Modular integrated construction (MiC), a game-changing design and construction method for certain types of buildings, has been gaining traction with the construction industry in Hong Kong. It is suitable for building types from low-rise healthcare facilities to high-rise mixed-use residential and hotel buildings. As a sub-group of design for manufacture and assembly (DFMA), MiC calls for a 3D design consideration for precast elements instead of the traditional 2D, which has been adopted by the construction industry for decades.
The InnoCell at Hong Kong Science and Technology Park (HKSTP) is the first multi-storey MiC project approved by the Buildings Department (BD) in Hong Kong. The structural construction of this 17-storey smart-living dormitory commenced in August 2019 and is expected to be completed by August 2020 and occupation permit will be issued by the end of 2020 according to schedule. MiC cuts down the project delivery time by a minimum of six months.
InnoCell (Image courtesy of Leigh & Orange Architects)
Design for MiC follows an approach different from the conventional one, and it must comply with PNAP ADV-36 issued by the BD. The first ever in-principle acceptance for a MiC system submission is successfully secured based on the prototype of the InnoCell. From the statutory submissions to the fabrication and construction stages, design details are continuously improved to cope with the construction difficulties without compromising the structural consideration already adopted.
The cores and structural frame of the superstructure are constructed in in-situ reinforced concrete (RC), which cover the corridor and the common areas of each floor. The shear walls of the two cores act together with the in-situ floor slabs to provide lateral stability for the building. Wind forces are progressively distributed to the shear walls and cores at each floor from the perimeter through the modules, and subsequently transmitted to the pile caps and socket H-pile foundations. At Levels One and Two, a transfer plate and transfer beams supporting the MiC modules above are linked to the cores. Together with their supporting columns, they help provide lateral stability.
Demarcation plan of MiC and in-situ RC
Each accommodation unit in the building is provided by a MiC module. It is a cuboid formed by structural steel sections with corner boxes along the perimeters and secondary beams at the top and bottom planes to support the floor slab and ceiling. The floor of the module is formed with 100 mm concrete and the walls along the long sides are formed with corrugated sheets. Each of them is fabricated in a factory with all mechanical, electrical and plumbing provisions, interior finishes and furniture installed before being transported to the site for installation.
The physical separation between the MiC modules results in discontinuity of the floor diaphragm. As a step in the design analysis, stiffness of the MiC floor slab is studied. Advanced computational structural analyses are performed to show the deformation and stress of the individual slabs, as well as the stiffness of the framing layout formed by the interconnection between modules, and how the lateral forces are transferred to the cores. The central communal areas formed by in-situ RC flat slabs tie all MiC modules together to form a compact floor diaphragm capable to transfer the loads back to the cores.
Hence, instead of adopting the conventional assumption of a rigid diaphragm at each floor, this structural scheme provides a more realistic structural simulation on the load paths and internal forces transmitted through the module connections, thus reflecting the actual construction details crucial to MiC.
Various fixity conditions and isolated modular diaphragms are analysed to precisely evaluate the internal forces at the connections due to different load cases. The entire model is developed with variation of connections at critical interfaces - between modules, modules to core walls and supports at the transfer levels. The system is subject to distinct load cases including wind load, disproportionate collapse cases, differential column shortening and thermal deformation between steel and RC cores.
A structural model of a MiC joint with multiple degrees of freedom is developed for a structural analysis that demonstrates the predicted behaviour of the joint. Components of the MiC splice connections are also studied using finite element modelling to justify the local stresses at critical bearing interfaces and chords for moment transmissions.
According to the Code of Practice for Structural Use of Steel 2011 (Steel Code), steel-framed buildings are assumed capable to withstand localised collapse if the following principles are satisfied by the structural details:
1. General tying;
2. Tying of edge columns; and
3. Continuity of columns.
In addition to the Steel Code, consideration of disproportionate collapse is also cited from British Standard BS5950 and relevant European codes.
In view of the disrupted continuity of ties at the individual MiC module connections (different from conventional designs with column splices continuously joined at each level), the above three principles are assessed during the design stage.
While the Steel Code only requires the provision of vertical and horizontal ties, the progressive collapse scenarios are examined carefully, and the details are designed to provide reassurance of the robustness of the structure.
To study the plastic hinges developed in the frame and connections, a non-linear staged analysis is carried out using NIDA, a software developed by The Hong Kong Polytechnic University. The results show that the frame is robust as it is strengthened by the truss action in the individual MiC unit, that is, the portal frames and the tensile tie action could pass through the MiC connecting plates and transfer the loads to the adjacent modules.
Individual module analysis
The building is also examined against the local removal of a column, which is an alternative approach to the provision of horizontal and vertical ties under the design codes. To assess the impact, an analysis model is developed by removing a column at different locations to explore the possible load distribution in each case.
The connection plate at each module column interface receives additional forces caused by the change in load path resulting from damage in a critical module column under accidental loading.
Types of steel
A variety of steel materials have been studied to suit the structural needs and market availability. Steel grades from S275 to S460 have been utilised at various locations to enhance the structural capacity while maintaining the overall size and weight of the module. Thin cold-formed panels are also used for typical modules to create the ideal framing.
Whilst the MiC system can expand laterally away from the cores, vertical thermal movement at the connection is allowed by introducing vertical slots with bolts to avoid residual stress caused by restrained movements between the modules and the RC cores.
In structural design, fire protection to the structure is an indispensable consideration. Other than applying cementitious spray/intumescent paint to the steelworks and providing adequate cover to the concrete elements, the MiC modules at InnoCell encounter a unique circumstance of having a physical separation between modules. Architects and engineers on the project have collaborated on the exploration for a cost-effective solution to form independent cavity barriers between modules with the application of fire stop sealants. A fire test has been conducted to simulate formation of a typical joint between modules in order to justify the amount of sealant required to be applied.
The Code of Practice for Precast Concrete Construction 2016 stipulates that critical load factors must be considered for different temporary stages. Design of the MiC modules follows this approach due to the similarity between MiC and precast RC elements. Therefore, the temporary stages, including transportation, lifting and installation, are examined to identify the critical loading condition for the detail design of the MiC module.
After the building layout was confirmed, the temporary storage plan for MiC modules was studied. For a reasonable number of modules to be stocked on site, they are designed to allow for temporary stacking into two layers maximum, doubling storage space on site.
To provide flexibility for the module installation sequence, the building is designed to allow independent installation at each wing. Each of them is designed to allow completion before the others.
Strict fabrication and installation tolerances are required to attain good contact between steel members. The module assembly is carefully developed by the designer and the MiC fabricator to achieve the required tolerance.
Future use of MiC in Hong Kong
For densely populated cities like Hong Kong, MiC provides an effective way to create living and working space in a much quicker, more economical and sustainable manner. It is anticipated to be adopted with increasing popularity for high-rise buildings in the future.
About the authors: Ir Philip K C Lai, Technical Director at WSP and the Registered Structural Engineer for the InnoCell; Ir W L Chan, Senior Associate at WSP and the Structural Lead for the InnoCell; and Ir Kevin H M Li, Senior Engineer at WSP and the Project Resident Engineer to supervise both MiC prefabrication and site construction for the InnoCell.