Introduction:
Modular buildings are prefabricated buildings which are essentially combined from sectional modules that are manufactured in plants and then transported to the site. The modules are then combined together to form a full or part of structure. Gibb (1999) identifies three types of prefabricated buildings, which are: non-volumetric, volumetric and modular buildings. However, he argues that there is flexibility in the line which divides each type. The difference between prefabrication and modularization, however, is that prefabrication is the process of joining various structural materials to construct a building component to its final installation. On the other hand, modularization can be defined as constructing a complete structure off-site to be then transported to the site to be installed. These components are generally larger than the prefabricated elements, (Haas et al 2000).
For steel modular buildings, there are two types of modular construction regarding loading support. In the first type, the walls are the controlling factor where the compression loads are transferred through the side walls. In this case, walls are the main structural elements - see figure 1. However, in the second type of steel modular building, the corner posts are the controlling factor where the compression loads are transferred using edge beams to these corner posts - see figure 2. As square hollow sections are found to be efficient to resist compression, they are generally used for the corner posts in the second type due to their high resistance to buckling, (Lawson and Richards, 2010).
Figure 1: continuously supported module Figure 2 Corner supported module
Source: (Lawson et al 2005)
Steel has been applied in construction for 70 years, and internationally, there are many good examples of using steel in construction. In modular construction, modern examples of using steel include light steel framing elements and panel systems, volumetric modular construction, hybrid and panel systems, hybrid modular panel and primary steel frame and open-building systems (Lawson, 2005). Because some of the steel modular blocks can be designed to be partly or fully open sided, this creates further advantages of using steel in modular construction. For example, the flexibility of open sides in modular units allows for the option of creating more space by placing theses units side by side. Therefore, it is important to understand the stresses' behavior of the modular units under the load effects, so that robustness and structural integrity is guaranteed. Based on several pieces of research, this review is set to identify the performance of steel modular units under load stresses when removing longitudinal supports or corner support to identify the implications of robustness under extreme cases. This may have comparable performance with steel shipping containers under load effects when using these containers as modular units. In addition, the history and characteristics of ISO shipping containers will be reviewed in order to evaluate their structure integrity when using them as modular units.
Light steel and modular construction
For modular construction, light steel frames are used in the form of C, Z or comparable shape sections. The steel is cold formed and the yield strength is designed to meet with steel grade S280 or S350. (1999 SCI publication) consider that the sections of light steel frames are comparatively not strong if there are no restraints along the members, as insufficiency in restraints lead to torsional buckling. However, the resistance to torsional buckling can be improved by using double sections back to back or by applying sufficient restraints along the sections. The edge beams of the floor are also important structural elements during the process of construction as they provide stiffness to the modular unit. They are normally provided with anchorage points at the corners to help the unit to be lifted.
(1999 SCI) identify six modes of failure for the floor of the light steel modular units when loads are applied:
Flexure failure will occur when the floor beams or joists are continuously restrained.
Lateral torsional failure will occur when the light steel sections are retrained infrequently.
Failure may occur when lifting the unit, as lifting my cause reversible loads.
Web crushing resulted from directly applied loads or reactions.
Excessive deflection which is affected by the natural frequency.
When designing for longer span floors, the natural frequency of the floor is found to be the controlling factor. Typically, the deflection for the floor joists should be limited to 15mm to have an allowable natural frequency. In addition, when the floor beams are exposed to a sufficient frequency in the connections between the beams and the floor, these beams should be fully restrained (1999 SCI).
The load paths can be influenced by the configurations of the modules and the way in which these modules are put together. Modules, in low-rise scales, should be designed for the critical loading case. When designing for high-rise modular buildings, for more than five storeys, the lower modules are designed with closer spaces for studs as they experience heavier loads (Gorgolewski et al, 2001)
The depth of the floor and ceiling depends on the span as increasing the span will increase the depth. Generally, the depths of the floor and ceiling are within the range between 100mm and 200mm. Nevertheless, when designing for open-sided modules, the depth of the floor can be increased to 500mm because of the edge beam required for the open sided zone (Gorgolewski et al, 2001). This could a disadvantage because increasing the floor depth will generate extra weight and cost. A typical floor design is illustrated in figure 3.
Figure 3: Typical design for the floor in modular construction
Source: (Gorgolewski et al, 2001)
The edge beams have two ways of supports as indicated in figure 4. They are either supported by load-bearing walls or corner supports that have local stiffeners. For the latter case, there should a consideration to the cumulative deflection during the assessment of serviceability. In terms of the design limit for the floor joists, the Steel Construction Institute proposes three limits for the deflection of light steel floor construction as follows:
Deflection limit for imposed load δ < span/450.
