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Revista ingeniería de construcción

versão On-line ISSN 0718-5073

Rev. ing. constr. v.24 n.2 Santiago ago. 2009

http://dx.doi.org/10.4067/S0718-50732009000200004 

Revista Ingeniería de Construcción Vol. 24 N°2, Agosto de 2009 www.ing.puc.cl/ric PAG. 167- 180

 

Technical and economical simulation of two roofing construction systems in southeast Mexico

 

Julio R. Baeza Pereyra**, Sergio O. Alvarez Romero*

* Universidad Autónoma de Yucatán, MÉXICO

Corresponding author:


ABSTRACT

The use of prefabrication in construction apparently is not a economic alternative if compared with more labor intensive construction methods in developing countries. This work presents a comparison study between two choices of roofing systems for massively housing construction in southeast Mexico. The traditional labor intensive roofing construction and the prefabricated roofing construction system coded L-18 were modeled using the computer program EzStrobe simulation software. A technical and economical analysis was performed on both construction methods, comparing the relationship between resources, direct cost, and indirect administrative cost. In reference to the direct cost, the prefabricated construction method cost was higher than the traditional method; however, the prefabricated system is capable of adjusting its selling price in the production factory according to a given construction volume.

Keywords: Computer-assisted simulation, direct cost, indirect cost, comparative analysis


 

  1. Introduction

The curren roofing construction system for housing in southeast Mexico, has been the same since long ago, the labor item that represents the highest percentage of the housing total direct cost, according to the study developed by Silvia Campos (Campos, 1985). As far as the execution time and the quality of this labor item are concerned, some deficiencies have been found, mainly derived from the lack of a proper supervision on the material delivered by the supplier, the lack of a managing control system, and construction precautions standards are not met (Gutiérrez, 1995). Besides, there are some other problems such as the waste amount of raw material, labor hand effort and the lack of productivity during waiting time while roof hardens and the centering support is employed. (Pinzón, 1990) (Figure 1).

Figure 1. Traditional system of ceiling hoist and vault in a ribbed slab

On the other hand developed countries have proved so far, that the trends for constructive processes are centered in precasting, since it is a more industrialized and supportable construction method; thus representing great savings, profits and benefits for construction companies, as well as for clients, society and the environment (Yee, 2001).

Figure 2. Precast roofing, code L-18 (assembly process)

 

The Engineering Department at Universidad Autónoma de Yucatán has made some efforts to create new diagrams and models to carry out a quantitative comparison between the traditional roofing construction method for massive traditional housing (Baeza, 2008) and the precast roofing method, coded L-18 (Sosa et. al, 2007). Both studies collected information about labor hand productivity and the equipment involved in the construction. A workflow study was made in order to determine the method that both processes followed. Such information was used to create simulation software to each individual model.

The referred models were run in a PC by using the EzStrobe simulation software (Martinez, 1998). Sosa et al. found that the system coded L-18 would take 299.18 minutes (app. 5 hours) to cover a lot of 4 roofs. On the other hand, Baeza found that using the same traditional roofing model in the region, the process would take 842 minutes (which is equivalent to 14 hrs in a long day's work or 1.75 eight hours in a day's work).

It is obvious that method L-18 represents a great advantage as far as execution time is concerned, since it is cheaper than the traditional method. However, this saving is not translated into an effective cost saving, especially in the southeast Mexico region, where labor hand is a unexpensive. The use of machinery has an initial investment cost higher for rented machinery than the use of labor hand only. (Sosa, 2005)

As described above, in order to compare these two systems, it is required to learn about the behavior at a massively construction level, beginning with an unproved theory: if the volume of massively housing construction is large enough, there would be a balance point between both constructive processes, where one of them will be more economic than the other in financial terms.

2. Methodology

In order to compare these two constructive systems the following steps were taken.

1. An adjustment to Baeza' s and Sosa's simulation models were made, so as to make them fit into a massively construction context1.

2.  In order to obtain the resources costs, Sosas thesis was employed (Sosa, 2005), which contains the information about direct and non-direct operation costs for each constructive systems.

3.  From such study the work diagrams and necessary resources where obtained for the simulation of both constructive systems.

4. The two constructive methods were simulated, first programming the models to obtain the roofing constructions execution time expressed in days work, for both constructive processes.

5.An analysis on unit prices per roofing constructed was made for both constructive processes. Another analysis was also made related to Non-direct Cost per Direct Production Cost in both cases.

3. Results

3.1 Simulation Models

Initially some modifications were made for the system Sosa et al., 2007, in order to make it fit into the massively construction simulation context. This modified model is shown in Figure 3.

Some improvements were made to the model presented by Soza, Baeza and Arcudia (2007); thus Figure 3 includes an entry point where information can be modified, such as number of houses, number of equipments, number of workers, as well as delays due to different kinds of nature.

Figure 3. Massively Housing Roofing Construction Model using the precast system L-18

Table 1. Details of model in figure 3, massive system L-18

The original model used for the traditional method is explained in Baeza (2008). Such model was modified as done with Sosa et. al., 2007 model, including nodes to register number of workers, concrete mixers, delays due to lack of raw material and inspections. The resulting model is shown in Figure 4. Table 2 shows detailed information of the model.

