering research—”Mixed-pixel de- composition and spatiotemporal location big data anal- ysis”—presented in the retrieval results of core papers  is representative of the development trend. Mixed-pixel decomposition and spatiotemporal location big data analysis include mixed-pixel decomposition, real-time precise point positioning, and spatiotemporal big data analysis. Mixed-pixel decomposition can be used to ob- tain high-spatial-resolution classification results from a coarse-spatial-resolution image. The real-time precise single-point positioning method makes use of a precise satellite ephemeris and precise clock product to obtain high-precision spatial location data with a pseudorange and carrier phase observations using the GNSS receiver. The aim of the spatiotemporal data acquisition method is to realize higher spatial resolution and spatial-temporal position accuracy in order to achieve the acquisition, stor- age, and reconciliation of high-dimensional spatial-tem- poral big data. Using the spatiotemporal big data analysis techniques for automatically discovering and extracting implicit, non-obvious patterns, rules, and knowledge from massive, multi-source spatiotemporal location big data, the values from the volume, velocity, and variety big data can be excavated.

(1)    Decomposition of mixed pixels

Owing to the spatial resolution of the remote sensor and the complex diversity of the ground, the mixed pixels are spread widely in the remote sensing image—several dif- ferent land cover types exist at a single pixel. The break- through sensor spatial resolution limit in mixed-pixel decomposition depicts the true nature of the mixed-pixel distribution at sub-pixel accuracy, and high-spatial-reso- lution image classification results can be obtained from a coarse-spatial-resolution image by scaling down in order to improve the accuracy of extraction of the image infor- mation. The key problem in mixed-pixel decomposition   is to determine the land cover types and their spatial pro- portions. According to the interaction of various objects in the mixed pixels, the current spectral mixture model can be categorized as a linear model or nonlinear model, and the linear model is the most widely used. The develop- ment trend of mixed-pixel decomposition mainly includes the following aspects: ① research on nonlinear unmixing based on various scattering models; ② development of the endmember extraction model under spectral variabili- ty; ③ investigation into spectral unmixing combined with spatial information; ④ multi-source image fusion based on the spectral mixture model; ⑤ decomposition of mixed pixels combined with machine learning technology such as sparse decomposition and deep learning; ⑥ unmix- ing accuracy verification and validation problems; and ⑦ application of decomposition of mixed pixels in the fields of vegetation, water-quality survey, environmental monitoring, etc. At present, the research on pixel unmix- ing is more focused on the solution of the existing spectral mixture model. Because this model lacks sufficient phys- ical basis, the testing data also have great limitations, and there thus exist some problems that are difficult to gener- alize in practice.

(2)    Real-time precise point positioning

Precise point positioning (PPP) is an absolute position method that makes use of a precise satellite ephemeris and precise satellite clock product to obtain a precise location with a pseudorange and carrier phase observa- tions. Compared with the multi-reference station-network real-time kinematic, real-time PPP has the advantage of a global-scale operation range, flexible station arrangement strategy, and simple operation mode. As compared with the double-difference observation data processing, the use of real-time PPP using non-difference data can improve the utilization of data. Real-time precise positioning is widely used in crustal deformation monitoring, near- real-time global positioning system (GPS) meteorology, satellite orbit determination, mobile carrier precision po- sitioning, sea level monitoring, tsunami and earthquake monitoring, and other fields. The development of PPP mainly includes the following aspects: ① research on the estimation theory and method of developing a precise satellite clock provides real-time high-precision satellite clock products; ② study on uncalibrated phase delay and separation method for undifferenced ambiguity, fixing of the undifferenced integer ambiguity, and exploring the ambiguity fixing method at the client terminal; ③ research on PPP using multi-system integration, speeding up of the PPP of the initial time, and enhancing the reliability of the positioning results.

