Productivity improvement of precast shop drawings generation through BIM-based process re-engineering

Automation in Construction 54 (2015) 54–68

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Automation in Construction journal homepage: www.elsevier.com/locate/autcon

Productivity improvement of precast shop drawings generation through BIM-based process re-engineering Tushar Nath a,⁎, Meghdad Attarzadeh a, Robert L.K. Tiong a, C. Chidambaram b, Zhao Yu c a b c

School of Civil and Environmental Engineering, Nanyang Technological University, N1-01a-29, 50 Nanyang Avenue, 639798, Singapore Centre for Construction IT, BCA Academy, Building and Construction Authority, 200 Bradell Road, 579700, Singapore HDB Building Research Institute, Housing and Development Board, HDB Hub, 480 Lorong 6 Toa Payoh, 310480, Singapore

a r t i c l e

i n f o

Article history: Received 20 August 2014 Received in revised form 5 March 2015 Accepted 6 March 2015 Available online 28 March 2015 Keywords: Building Information Modelling Process re-engineering BIM-based technologically-enhanced workflow Precast shop drawing generation Parametric BIM components Value stream mapping and analysis Lean construction Processing time Total time

a b s t r a c t Building Information Modelling (BIM) has been identified as a key computer aided technology that facilitates construction productivity enhancements through the elimination of various construction inefficiencies. Identifying these inefficiencies, their source, and their potential remedies offers a foundation for process re-engineering. The objective of this research was to identify the constraints in the present workflow for precast shop drawing generation and to propose a BIM-based technologically-enhanced workflow that would address those constraints. In order to facilitate in shortening the shop drawings generation process, (1) key precast elements were developed as set of parametric BIM components; and (2) value stream maps spanning the construction disciplines were used to recognize the mechanisms that drive the prospective changes leading to potential productivity enhancement through the implementation of lean construction principles. The value-added, non-value added, and queue times for the present and the proposed future workflows were determined from the analysis of the value maps. Through the implementation of the proposed future workflow, for the generation of precast shop drawings, the research concluded that there would be a significant overall productivity improvement for processing time and total time. © 2015 Elsevier B.V. All rights reserved.

1. Introduction 1.1. Background The construction industry in developed countries is experiencing a major shift from the traditional methods of intensive manual labour towards the utilization of automation that have been made possible through the use of information technology. This trend results in enhanced construction efficiency leading to improved productivity in terms of reduction in wastage, errors, and rework [2,37]. There is an increased awareness amongst the precast industry and also the government of Singapore regarding the adoption of advanced engineering technologies that might prove imperative as far as increasing productivity of the construction sector is concerned [6–11]. In Singapore, for sustained economic growth to be achieved, the Building and Construction Authority (BCA) of Singapore has set up a productivity improvement target of at least 20–30% by the year 2020. To improve the situation, BCA has identified and mandated Building Information Modelling (BIM) as a key technological tool to improve ⁎ Corresponding author. Tel.: +65 97568463. E-mail address: [email protected] (T. Nath).

http://dx.doi.org/10.1016/j.autcon.2015.03.014 0926-5805/© 2015 Elsevier B.V. All rights reserved.

productivity. One measure to achieve the target is for the public sector agencies like the Housing and Development Board (HDB) to take the lead in order to facilitate the adoption of BIM [6–11]. For a typical HDB project, approximately 70% of the job involves precast construction. The precast process facilitates in reducing the construction time and labour requirement, provides better quality control, and guarantees precise management of material [19]. In Singapore's context, the current challenges for precast construction are: 1) issuance of tender drawings in 2D PDF format, 2) longer shop drawing submission and approval time, 3) receipt of details regarding the mechanical, electrical and plumbing services at a later stage of the project by the precasters, and 4) project coordination at a later stage [14,20,27]. In order to facilitate better construction management, HDB seeks to leverage the BIM capabilities to improve the construction productivity through increased support for standardization of precast elements which leads to error-free generation of shop drawings. 1.1.1. Building Information Modelling According to the United States National Institute of Building Sciences (NIBS), BIM is a process that represents the geometrical and functional properties of a building. It serves as a resource for gathering relevant information required for collaboration by the project stakeholders during

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different phases of the project [18,37]. In other words, BIM is the process of creating, saving, and reusing the design models and drawings in 3D format that facilitates in updating, editing, and modifying the information in BIM process [5,18,23]. The idea of “pre-building” a precast project in the virtual world of BIM software is valuable, and BIM technology is needed for precast industry to ensure that the geometry, details, and connections within the model are correctly placed and coordinated. The various BIM platforms facilitate the construction of a virtual digital building that contains an unambiguous geometric description of the architectural and structural design intent, guarantees that all documents, including drawings, are spatially consistent and eliminates most spatial conflicts [15,17]. Some of the expected benefits of utilizing BIM technology for precast industry are [33]: • Improvement in project definition during tender submission — improved 3D visualization enhances the presentation capabilities of project proposals to users/clients which results in properly defined projects. • Enhanced accuracy in cost estimation — the estimation of projects can be done in a more detailed and an accurate manner which may result in reduced contracting risks. • Customer services enhancement — better responsiveness to changes or alterations, referred as change management, in the facility as per the requests from clients/users which helps in significantly reducing the lead time. • Streamlined logistics — by integrating the BIM model with resource planning, it helps in reducing internal communication errors and costs leading to better coordination, and helps in enhanced control over management of components inventories. • Reduction in errors — it enhances productivity by facilitating reduction in errors due to 2D drafting and design.

