Earthquakes cause serious damage to buildings and result in heavy losses to society, therefore, it is necessary to enhance the seismic capacity of existing buildings via structural retrofitting. The traditional retrofitting approaches are based on the component-level, but their improvement effect for the overall structure is not obvious. The ultimate goal of seismic retrofitting is to improve the overall seismic performance of the whole structure, thus a variety of external sub-structure retrofitting methods have been developed at home and abroad since the 1970s. The external sub-structure is connected with the existing structure as a whole on the structural -system-level, and it is of great significance for lifeline projects or non-interrupted buildings. At this stage, the external sub-structure retrofitting technology has received wide attention in the seismic community and is still developing in bloom. This paper gives a state of the art review of the advances and research interests of the external sub-structure retrofitting technology. First, the general concepts of the external sub-structure retrofitting technology are given, including (1) retrofitting principle and (2) retrofitting superiority. Then, the typical types of the external sub-structure retrofitting technology are summarized, including (1) external frame sub-structures, (2) external frame-brace sub-structures, (3) external wall sub-structures and (4) other external sub-structures. Finally, some critical issues of the external sub-structure retrofitting technology are extracted, including (1) interfacial shear transferring mechanism, (2) joint property and connection performance, (3) combination with precast-assembly technology, (4) combination with prestress technology, (5) numerical approach and assessment indicators, (6) optimization strategy and design procedure, (7) environment interaction and maintenance cost, and (8) application in practical engineering. The future perspectives of the external sub-structure retrofitting technology are also pointed out, and the contents can provide some reference for the subsequent research as well as the developing trend in the future.

Prefabricated 3D modules are widely adopted in building construction for better quality, lower demand on labor and construction space, faster construction, and less emission. Various types of modular structures and connections have been tested and are commercially available. A large number of actual applications demonstrate that the structural system of a modular building can be flexibly organized to suit for the topmost utility architecturally. However, in terms of the stability of the structural system, the role of prefabricated modules widely varies. A systematic summary and categorization of different structural stability systems and layout patterns, especially in multi-story modular buildings where load sharing and transfer between the units are necessary, has yet to be made. This paper provides a state-of-the-art review which focuses on four major structural aspects, e.g., module connection, individual module structure, layout pattern, and additional stability unit. Recommended combinations of structural parts of the system are provided. Nonlinear static or pushover analysis is carried out with the numerical model based on available experimental data to demonstrate the roles of individual modules and additional stability units on system stability. A rating procedure for the modular structural system is proposed to briefly evaluate several aspects of the multi-story modular building structures. Trends in design and usage of modular structure systems are presented using examples of 20 unique applications.

In recent decades, fibre-reinforced polymers (FRPs) have received extensive attention from the civil engineering field due to their light weight, high strength, corrosion resistance, fatigue resistance, and designability. The forms of FRP products used in civil engineering structures include FRP sheets/plates, FRP bars/cables, FRP profiles, FRP grids, and FRP tubes. Among them, FRP tubes can not only be used as formwork for concrete pouring but can also provide lateral confinement similar to steel tubes; moreover, they have the advantages of corrosion resistance and nonmagnetic properties. Therefore, it is particularly suitable for newly built structures under harsh service environments. In recent years, extensive research on traditional concrete-filled FRP tubes (CFFTs) has already been widely carried out in terms of experimental tests and theoretical analysis, and scholars have turned their attention to innovations in the types of internally filled concrete. The concrete type has extended from ordinary concrete to sea-sand concrete (including seawater sea-sand concrete), lightweight aggregate concrete, recycled aggregate concrete, ultrahigh performance concrete, and alkali-activated concrete. In addition, in terms of the tube shape, a new type of curved FRP tubular arch has been innovatively developed. In light of the above, this paper comprehensively reviewed the latest innovative studies of FRP tube-reinforced concrete structures in terms of the filled concrete type and the tube shape in the literature, which can provide a reference for follow-up research and their actual application.

In recent decades, fibre-reinforced polymers (FRPs) have received extensive attention from the civil engineering field due to their light weight, high strength, corrosion resistance, fatigue resistance, and designability. The forms of FRP products used in civil engineering structures include FRP sheets/plates, FRP bars/cables, FRP profiles, FRP grids, and FRP tubes. Among them, FRP tubes can not only be used as formwork for concrete pouring but can also provide lateral confinement similar to steel tubes; moreover, they have the advantages of corrosion resistance and nonmagnetic properties. Therefore, it is particularly suitable for newly built structures under harsh service environments. In recent years, extensive research on traditional concrete-filled FRP tubes (CFFTs) has already been widely carried out in terms of experimental tests and theoretical analysis, and scholars have turned their attention to innovations in the types of internally filled concrete. The concrete type has extended from ordinary concrete to sea-sand concrete (including seawater sea-sand concrete), lightweight aggregate concrete, recycled aggregate concrete, ultrahigh performance concrete, and alkali-activated concrete. In addition, in terms of the tube shape, a new type of curved FRP tubular arch has been innovatively developed. In light of the above, this paper comprehensively reviewed the latest innovative studies of FRP tube-reinforced concrete structures in terms of the filled concrete type and the tube shape in the literature, which can provide a reference for follow-up research and their actual application.

Prefabricated 3D modules are widely adopted in building construction for better quality, lower demand on labor and construction space, faster construction, and less emission. Various types of modular structures and connections have been tested and are commercially available. A large number of actual applications demonstrate that the structural system of a modular building can be flexibly organized to suit for the topmost utility architecturally. However, in terms of the stability of the structural system, the role of prefabricated modules widely varies. A systematic summary and categorization of different structural stability systems and layout patterns, especially in multi-story modular buildings where load sharing and transfer between the units are necessary, has yet to be made. This paper provides a state-of-the-art review which focuses on four major structural aspects, e.g., module connection, individual module structure, layout pattern, and additional stability unit. Recommended combinations of structural parts of the system are provided. Nonlinear static or pushover analysis is carried out with the numerical model based on available experimental data to demonstrate the roles of individual modules and additional stability units on system stability. A rating procedure for the modular structural system is proposed to briefly evaluate several aspects of the multi-story modular building structures. Trends in design and usage of modular structure systems are presented using examples of 20 unique applications.