Gantt Chart In Construction

Created by Peter Birkholm, February 2022

The Gantt chart is a type of work scheduling tool in which processes are visualised. It is based on the Harmonygraph, which was developed by Karol Adamiecki at the end of the 19th century. The American engineer Henry Gantt was the one who successfully introduced the method to a wider audience and thus became the namesake for the method. The Gantt chart has been used in many well-known construction projects after its conception, for example, Hoover Dam and the Interstate Highway System.

Gantt charts are frequently used for construction projects to illustrate tasks along a timeline. They are effective for several reasons, though primarily due to their capability in showing a wide array of relevant project data in a simple fashion. At its core, the Gantt chart is built around illustration of tasks that are to be completed along a timeline. Each task is represented by a bar, with the location and length indicating start dates and durations. Furthermore, these tasks can have their dependencies illustrated through connecting lines. Being able to illustrate each type of dependency associated with a task (Finish to start, Start to Start etc.) means that the tool can also be used to assess feasibility of, as well as attempt to optimise, workflows before the start of a project. After the construction project is underway, the Gantt chart can be used to track milestones and assess whether timelines are being followed.

There are a myriad of advantages and disadvantages to utilising Gantt charts for construction projects. A key benefit is the visualising of dependencies – as construction tends to be ordinal in nature. One disadvantage that is commonly seen is the Gantt chart becoming less usable as the size and scope of the construction project increases.

Gantt Chart Overview

The Gantt chart is a visual overview and representation of all planned tasks that are necessary for the completion of a project. These tasks are usually represented by a rectangle that indicates the expected duration of the task. When these rectangles are placed on a timeline, they can also represent the start and expected times for the tasks. As this timeline is expanded into 2 dimensions, the tasks can be sub-categorised. These subcategories can be defined in various ways depending on the project at hand. Usually for construction, it will be the breakdown of larger tasks. An example being “Installation of Windows” being subdivided into “Mounting Frame”, “Mount glazing”, “Apply Caulk” etc. This is formally known as a "Work Breakdown Structure" which is defined by the Project Management Body of Knowledge as a "hierarchical decomposition of the total scope of work to be carried out by the project team to accomplish the project objectives and create the required deliverables". The 2 dimensionality of the timeline also allows for visualisation of concurrent tasks. This means that, especially for construction, it is evident which work will be ongoing during any specific day – a critical aspect in planning and allocation of the workforce. The timeline can be split into equidistant boxes. This standardisation of the timeline range gives a digestible overview of the tasks and their relative length. The time range can vary depending on the project. In cases of very complex construction projects and therefore planned to a high degree of detail, there can be Gantt charts with weekly time units and down to hourly or less.

Example of a Gantt Chart

Dependencies

Another information aspect that can be illustrated in Gantt charts are the dependencies across the tasks. In short, this is an illustration of which tasks need to be completed in order for others to be started or completed. These, as with most projects, typically occur in construction as many phases are completely contingent on other earlier ones. Understanding these dependencies allows for more effective planning of construction projects. This is especially true when setbacks do occur, as they allow for an overview of where accelerations can be made, and perhaps more pertinent, where they cannot. There are 4 types of dependencies.)

Finish-to-Start Task Dependency

The finish to start dependency is likely the most common. As suggested by the name it refers to a a dependency in which one task needs to be completed for another to start. This type of dependency means that a delay in task A will postpone task B and therefore delay the schedule. This can cause a domino effect in which a delay of one task can propagate through successive tasks and delay these.

Start-to-Start Task Dependency

Start to start is the second type of dependency. Once again, the dependency is aptly named, as it refers to a task relationship in which one needs to start in order for another to start. It is important to understand that these tasks do not necessarily have to begin at the same time (and in most cases do not). These types of dependencies usually pose a lesser threat of compounding delays as they can, in theory, run concurrently.

Finish-to-Finish Task Dependency

The third type of dependency is the Finish-to-Finish category. These dependencies occur when one task needs to be completed in order for another to be completed. As with the aforementioned start-to-start, finish-to-finish tasks can run concurrently. The two tasks are only contingent in regard to their completion (though this does not have to be at the same time).

Start-to-Finish Task Dependency

Start-to-finish task dependencies are perhaps the rarest form. This dependency is the inverse form of the first category. Here task A cannot be completed until task B has been started.