(3)    Spatiotemporal big data analysis

Spatiotemporal big data is the result of the integration of big data and spatiotemporal data, which considers the earth as the object, and the spatiotemporal location data and activity data are acquired using a unified space-time reference. Spatiotemporal big data analysis can be used to automatically discover and extract the implicit, non-obvi- ous patterns, rules, and knowledge from massive, multi- source spatiotemporal location big data; pay more atten- tion to discovery rather than demonstration; put more emphasis on relationships rather than causes and effects; and focus more on prediction. The development trend of spatiotemporal big data is the study of basic theories and methods of spatiotemporal data representation, measure- ment, and understanding, and revealing the correspond- ing laws between spatiotemporal big data and real world objects, behaviors, and events.

This subject of engineering research—“Mixed-pixel decomposition and spatiotemporal location big data anal- ysis”—has a total of 95 published core papers, and each article has been cited 40.92 times on average (Table 1.1.1). The top five countries or regions that published these core papers include China (excluding Taiwan of China), Australia, America, Germany, and the Netherlands, and the proportion of papers published by Chinese authors was 56.84% (47.37% from the Chinese mainland and 9.47% from Hong Kong and Macao; Table 1.2.9). China has the greatest proportion in the statistics and is one of the most important countries or regions in this area. The top five countries or regions producing the most core papers in this subject include Portugal, Spain, France, the USA, and Belgium, and the citation frequency of the papers pub- lished by Chinese authors is 26.3, which is approximately 60% of the average value (Table 1.2.9). This shows that Chinese scholars are increasingly focusing on this research focus. As can be observed from the cooperation network shown in Figure 1.2.5, there exists a close cooperative rela- tion between the top five countries or regions.

According to the statistics of institutions publishing these core papers (Table 1.2.10), the top five institutions include the Wuhan University, Chinese Academy of Sci- ences, Curtin University, University of Extremadura, and Delft University of Technology. As can be observed from the cooperation network of the top 10 paper publishing institutions shown in Figure 1.2.6, there exists a closer co- operation between foreign institutions.

According to the statistics of the cited papers (Table 1.2.11 and Table 1.2.12), the top five countries or regions that published the most core papers on this subject in- clude China, Australia, the Netherlands, Germany, and England, and the proportion of Chinese core papers cited was 45.59% (the Chinese institutions among the top five

《Table 1.2.9》

Table 1.2.9 Major producing countries or regions of core papers on the engineering research focus “Mixed-pixel decomposition and spatiotemporal location big data analysis”

《Table 1.2.10》

Table 1.2.10 Major producing institutions of core papers on the engineering research focus “Mixed-pixel decomposition and spatiotemporal location big data analysis”

institutions included Wuhan University, Chinese Acade- my of Sciences, and Hong Kong Polytechnic University), which shows that Chinese scholars paid relatively close attention to the dynamics of this subject.
Based on the presented statistics, in the case of this engineering research subject—”Mixed-pixel decompo- sition and spatiotemporal location big data analysis”— Chinese scholars are keeping abreast of their foreign counterparts and gradually developing into world leaders in this research subject area. We suggest that the Chinese government should continue supplying funds to this re- search area in order to accelerate its development to the world-leading level.

《2    Engineering development hotspots and engineering development focus》

2    Engineering development hotspots and engineering development focus

《2.1    Development trends of engineering development hotspots》

2.1    Development trends of engineering development hotspots

The top 10 engineering development hotspots in the fields of civil, hydraulic engineering and architecture are listed in Table 2.1.1, and they include “Green city and green building,” “New materials for civil engineering,” “Intelligent building construction,” “Intelligent urban transportation systems,” “Digital urban planning,” “Seis- mic and vibration control,” “Monitoring, inspection, and informatization of geotechnical and underground

《Figure 1.2.5》

Figure 1.2.5 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “Mixed-pixel decomposition and spatiotemporal location big data analysis”

《Figure 1.2.6》

Figure 1.2.6 Collaboration network of the major producing institutions of core papers on the engineering research focus “Mixed-pixel decomposition and spatiotemporal location big data analysis”

《Table 1.2.11》

Table 1.2.11 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Mixed-pixel decomposition and spatiotemporal location big data analysis”