تجزیه و تحلیل کمی از جریان کاری به وسیله ساختار موقت، و بهره وری با استفاده از مدل 4D

This paper presents time–space analyses of construction operations supported by quantitative information extracted from 4D CAD models. The application of 4D models is a promising approach to help introduce construction innovations and to evaluate construction alternatives. Current analyses of 4D models are mainly visual and provide project stakeholders with a clear, but limited, insight of construction planning information. This practice does not take advantage of the quantitative data contained in 4D models. We use two 4D models of an industry test case to illustrate how to analyze, compare, and present 4D content quantitatively (i.e., workspace areas, work locations, and distances between concurrent activities). This paper shows how different types of 4D content can be extracted from 4D models to support 4D-content-based analyses and novel presentation of construction planning information. We suggest further research aimed at formalizing the contents in 4D models to enable comparative quantitative analyses of construction planning alternatives. Formalized 4D content can enable the development of reasoning mechanisms that automate 4D-model-based analyses and provide the data content for presentations of construction planning information.

The application of 4D CAD models is a promising approach to help introduce construction innovations and to evaluate construction alternatives. 4D modeling combines schedule data and spatial data. The method visualizes 3D CAD models in a 4-dimensional environment (i.e., time–space environment), facilitating analyses of different production strategies before work on site is initiated [1]. 4D models are typically created by linking building components from 3D CAD models with activities from activity-based scheduling methods, such as the Critical Path Method (CPM) [2]. Building components that are related to an ongoing activity are highlighted, providing users with spatial and temporal insights of the construction process. Simulating production options with multiple 4D CAD models from different perspectives allows project stakeholders to compare construction alternatives. However, today these analyses are mainly based on visual analyses where experienced practitioners may or may not detect constructability issues, such as time–space conflicts, that make certain alternatives more or less feasible. It is relatively easy to spot potential time–space conflicts through visual inspections, and some 4D modeling tools provide automated time–space conflict analysis, i.e., they alert the user if two activities are scheduled to use (part of) the same space at the same time. However, it is more difficult to identify opportunities to improve workflow on site and improve space usage and productivity through purely visual inspection or analysis of 4D models. Nevertheless, planning supported by visual analysis of 4D CAD models is considered more useful and better than traditional planning [3], [4], [5] and [6]. However, visual analysis does not take advantage of the quantitative data contained in 4D CAD models. We present three types of analyses to illustrate the usefulness of quantitative analyses from 4D CAD models for planning of construction operations. The first analysis addresses workflow, workspaces and space buffers. The second analysis concentrates on the planning of temporary structures. The third analysis focuses on crew productivity and production costs. These three analyses are based on temporal and spatial data extracted from 4D models. 1.1. Planning time and space buffers for construction operations Planning workspace for crews or trades and space between different crews (i.e., space buffers) is a challenging task. Construction planners need to carefully design time–space buffers between activities so that on one hand the productivity for each crew is not slowed by time–space conflicts and lack of workspace and on the other hand the overall schedule is not lengthened due to excessive use of time–space buffers. This planning task is particularly challenging because the space usage on construction sites changes dynamically. Different crews move across the site from one work location to another. To execute their work, construction crews use, among other things, temporary structures. Temporary structures are one of the important factors in planning workspace and time–space buffers as they occupy space, but also provide workspace depending on the stage of a construction project. The productivity and therefore the output of crews is strongly dependent on available workspace and affects the progress on projects, and ultimately the project cost. Traditional scheduling techniques, such as the Critical Path Method (CPM), used in combination with 2D drawings, do not provide planners with the spatial insight that is required for the planning of efficient workspace use and allocation of optimal time–space buffers. In the traditional approach using CPM and 2D drawings users are required to look at 2D drawings to conceptually associate building components with the related activities [5]. Different actors may develop inconsistent interpretations of the relations between activities in a schedule and project components. This practice is prone to errors and limits the understanding of the spatial context of the flow of construction work on projects. Virtual environments, such as virtual reality (VR) and 4D CAD, promote improved understanding of construction operations [7]. Akinci et al. [8] formalize and model space usage for construction activities in 4D CAD models, resulting in space-loaded 4D CAD models. The formalization and modeling methods provide insight into various types of spaces that are related to specific types of construction activities and potential conflicts (e.g., material space, labor space, building component space, etc.), but do not provide insight into the efficiency of designed time–space buffers between different activities. The Line-of-Balance (LoB) method [9], also known as Location-based Scheduling (LBS), provides spatial insight into the planning of time–space buffers and workspaces in the construction process. This technique uses lines to represent the production of crews over time in diagrams. LoB diagrams are based on a well-defined spatial sub-system, such as a floor consisting of apartments, divided into rooms. This type of static spatial hierarchy of workspaces is not present during all stages of the construction process. The distribution of and boundaries between workspaces are generally much more complex and less clear. Projects can have complex workflow directions of crews as a result of variations in the spatial configuration of a project [10]. Akbas [11] proposes a geometry-based process model (GPM) that uses geometric models to model and simulate workflows and work locations. This method provides spatial insight into the planning of workspaces and space buffers for repetitive crew activities, but has not been applied for quantitative analyses of 4D CAD models. It has rather been used as a simulation method for workflow and work locations with detailed 4D CAD models as an output. 1.2. Quantitative analyses using 4D models In this paper we show the value and method of extracting data from 4D CAD models to enable quantitative analyses of 4D CAD models. The basis for analysis is the 4D CAD model and not the underlying scheduling and modeling method for these models, such as the CPM schedule, LoB diagram, or GPM simulation. This paper first introduces 4D CAD models of an industry test case that we use as a case example. Then, the paper describes quantitative analyses of the 4D CAD models, in which we analyze workflow and planning of temporary structures. In addition, we perform an analysis in which we relate crew productivity and production costs to extracted data from the 4D CAD models. The paper concludes by suggesting research, including determining the content of 4D models needed for quantitative analyses, standardizing the representation of that content, and formalizing automated methods for quantitative analyses of construction concerns on the basis of 4D models.

