Bolt-free post-tensioned connection for steel-framed modular buildings and design for optimal preloa2024_Lee 等

Journal of Constructional Steel Research 218 (2024) 108703 Available online 2 May 2024 0143-974X/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Bolt-free post-tensioned connection for steel-framed modular buildings and design for optimal preloading Keunwoo Lee a,* , Kim J.R. Rasmussen a , Bong-Ho Cho b a School of Civil Engineering, University of Sydney, Sydney, Australia b Department of Architectural Engineering, Ajou University, Suwon, Republic of Korea ARTICLE INFO Keywords: Prefabricated construction Modular steel building Inter-module connection Post-tensioned connection Design ABSTRACT This paper introduces a novel bolt-free preloaded column-to-column connection, termed the ‘AJ connection’, for steel-framed modular buildings. The AJ connection is classified as a post-tensioned inter-module connection, and modules are connected vertically by preload, using specially designed couplers and high-tensile steel rod bolts (SRB) within the columns. The AJ connection enables easy assembly and disassembly processes of modular buildings by inducing preload on top of the modules using a torque or impact wrench, thus improving construction speed and reusability. The paper also establishes a design method, not previously attempted in the literature, to determine the minimum necessary preload for the connection. The primary aim of this method is to design against potential failures by gap-opening and slip between modules, commonly associated with existing post-tensioned inter-module connections. The effectiveness of the suggested method is verified through numerical simulations using FE joint models that have been calibrated against monotonic loading test results. Furthermore, the study investigates the adverse effects of gap-opening and slip at the connection on the performance of the beam-to-column joint and the SRBs within the columns. The findings indicate that (i) minimising the difference between the flexural stiffness of the floor and ceiling beams reduces the impact of gap-opening, and (ii) the rotation of the coupler caused by plate bearing due to slip prior to gap-opening affects the induced preload in the SRBs and leads to premature gap-opening at the connection. 1. Introduction Modular construction is a type of prefabricated construction system using 3D volumetric modules. The modules are manufactured in a factory and then transported on-site for assembling to form a building. Modular construction has many advantages, such as high-quality workmanship, speedy construction and associated cost savings, reduction of waste as well as recyclability [1–3]. In recent years, the demand for multi-rise steel-framed modular buildings has grown rapidly in urban areas for addressing severe urban housing shortages sustainably as an alternative to conventional construction [4]. To maximise the benefits of modular construction, effective intermodule connections are most critical. The connections should not only ensure sufficient rigidity and strength, but also facilitate easy on-site installation for rapid construction, cost savings, and recyclability. Generally, the bolted connection is preferred as an easy and costefficient on-site method, so various shapes of the bolted connection have been developed [5–15] (see Table 1). However, most of these prevent modules from being manufactured as complete finished products, as bolt installation requires external access space on site, resulting in additional on-site finishing works. Moreover, most existing bolted connections have complex shapes due to the numerous bolts required, making it difficult to assemble and disassemble modules on site. To address these challenges, the post-tensioned type connection has been introduced, as shown in Table 2. Chen et al. [16,17] proposed a prestressed connection with concrete-filled RHS columns, where strands were stretched first, and the concrete was poured into the columns. The two-storey modular frame was tested to investigate the momenttransferring mechanism and seismic performance of the connection under cyclic loading. Sanches et al. [18] suggested a post-tensioned connection consisting of a shear steel box and a threaded rod passing through upper and lower HSS columns, where the threaded rod was post-tensioned using a hydraulic jack. Exterior joint tests were conducted under cyclic loading to investigate its seismic performance. Adopting an alternative design, Jung et al. [19] used a steel strand instead of a threaded rod passing through the upper and lower columns, which could connect several modules vertically at once. The strand was * Corresponding author. E-mail address: [email protected] (K. Lee). Contents lists available at