Analysis and design of cold-formed steel modular building frame with SCIA Engineer

The Industrialised Building System (IBS) is a well-known construction method with many advantages for future construction. However, IBS adoption in Malaysia remains low, particularly when coldformed steel (CFS) is used. A study on structural analysis and design of a CFS container module for the IBS system is conducted. The research is comprised of two parametric studies and one comparison study. The first parametric study was carried out to determine the cost-effective model from three parameters, including module with various layouts, the strength of steel, and the size of the CFS steel section. The analysis and design results from SCIA Engineer software of the chosen model are compared with manual calculations for validation. The second parametric study was carried out with the CFS container subjected to various angles of cable-lifting during transportation and installation. From the findings, an optimised CFS container module was selected from 64 models from the SCIA Engineer analysis result. Furthermore, it is found that certain cable-lifting angles would cause excessive stresses in the CFS container module that could lead to premature failure. Thus, it is recommended that professional measures are necessary while lifting the CFS container module.


Introduction
Industrialised Building System (IBS) construction is a type of construction method which allows a building to be constructed off-site. This construction method is widely known to be used in construction to finish a project in a short duration at a meagre cost. According to Prewer et al. [1] typical design properties of a steel section in a Cold-Formed Steel (CFS) Container Module include yield strength of S280 to S350 N/mm2; thickness of 1.2 to 3.2 mm; section depth of 300 mm; and spacing between joists and studs of 400 to 600 mm.
With the increasing popularity of the IBS construction, much research has been done to improve the structural performance of each module in modular construction. In short, these studies have conducted parametric studies to determine the modular building's response [2], to form an interaction diagram of axial load and bending moment capacity of connection [3], and to investigate the stability performance of multi-column modular wall [4] with their chosen parameters. It was discovered that these studies focussed on the module connectivity, building collapse reaction, and the bearing capacity of the wall panel in modular structures. According to Kamar et al. [5], implementation of IBS in Malaysia is low due to cost concerns and lack of familiarity with the IBS construction methods. Hence, this study aims to explore the optimum design of the CFS container module with the help of a structural analysis software.
Cable-lifting of the volumetric modular unit is part of the IBS construction process, i.e. during transportation and installation. Being influenced by conventional construction method in Malaysia, design of lifting procedures often come after the structural design and during the construction process, usually proposed by the appointed contractor. A concern of the lifting procedure should be included during structural engineering analysis and design of the raised container module, as inappropriate lifting techniques will severely damage the structure in the container module. Different lifting techniques can be used to lift the CFS container module during construction. Among the techniques, lifting with a container spreader frame and lifting with various cable angles are focussed on in this study.
The research's main objective is to perform parametric analysis on the CFS container module, validate the results with manual calculations, and determine the module's behaviour at various cable lifting angles. SCIA Engineer software is used to design and analyse a 3 m × 3 m × 6 m four-sided structural CFS Container Module, which consists of a CFS channel section (C-sections). Two parametric studies and a comparison study are performed on the model in SCIA Engineer applying normal gravity load or lifting load. Furthermore, as the openings in the CFS Container Module are small, the self-weight of the module is assumed to be evenly distributed throughout the model.

Methodology
This research consists of three key activities, which are the design of cold-formed steel, model setup in SCIA Engineer, and parametric investigation.

Design of cold-formed steel
In this research, the design of CFS with manual calculation and SCIA Engineer will be performed based on Eurocode 3 [6][7][8]. The manual design procedure of CFS in this study is based on the procedure suggested in the report done by Way and Heywood [9]. In the design of CFS sections, the gross and effective section properties of CFS are determined with considering the rounded corner influence check in accordance with EN 1993-1-3: 5.1(3) [7], followed by steel member resistance, and finally, the deflection check. Usually, CFS sections are thin and slender, so buckling resistance should be checked during design. These includes flexural, torsional, torsional-flexural, and lateral-torsional buckling.
Few sets of load combinations are established to analyse the ultimate forces and deflection on the CFS section. The load combinations are divided into two groups which are Ultimate Limit State (ULS) and Serviceability Limit State (SLS). Equation 1 shows the load combination of ULS for in Eurocode 3.
Where G k is the permanent load action in the structure; Q k is the variable load action in the structure.
The load combinations of SLS used in this study are based on the four criterions based on the report by Way and Heywood [9]. Table 1 shows the four criterions need to be considered while designing a CFS Container Module.

Model setup in SCIA Engineer
The concept of structural modelling is based on Chong [11] thesis study. In this study, a 3 m × 3 m × 6 m CFS container module is modelled in SCIA Engineer. The modelling consists of four main columns, eight main beams, and wall studs and floor joists with the sides of the CFS container module surrounded with the truss structure accompanied with small openings. The size of the CFS channel section is depicted as "C-depth-thickness", e.g. C15012 for 150 mm depth and 1.2 mm thickness channel section. All steel section sizes used in this study are based on the local manufacturer's catalogue [12,13]. The modelling assumes that every 1D member has six degrees of freedom, similar to a 3D frame structure. Secondly, the structure's support is assumed to be four-corner pinned, with only axial load transferred to the foundation to avoid soil tension. Third, the inclined bracing members in the module is set to "Axial Load only", so the member reacts like a truss. Lastly, the floor joists and bracing members' end connections are pinned, as the members are assumed not to provide enough bending moment restraint.
By referring to Lysaght's Walling Solutions Catalogue [14], BS EN 1991-1-1 [15] and Cobb's Structural Engineer's Pocket Book [16], the steel weight and the loading acting on the CFS container module is calculated and applied to the structural model in SCIA Engineer.

