Modular building system design integrating prevention and safety features

Introduction

The building industry is one of the working environments with the highest risk [1], [2]. European statistics show that the construction sector has the highest accident rates [3]. Specifically in Spain, the construction sector has double the number of accidents of all the remaining production industries as a whole [4]. Falls from heights [5] and handling heavy loads are the most serious dangers in the construction sector. In particular, handling heavy loads is becoming an increasingly important hazard factor and it is expected to account for a significant percentage of all work accidents [6].

According to the European Directive [7], more than half of all building accidents are related to architectural decisions, bad organization and planning deficiencies in design phases. Previous studies have reported a significant reduction in accidents when the design phases include properly assessed and planned work procedures [8]. How the design phase impacts on the number of construction accidents is analyzed in [9] and [10]. Although previous studies have attempted to define a quantitative and qualitative method of evaluating the features of construction projects (FCP), no final conclusion has yet been reached [11]. Therefore, no direct relation can be claimed between the European Directive’s application and a drop in the accident rate [12].

As stated by the European Directive, hazard prevention in building construction must be analyzed in construction projects. In Spain, this is done through a health and safety appendix to projects. The health and safety appendix must include the risk to which construction workers will be exposed, as well as risks resulting from machinery uses, such as load lifting or mortar mixing. However, in many cases these are generic documents that do not coincide with prevention needs and they expose workers to unnecessary risks. Although the aim of the policy is right, it needs to be implemented in an effective practical way to have a real impact on worker safety [13].

The current paper presents a new prefabricated building system, protected by patents (U201131300, U201330132) and a comparison with an equivalent conventional building system. Conventional construction methods, defined as those that use load-bearing brick walls or load blocks and unidirectional slabs for frameworks and enclosures, usually lack preventive measures implemented during the design phase, this being one of the reasons for construction workers’ permanent exposure to hazards. This lack of prevention during the product design phase is barely remediable in further project implementation phases, and it affects worker safety in terms of two main hazards: exposure to falls from heights and muscle fatigue due to load handling.

Time consumption, assembly costs and environmental sustainability are also taken into account during design phases. The high water consumption and high level of CO2 emissions characteristic of conventional construction systems are easily minimized with prefabricated modular systems, due to the possibility of waterless assembly and manual load lifting, which also reduces exposure to suspended load risks.

PtD (Prevention throughout Design) [14] – that is, the reduction of hazard-related injuries by applying safety to the design process – and Design for Construction Safety (DfCS) [15] are basic in ensuring a significant reduction in injuries, both in construction phases and on-site assembly. Given the lack of regulatory jurisdiction over PtD and PfCS and scanty knowledge or interest by project designers, integrating preventive measures in early project phases – the design of prefabricated elements – has a bigger impact on hazard prevention and planning costs.

Previous projects have made use of the PtD methodology, such as “Life Cycle Safety” (LCS) [16] or projects by the international design firm Foster and Partners, which is based in London. Both LCS and projects by Foster and Partners have shown that the early implementation of proceedings and policies in design and prevention’s consideration as a core value not only considerably improve worker safety, but they also have an important impact on project execution costs, the end quality of projects, man-hour productivity and reliability. These benefits make the implementation of the PtD methodology in prefabricated modular buildings such as the one described in this paper really interesting. [17] and [18] are other examples of the implementation of preventive measures through software applications for 4D CAD modelling.

The new construction system applies PtD to a prefabricated modular system, saving on time and assembly costs and contributing to environmental sustainability (water consumption and CO2 emissions). The implemented hazard prevention policy focuses on two main hazards: falls from heights and worker muscle fatigue due to manual load lifting.

Objectives

The main goal of the project is to integrate prevention into a prefabricated construction system, maintaining a high quality end product. PtD must be present during all construction phases of the project: components manufacturing, the building construction project/plans and final assembly work. A final real project that incorporates all the PtD methodology will check and confirm correct fulfilment of the implemented preventive measures.

The preventive objectives focus on two main aspects of the project: manual load handling optimization and a reduction in the risk of falls from heights during the assembly process. The implementation of PtD has a direct impact on the final design and assembly process of the prefabricated module presented in this paper. The quality of the end product is represented by its structural stability and thermal insulation, minimal values for which are established in the CTE (the Spanish Building Code). The structural stability for wood is defined by the SE-M document and the limitation of the demand for energy by HE-1.

The choice of materials must meet regulations set forth in the CTE, together with system requirements such as time consumption, assembly costs and environmental sustainability. The selected materials must be sufficiently strong and light to meet structural performance requirements, the insulation sufficient to meet energy demand requirements, and the system must be environmentally sustainable so as to minimize water consumption and CO2 emissions in component manufacturing phases.