ScienceDirect Journal of Constructional Steel Research journal homepage: www.elsevier.com/locate/jcsr https://doi.org/10.1016/j.jcsr.2024.108703 Received 4 November 2023; Received in revised form 10 April 2024; Accepted 21 April 2024 Journal of Constructional Steel Research 218 (2024) 108703 2 post-tensioned using a hydraulic jack. The seismic performance of the connection was investigated experimentally with the exterior joint under cyclic loading. Shin et al. [20] suggested a similar post-tensioned connection using a post-tensioned strand through the hydraulic jack. Its structural stability and lateral-load carrying capacities were investigated experimentally and numerically. Lacey et al. [21,22] proposed a connection consisting of a shear key and a post-tensioned tie rod in which preload was applied using a torque wrench through the access opening provided in the columns from inside the module. Experimental and numerical investigations were conducted on the load-slip behaviour of the connection, considering the impact of preload, slip factor, and contact area. Nomenclature AT and AC Tensile and compressive axial force acting on the column due to the lateral force F, respectively be Width of endplate C (C1, C2 and C3) Clamping force between plates at the connection Ce Uniform compressive force on the endplate due to preload CF and TF Contact force and uplifting force at the ends of the column endplate, respectively Cg and Tg Contact force at the end of the endplate and tensile force applied to the SRB, respectively E Young's modulus F Lateral force Fcr Critical lateral force that the column supports without gapopening, related to the Pb,min Feff Effective lateral force that causes Cg and Tg FN,c1 and FN,c2 Normal forces on the contact surface-1 and -2, respectively Fs,Rd1 and Fs,Rd2 Slip resistance on the contact surface-1 and -2, respectively Fv,Ed1 and Fv,Ed2 Design ultimate shear loads on the contact surface-1 and -2, respectively fy Measured yield stress fy,SRB Measured yield stress of SRB fu Measured ultimate tensile strength G (G1 and G2) Gravity load (acting on the column-1F and column2F, respectively) h1 and h2 Height of module-1F and module-2F, respectively Ki Initial rotational stiffness kB Axial stiffness of bolt part consisting of the SRB and the coupler kM Axial stiffness of column part consisting of the column and the connected plates L Width of module (column centre to centre) LM Width of module (including column endplate) Mg and Ms Moment for the occurrence of gap-opening and slip at the connection, respectively Mov and Mst Overturning moment and Stabilizing moment, respectively Mp Expected plastic moment capacity of the beam-to-column joint at the column centreline Mpl,fb and Mpl,cb Plastic flexural capacities of floor and ceiling beams, respectively Mu Ultimate moment strength My Yield moment strength Nc1 and Nc2 Axial forces acting on the column-1F and column-2F, respectively Nb1 and Nb2 Axial forces acting on the ceiling beam and floor beam, respectively Pb Preload applied to the SRB Pb,min Minimum demand preload to prevent the column from gap-opening Pb1 and Pb2 Preload applied to the SRB-1F and -2F, respectively Pb1,g and Pb2,g Minimum demand preload to prevent gap-opening at the column-1F and -2F, respectively Pb1,s and Pb2,s Minimum demand preload to prevent slip at contact surface-1 and − 2, respectively Pb2,A and Pb2,B Demand preload for preventing gap-opening at points A and B on contact surface-2, respectively Pb2,C and Pb2,D Demand preload for preventing gap-opening at points C and D on contact surface-1, respectively Pc1 and Pc2 Uniform compressive forces on the column-1F and column-2F due to preload, respectively PF,g and PF,s Final demand preloads to prevent gap-opening and slip at the connection including the weight effect of upper modules, respectively Pg and Ps Demand preload for preventing gap-opening and slip at the connection, respectively Pi Initially induced preload at the connection Pn Remaining or net preload at the connection due to the weight of upper modules PR Reduced preload at the connection due to the weight of upper modules SRB Steel rod bolt Rx and Ry Horizontal and vertical reaction forces tp Thickness of plates at connection VB Total shear force acting on the beams VB,G and VB,S The analytically estimated strength for the occurrence of the gap-opening and slip at the connection, respectively VFEA,G and VFEA,S The FE simulation strength causing the gapopening and slip at the connection, respectively Vb1 and Vb2 Shear forces acting on the ceiling beam and floor beam, respectively Vc1 and Vc2 Shear forces acting on the column-1F and column-2F, respectively Vcb,g Additional shear force acting on the ceiling beam due to the effect of gap-opening at the connection Vfb and Vcb