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Construction Duration Forecasting
"This chapter deals with construction duration forecasting. Definitions of construction duration are first given. Roles of schedulers or planners are described. "
Introduction

This chapter deals with construction duration forecasting. Definitions of construction duration are first given. Roles of schedulers or planners are described. Next, factors affecting the forecasting of construction duration are summarized. Following that, methods of forecasting construction duration are briefly reviewed before delays and causes of delays are discussed. Then, relationship between the construction cost and duration or time is introduced. Even though the studies and researches concerning the forecast of construction duration of building projects are limited, this chapter presents the possible approach to build a project time or duration forecasting model based on almost all the inputs or variables used in pre-design cost forecasting model. The forecast time frame will inform the owner on the proper duration to allow for the contractor to complete the project. On the other hand, the contractor could prepare realistic and practical detailed schedule at the minimum costs within the limited time frame

Definitions
Construction duration can be defined as one or a combination of the following:

1) The construction duration arising from critical path in which duration for items of work or activity in sequence cannot be reduced further (Barrie and Paulson, 1992).
2) Duration means the time required to complete a specified task or activity. And, construction duration is the time determined by the owner's needs to occupy, utilize, or rent the completed space of the project (Callahan et al., 1992).
3) Construction duration is a duration resulting from an examination of one or more methods of carrying out the works on the basis of minimum cost, it is usually estimated in the first instance for normal condition (Pilcher, 1992).
4) Construction duration refers to a given time to execute and complete item(s) of work using all project information and resources within an estimated or predicted cost (Kwaku, 1994).
5) Construction time can be defined as the elapsed period from the commencement of site works to the completion time of building to the client. It is usually specified prior to the commencement of construction (Nkado, 1995).

In this research, construction duration is defined as the time frame given by the owner for the contractor to complete the project under normal work conditions, normal practice of construction, and based on the minimum costs. It starts when the contractor receives the instruction to proceed and ends at the completion of construction works on site. It also includes delays caused by unanticipated circumstances, e.g. alteration of works (changed conditions and change orders), extra works, supply of materials, location, weather, and site work conditions. Major changes that after the scope of work significantly are not included.

Scheduling and Schedulers
Control of construction duration needs a clear systematic plan and commitment on the part of the people involved (McNulty, 1982). The systematic plan is known as schedule. The scheduling is the determination of the timing of activities and follows logically from the planning process (Callahan et al., 1992). It is normally used for controlling construction duration (Callahan et al., 1992). To both the owner and contractor, scheduling plays an important role in financial proposal and budgeting (Peurifoy and Ledbetter, 1985; Kaka and Price, 1991). The schedule is prepared by the scheduler and/or planner. In preparing a schedule, the scheduler or planner may meet or discuss with some people for crucial information, e.g. estimator, manager, superintendent, sub-contractor, architect, engineer, owner, and materials' suppliers (Callahan et al., 1992). They may need to study the contract, drawings, specification, and perhaps, conduct site reconnaissance. Further, they need to know about manpower and productivity. (Callahan et al., 1992; Pilcher, 1992). Apart from the schedule, it is also necessary to prepare systematic monitoring to provide early warning of restraints as well as imaginative action to overcome them (McNulty, 1982).

Nowadays, computers may assist the scheduler or planner by storing and sorting the information, as well as performing mathematical calculations but they do not provide the intellectual direction (Callahan et al., 1992). Human skill and experience are still necessary, i.e. the scheduler or planner may use experience of similar projects in estimating the construction duration (Pilcher, 1992). The scheduler or planner may use their judgement according to various constraints, e.g. location and access, weather, space and site work conditions, complexity of the project, quality of workmanship, delivery of materials, and economic or marketing conditions (Pilcher, 1992).

Forecasting Construction Duration
1. Inputs
The basic inputs for project scheduling are: 1) contract; 2) drawings; 3) specification; 4) resources (materials, manpower and productivity); and 5) other constraints, e.g. site conditions and weather (Burgess and White, 1979; Fisk, 1982; McNulty, 1982; Peurifoy and Ledbetter, 1985; Ashworth, 1988; Barrie and Paulson, 1992; Callahan et al., 1992; Pilcher, 1992). The contract may clearly specify the completion date. Therefore, the scheduler or planner may use this time frame to prepare the schedule together with other factor constraints. First, it is usual to prepare the schedule for normal conditions by assuming one or more methods of carrying out the works on the basis of minimum cost (Nkado, 1992; Pilcher, 1992). When it is necessary to shorten the duration for an activity or a project, crashing may be done, e.g. increase manpower and overtime working. This process leads to increase in construction cost (Puerifoy and Ledbetter, 1985; Barrie and Paulson, 1992; Pilcher, 1992).

Drawings contain physical features of the project: 1) function; 2) height; 3) systems (e.g. plumbing, fire fighting, and lighting); and complexity (Ireland, 1985; Ashworth, 1988; Pilcher, 1992; Nkado, 1992). In other words, the information on the drawings is presented in form of dimensions and descriptions (Ashworth, 1988). Materials and installed equipment for the project, as well as the plant and construction equipment can also be known from the drawings (Ashworth, 1988; Peurifoy and Ledbetter, 1992). Meanwhile, the contract may specify the constraints of work, e.g. construction cost, duration, payment, inspection, method or conditions of works, delays, and damages. In addition, the specification may address the quality of materials, workmanship, and method of working (Barrie and Paulson, 1992).

2.Resource scheduling
The scheduler or planner has to allocate the resources, i.e. materials and manpower (or working team) to each activity or task at the proper time they are needed. Thus, the resource schedule deals with levelling and allocation of all necessary resources (Peurifoy and Ledbetter, 1985). The former smoothes out the peaks and valley in resource use within the project duration. The latter determines the shortest project duration consistent with the limited resources.