1.1.2. Shop drawings generation During the last three decades there have been significant transformations leading to enhanced accuracies for generation of construction drawings. The initial change was from hand drafting to 2D CAD (Computer aided drafting/design) process. Recently, by introducing BIM, this process has been further improved [5,36,38,39]. For precast construction, automated preparation of drawings, bills of quantities, and other reports are of high priority. Hence, for a system to be compliant, it must be easily configurable by the end users for the way the drawings are composed, dimensioned, and annotated [35]. Shop drawings illustrate the procedure in which the precast firms intend to implement the concepts of consultants tender design drawings for prefabrication. A shop drawing serves the purpose of eliminating the deficiencies in a construction project. Delay in generation of shop drawings may eventually lead to delay in precast element production [1]. In Singapore's context, as per the current processes, the consultant forwards the tender drawings to the main contractor who further passes it on to the appointed precast firm. The precaster then generates the shop drawings and is obligated to pass it back to the main contractor and consultants for comments and approval. Only after the approval is obtained on these shop drawings, the production of the precast elements can commence [14,20,27]. 1.1.3. Process re-engineering and lean construction principles According to Champy [13], the term process re-engineering is defined as “the science of using technologically enabled processes in order to connect various disciplines to achieve efficiency enhancements leading to creation of value for everyone involved”. One of the major hindrances, as identified by Ohno [28], which prevents lean construction, is the wastage occurring in terms of time. As identified by Koskela

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[24] lean construction can be achieved by eliminating wastage in the following ways: • • • • •

Reduce time taken for the non-value-added activities Lead time reduction Variability reduction Minimizing the number of steps, parts, and linkages Flexibility enhancement.

The transformation of the present workflow towards a lean process requires restructuring and elimination of wastage from the system of interdependent activities performed in order to generate the precast shop drawings. These sets of activities make up the value stream. A value stream perspective pans across individual activities, functions, and disciplines and focuses on the overall performance of the workflow. Value streams are mapped and analyzed using value stream mapping (VSM), which is a technique developed at Toyota to counter wastage [32]. VSM includes creation of a systematic workflow with associated information for the production process. The present workflow serves as a basis for the development of the proposed future workflow which is expected to streamline the precast shop drawing generation process while eliminating wastage in terms of time. The differences between the present workflow and the proposed future workflow provide a roadmap that may lead to productivity enhancements through the implementation of BIM technological tools. One of the major factors that influences the lead time is the queue or the wait time [21]. Queue time is greatly impacted by the decision making time [3] which is the time needed to grant approval. Thus, in order to compress the lead time it is of utmost importance to identify and reduce the causes that delay the decision making process. A shorter lead time leads to reduced disruption due to design changes, enhanced collaboration, and reduction in wastage of time and material [24]. 1.2. Research objectives The objective of this research was to conduct process mapping for the generation of shop drawing as per the workflow followed by the construction industry in the present scenario, and identify the areas of improvement where BIM tools can be implemented in order to propose a streamlined and a coordinated workflow leading to productivity enhancement. This was achieved by leveraging BIM capabilities which facilitates in easing the level of integration and collaboration across the various disciplines in the precast construction industry value chain. As part of the proposed workflow, a set of key HDB precast elements were developed as standardized parametric BIM components on top of the three commonly used BIM platforms in Singapore i.e. Autodesk Revit, Tekla Structure, and Nemetschek Allplan Precast. These components, developed based on HDB's standard guide drawings (sample of a guide drawing is shown in Fig. 2), facilitate the automated generation of shop drawings and reinforcement schedules in accordance with HDB's requirements. The implementation of the proposed future workflow is believed to facilitate productivity enhancements through lean construction. Through the comparison of the present and the proposed future workflows, the significance of this research was to evaluate the key performance indicator (KPI) — expected time reduction for the activities related to precast shop drawing generation. The project was done based on the following assumptions. 1) In Singapore, only a few precasters are well acquainted with the BIM technology, however, the three precast firms who were involved in this research have been using BIM tools in their organization for more than two years. Hence, they have a capability to judge on the expected productivity enhancements through BIM implementation in the proposed future workflow. 2) In order to validate the productivity gains for the entire workflow, the proposed future workflow needs to be adopted

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fully by Singapore's construction industry. The adoption of the proposed future workflow needs to be mandated by the construction authorities in Singapore which is not in the scope of this research project. This research provides a recommendation in terms of the usage of BIM which may lead to productivity gains in the future. 3) In terms of the software platforms, Autodesk Revit is employed by the consultants in Singapore, whereas the precast firms may utilize the capabilities of either Tekla Structures or Nemetschek Allplan for generation of shop drawings.