Shear forces acting on the floor beam and ceiling beam, respectively W Self-weight μ Friction coefficient between clampled plates Δfb and Δcb Deflection of the floor and ceiling beams, respectively Δf,g and Δc,g Lateral displacement which reflected in the floor beam's displacement and the ceiling beam's displacement, respectively δH Lateral deflection at the tip of the column δB,G Stretched length of the SRB δM,G Compressive axial deformation of the column γov and γsh Material overstrength factor and hardening overstrength factor, respectively θg and θs Interstorey drift for the occurrence of gap-opening and slip at the connection, respectively θgap Degree of gap-opening at the connection θu Ultimate interstorey drift θy Yield interstorey drift σp Initial tensile stress subjected to the SRB due to induced by preload σf, σs, and σg Stress subjected to the SRB due to the flexural deformation of columns, the occurrence of slip at the connection, and the occurrence of gap-opening at the connection K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 3 Thus, over the last few years, several post-tensioned connections have been developed and studied to enhance the constructability and structural performance of modular buildings. However, they still have drawbacks: the necessity of a hydraulic jack to assemble and disassemble adjacent modules on site results in associated work-related risks and a decrease in reusability; also, the loss of column cross-section to accommodate connection installation prevents the columns from utilizing their full load-bearing capacity. Furthermore, despite the various post-tensioned connections documented in the literature, no explanation is provided regarding the process for determining the minimum required post-tensioning load. It is considered that a theoretical gap remains concerning the determination of adequate post-tensioning load for column-to-column connections to ensure their optimal performance. This paper proposes a novel bolt-free post-tensioned inter-module column-to-column connection for steel-framed modular buildings that optimises efficiency and reusability by simplifying the assembly and disassembly of modular buildings. In addition, this paper clarifies the potential failure modes associated with post-tensioned connections and establishes a design method to determine the minimum demand preload (or post-tensioning load) for column-to-column connections. This design method could serve as a basis for determining the necessary preload for most post-tensioned type connections. The effectiveness of the suggested method is demonstrated by undertaking numerical simulations using FE joint models calibrated against monotonic loading test results, and investigating the impact of gap-opening and slip at the connection on the stiffness and strength of the beam-to-column joint. Table 1 Existing bolted connections. Lee et al. [5] Lacey et al. [7] Cho et al. [8] Ma et al. [9] Chen et al. [10] Zhang et al. [11] Table 2 Existing post-tensioned connections. Chen et al. [17] Sanches et al. [18] Lacey et al. [21] Jung et al. [19] Shin et al. [20] K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 4 Fig. 1. Details of the AJ connection. K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 5 Fig. 2. Clamping forces (C) at the AJ connection by preload (P) according to the assembling process of upper and lower columns. K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 6 2. Innovative preloaded connection 2.1. Details of the AJ connection The proposed connection (referred to as the ‘AJ’ connection) and its assembly process are shown in Fig. 1. The AJ connection was jointly developed by the University of Sydney in Australia and Ajou University in South Korea. Modules are connected horizontally by plates and vertically by preload, using a specifically designed coupler and hightensile steel rod bolts (SRB) inserted into the hollow section columns during the manufacturing process in the factory. The AJ connection enables easy assembly and disassembly processes of modular buildings with its streamlined installation process, improving construction speed and reusability. Preload is applied to the SRB using a torque or impact wrench on top of the modules, thus eliminating the need for access space to install the connection from inside the module. This contrasts existing bolted connections which require additional on-site finishing work after connecting the modules. Consequently, the modules can be manufactured as completely finished products in the factory, resulting in improved module quality and increased construction speed on site. Also, the connection method eliminates the need for access holes from columns for connection installation, which preserves the full strength of the columns. Lastly, the AJ connection improves worker safety and saves costs by replacing the hydraulic jacks required for existing on-site posttensioned connections with a torque or impact wrench and by enabling all tasks to be carried out on top of the modules, thereby eliminating the Fig. 3. Free body diagrams for clamping forces (C) at the AJ connection depending on the relative magnitudes of preloads P1 and P2. Fig. 4. Deformed shapes of a modular building depending on the AJ connection behaviour. K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 7 need, and associated risks, for work on the exterior of the building and installation of scaffolding. 