Following PtD implementation studies [19], a wood structure made of GluLam and OSB (Oriented Strand Board) is chosen. GluLam is used as perimeter frame providing structural stability while OSB wood is used as enclosure for the insulating material. In addition to material selection, preventive suggestions from the previously referenced PtD study has been included such as quick panels/sections connexions and symmetric ends which simplify assembly procedures.

Manufactured panels/sections must allow for on-site waterless assembly and easy manual lifting by workers, both ergonomically and in terms of weight limitations. The assembly process must facilitate the handling of loads by two workers, with the integration of PtD. The weight of the panels/sections must be low so as to prevent muscle fatigue. The maximum load per trained worker is 40kg [20], [21] which restricts the panels/sections to a maximum total weight of 80Kg. The final weight of the panels/sections is 75kg; a weight that prevents muscle fatigue, minimizes risks from suspended loads and eliminates CO2 emissions and risks associated with the use of lifting machinery.

Methodology

The implementation of PtD not only affects manufacturing processes but all necessary phases in the achievement of a final working product, from product design to on-site assembly. An assembly guide is a method of analyzing and minimizing all possible risks that might occur during assembly procedures by means of a simulated working environment. The simulated working environment uses a typical prototype building system, Figure 1.

Figure 1. Typical prototype building system.

The basic module of the prefabricated building is made up of panels/sections that provide the desired structural stability and thermal insulation. As indicated in the section on objectives, each panel/section weighs 75kg and it is lifted by two workers, ensuring a minimal risk of muscle fatigue.

The first assembly phase is to gather the panels/sections, beams and other required items on the work level. At the start of the assembly process, this will be at ground level. In a multilevel building, material will need to be gathered on the next floor level in order to assemble the second and any subsequent levels.

The assembly process of the current prototype building system will entail four different phases in which the workers are exposed to different risks. Therefore each phase will be analyzed separately. The four assembly phases are: Foundations, Floor, Enclosure and Ceiling.

Foundations

During the foundations phase, all procedures needed to ensure a stable base and adequate load capacity are performed. This phase includes earthmoving, making the foundations and the anchoring of wood pads. This phase is not studied in the assembly guide as many factors influence a construction project’s particular needs. The health and safety appendix of the construction project takes a close look at these needs and contains a suitable prevention plan.

Floor

During the floor phase, all the ground-level building components are assembled. These include perimeter beams, support beams and floorboards. If the building has more than one level, a reduced ceiling phase will entail the creation of a new floor level. All operations performed during the floor phase are at ground level, which eliminates the risk of falls from heights.

The perimeter beams are placed on formerly anchored wood pads. The perimeter beams are made of glued laminated timber (GluLam GL24h) with dimensions of 27 x 12 x 270 cm and a weight of 31 kg. These beams should be positioned by two workers, as shown in Figure 2. The beam’s weight and the assembly instructions should minimize the risk of muscle fatigue suffered by workers during the beam assembly process.

Figure 2. Assembly process for perimeter beams.

Support beams made of GluLem, measuring 20 x 10 x 540 cm and weighing 41 kg, should be positioned on braces anchored to the perimeter beam. As with the perimeter beam, the assembly of the support beam should be performed by two workers.

The floorboards, made of OSB (Oriented Strand Board), measuring 180 x 90 cm and weighing 42 kg, should be placed in rows, as shown in Figure 3, to ensure ground level work during assembly.

Figure 3. Floorboard assembly process.

The risks to which a worker is exposed during the floor phase and the applied preventive methodology are shown in Figure 4.

Figure 4. Floor phase risks and preventive measures.

Enclosure

The prototype walls are constructed by erecting modular panels/sections. These are the heaviest components in the building assembly process. A correct ergonomic assembly process is important in minimizing the risk of entrapment or crushing. The assembly of the panels/sections should be performed by three workers, as seen in Figure 5, two of them in charge of load lifting and the third guiding and inserting the module into position.

The third ‘guider’ worker substantially increases stability during lifting procedures, which considerably reduces the risk of entrapment or crushing. Coordination by the two workers in charge of load lifting reduces muscle fatigue and the risk of material falling from a height.

Figure 5. Wall assembly procedure.

The risks to which a worker is exposed during the enclosure phase and the applied preventive methodology are shown in Figure 6.

Figure 6. Enclosure phase risks and preventive measures.

Ceiling

All the upper enclosure components will be assembled during the ceiling phase, such as perimeter bands, roof beams and roof boards. During the ceiling assembly phase, risks of falls from a height must be taken into consideration and measures should be introduced to minimize this hazard. The first task during this phase is the erection of auxiliary scaffolding, with a platform whose height ensures that ceiling elements can be assembled in comfort. In the current building, with a platform 150cm high, the workers do not have to lift any load above shoulder level and the high walls protect workers from falling.