2. Research methodology This project adopts a quantitative research approach [30] to determine the impact of BIM on the productivity of precast companies working on HDB projects. The procedure included the following steps as shown in Fig. 1. First, an online survey was conducted through an open-ended selfcompletion questionnaire to acquire initial information and determine the prevalent issues and constraints on the present workflow from the precast firms involved in this research. The project data was collected over a period of one year by conducting in-depth technical interviews which included face-to-face meetings, telephonic interviews with follow-up calls and corresponding email exchanges. Discussions were structured around the available project data and also based on the familiarity of the precasters' through implementation of BIM in their organization. Second, a set of key HDB precast elements were developed as standardized parametric BIM components. The components were prepared to a level of details that met HDB's standard guide drawings' requirements e.g. the geometric details, connection details, and rebar details. Based on the information gathered through surveys and interviews, the present workflow was mapped and a BIM-based technologically-enhanced workflow was proposed to address the constraints in the present workflow. Value stream mapping and analysis

were performed in order to determine the value added and nonvalue added times. Third, a technical workshop was conducted over a period of 1.5 days to share the project deliverables with the technical managers, design engineers, BIM modellers, and draughtsmen of the three precast firms as well as representatives from HDB and BCA. The established targets for the workshop were, 1) to explore best practice for the application of BIM tools for collaboration between consultants, main contractor, MEP sub-contractors, and the precast fabricators, and to highlight the shortcomings of the present workflow, 2) to document the processes and productivity achieved through value stream mapping, and 3) to measure, from the standpoint of the precaster, the saved time related to the activities as per the BIM-based technologically-enhanced workflow. The data collected were based on the “estimated duration” observed from the previous precast projects for the present workflow and “expected duration” for the proposed future workflow. However, the data collected for the proposed workflow was approximate and was based on the experience of the interviewees. Fourth, the parametric BIM components were utilized in precasters' pilot projects in order to verify the productivity improvements for shop drawing generation. The other activities as per the workflows were not considered for validation through this pilot project because in order to validate the productivity improvements for the entire workflow, the proposed future workflow needs to be mandated by the construction authorities in Singapore, however, based on the experience of the precast firms, the expected values for the activities were obtained (through mock exercises for certain activities during the technical workshop) which facilitated in evaluating the potential productivity improvement for the entire workflow. 3. Parametric design, parametric BIM components The functionality of parametric modelling has been increased over the past decades [25,26]. Parametric design and modelling was

Fig. 1. Research project methodology.

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originally developed as a solution for reuse of existing designs. BIM software, which is powerful in terms of its ability to manipulate parametric model, is utilized to create 3D parametric models that includes both geometric and non-geometric designs and construction information. Every alteration and modification made to an element in BIM model is automatically propagated through the model to keep all components, views and annotations consistent. Modelling buildings in fully parametric 3D computer-aided design offers numerous benefits in terms of productivity, the ability to rapidly generate design alternatives, and elimination of errors that result from the disparity between different drawings in current practice [33,34]. The automated modification is usually termed as the “behaviour” of the parametric components. This behaviour differentiates the parametric BIM process from the traditional 2D process which is the main source for productivity gains [33]. Most of the former dimension-driven CAD systems were just able to deal with situations where dimension values do not affect a variation of the topology with respect to the master model, the model that covers the complex structural configuration. Topology in this context means the adjacent relationship of geometric elements. Roller [31] presented an advanced method for the creation of parametric models in computer-aided design systems by using automatic storage of geometric constraints during the design input, and the support of topology parameters. According to him, together with a record of the construction sequence, key information about the designer's intent can be captured, and a more comprehensive description of a design can be achieved. He introduced a method for the interactive generation of parametric design with structural parameters in addition to dimensional parameters using constrained geometry which resulted in a substantial increase in the efficiency of interactive design with CAD systems, as significantly more designs can be generated automatically out of a master model. Sacks et al. [34] surveyed technical issues associated with the use of parametric solid modelling to design buildings at construction level of details by focusing on the specific characteristics of building design. For instance, the structural connections were modelled with parametric relations, such that any changes in one element would be propagated to change the corresponding elements appropriately. 3D parametric building modelling, with embedded assembly, piece and component function and behaviour provides a new level of support for building design automation. Lee et al. [25] explored the extent to which design and engineering knowledge can be practically embedded in production software for BIM. They focused on a building object behaviour (BOB) description notation and method, developed as a shorthand protocol for designing, validating and sharing the design intent of parametric objects.