4. Discussion and further research Although based on a real case and realistic production data, the specific numeric results of the presented analyses are hypothetical, but show a type of reasoning or analysis that does not occur in today's 4D CAD simulations. Reasoning, such as analyzing distances between crews and re-routing formwork crews, is done on the construction site where there is a limited opportunity to change construction execution strategies. The distance between different types of work is an important factor in safe and productive execution of work. This paper showed that early analyses of 4D content can limit the risk for time–space conflicts in production. In addition to minimizing risks, there is also a potential to use analyses of 4D content to improve construction processes on aspects such as workspace usage and resource usage. The analyses of 4D content that were performed in this study showed, for example, disruptions in workspace usage that did not become clear from CPM schedules or from 4D CAD models. Representation of workspaces of different crews in a graph made these disruptions explicit and immediately visible and obvious and illustrated the usefulness of extracting data from 4D CAD models. In the process of manually extracting the 4D content we made certain assumptions about how to measure, for example, distances between crews. We measured distances in the horizontal (XY) plane and did not consider differences in height. In addition, we assumed that the 3D CAD components represented the workspace used by crews, but in reality the workspace used by crews is not limited to these components [13]. Representation and interpretation of extracted 4D content calls for an accompanying 4D CAD model to understand the meaning of these data and to put the data into their spatial and temporal context. 4D CAD models can be built in different levels of detail and with different contents. Certain 4D CAD models are, for example, workspace loaded [8], and other 4D CAD models contain a temporary structure plan [14]. How to extract 4D content from a 4D CAD model therefore depends on the type of 4D CAD model that is available. As a result, the process of querying 4D content is currently not a straightforward database query in time, since there is no agreed upon standard for defining and managing 4D content of 4D CAD models. To illustrate this, 4D CAD objects that represent shoring in the 4D CAD models of the industry case study have the same object property definitions as objects that represent formwork. Although the IFC [15] provide standardized definitions for 3D objects and construction schedule information, it lacks specific definitions for the 4D contents that we discussed in this paper. The lack of specific definitions for these objects and the lack of instruction for the use of these definitions for modelers require custom-built reasoning mechanisms to extract 4D content such as distances between crews automatically from a 4D model. Certain 4D CAD software tools [3] offer functionality to specify the type of activity in 4D CAD models (i.e., activity types), which works in a similar way as CAD layers in 3D CAD tools. Using activity types can be one way of defining specific 4D contents, which in turn could enable the automatic extraction of these contents. However, activity types are not always used in 4D CAD models, nor are they available in all 4D CAD software tools. In the 4D CAD software tools that support activity types there is currently no functionality available to query the 4D CAD model for specific types of activities. The technical challenges of developing such functionality are minimal. However, the process to input 4D content and to extract this content requires further research. In this paper we illustrated a number of 4D contents, with different analyses, but there are most probably more 4D contents that can be of interest for the design, planning, and control of the construction process. We suggest further research where different types of 4D contents are extracted and analyzed from 4D CAD models. Combined, these studies can eventually result in sets of 4D contents, reasoning mechanisms, and information presentations that are standardized for different types of analyses. The standard 4D contents and subsequent analyses can be expressed as a set of metrics that can provide planners with a basis to compare construction planning alternatives for different aspects. We believe that quantitative analyses of 4D models can benefit different types of construction projects, but recommend starting the application for critical stages of a construction planning in complex projects (e.g., locations where work is scheduled for multiple crews, hazardous operations, etc.).