The positioning of the perimeter bands and roof beams follow the same procedure as for the floor process: loads are lifted by two workers, ensuring correct ergonomic load lifting, and care is taken during load release. The components should be gathered at floor level and then the two workers should transfer the material to the scaffold platform as required and proceed with the assembly process.

Figure 7. Auxiliary scaffolding.

The risks to which a worker is exposed during the ceiling phase and the applied preventive methodology are shown in Figure 6.

Figure 8. Ceiling phase risks and preventive measures.

Results

The fulfilment of the PtD methodology was checked and confirmed by means of the construction of a real building project. This real assembly process observed the previous guidelines and any event not covered by the guide that occurred during the assembly process was pinpointed. Another important feature of the project’s practical implementation was the collection of time and material-related performance data, as well as the final weights lifted per worker. This then offers a reliable comparison with conventional construction methods. Outlined below are the most significant changes that were observed during the real assembly process.

Foundations

This phase did not display any differences when compared with the simulated procedure. The guidelines for the assembly process were followed without any incident of note. All the assembly procedures were performed at ground level. The time needed to gather the materials and assemble them during this phase was 12 hours. The heaviest weight that workers had to lift were the perimeter beams, with a weight of 59 kg. They were lifted by two workers, thus minimizing any risk of muscle fatigue.

Figure 9. Foundation phase.

Enclosure

During the enclosure phase, all the planned proceedings for the assembly of the walls were followed without incident. The heaviest weights in this phase were the 75kg panels/sections and the process lasted approximately 16 hours. There was a minimal risk of falls from heights during the enclosure phase since all the assembly processes were at ground level or a low level.

Figure 10. Wall assembly.

Ceiling

In the practical application of this phase, some differences from the simulated guidelines were observed. The scaffolding was erected outside the building, as seen in Figure 11. This increases the height of the platform and thus the risk of falls from heights, although it significantly increases stability. This assembly phase takes approximately 5 hours and the maximum load lifted per worker (the upper perimeter beams) was 59kg.

This phase is still unfinished, pending the assembly of roof boards and its related safety study.

Figure 11. Ceiling assembly.

Discussion

The designed system is very different from conventional construction methods. Figure 12 analyzes the building components that were involved, the required assembly time and the weight lifted per worker during each construction phase, recording both the average and maximum figure. Finally, a comparison was made of the risk of falls from heights and muscle fatigue between conventional construction systems and the prefabricated modular system presented here.

For the weight lifted per worker with a conventional construction system, the building materials normally handled by operators were taken into consideration, even if lifting machinery is available.

Figure 12. Comparison between traditional and designed systems.

The total weights that are lifted with conventional construction methods are higher than those lifted with the PtD-designed construction method. The weight in each assembly phase, weight per worker and maximum weight lifted per worker (68.5 kg versus the 37.5 kg of presented method) are higher when conventional methods are used. The assembly time - which leads to a reduction in worker exposure time to muscle fatigue - is considerably reduced with the presented method.

Ground level work with the prefabricated construction system and auxiliary means during the ceiling phase minimize the risk of falls from heights, unlike conventional construction methods where 50 percent of the work is done at a height. Unavoidable risks, such as suspended loads or entrapment, that are only found with conventional methods are mainly due to risks inherent in using lifting machinery. With the presented method, these risks have been analyzed to prevent their incidence.

A comparison of the risks to which workers are exposed highlight fundamental differences in both methodologies, with a substantial improvement being observed in comparison with current building methods.

Figure 13 offers a summarized comparison of the assembly hours of both conventional systems and the one presented in this paper, as well as the time needed to lift loads of over 40 kg and the total weight lifted per worker.

Figure 13. Summary of worker and load lifting times.

Conclusions

A modular prefabricated system has been designed, using patented panels/sections and the application of the PtD methodology. The integration of preventive measures into a prefabricated assembly system has an impact on the assembly process, reducing the risk of muscle fatigue and falls from heights. By integrating safety measures into design phases, times and assembly costs (worker efficiency, safety measures etc.) were considerably reduced.

Assembly guidelines have been proposed, aimed at analyzing and planning safety measures prior to the on-site construction process. These guidelines try to systematize the assembly procedure, eliminating the element of improvisation and a lack of safety measures that can lead to unexpected risks. By limiting the total weight lifted per worker, the risk of muscle fatigue inherent in conventional construction methods is greatly decreased. By planning work areas during all assembly phases and guiding the workers in tasks, the risk of falls from heights has been reduced.