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The following components were developed during the course of this research project, Duct, Plank, Infill Wall, Gable End Wall, Staircase, Sunbreaker, Air-Con Ledge, Parapet Wall, Household Shelter, Façade Wall, and Refuse Chute. In this paper, as a sample, the concept of parametric BIM components is briefly represented through the case of a façade wall. In the specific context of precasting, BIM technological tools allow users to create precast elements as parametric components with defined parameters and attributes. In other words, it enables precast elements to be parametric in the sense that any alterations in the preprogrammed components reflect the changes as per the design requirements to maintain consistency between the elements automatically. The parametric BIM components, as pre-defined parametric objects, are an effective, extensible and flexible means to embed expert knowledge and domain semantics in a parametric modelling system. As a result, the system is more interactive and automated towards engineering design leading to a well-defined configuration which further leads to standardization [16]. It enables users to create customized components and define their detailing rules using parametric objects and constraints. A new case of a component could be produced via a userinterface by allocating new values to a set of pre-defined parameters. This facilitates cooperation and collaboration between the parties involved and ensures that all information such as areas, volume and rebar schedules etc. are updated dynamically when changes in the model are made. The following illustrative example shows the complex parametric behaviour required to support the typical operations a precast modeller must perform in order to build and maintain a precast parametric BIM component. As shown in Fig. 2, the parameter “OpeningHeight” is dependent on parameters “BeamPositionFromFloorLevel” and “OpeningSillHeight” via Eq. (1): OpeningHeight ¼ BeamPositionFromFloorLevel–OpeningSillHeight: ð1Þ Hence, changes in the dimensions of any one of the two parameters on the right hand side of the above equation will automatically propagate the changes in the opening height. The integration in geometry is achieved based on the capability of BIM software that allows connecting/binding different portions of component with the referencing lines/planes. The complete parametric definitions for this component are described in Table 1 and Fig. 5. In a 2D environment, there are multiple drawings to address the geometric details of the precast façade; a sample HDB guide drawing in 2D format for a typical case of a precast façade is as shown in Fig. 3. Based on the 2D guide drawings for key HDB precast elements, the 3D parametric BIM components were developed for the Revit, Tekla, and

Fig. 2. Geometric integration — façade wall with one opening.

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Table 1 Parameters/attributes definition — façade wall with one opening in Autodesk Revit. Parameters/attributes name

Parameters/attributes definition

Graphics HasTopConnection HasSideTopConnection2 HasSideTopConnection1 HasSideConnection2 HasSideConnection1 HasBottomConnection

Toggle On/Off; This parameter determines if the connection on the top is required or not Toggle On/Off; This parameter determines if the top connection on the right side of the wall is required or not Toggle On/Off; This parameter determines if the top connection on the left side of the wall is required or not Toggle On/Off; This parameter determines if the side connection on the right side of the wall is required or not Toggle On/Off; This parameter determines if the side connection on the left side of the wall is required or not Toggle On/Off; This parameter determines if the connection on the bottom of the wall is required or not

Dimensions TopConnection⁎

[= if(HasTopConnection, (WallLength), 0 mm)]; This is a dependent parameter, it is provided to control the visibility of the top connection of the wall SideTopCutWidth Determines the width of the side cut on the top of the wall SideTopCutDepth Determines the depth of the side cut on the top of the wall SideTopConnection2⁎ [= if(HasSideTopConnection2, (WallThickness), 0 mm)]; This is a dependent parameter, it is provided to control the visibility of the right side top connection of the wall SideTopConnection1⁎ [= if(HasSideTopConnection1, (WallThickness), 0 mm)]; This is a dependent parameter, it is provided to control the visibility of the left side top connection of the wall ⁎ SideConnection2 [= if(HasSideConnection2, (WallHeight), 0 mm)]; This is a dependent parameter, it is provided to control the visibility of the connection to the right side of the wall SideConnection1⁎ [= if(HasSideConnection1, (WallHeight), 0 mm)]; This is a dependent parameter, it is provided to control the visibility of the connection to the left side of the wall OpeningStartDistance Determines the starting position of the window from left wall edge WallThickness Determines the thickness of the wall WallLength Determines the length of the wall WallHeight Determines the height of the wall TopCutWidth Determines the width of the cut on the top of the wall TopCutDepth Determines the depth of the cut on the top of the wall OpeningSillHeight Determines the opening position of the window from the bottom of the wall OpeningHeight⁎ [= BeamPositionFromFloorLevel — OpeningSillHeight]; This is a dependent parameter, it is provided to highlight the height of the window opening OpeningEndDistance Determines the starting position of the window from right edge of the wall EndSideCutWidth Determines the width of the cut on left side of the wall StartSideCutWidth Determines the width of the cut on right side of the wall SideCutDepth Determines the depth of the cut on the sides of the wall CopingWidth Determines the width of the coping CopingFaceDepth Determines the depth of the coping at its face CanopyWidth Determines the width of the canopy CanopyStartDistance Determines the starting position of the canopy from left edge of the wall CanopyFaceDepth Determines the depth of the canopy at its face CanopyEndDistance Determines the starting position of the canopy from right edge of the wall CanopyAngle Determines the angle of the canopy slope from the horizontal CalculatedTopCutWidth⁎ [= TopCutWidth — 5 mm]; This is a dependent parameter, it is provided to control the width of the cut on the top of the wall. CalculatedTopCutDepth⁎ [= TopCutDepth — 10 mm]; This is a dependent parameter, it is provided to control the depth of the cut on the top of the wall. CalculatedSideTopConnectionWidth⁎ [= SideTopCutWidth — 5 mm]; This is a dependent parameter, it is provided to control the width of the side cut on the top of the wall. CalculatedSideTopConnectionDepth⁎ [= SideTopCutDepth — 10 mm]; This is a dependent parameter, it is provided to control the depth of the side cut on the top of the wall. CalculatedBottomProjectionWidth⁎ [= BottomProjectionWidth — 5 mm]; This is a dependent parameter, it is provided to control the width of the cut on the bottom of the wall. CalculatedBottomProjectionDepth⁎ [= BottomProjectionDepth — 8 mm]; This is a dependent parameter, it is provided to control the depth of the cut on the bottom of the wall. BottomProjectionWidth Determines the width for cut on the bottom of the wall BottomProjectionOffset Determines the offset between the Wall and the Reference Line BottomProjectionDepth Determines the depth for cut on the bottom of the wall BottomConnection⁎ [= if(HasBottomConnection, (WallLength), 0 mm)]; This is a dependent parameter, it is provided to control the visibility of the bottom connection of the wall BeamWidth Determines the width of the beam attached to the façade wall BeamStartDistance⁎ [= StartSideCutWidth]; This is a dependent parameter, it is provided to control the distance of the beam from the edge of the wall. It is made equivalent to the side cut of the wall. BeamPositionFromFloorLevel Determines the bottom position of beam from the bottom of the wall BeamDepth Determines the depth of the beam attached to the façade wall BeamEndDistance⁎ [= EndSideCutWidth]; This is a dependent parameter, it is provided to control the distance of the beam from the edge of the wall. It is made equivalent to the width of the side cut of the wall. ⁎ Dependent dimensions — These parameters are assigned with mathematical formulas to aid in computation. The users need not assign values to these parameters; however, based on their discretionary judgement, they can edit the formulas to suit their requirements.