Figura 4. Massively Roofing Housing Construction Model, using the traditional ceiling joist and vault system

Table 2. Details of model shown in Figure 4, massively traditional method

3.2 Simulation costs

Considering a given 45 m2 roofing lot per housing unit, the average direct cost used for this work was taken from Sosa s mastery thesis. (Sosa, 2005). Prices are expressed in mexican pesos of 2005.

Table 3. Statistical Costs for traditional roofing system simulation

 

Tabla 4. Statistical cost for precast system L-18 simulation

A middle size local construction company, that builds an important number of housing units per year, provided the non-direct costs which were processed according to Sosas thesis calculation (Sosa, 2005); thus obtaining the unit index. Regarding the non-direct costs involved, the most important non-direct items for a construction company also vary as far as time is concerned (office and field salaries, services payment and operational costs). The amount of non-direct cost provided by the company researched by Sosa is $507.27

3.3 Work Plan

In an interview the building supervisor explained which would be the supply chain methods required to develop several construction volumes. Valuable data obtained from such interview were used on the resources study required for the simulation. Tables 5 and 6 show the resources diagrams used for the simulation on both construction systems.

Table 5. Productivity analysis for the traditional roofing system

Table 6. Productivity analysis for the precast L-18 roofing system

3.3 Cost Analysis for Both Systems

In this stage a cost analysis was made in order to determine the Unit Price ratio per each roofing system. The results are shown in Tables 7 and 8.

Table 7. Unit Costs Analisys for the traditional system

Table 8. Unit Costs Analysis for the the precast L-18 roofing system

The results of combining data from column Total Unit Prices per Roof in Tables 7 and 8, versus the corresponding roofing number, are shown in Figure 5.

Figure 5. Unit Prices Massively Construction Housing Roofing System, using the traditional system and precast L-18system

According to this graph, a point of balance seems to be found. Regarding unit cost, Figure 5 shows that the cost of the L-18 system becomes lower than traditional system when the company builds 65 roofing systems ot more.

The percentage ratio from Non-Direct and Direct Cost Columns in Tables 7 and 8 (DC/NON-DC), versus the corresponding roofing number is shown in Figure 6.

Figure 6 shows that the percentage relation DC/NON-DC keeps below 60% for precast L-18 system. Besides, this index tends to increase for traditional ceiling hoist and vault system.

Figure 6. Percentage Ratio between Direct and Non-Direct Costs

4. Discussion

Proposed models can be improved, but they would become the next step for comparing traditional systems to others expressed in the same way as the ones studied by (Sosa et. al, 2007). However, models shown in this study were based on previous works. These models already take into consideration data related to resources and non-direct costs.

The information about resources for both systems was obtained from an interview with the building supervisor. However data can be improved by interviewing more workers in order to count with the exact and complete information for the simulation of two roofing systems.

Even though the use of precasting in the housing roofing units construction is quite expensive, savings can be made when the construction volume is high. This study can be taken into consideration, as a starting point to improve processes in construction companies at the southeast Mexico region. The balance point found between two systems determines the financial profit a construction company obtains.

Although traditional system has the advantage of being more economic (compare column E in Tables 7 and 8), this saving is only apparent for the construction company (See Figure 6). The relation direct costs and non-direct costs indicates that the company could even pay more in administrative issues than the production direct cost (close to 100% for 65 roofing systems).

5. Conclusion

Execution period for roofing systems in a housing construction project, happened to be more productive when using the precast system than the ceiling joist and vault system for massively construction volumes. The total unit price per roofing system, for a construction company, during massively housing construction involves the addition of two partial unit costs: direct cost that includes labor hand and raw material, and non-direct administrative cost that includes office and field expenses, services payment and operational costs.

As far as direct cost is concerned, the precast system is higher than the ceiling joist and vault system, however, the precast system is well able to adjust and to adapt its selling price based on a given construction volume, as seen in Figure 5, thus resulting a decrease in the direct percentage (Figure 6).

6. References

 

Baeza Pereyra y Julio R. (2008), Modelo de simulación para fabricar losa tradicional en la región sureste de México, Ingeniería y Universidad, Pontificia Universidad Javeriana, Vol. 12, num. 1         [ Links ]

Campos Silvia (1985), Costo de una vivienda de interés social, monografía inédita, Universidad Autónoma De Yucatán.         [ Links ]

Gutierrez Aída (1995), Defectos de procesos constructivos, monografía inédita, Universidad Autónoma De Yucatán.         [ Links ]

Martínez J. C. (1998), "EZStrobe - General-Purpose Simulation System Based on Activity Cycle, Diagrams", Proceedings of the 1998 Winter Simulation Conference held in Washington, D.C., December 13-16.         [ Links ]

Pinzón José (1990), Estudio de rendimientos de mano de obra en la construcción de viviendas de interés social, tesis inédita, Universidad Autónoma De Yucatán.         [ Links ]

Sosa Canto Jorge A., Baeza Pereyra Julio R. y Arcudia Abad Carlos E. (2007), Modelos para la simulación computarizada del proceso constructivo de losa prefabricada L-18, Ingeniería y Universidad, Pontificia Universidad Javeriana, Vol. 11, num. 1.         [ Links ]

Sosa Canto Jorge A. (2005), Evaluación técnico-económica entre dos tipos de losa mediante un proceso de simulación, tesis inédita, Universidad Autónoma De Yucatán.         [ Links ]

Yee Alfred ( 2001), Structural and economic benefits of precast/prestressed concrete construction, Pc Journal (Illinois) 46:, num 4.        [ Links ]

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