1. Introduction
Modular steel construction (MSC) has been used widely due to its exceptional advantages: minimum of on-site work, and higher construction speed and qualities. Different from other types of construction, modular steel construction is highly integrated prefabricated. Due to these features, MSCs are an ideal construction for emergency situations (e.g., emergency hospital centers). Structural components and building services, i.e., electrics, pipelines, and decorations, are integrated into room modules, which are the basic units of modular steel construction. Each room module combines structural and architectural functioning, e.g., a living room module, a bathroom module, or a staircase. These room modules are off-site prefabricated and can be replaced easily. This feature of MSC provides the potential for removability and reuse. Thus, MSC can be adapted to temporary and movable buildings.
The connecting joints of modular steel construction are very different from that of regular steel structures [
1].
Figure 1 compares the joints in a regular structure and MSC. In joints of a regular structure, there are four beams and one continuous column. In joints of MSC, the numbers are sixteen and eight. Additional special requirements should be fulfilled for the MSC joints: good cooperation with other installed structure components and building services, minimum need of construction spaces and time, and potential to be demounted. Joints with mechanical connections are recommended due to good detachability. One of the key issues of MSC joints is the operation spaces for installation or uninstallation. As shown in
Figure 2, there are three types of joints for modular steel construction, i.e., side, corner, and middle joints. Among these joints, the side and corner joints will not be limited by operation spaces. But for the middle joints, there is no access from outside. As the walls, doors, floors, and ceilings were installed before the assembling, there are no operation spaces for the connection of joints.
Previous research has provided various types of mechanical-connecting joints for modular steel construction. Lawson proposed bolt-connecting joints for a square hollow section or open section column, as shown in
Figure 3a [
2]. These types of joints can be used as side or corner joints for modular steel construction, but cannot be used as middle joints. The end plates of the upper and lower module columns are connected with bolts. For square hollow section columns, access holes are set for installation or uninstallation. Chen developed bolt-connecting joints with a plug-in for square hollow section columns [
3,
4]. These types of joints can be used as middle or side joints for modular steel construction. Upper and lower modules are connected by long penetrating bolts at the end of the beams. A plug-in component is set between the upper and lower square hollow section columns. The vertical load cannot be transferred between the upper and lower columns directly. Moreover, the penetrating long bolts will interfere in the installation of other structure components, e.g., wall plates. Ding developed blind bolt-connecting joints with a plug-in for square hollow section columns, as shown in
Figure 3b. These types of joints can be used as side joints for modular steel construction, but cannot be used as middle joints. Similar to the joints of Chen, a plug-in component is set between the upper and lower square hollow columns. The columns are connected to the plug-in component with blind bolts, which only require one-side operation. Though many other joints were proposed, analyzed, and tested, the problems of joints for modular steel construction still exist [
5,
6,
7,
8,
9]. Joints for modular steel construction still need to be improved.
The most common modular buildings also include the steel frame modular, which consists of the full length of beams and girders, and column ends and requires beam-column bolt connections at each floor. Chen developed a new type of f site-bolted assembled joint for prefabricated modular H-shaped steel-beam-column joints [
11]. The assembly diagram of joints is shown in
Figure 4a. The developed joint assembles the columns and beams at the connector region through on-site bolting of the column base plate and beams’ splice plate, avoiding direct welding of the structural components to each other. Du proposed a beam-through moment-resisting joint for H-section (wide-flange) beams and columns that can be prefabricated [
12]. The joints erect an entire floor by bolting the upper-level columns to the lower-level beams, as shown in
Figure 4b. With this type of beam-through beam-column joint, it is feasible to offset columns on adjacent floors and increase the flexibility and versatility of the framing plan layout.
Self-lock joints for modular steel construction were proposed to solve this middle joint connection issue [
13]. These self-lock joints need no operation spaces during both the connection and disconnection processes, and can be demounted easily due to the unlocking device. Meanwhile, the joints have no interruptions with other integrated structural components and services, and no limitations to sections of beams or columns. Thus, this type of joint can be used as a middle joint for modular steel construction.
Although more and more new modular joints have been developed in recent years, there are still considerable difficulties in realizing the overall disassembly of module buildings and the rapid off-site reconstruction after disassembly. In this follow-up research, the self-lock plug-in connector was improved. A set of unlocking devices was added for easier disconnection of the joint. This improvement could increase the speed of the demountable function for the joints, which helps to further realize the overall disassembly of module buildings. A full-scale test consisting of four specimens is reported in the following part of this paper. The demounting processes of the joints were verified through a functional test. Then, the full-scale specimens were applied with cyclic load to investigate the seismic behavior. Finite element (FE) models of these novel joints were developed and validated with the test results. Parameter analysis was conducted based on the FE models.