3.Other judgements
They also have to provide timing for preliminary works, e.g. construction plant, and mobilization (Peurifoy and Ledbetter, 1985). This often includes time for remedial works and site clearing after completing the construction.

Site reconnaissance enables the scheduler or planner to adjust the schedule against various constraints, e.g. location and access, weather, space and site work conditions, complexity of the project, quality of workmanship, delivery of materials, and economic or marketing conditions (Barrie and Paulson, 1992; Pilcher, 1992).

Some Methods for Scheduling

There are many forms of schedules, e.g. Gantt or bar chart, Critical Path Method (CPM), and Program Evaluation and Review Technique (PERT). Each has its own advantages, disadvantages, and application areas for which it is most appropriate. They are means of visual presentation of a construction program (Pilcher, 1992), and used for project planning, management, and control (Burgess and White, 1979; Fisk, 1982; McNulty, 1982; Barrie and Pualson, 1992).

Gantt chart simply represents the activity and its duration by a bar. It is sometimes called Bar chart. Gantt or bar chart can include a great deal of anticipated and actual information for; 1) cashflow; 2) manpower and/or manpower by trade; and 3) productivity (Barrie and Paulson, 1992; Callahan et al., 1992).

In CPM scheduling, a construction project is sub-divided into several activities. An activity is a single work step that has a recognizable beginning and finishing or ending (Callahan et al., 1992). In other words, the activity is a time-consuming task. The basis of CPM is network diagram, i.e. which needs nodes and arrows. It deals with four aspects: 1) activities identification; 2) logical sequence,; 3) network construction; and 4) allocation of resources (Barrie and Paulson, 1992). Callahan et al. (1992) divided the development of CPM schedule into six phases: 1) understanding the project; 2) conceptual approach definition; 3) physical creation of the schedule; 4) computerization; 5) refinement; and 6: reproduction. Further, physical creation of CPM scheduling is divided into eight steps: 1) select software; 2) divide project into several activities and sub-networks; 3) develop responsibility code; 4) develop information code; 5) develop specific sub-networks; 6) build or plot the logic diagram; 7) numbering the activities; and 8) linking the sub-networks together. The longest interconnected path of activities through the network called "critical path" becomes the project duration. On the critical path means that the duration for items of work or activity on the sequence cannot be reduced further without paying extra costs (Peurifoy and Ledbetter, 1985; Barrie and Paulson, 1992).

PERT is like CPM scheduling. It uses logical diagram to analyze performance time. It overcomes difficulties associated with duration of activities which could not be estimated reliably. PERT enables the scheduler to estimate the most probable project duration and the probability that the project or any portion of the project will complete at particular time. PERT focuses on events or nodes, called event-oriented. Unlike the CPM, it requires three estimates of duration for each activity: 1) optimistic (high productivity); 2) pessimistic (low productivity); and 3) most likely duration (Peurifoy and Ledbetter, 1985; Barrie and Paulson, 1992; Callahan et al., 1992; Pilcher, 1992).

Factor Affecting Construction Duration

The following are some factors affecting construction duration and its estimate.
1. Size of project
Size of the project can be represented in terms of functional or floor area, i.e. in ft2, or m2. The larger the building size, the more complex the construction, thus needing longer duration to complete (Sadashiv, 1979; Ireland, 1985; Ashworth, 1988; Pilcher, 1992; Nkado, 1992).

2. Function
Function implies type of building and required engineering systems, e.g. plumbing, fire fighting, and lighting (Ashworth, 1988; Pilcher, 1992). It is an important facet in designing of construction project (Ashwoth, 1988). Function of a building implies business target that the building serves. It can be considered as qualitative variables, e.g. office, retail, and other buildings (Nkado, 1992).

3. Height
Height of building, represented by number of floors (or storeys) affects the construction duration (Sadashiv, 1979; Ireland, 1985). The height of building indicates construction technique, major equipment used, and construction sequence (Sadashiv, 1979; Callahan et al., 1992).

4. Complexity
Complexity implies unfamiliarity with work (Pilcher, 1992). The complexity of building impacts the form of construction, i.e. building frame, foundation, and systems (Ireland, 1985; Ashworth, 1988). Complexity can be represented in form of construction equipment, method and sequence (Sadashiv, 1979; Callahan et al., 1992; Chan and Kumaraswamy, 1995).

5. Quality
Quality can be classified by variables or attributes, i.e. appearance, strength, stability, materials used, performance finish. Appearance of the building, e.g. external facing is one aspect of quality (Ashworth, 1988). Sadashiv (1979) considered number of major finishing works in duration forecasting instead of a defined quality index.

6. Location
The location of the building has a significant effect on the construction duration (Chan and Kumaraswamy (1995). It reflects restrictions or easements that exist and availability of services (Burgess and White, 1979). It effects supply of resources, e.g. materials, and equipment (Sadashiv, 1979). Consequently, it also effects the use of major equipment (Sadashiv, 1979), and productivity on site (Callahan et al., 1992).

There are other possible factors affecting the construction duration, e.g. construction planning (Sadashiv, 1979; Ireland, 1985), design-construction interface coordination (Ireland, 1985), dispute per unit of time (Ireland, 1985). Type and/or variation to the contract refers to risk allocation management structure and payment modalities (Burgess and White, 1979; Ireland, 1985; Chan and Kumaraswamy, 1995). By contrast, Walker (1994) concluded that client related factors have more significant affect on speed of construction, or construction duration, than the contract type. Callahan et al. (1992) pointed out that quality of supervision, labour training and motivation, can also be affecting factors. Al Tabtabai et al. (1997) developed the models for expert judgment in forecasting construction project

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