Allplan. As a sample, the different views (Floor Plan: Reference Level; Elevation: Front View; Elevation: Left View; and 3D View) of a parametric BIM component (façade wall) in Revit platform, are as shown in Fig. 4. The various parameters and attributes defined for the façade wall with one opening in Autodesk Revit are represented through Table 1 and Fig. 5. The same parameters and attributes are represented in pictorial form for Tekla Structures as shown in Fig. 6. The necessary parameters and attributes can be modified as per the design

requirements to produce the required shapes. This paper does not deal with the explanation of parametric capabilities of the software's. Once the necessary modifications are implemented, the shop drawings can be generated automatically using the parametric BIM components as shown in Figs. 7, 8, and 9. As per the interviews conducted in this research, it was suggested by the precasters that the standardization of the key HDB precast elements would support the designers to avoid variations in the design during the

DETAILS

TYPICAL DETAILS OF PRECAST FAÇADE WITH BEAM INTEGRATED – CASE 2

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NOTES: 1. “B” AND “D” DENOTE BEAM WIDTH AND BEAM DEPTH RESPECTIVELY 2. REINFORCEMENT WITH * DENOTES: DRAWING TITLE A) 2T16 LOOP BAR FROM FAÇADE FOR BEAM WIDTH OF 250/300mm WITH MAX. BEAM BOTTOM REINF. OF 3T25 PRECAST FAÇADE (150mm THICK WITHOUT BEAM POCKET) B) 2T20 LOOP BAR FROM FAÇADE FOR BEAM WIDTH OF 300mm (Min.) WITH MAX. BEAM BOTTOM REINF. OF 5T25

SCALE: 1:25 & 1:10 DRAWING N0. HB-REF-JULY2013-12-24

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Fig. 3. Typical arrangement details in 2D of a precast façade wall. Source: HDB Guide Drawings, HB-REF-JULY2013-12-24.

CONTRACT: BUILDING

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Fig. 4. 3D Parametric BIM component — façade wall with one opening (Autodesk Revit) — component views.

later stages of the project which affects on-time generation of shop drawing. In order to standardize the precast elements, the element configurations must be defined with minimum variations which lead to fewer mistakes and avoid rework [4,14,20,27].

Other interesting issue in this context is parametric design for precast construction with more detail on parametric BIM components, which is, however, beyond the scope of this paper and it will be considered in a separate paper by same authors.

`

Fig. 5. Graphical representation of parameters/attributes (described in Table 1) for façade wall with one opening in Autodesk Revit.

T. Nath et al. / Automation in Construction 54 (2015) 54–68

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Fig. 6. Dialogue boxes for modification of relevant parameters in Tekla Structures.

4. Workflows for shop drawing generation

4.1. Value stream mapping and analysis results — present workflow

In this section, the activities performed in order to generate the shop drawings are elaborated. These activities are represented in Table 2. Value stream mapping is introduced to analyze the value-added and the non-value added times in order to emphasize on the concepts of lean construction and process re-engineering.