Deflection limit for total load (15mm < δ < span/350)
The natural frequency should be > 8Hz
Figure 4: Corner and continuously supported modules
Source: (Gorgolewski et al, 2001)
According to Lawson et al (2005), when designing floors of light steel modular construction, the controlling factor is the serviceability performance. The transverse stiffness of the floor has significant impact on the floor reaction. This stiffness can be used to determine the number of floor joists, Neff, which are needed to support the floor using the following equation:
Neff = (L/S)*(Dx/Dy) 0.25 ………………………………………………………………………….. (1)
Where: L is the floor span
S is the spacing of the floor
Dx is the transverse stiffness of the material of the floor/ unit width
Dy is the longitudinal stiffness of the floor joists / unit width which is
Equal to EIy/S
Iy is the second moment of area of the floor joists
Dy/Dx is dependent on the depth and number of the boards of the floor.
Lawson et al (2005) also used formula 2 to determine the deflection limit for the floor for
1 kN concentrated load.
lim 6/L (mm)…………………………………………………………………………………………….. (2)
Where L is the joists span (m).
In his tests, Lawson et al (2005) concluded that there are several factors that control the level of stiffness of the floor which are as follows:
Stiffness can be enhanced by restraining the ends of the floor either by bolts or straight connections with the walls.
Stiffness can be increased by 8 to 29% by fixing the joists in the floor.
The closer the spaces between bolt connections the higher the stiffness.
The stiffness can be increased by 8% by increasing the depth of the floor from 22mm to 38mm for the floor.
To ensure that the floor is adequately stiff, Lawson et al (2005) recommends that the maximum allowable spacing for the joists of the floor can be taken from the next formula:
L=0.083(IyNeff) 0.25 meters………………………………………………………………………….. (3)
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Figure 5: Deflection of a pair of modules with removed support in the corner
When Lawson et al (2005) tested the robustness of a modular structural unit constructed using light steel framing, he found that the connected elements act as a distributor for the tensile forces where these tensile forces can be calculated from 0.7 x design factored shear force in normal condition. The test included two cases of a pair of models 7.5m x 3.6m located above each other, as shown in figure 3. In the first case, the support on one side of the models is removed except for the corner supports. In this case, the side, where the supports are removed, was able to act as a deep beam with maximum deflection of 23mm. In the second case, the removed supports were at the corner and one half of one side which led to torsional action. The deflection of the floor when subjected to 3.2kN/m2 was 19mm. The analysis of this test indicates an adequate stability of the models which are connected horizontally and vertically from their corners. This stability relies heavily on the connections in the corners. However, Lawson et al stated that there are cretin areas regarding modular construction which are still uncovered by codes, such as 'composite action with other materials, detailed aspects of design and whole building performance'.
Lawson and Ogden (2008) refer to high-level prefabricated buildings as a modern method of construction (MMC), which aims for significant advantages such as reducing the amount of time required for construction, providing higher quality and more adequate and flexible use of the space. Their research introduced a combined volumetric hybrid structure as a new idea for modular construction which can be constructed more economically. In addition, this hybrid modular system aims to achieve some of the basics of open-building systems by providing adaptability in internal planning and interchangeability of inside components. Lawson and Ogden's concept was a hybrid modular-panel with modules for the kitchen, bathrooms, stairs and the lift. These modules are used as the central part of the building. Then floor cassettes, which are supported by the modules and external walls, are extended with long spans to provide flexible areas which can serve various requirements as illustrated in figure 4
Figure 6. Hybrid building 2D and 3D. Source: Lawson and Ogden (2008)
The research tested the light steel wall panels to identify the shear capacity for these walls using different sheathing boards and brickwork cladding which were: plasterboard (internally); plywood (externally) and plasterboard (internally); cement particleboard (externally) and plasterboard (internally); steel sheeting (externally) and plasterboard (internally). Because the effect of the vertical load is relatively smaller than the horizontal one in light steel framing, only increasing horizontal load was applied at the top corner of the frame until failure occurred where the failure load was twice as much as the serviceability load. As a result, it was observed that the controlling factor for the design of the panels was the stiffness. One of the factors affecting the stiffness was found to be the spacing between the connections of the frame and walls. From the four types of walls included in this test, the cement particleboard performed the best at resisting the shearing forces. In general, the light steel frame performed adequately at shearing resistance and overall stability.
However, because the structurally bearing elements in this concept are the modules and the walls, this may constrain the flexibility of wall openings. Furthermore, this method, as indicated in the research, has a limit to the number of storeys that are allowable.
