Axial mechanical behavior of an innovative liftable connection for modular steel construction

The contemporary construction industry demands innovative construction approaches best suitable to solve the challenges facing the industry today. One such approach is modular construction. It allows offsite construction of various building components/units before they are transported and assembled at the actual construction site. Modular steel construction (MSC) has particularly grown in popularity. Compared with conventional steel construction, MSC offers a range of benefits, including remarkable quality, decreased construction times and more environmentally friendly.

Generally, MSC systems comprise intra- and inter-module connections. Intra-module connections primarily consist of rigid joints welded together offsite. They significantly influence the MSC performance, which highly depends on the welding quality. Similarly, inter-module connections also play a critical role in determining the practicability and safety of MSCs. While the research on intra- and inter-connection has intensified in recent decades, most existing connections have complex geometry and neglect the module unit lifting, thus reducing the convenience and speed of onsite installation. Additionally, there has been limited research on the axial mechanical behavior of inter-module connections and there are only a few studies on their tensile and comprehensive performance.

On this account, a team of researchers at Zhengzhou University: Dr. En-Feng Deng, Mr. Jun-Yi Lian, Professor Zhe Zhang, Professor Hui Qian, Miss Guang-Cao Zhang, Professor Pu Zhang and Professor Shamim Ahmed Sheikh proposed an innovative and fully prefabricated liftable connection (FPLC) for MSC. A series of axial compressive loading tests were conducted on three full-scale specimens to determine the axial mechanical behavior and compressive performance of the FPLC. Their work is currently published in the journal, Thin-walled Structures.

In their approach, the research team developed and validated experimentally a nonlinear finite element model (FEM) for the FPLC. Using the validated FEM, a series of parametric studies and simulations were performed to evaluate the influence of various parameters on the damage indices and tensile performance of FPLC for all the specimens. The representative failure modes and the damage mechanism under tensile loading were identified. Finally, a theoretical model is proposed to determine the ultimate tensile capacity of the proposed MSC connection.

The authors demonstrated the capability of the validated FEM in predicting the ultimate loading bearing capacity as well as the failure mode of the FPLC. All three tested specimens experienced local buckling in their modular column. The proposed FEM also provided reasonable estimates of the failure modes and the ultimate resistance. The failure modes were grouped into three: bearing and bending failure (of both the bolt holes within the column and bearing failure of the inner bolt plate connector) and the shear failure of the long stay bolt.

The parametric study revealed that increasing gusset plate thickness increased the ultimate bearing capacity of the FPLC, while increasing the thickness of the inner plate significantly increased its tensile capacity. Although increasing the thickness of the modular column and the diameter of the long stay bolt increased the ultimate load resistance of the FPLC, the grade of the column strength had minimal impact. By comparing the distribution indices, it was established that an increase in the strength grade of the column, the thickness of the gusset and inner plates, thickness of the column and the diameter of the long stay bolt resulted in a safer stress state as they potentially transformed the connection failure mode under tension.

In summary, the study proposed and tested the feasibility of an innovative FPLC for potential application in modular steel construction. The influence of different parameters on the damage indices and axial tensile resistance was determined along the critical lines of the tested specimen. The proposed theoretical model was proved to be reliable and conservative. In a statement to Advances in Engineering, Professor En-Feng Deng said that their study provided valuable insights that would advance the much-needed design guidance for MSC.