Parametric investigation
Two parametric studies were carried out to design and analyse the CFS container module using SCIA Engineer.

Parametric study I: Design of CFS container models with various layouts, materials and sections
Parameters such as floor/roof joists sizes, wall studs sizes, steel grades, centre to centre distance between floor/roof joists and wall studs are introduced in this parametric analysis. Table 2 below shows the parameters and the values of the parameter used in this study.

Parametric study II: CFS container models under different lifting conditions
This parametric study investigates how lifting in construction affects the CFS container module on site. Various cable lifting angles are represented by the total loading from the container divided by the four corners with varying decline angles measured from the roof plan [17]. Steel grades and the centre-to-centre distance between joist and studs are also included in this study to assess the influence of these parameters on the model during lifting. Table 3 below shows the various parameters and the values used in this study.

Result and discussion
The analysis and design of SCIA Engineer software in the CFS container module are discussed herein. Detailed results can be referred to the complete text thesis [17].

Parametric study I: Design of CFS container models with various layouts, materials and sections
In this parametric study, 64 models of CFS container models have been tested in SCIA Engineer software. The spacing between roof joists, floor joists and wall studs are represented by Parametric Layout 1, and Parametric Layout 2, which have their spacing between joists are 600 mm and 900 mm, respectively. This analysis found that Model 36 stands out as the most economical model of all models, which also passes the ULS and SLS Check in the CFS design. Table 4 below shows the combination of the parameters for Model 36, while Figure 1 shows the detailed drawing for the same model.  This study also proves that the truss structure is appropriate for designing the CFS container module as all studs in the study have resulted in a pass [17]. As a result, smaller wall studs may be used.

Comparison study: Difference between manual calculation against SCIA Engineer calculation for Model 36
The design findings of floor joists and wall studs in CFS Container Model 36 from SCIA Engineer are utilised to compare with the results of manual calculations for section properties, i.e. mid-line theory based on EN1993-1-3 [7]. Table 5 and Table 6 provide the comparison tables for members C15015 and C10012, respectively. According to both tables, the gross and effective section properties calculated by manual calculation are smaller than that of SCIA Engineer. Hence, the resistance of the member calculated by manual calculation will be smaller than SCIA Engineer. As a result, it is shown that the design of the particular CFS member is failed based on the manual calculation. To prove the conservativeness of manual calculation, the MASSPROP function in AutoCAD is used to compare the gross section properties of the steel sections. The table of summary of comparison is shown in Table 7 below. Based on Table 7, The section properties calculated in AutoCAD have more negligible differences than the results obtained in SCIA Engineer, implying that SCIA Engineer's calculation is more accurate than manual calculation. As a result, the manual calculation of the design of the CFS section is proved to be more conservative.

Parametric study II: Parametric study of CFS container models under different lifting conditions
For this analysis, models that pass ULS check and SLS check in Parametric Study I are used to conduct further lifting load analysis. Figure 2 shows a concept design of the lifting model used in SCIA Engineer, where P is the applied cable lifting force, and α is the declined angle measured from the roof plan. This study is initiated by the calculation of dead load and live load of the lifting load. The assumption is made that the lifting will take place when the CFS container model is fully furnished. According to the lifting load calculation, when the declined angle decreases, the dead load and live load increase [17].
The reduction of the declined angle represents the application of a short cable for lifting where 90° lifting load are excluded. It is also assumed that the model with a 90° lifting load represents a CFS container module lifted with the provision of a container spreader frame. As a result, when the declined angle is small, the internal force carried by the cable increases. Four models with the overall maximum internal compressive force at the main beam are chosen for further investigation (Models 16, 32, 48, and 64) [17]. The stability check ratio and section check ratio in ULS Check will be reviewed for these four models. Figure 3 shows the graphical representations of the four models' results where Section Check Ratio represents the ratio value of Combined Compression and Bending Check of the steel member based on EN1993-1-3: 6.1.9 [7]; and Stability Check Ratio represents the ratio value of Combined Bending and Axial Compression Check of the steel member based on EN1993-1-1: 6.3.3 [6]. Based on Figure 3, it is observed that when the declined angle reduces, the compressive force of the main beam increases. As the compressive force in the beam increases, so does the section check ratio and the stability check ratio increases until they pass the 1.0 limit, suggesting that the member failed the ULS check. Furthermore, the spacing of joists and wall studs and the steel strength also influenced the result, though the influence of these factors is insignificant. As a result, this analysis concludes that lifting of CFS container module at a 90° declining angle is the safest choice without damaging its structural members.

Conclusion
The following are the findings that correspond to this study's objectives: 1. The most cost-effective model that passes the ULS and SLS checks in SCIA Engineer is Model 36. Furthermore, it is proven that the truss arrangement of the wall studs allows the wall studs to pass ULS and SLS checks with smaller section sizes. 2. When the design results are compared to the values in AutoCAD, it is discovered that SCIA Engineer is accurate. As a result, steel section properties based on manual calculation is found to be conservative. 3. During the construction lifting process, a short lifting cable will cause a small angle of lifting. The CFS container module will be induced with a tremendous compressive force around the top beams of the container frame where it is being lifted. With inappropriate lifting techniques, premature damage could happen to the module, as shown in this study. Lifting the container module at a 90° declined angle with the help of a spreader frame is the safest alternative. It is recommended that professional measures be necessary to lift the CFS container module and be included as part of the structural engineering analysis and design stage.