Light steel modular frames are considered to be a new form of construction by Lawson et al (2008), because this method has different ways of interconnecting elements compared with other forms of steel frames. As there is an increasing desire to understand the performance of such forms of structure, especially those under extreme cases such as sudden events that my cause failure, Lawson at el (2008) conducted a robustness test for a module of modular structure to evaluate the robustness and the structural behavior. The modular unit is fabricated using a series of c sections joined in-plan with 'top-hat' elements. A plasterboard is attached to the 'top-hat' elements and the floor. The c sections (150x40x1.6mm) were located at 400mm intervals. In general, the elements are connected using welding. Twelve water containers were used to distribute loads on the floor area located as shown in figure 5
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Ground floor with support removed at six points First floor (Points 16, 17 and 18 are horizontal transducers)
Figure 7: Water containers distribution over ground and first floor
Source: Lawson et al (2008)
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Figure 8: Maximum vertical and horizontal deflections when removing longitudinal support and corner support
Lawson at el used two modules placed as shown in figure 7 and the test included removing the longitudinal support on only one side (7.5m) in the first case where the side wall acted as a deep beam. In the second case, the support for the ground level corner over half of one side was removed to consider the performance under extreme cases. In normal cases, the four sides of the module are supported and the vertical load is resisted by the side walls. When removing the supports on one side, the maximum shearing force which acts on this side was derived from the following formula:
R = (wa b L)/4 ………………………………………………………………………………………………. (4)
wa : The accidental load+ self weight of the module.
b: The width of the module (3.6m).
L: The length of the module (7.5m).
However, in the second case when removing the support in one corner, the maximum shearing force was derived as follows:
R = (wa b L)/16 ……………………………………………………………………………………………… (5)
From the test conducted by Lawson et al (2008), it was concluded that the 'torsional stiffness' can contribute high robustness for modular units constructed using light steel frames by playing a role in redistributing loads if one element is damaged. This robustness is ensured by designed inter-connections that offer tying forces which provide alternative options for load redistribution if one of the structural elements is damaged. The load redistribution progression was associated with only small vertical displacement which did not affect the structural integrity. However, in this test, the structure has to work as a whole including the side walls and any openings in the side walls may affect the structural integrity.
Lawson and Richards (2010) in their recent research have studied the sway mechanisms of modular units constructed using light steel, so that constructing high-rise modular buildings can be evaluated. In their research they found that the corner posts of a modular unit can increase the level of compression resistance. However, the sway stiffness of the side walls is the controlling factor for posts buckling resistance.
According to Lawson and Richard (2010), the main points that should be taken into account when designing a high rise modular building are as follows:
The out of eccentricities due to construction that create extra forces and moments at base of walls.
The effect of national horizontal forces which are used to assess the sway stability for steelwork, as given in BS 5950-1.
The effect of sway stability of combined modules due to force transfer.
Corner posts' stability due to sway.
Stability of modules due to accidental damage, which can be referred to as robustness of modules.
The resistance to horizontal loads is important because if a module is unable to resist such forces, the module unit must be fixed to other modular units horizontally to achieve a stable system. This may be the case for an open sided system, because removing one of the sides can decrease the stability of a module.
Considering vertical stresses, Lawson and Richard (2010) found that corner posts for a modular unit can sufficiently enhance the resistance to compression forces when these posts are included within the unit. The corner posts can be restrained by the side walls or unrestrained in the case of open sided modules. Regarding the stability of the corner posts, increasing the vertical load can increase the out of eccentricity as illustrated in figure 8. This out of eccentricity is given by the next formula, as Lawson and Richard (2010) observed.
= δ0/ (1-(2p/pcrit) ………………………………………………………………………………. (6)
Where P is the vertical load acting on one post; δ0 is the initial out of eccentricity of the post; pcrit is the critical resistance of buckling = 0.5*k*h, where k is the shear stiffness of the wall, h is the height of the modular unit.
Corner posts and sway stability Out of eccentricity due to vertical loads
Figure 9 : Source Lawson and Richard (2010)
It was found by Lawson and Richard (2010) that to achieve stability, it is important to make sure that p is less than pcrit . As p comes close to pcrit, the shear deflection soars rapidly. As a result, the stability of the post can be examined by the next formula:
p/pc + M/Mc ≤ 1.0 …………………………………………………………………………… (7)
M is the moment acting on the floor as illustrated in figure 9 which is equal to p Mc = pc *δ
Therefore, the moment resulted from sway must be checked when designing modular units with corner posts, particularly in high rise scales.
Shipping container history
The use of standardized shipping containers was found for decades. In the late nineteenth century, British and French railway companies used large boxes made from wood to move domestic furniture. After the end of the Second World War, there was a need to develop a container with standard dimensions which could be loaded and unloaded easily between warehouse floors, vessels and rails using cranes. Around 1920, the American railroad first adopted the concept of introducing steel shipping containers with sides that dropped down. This led to a reduction in theft and goods being damaged and, by using these containers for shipments, costs decreased significantly (Levinson, 2006).