In this section, value stream mapping (VSM) and value stream analysis (VSA) are done for the workflow followed in the present scenario for precast shop drawing generation in Singapore's context as shown in Figs. 10 and 11 respectively. The map includes the activities performed by the consultants, the main contractors, the MEP sub-

Fig. 7. Sample shop drawing assembly for a typical façade wall. Source: HL Building Materials Pte. Ltd — Page 1.

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contractors, and the precasters. However, for this research the precast process is focused in detail. The VSM and VSA performed correspond to the generation of shop drawing for a typical HDB building in Singapore. Fig. 11 shows the value stream of a typical HDB project for the generation of shop drawings which represents a series of linked activity boxes with triangles in between them. The activity boxes, with processing time below them, represent the time taken for each activity before it moves on to the next step. This processing time provides estimates for the value-added time. The triangles, which do not have any specific duration, represent the queuing time before the processing of the subsequent activity. VSM shows the total duration between tasks or the lead time (represented by the arrows above each one of the task and spanning between two subsequent triangles). The difference between the total time (sum of lead time represented by the arrows above the activities) and the activity processing time (sum of times mentioned under each activity box) represents the total non-value added time or waste [4]. The unit of time for the entire workflow is considered as week, and each week corresponds to 40 hours of work per person. The unit of value added time considered is man-hours. It is possible for more than one person to contribute for the completion of activities, so the real time needed for each activity completion may differ from the value added times as shown. The VSA results show that, for the generation of shop drawings as per the present workflow, it takes 20 to 30 weeks for a typical HDB project. The reason for this range of the values is due to the diversity and the complexity of the precast elements involved. The analysis also represents that about 25% of the total time is the value added time and the remaining 75% is the non-value added time, or wastage.

4.2. Value stream mapping and analysis results — proposed future workflow The flowchart shown in Fig. 12 highlights the proposed process that is presented as a recommendation to be followed by construction industry for shop drawing generation. Fig. 13 shows the value stream of the proposed future workflow. It is expected that adoption of the enhanced process will eventually result in a significant amount of savings in terms of time. As per the proposed future workflow, it was recommended that the MEP-SC shall be involved during the early stages of the project and produce a coordinated BIM model which will facilitate in the elimination of certain repetitive activities, like RFI-R, RFI-C, RFI-A, SD-G, SD- C, and SD-A, leading to generation of shop drawings in a shorter time frame. As a result, wasteful work can be eliminated and total lead time can be compressed by 7 to 12 weeks. Due to the implementation of BIM, the duration of most of the activities in the proposed future workflow will also be reduced significantly. The new lead time ranges from 13 to 18 weeks which is approximately 35% reduction as compared to the workflow followed by the precasters in the present context. 5. Results and discussion 5.1. Present workflow vs. proposed future workflow The process transformation from a time consuming present workflow to a BIM-based technologically-enhanced workflow is expected to reduce the number of cumbersome activities involved which in turn enhances the productivity for shop drawing generation and

Fig. 8. Sample shop drawing assembly for a typical façade wall. Source: HL Building Materials Pte. Ltd — Page 2.

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63

TYPE K1F1AX CONCRETE GRADE – 40 REBAR BENDING SCHEDULE BAR MARK

QTY.

SIZE

1

3

T16

2

1

T13

3

11

R10

4

15

R10

5

1

R6

BAR MARK

QTY.

SIZE

11

1

T10

6

1

T13

12

15

R6

7

1

T10

13

18

R6

8

2

T13

14

15

R6

9

4

T13

15

18

R6

10

1

T13

PROFILE

PROFILE

Fig. 9. Sample shop drawing assembly for a typical façade wall. Source: HL Building Materials Pte. Ltd — Page 3.

approval. This section discusses the various potential benefits of using BIM tools in the proposed future workflow but the detailed explanation regarding the functionalities of the tools are beyond the scope of this paper. Quantity take-off is performed before the award of tender, as per the present workflow the precaster receives the project information in 2D format for tender preparation, but in the proposed future workflow it is recommended that the precaster receives a detailed design BIM model generated by the consultant. The BIM model shall be generated using the parametric BIM components created for this project. Through this BIM model the time taken for finalization of budgetary quotation can be reduced due to automated volumetric quantifications. The data

regarding the time taken for QTO based on 2D (manual) format was collected from precasters' past projects, using the BIM model it was observed during the workshop that considerable improvement in productivity is possible for QTO. Once the precaster receives the letter of award (LOA) from the main contractor, as part of the initial study regarding the generation of shop drawings, design review is done on the consultant's architectural and structural 2D drawings for the identification of critical criteria as per the present workflow, but according to the proposed workflow, clash detection capabilities of BIM can be utilized to automate the checking process which has a higher potential to yield accurate results in a shorter time frame. Presently, the consultants provide the component

Table 2 Activities performed in order to generate the shop drawings. Activity

Abbreviation Descriptions

Quantity Take-Off Design review and verification Determination of component types and markings Production planning/mould grouping/delivery planning Mechanical, electrical, and plumbing services Request for information-raised