2.2. Preload mechanism and clamping force Fig. 2 presents the process of generating clamping forces at the AJ connection by applying preload to the SRBs. The assembly process is as follows: When preload P1 is applied to SRB-1F by Coupler-1F, the interface between Connecting plate and Endplate-1F has the clamping force of C1 equal to P1 (Fig. 2(a)). Then, Cover plate is placed on the Connecting plate, and Column-2F is placed on Cover plate (Fig. 2(b)). After installing SRB-2F, Coupler-2F is tightened to give preload to SRBFig. 5. Two types of gap-opening at the AJ connection due to lateral force. Fig. 6. Overturning and stabilizing moments. K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 8 2F, resulting in the interface between Connecting plate and Cover plate and the interface between Cover and Endplate-2F plate having the clamping forces of C2 and C3, respectively, both equal to P2. Depending on the difference in magnitude between preload P1 in SRB-1F and preload P2 in SRB-2F, the clamping forces (C1, C2, and C3) at the connection can be different: • In case P1 ≥ P2 (Fig. 3(a)), when P2 has been applied to SRB-2F, Coupler-1F still remains in contact with Connecting plate. The preload P1 in SRB-1F determines the clamping force of C1 for Column-1F at the connection. The preload P2 in SRB-2F determines the clamping forces of C2 and C3 for Column-2F at the connection. If P1 = P2, the clamping forces at all the contact surfaces are the same (C1 = C2 = C3). • In case P2 > P1 (Fig. 3(b)), when P2 exceeds P1, Coupler-1F is subjected to a larger upward pulling force of P2 than the downward pulling force P1, resulting in Coupler-1F becoming unattached from Connecting plate. From that moment, SRB-1F, Coupler-1F, and SRB2F begin to act as one long bolt, so when tightening Coupler-2F, they are elongated altogether. Therefore, all contact surfaces (CB, C1, C2, C3, and C4) in the columns from the base plate to the top of Column2F have the same clamping force, P2. It is desirable to make the connection have a uniform clamping force by applying the same amount of preload to SRB-1F (P1) and SRB-2F (P2) for reliable performance of the connection. 3. Failure mode at the AJ connection The ideal behaviour of the AJ connection is a rigid column-to-column connection (Fig. 4(a)). However, if preload is insufficient, two types of failure are possible: gap-opening and slip. They affect the overall stability of modular buildings, as shown in Fig. 4(b) and (c), respectively. 3.1. Gap-opening If the tensile force acting on the AJ connection, as induced by external loads on the building, is larger than the clamping force induced by preloading, an uplift of the column occurs (Fig. 4(c)). In this study, this phenomenon is called gap-opening. The gap-opening causes a large displacement at the top of the building, reducing the overall stiffness of the modular building and severely affecting its stability. As the lateral force is applied to the modular building, the columns and beams are simultaneously subjected to axial and shear forces (Fig. 5(a)). Depending on which force is dominant, the gap-opening can be classified into two types. When the axial tensile force acting on the column becomes dominant, the column separates (Fig. 5(b)). On the other hand, if the shear forces acting on the columns and beams are dominant, contact between plates in the connection is retained as the gap-opening develops (Fig. 5(c). 3.2. Slip When assembling two modules on-site, accurately aligning the columns is challenging due to unavoidable manufacturing tolerances resulting from module production. Therefore, it is essential to provide Fig. 7. Preloaded column subjected to the lateral force F. Fig. 8. Additional tensile force acting on the SRB in the preloaded column due to gap-opening. K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 9 Fig. 9. Deformed shape of the beam-to-column joint with applied external and internal forces. K. Lee et al. Journal of Constructional Steel Research 218 (2024) 108703 10 Fig. 10. Design forces for calculating the demand preload to prevent gap-opening (Pg) and slip (Ps) at the AJ connection. K. Lee et al.