Malcoln McLean was the founder of the concept of the development of containerization. He was in the shipping business in the 1950s when he first proposed the idea of using a 33ft long container. His idea was to use strong steel posts at the corner of the container with appropriate holes at the bottom to help cranes to upload and unload these containers. In 1956, the first 58 shipping containers made their journey from New Jersey to Texas. These containers were seven times bigger than any container used normally.
Setting a high standard for ISO shipping containers was essential as it was very important to have these containers working on ships, trucks, trains and also that they were easy to exchange internationally amongst shipping companies. Consequently, the International Standard Organization (ISO) has set standard dimensions to produce standard shipping containers using high strength steel to meet with its requirements. For these requirements, the containers were standardized at 20 and 40 feet long (Lowe, 2005). However, the 24 foot container was identified to be the most suitable one by the Matson company to trade between the west coast and Hawaii (America on the move, n. d.).
The ISO shipping containers had a significant impact on the shape of the shipping industry by helping shipping to become cheaper and safer (Levinson, 2006). Nevertheless, because of the unbalanced shipping trades between eastern and western countries, there are increasing numbers of empty shipping containers available for reuse. Therefore, these containers have been used as low cost housing units, emergency shelter, military re-locatable accommodation and even for larger scales, such as schools and student accommodation (Smith, 2005). Shipping containers can provide alternative modular construction for future developments and there are good examples of using shipping containers as building components such as the two container cities in London (containercity, n. d.).[expand]
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Figure 10: the container city I which was built in 2001
Source: http://www.containercity.com/container-city-one.html
The Properties of steel used for ISO shipping container production.
As specified in the British standard (BS EN 10025- 5:2004), the ISO shipping containers are produced using a 'weathering steel' which has high level of resistance to corrosion. This steel is also known as Cor-ten steel. The steel is design to resist corrosion due to the high exposure to sea corrosive environments. Considering the strength of Cor-ten steel, this steel has been designed to have similar strength properties to steel grade S355.
How does Cor-ten steel prevent corrosion: explain?
1.4 The characteristic of ISO shipping container
There are several standards of ISO shipping containers, as identified in the British standard BS ISO 668:1995, series 1 and ISO 1496- 1: 1990 (ISO 1990). However, the most common two standards are the 20 feet (6m) and 40 feet (12m). The most widespread height is 8.6 feet (2.59m) and 9.6 feet (2.896m). The standard width of the ISO shipping containers is 8 feet (2.438m).
The main elements which give the strength of a shipping container are the four corner posts and the floor. Shipping containers can be stacked to up to nine containers high where the corner posts are used to transfer loads to corner castings at the bottom. Steel cross members (150mm) are used for the floor construction to provide strength and robustness. Welding is used at each end of the cross members to connect them to longitudinal beams that are welded to the corner posts. Consequently, the floor can sufficiently transfer loads to the corner posts. Therefore, the strength of shipping containers relies on the floor and the corner posts and any damage to these elements will seriously affect the strength of the container.
Figure 11: the corner post of a shipping container
Concerning the load capacity, in the British standard ISO 1496- 1: 1990, two loading conditions are tested. In the first condition, the four corners are loaded simultaneously where the container can carry a total vertical force of 3767kN. In the second case of the loading test, only a pair of end corners is loaded simultaneously. The vertical loading capacity in this case is 1883 kN. Figure 11 and 12 illustrates the two conditions of loading.
Expand more details:
Figure 12: Load capacities when the container is loaded at four corners
and when it is loaded at a pair of corner posts.
In addition, the roof for any container is designed to carry an imposed load for expected people working on it. Therefore, a load of 300kg is distributed on 600mm x 300mm, which is equal to 16.35 kN/m2, and it is assumed to be carried adequately by the roof (ISO 1496-1: 1990).
ISO shipping containers are found to have a sense of structural integrity as they are designed to carry heavy loads and they can be stacked to up to nine units, as indicated in the British standard (ISO 1496-1:1990). They are already transportable and available worldwide. The use of these shipping containers as structural modular components is a new idea as they can be used as low cost building units. Consequently, ISO shipping containers can be a platform for future developments in the modularization and prefabrication industry.
To conclude, although all the research detailed above were successful in addressing the feasibility of using light steel for modular construction as a basic unit, using a steel modular unit with one side fully removed has not been examined. Therefore, this project is aiming to develop a modular building constructed as a basic unit from two ISO shipping containers with fully removed sides. This will allow for more adaptability in the use of space and internal space planning. A framing system will be designed to link the two shipping containers together and to carry the redistributed loads that will have resulted from removing the sides of the containers. The framing system should aim for both ease and simplicity in construction and robustness. Therefore, the modular building constructed using this steel frame should meet with the structure integrity requirements for its use.