QTO DRV DCT&M

Request for information-coordination

RFI-C

Request for information-approval Shop drawing-generation Shop drawing-coordination Shop drawing-approval

RFI-A SD-G SD-C SD-A

PP/MG/DP MEP RFI-R

Precaster performs volumetric quantifications for budgetary quotation submission. Precaster reviews and verifies the 2D drawings or 3D model. The marking and type of the components shall be studied by the precaster in order to proceed with shop drawing generation. Planning for production and delivery along with mould grouping needs to be done by the precaster after the receipt of initial project data. MEP sub-contractor provides specific MEP details based on which shop drawings has to be generated. The precaster raises RFI's after properly reviewing the design drawings and determining the types of precast elements in order to clarify certain issues. The main contractor collects the RFI's raised from the precaster and coordinates between the consultant and the precaster in order to maintain a streamlined flow of information. The consultant reviews the raised RFI's and provides clarifications, comments and approval on the raised issues. The precaster generates the shop drawing which is with/without MEP incorporations. The main contractor coordinates the shop drawing and passes it on to the consultant for approval. The consultant reviews the shop drawing generated by the precaster and provide their comments and approval.

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Fig. 10. Present workflow for precast shop drawing generation.

markings in the issued drawings, the precaster manually reviews the design drawings and sorts out the component markings for individual precast elements. In the future, the detailed BIM model generated by consultant shall be utilized by the precaster for review and verification and determination of components types and markings. Due to the enhanced visualization capabilities of BIM, the organization of the components within BIM model with proper sequence of marking can be done in less error prone manner as compared to the current 2D process. In this research, BIM was not used to examine the productivity gains for the following activities, production planning, mould grouping, and

delivery planning; hence, it was assumed that the amount of time taken for the aforementioned activities is the same for both the workflows. As per the present workflow, the precaster raises requests for information (RFI's) based on the design review and verification to clarify the identified discrepancies in the issued drawings in a paper-based format. After the receipt of reply on RFI, the first set of shop drawings is generated and submitted to the main contractor for approval. The first generation of shop drawings, which is without the MEP incorporations, is forwarded by the main contractor to the consultant for their comments

Fig. 11. Value stream mapping and analysis for precast shop drawing generation, present workflow.

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Fig. 12. Proposed future workflow for precast shop drawing generation.

and approval, and at the same time, the main contractor also forwards it to the MEP sub-contractor for their inputs on the MEP markings and detail. After the receipt of the complete set of MEP information, along with the comments from the consultants, the precaster regenerates the shop drawings, which includes the MEP details and markings, and resubmits the new set of shop drawings for further approval. If there are discrepancies, the shop drawings are modified again, if not a final approval on the shop drawings is obtained for further course of action by the precaster. But, according to the proposed future workflow, in order to enhance the shop drawing generation and approval process, it was

recommended through this research, that the MEP-SC shall be involved during the early stages of the project to provide the relevant MEP markings and details and produce a coordinated MEP BIM model. Due to the early involvement of MEP-SC, repetition in certain activities corresponding to RFI and the shop drawing can be eliminated. Additionally, the shop drawing generated will include MEP incorporations. As shown in Fig. 13, the lead time in the future workflow can be greatly reduced by eliminating the repetition in the aforementioned activities. It also represents the activities which will encounter reduction in time if the BIM tools are adequately applied.

Fig. 13. Value stream mapping and analysis for precast shop drawing generation, proposed future workflow.

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Fig. 14. Time reductions in the value stream and eliminated activities in the present workflow.

information amongst each other. The timeline for activities RFI-R, RFIC, and RFI-A as shown in Fig. 13 is estimated based on this exercise. However, the obtained values may vary according to the requirements of live precast project. The BIM components generated for this project were utilized by the precasters in an ongoing precast project and the components' parameters/attributes were modified accordingly to fit the projects design requirements. Shop drawings were generated through these components which proved to take considerably lesser time and efforts. Even though it may take just a few minutes to generate the shop drawings through BIM, the precaster needs to adjust the drawing layout according to HDB's requirements which is a major time consuming process. Fig. 14 represents the group of activities which will experience reduction in time as well as total elimination due to the advent of BIM.

Through this research it was suggested that the RFI submission and approval process may be improved by using BIM capabilities, such as, BIM Collaboration Format (BCF). BCF is an open BIM standard which permits the users to share the encoded messages capable of informing the different BIM platforms about the issues identified in the BIM model generated in another BIM platform. The BCF format stores location, image, mark-up, comments, source, date, and objects for each issue. The process of sharing relevant data between the different BIM platforms helps to enable a greater degree of collaboration between the stakeholders. The objective of this data sharing is that only the issues highlighted will be shared across the BIM platforms and the entire project data need not be shared [12]. A mock exercise regarding the sharing of project information through BCF was conducted during the technical workshop wherein the participants utilized BCF to share the project

PRESENT WORKFLOW vs. PROPOSED FUTURE WORKFLOW

32%

SD-G; SD-C; SD-A

46% 61%

Activities

RFI-R; RFI-C; RFI-A

44% 20%

MEP

23% 21%

DRV + DCT&M

33% 64%

QTO

72% 0%

10%

20%

30%

40%

50%

60%

70%

Productivity Improvment TOTAL TIME PER ACTIVITY

PROCESSING TIME PER ACTIVITY

Fig. 15. Productivity improvement percentage of workflows' activities.