Critical Path Method

The “Critical Path Method”, is a process used to determine the longest stretch of activities that are dependent on each other and calculating the time required to complete them all. The method was developed in 1950s by Morgan R. Walker and James E. Kelley Jr. and is built upon 4 basic requirements:

1. A work breakdown structure
2. Duration of each activity
3. Dependencies of the activities
4. End points (milestones or deliverables)

As can be seen, these are components that naturally stem from or are needed for the implementation of a Gantt Chart and thus the critical path method harmoniously incorporates into Gantt charts. The establishment of the critical path means the Gantt chart can also function as a way to manage delays as it can give an overview of the float available for tasks (i.e., Tasks that can be delayed without further delaying the project itself). Furthermore it strengthens the capabilities of the chart in regard to allocation of resources effectively (i.e. Assigning more people to critical tasks).

Advantages and limitations

Advantages

• It is easy to present and understand the project schedule and required activities
• Tasks can be divided into sub-tasks visually.
• Can be responsive if required labour is put in.
• Milestones can be visually graphed on the timeline.
• Time frames and key dates can easily be labelled and planned for.
• The timeline can be fragmented into relevant intervals (Days,Weeks,Months etc.).
• Allows for assessment of feasiblity of timeline (given suitable time estimates).
• Stakeholders and contributors to the project can see an overview of when and where they are needed.
• The state of the project can be seen and evaluated easily. The completion status can also be assessed.
• Allows for easier coordination between disciplines.
• Dependencies are effectively visualised and tracked

Limitations

• Charts can become large and complex as project size and complexity increases.
• As charts become complex it can become time consuming to keep the chart accurate and up to date.
• Can become inflexible and outdated as project sizes increases.
• There is mostly a focus on time restraints and the resources associated therein.
• Can be harder to plan according to other resources, for example workspace constraints.
• If other restraints are incorporated into the chart, this information can be obfuscated (for example, the requirement of a crane to move materials)

Applications in Construction Projects

Gantt charts are frequently used for construction projects. This spans the spectrum of construction project scopes from the largest infrastructure projects to the smallest renovations. This is due to a myriad of reasons. Construction projects fall under most of the general theories of projects, but they do exhibit a few traits that make applications of certain techniques more effective. A few of these differences are:

1. The usually fixed location of construction. This adds certain logistical complexities. This can take the form of procuring building materials which have to be transported to the site. This may also include traffic management, security and various other extraneous factors.
2. Construction projects are more prone to be affect by the uncertainty of weather and thus need to be more flexible in this regard. Extreme weather can have a negative impact on timelines and feasibility of certain work.
3. The high level of focus on the green transition within construction has added several layers of risk and complexity to what used to be regarded as simpler projects.
4. The time scope of construction projects tends to be measured more in years than days. This means that finer details or points may not be in place before construction commences.

The added logistical complexities that can be associated with construction play into the benefits of a Gantt Chart. The overview of the tasks to be completed and their dependencies allows for more effective planning and organisation of needed prerequisites. For example, the knowledge of when supplies and materials will be needed, allows for planning and management to minimise the losses that can occur under such constraints. As the Gantt chart can be dynamically updated, it can be effective when operating under the uncertainty of weather in construction. Furthermore, it allows for an overview of whether bad weather impedes the flow of the critical tasks that can extend the project length. As mentioned, larger construction projects can take years or decades and thus would not always have all minutiae delineated from the onset of the project. The Gantt Chart can help by allowing for rougher “blocking” of time slots that can then be detailed and have their effect on the construction evaluated at a later date.

Further Perspectives

Gantt charts remain very popular management tools despite being conceived over 100 years ago. In their current state they mostly provide a visual and effective way of displaying and communicating important information in regard to a construction project. Although the rapid technological advancements that have occurred have meant that computer science can now offer more powerful techniques in modelling solutions to some of the more fundamental time scheduling conundrums - Gantt charts can continue to play a role by providing an easy-to-use interface for stakeholders to identify and define problems on top of aiding a better understanding of chosen solutions.

Current Gantt tools primarily allow for focus on only one schedule at a time creating obstacles in comparing and testing alternative versions of schedules. Future tools will likely improve upon this and allow for simpler methods of testing feasibility of different timelines.

The next evolutionary step for construction timelines will likely be that of 4D BIM . Gantt charts can naturally be a part of this evolution as the links between parametric object models and their scheduling data becomes more widespread. These new forms of visualisation will improve the efficiency and accuracy of construction schedules.

Annotated Bibliography

• The Gantt chart, a working tool of management By Clark, Wallace; Polakov, Walter Nicholas; Trabold, Frank W (1922)

An overview of both the history and origins of the Gantt chart as well as a description of the set up and implementation of a chart. A publication of older character though still useful source of information.

• Gantt Charts: A Centenary Appreciation by James M. Wilson (2003)

A journal article with a description and overview of the development and applications of Gantt Charts with a more modern perspective.