Lastly, the construction of the modular prefabricated system was put into practice. Through this real assembly process, the efficient integration of preventive measures can be checked and confirmed while also highlighting possible unexpected eventualities during the definition of the guidelines.

References

• 1. Jannadi, O. A., BuKhamsin, M. S. (2002); Safety factors considered by industrial contractors in Saudi Arabia. Building and Environment, 37(5), 539–547. doi:10.1016/S03601323(01)000567
• 2. Tam, C. M., Zeng, S. X., Deng, Z. M. (2004); Identifying elements of poor construction safety management in China. Safety Science, 42(7), 569–586. doi:10.1016/j.ssci.2003.09.001
• 3. EUROSTAT (http://appsso.eurostat.ec.europa.eu/nui/show.do?dataset=hsw_aw_inaws&lan..., consulted el 22/02/14)
• 4. De la Orden, M. V., D. A., C., Zimmermann, M. (2010); Estudio sobre perfil demográfico, siniestralidad y condiciones de trabajo (p. 44). Spain: National Institute for Occupational Health & Hygiene.
• 5. Adam, J. M., Pallarés, F. J., Calderón, P. A. (2009); Falls from height during the floor slab formwork of buildings: Current situation in Spain. Journal of Safety Research, 40(4), 293–299. doi:10.1016/j.jsr.2009.07.003
• 6. EUROSTAT (http://appsso.eurostat.ec.europa.eu/nui/show.do?dataset=hsw_aw_pa1&lang=en consulted  22/02/14)
• 7. COUNCIL DIRECTIVE 92/57/EC of June 24th 1992 on minimum health and safety requirements at temporary or mobile construction sites.
• 8. Frijters, A. C. P., Swuste, P. H. J. J. (2008); Safety assessment in design and preparation phase. Safety Science, 46(2), 272–281. doi:10.1016/j.ssci.2007.06.032
• 9. Gambatese, J. A., Behm, M., Rajendran, S. (2008); Design’s role in construction accident causality and prevention: Perspectives from an expert panel. Safety Science, 46(4), 675–691. doi:10.1016/j.ssci.2007.06.010
• 10. Hatipkarasulu, Y. (2010); Project level analysis of special trade contractor fatalities using accident investigation reports. Journal of Safety Research, 41(5), 451–457. doi:10.1016/j.jsr.2010.08.005
• 11. Manu, P., Ankrah, N., Proverbs, D., Suresh, S. (2010); An approach for determining the extent of contribution of construction project features to accident causation. Safety Science, 48(6), 687–692. doi:10.1016/j.ssci.2010.03.001
• 12. Martínez, M., Rubio, M., Gibb, A. (2010); Prevention through design: The effect of European Directives on construction workplace accidents. Safety Science, 48(2), 248–258. doi:10.1016/j.ssci.2009.09.004
• 13. Baxendale, T., Jones, O. (2000); Construction design and management safety regulations in practiceprogress on implementation. International Journal of Project Management, 18(1), 33–40. doi:10.1016/S02637863(98)000660
• 14. DEPARTMENT OF HEALTH AND HUMAN SERVICES; Centers for Disease Control and Prevention. National Institute for Occupational Safety and Health November 2010; Prevention through Design. Plan for the national initiative. DHHS (NIOSH) Publication No. 2011–121.
• 15. Szymberski, R. (1997); Construction Project Safety Planning, TAPPI Journal, 80(11), 69 – 74.
• 16. Hecker, S., Gambatese, J., and Weinstein, M, 2004; Life cycle safety:  An intervention to improve construction worker safety and health through design. UO Press, Eugene, Ore.
• 17. Benjaoran, V., Bhokha, S. (2010); An integrated safety management with construction management using 4D CAD model. Safety Science, 48(3), 395–403. doi:10.1016/j.ssci.2009.09.009
• 18. Hu, Z., Zhang, J. (2011); BIM and 4Dbased integrated solution of analysis and management for conflicts and structural safety problems during construction: 2. Development and site trials. Automation in Construction, 20(2), 167–180. doi:10.1016/j.autcon.2010.09.014
• 19. Tymvios, N., Gambetese; J. (2013); Prevention through Design (PtD) in Wood Design and Construction. Wood Design Focus 23(1). ISSN 10665757
• 20. COUNCIL DIRECTIVE 90/269/EC of May 29th 1990 on the minimum health and safety requirements for the manual handling of loads where there is a particular risk of bad injury to workers.
• 21. Ministry of Labour. National Institute for Occupational Health & Hygiene, Spain; Guía técnica para la evaluación y prevención de los riesgos relativos a la Manipulación manual de cargas (2003)