80%

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5.2. Productivity improvement measurement The potential productivity benefits through the change in the workflow were measured in terms of saved time. The activities were extracted based on the present workflow and the proposed future workflow, and the data was collected in order to estimate the duration of the activities [22,29]. For this research, the data for the present workflow is taken from the previous projects of the precasters (actual data is confidential which cannot be represented in this paper). As discussed earlier, the productivity gains for activities quantity take-off and shop drawing generation are validated through workshop and precasters' pilot project respectively. In order to validate the productivity gains shown in this paper for the other activities, the proposed future workflow needs to be mandated by the Singapore's construction authorities. However, the data for the proposed future workflow is estimated based on the expectation from the precasters. The precasters involved in this project are well versed with BIM tools as well as have enough experience for shop drawing generation through the traditional 2D process, so it was possible for them to evaluate the workflows. The productivity improvement results, which were corroborated as per the precasters' expectations, is based on the comparison between the ‘estimated values’ for the present workflow and ‘expected values’ for the proposed future workflow. Fig. 15 shows the productivity improvement for each activity in terms of total time and processing time which facilitates to plan for the critical activities that need to be considered during the relevant stages of the project. The results show that the activity QTO has maximum productivity gain (72% for processing time and 64% for total time). The results represent a benchmark for adoption of the BIM-based technologically-enhanced workflow to identify the critical activities through process re-engineering. In terms of overall productivity improvement by adoption of the proposed future workflow, it was concluded that there would be a substantial time saving of 380 man hours leading to an overall productivity improvement of approximately 36% for processing time and 38% for total time. To validate the research findings, the project data was reviewed by the members of the project team which consisted of the precast firms along with the resource persons from the side of the project leaders i.e. HDB. 6. Conclusion The major objective of this research was to map and re-engineer the current practices for the generation of shop drawings and to propose a BIM-based technologically-enhanced workflow in order to streamline the process for the purpose of achieving lean construction. This paper represented the potential productivity improvements that can be obtained if the proposed workflow is followed in the future. A set of key HDB precast elements were developed as parametric BIM components which enables the automated shop drawings and reinforcement schedule generation. Value stream mapping and analysis technique was utilized to identify processing time and queuing time. The expected productivity improvement through the adoption of the new workflow was identified and recorded, along with the characteristics of each activity and the problems encountered in each of the workflows. The results show that if the proposed workflow is followed there would be an overall productivity improvement of approximately 36% for processing time and 38% for total time. In order to further improve the process and reduce the lead time, the industry participants should consider some tactics that may be used to facilitate major process improvement, such as, 1) Communication and collaboration between the projects stakeholders — usage of BIM tools may help to significantly enhance the collaboration between the stakeholders of the project. Cultivating enhanced coordination and collaboration between the different stakeholders of the project will help to increase flexibility and transparency in order to streamline and properly

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synchronize the workflow. The precast firms involved in this research had a very positive view regarding the usage of BCF, as a tool which facilitates collaboration. 2) Standardization of precast elements and precast processes — standardization facilitates in reducing the variability of the precast elements from project to project basis. It is advantageous in terms of design and detailing of a precast element and it reduces complexity.

Acknowledgements This Research project was funded by the Building and Construction Authority (BCA) of Singapore under the Productivity Improvement Project (PIP) scheme [BCA PIP Case No. 2013-07-00343]. It was led by the Building Research Institute (BRI) of the Housing and Development Board (HDB) of Singapore. Special thanks go to Dr. Johnny Wong (GDBRI — HDB), Mr. Larry Cheng (DBDS — HDB), Mr. Alvin Chong (DDIDS — HDB), Dr. Kee Wee Tan (Director, IT Department and Director, Centre for Construction IT, BCA) and Ms. Fan Suet Lay (EM, CPC, PD(I)D, BCA). The work was conducted at the Centre for Infrastructure Systems (CIS), NTU. The authors would like to express their gratitude towards Dr. Wong Yiik Diew (Associate Professor, Director, Centre for Infrastructure Systems (CIS), NTU) and Dr. Soh Chee Kiong (Professor, School of Civil and Environmental Engineering, NTU). The authors would specially like to thank and acknowledge the contributions of the three precast firms, Sunway Concrete products Pte. Ltd, Excel Precast Pte. Ltd, and HL Building Materials Pte. Ltd for providing project data and relevant feedbacks for the deliverables of this research project. The contribution of Tekla (SEA) Pte. Ltd and Nemetschek Engineering GmBH for development of the parametric BIM components in Tekla Structures and Nemetschek Allplan platforms respectively is sincerely appreciated.

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