## 1 Introduction  
Modular construction is an industrialized construction system, of which process mainly consists of the factory production of pre-engineered building units, the delivery of the fabricated units to the construction site and their fast assembly [1; 2; 3; 4]. In recent years, modular construction has expanded its applications, due to its advantages, such as mass-fabrication, excellent quality, speed-up of the construction period. For example, there have been high-story modular buildings of more than 20 stories constructed in the US and Great Britain [2; 5; 6], and a 30 story hotel building has been completed using modularized prefabricated frame members in China [7; 8; 9; 10]. It is highly important to develop effective connection details between unit modules, which allow the rapid assemblage of modular units without having any interference between their furnishing materials. In general, pre-fabricated unit modules include all of the final finishing materials, and thus it is not easy to achieve the simplicity of connection methods and to guarantee the structural safety of the completed modular building. Most commonly, unit modules are connected using high-tension bolts through access holes located at the bottom and top of the steel box columns [7; 11]. Alternatively, it is also possible to pre-install nuts inside the column member and to connect unit modules by applying steel plates and bolts outside the member. Although the connection of unit modules by welding [12] is conceivable, it may cause fire during this process, and therefore is not frequently used on construction site. Another critical issue with the development of effective modular construction is to create a modular structural system that is based on the so-called strong column-weak beam concept [11]. The strong column-weak beam behavior of the modular structural system is desirable since it can retain sufficient ductility under extreme loading conditions, such as earthquake, tsunami, flood and hurricane. In general, the modular building unit has both ceiling and floor beams, and most of the vertical loads are supported by the floor beams. Although the ceiling beams are barely functional as main structural members except when transporting and assembling modular units, the combination of these two components may result in the weak column-strong beam behavior, which is not desirable in the viewpoint of earthquake resistant design. In order to handle this issue, the concept of the special truss moment frame discussed in [13; 14; 15; 16] may be applied to the design of the modular structural system. Its main idea is to install a Vierendeel opening between steel truss moment frames and use it as an effective energy dissipation device, thus the strong column-weak beam behavior can be guaranteed. However, this concept requires complicated connection details, which are difficult to be implemented in the assemblage process of modular units, thus a simpler solution than this needs to be developed. The development of a reasonable finite element analysis model of module-to-module connections is also an important issue for the structural design of modular buildings. Generally, the joint between the column and beam is assumed to be rigid, while the horizontal and vertical connections between the modules to be an internal hinge [17]. For the detailed analysis of modular structures, a more refined model is required, but no well-validated research has been performed regarding this issue. Considering all of the issues discussed above, this study proposes an effective steel frame modular system and evaluates the structural performance of its beam-column connection through experimental and analytical work. The new steel frame modular system utilizes the blind bolts, which allow free access to the structural members of the closed cross-section. Although the blind bolts have been used in the connection of steel frames [18; 19; 20; 21], they have been very rarely used in the modular construction.