“Human history teaches us, however, that economic growth springs from better recipes, not just from more cooking.”
–Romer, Paul M. “Economic Growth.” In the Concise Encyclopedia of Economics, 2008
Back in 1965, the Gantt Chart was viewed with some skepticism.
In the 1965 published book, “CPM in Construction Management”, the author, J. O’Brien, challenges the notion of a new visual planning aid. If the bar graph is so well suited to construction activity, why look for another planning aid? The reason lies in the fact that the bar graph is limited in what it can present. In the preparation of the bar chart, the scheduler is almost necessarily influenced by desired finish dates, often actually working backward from the completion dates. The resultant mixture of planning and scheduling is often no better than wishful thinking.
If a bar graph is carefully prepared, the scheduler goes through the same thinking process that the CPM planner does. However, the bar graph cannot show (or record) the dependent tasks which control the project progress. Later, even the originator is often hard-pressed to explain his plan using the bar graph.
Nowadays, the Gantt Chart has many supporters. Gantt chart has evolved to become the primary means of communicating project plans and for good reason. The Gantt chart is the primary visualization method in many of the project scheduling software options available today. Gantt chart software was created to automate the planning and scheduling process to create task dependencies, add milestones, or identify the project’s critical path.
But then a Gantt Devotee Broke Rank. In 2012, Dave Garrett, founder of the online community ganttheads.com sent an email to his network with the subject of “Subject: Project Management is Changing (and so are we…)”. The well-known project manager recognized the coming changes saying “…times change and many project leaders are much more focused on their KanBan Boards than on a linear schedule.” He changed the online community name to Project Management, noting changes to tools and approaches.
“…times change and many project leaders are much more focused on their KanBan Board than a linear schedule.”
–Dave Garrett
A Century Worth of Gantt Charts
Mid-1910s Henry Gantt creates Gantt charts as a tool for production scheduling in the job shop environment
1931 Construction begins on the Hoover Dam using Gantt charts
1956 The Interstate Highway System begins construction; Gantt charts are used as the scheduling method
1957 CPM improves on Gantt charts by expressly recognizing the logic ties or task dependencies 1957 to Mid-1980s Gantt charts survive CPM as one of the formats often used to display CPM schedules
Mid-1980s Gantt charts with logic ties aka Logic Gantt charts become the CPM software graph of choice
2008 Dr. Gui introduces the graphical path method or GPM® at the PMI College of Scheduling Annual Conference in Chicago
2012 Patrick Weaver debunks the notion that Bar Graphs were invented by Gantt
Construction Gantt Chart Examples
Three Perspectives on Gantt Charts for Construction
A simple visualization of a project timeline that engenders the ability to see where project activities fall in relation to each other. They are easily understood by project executives and foremen alike. A staple of project management for many practitioners.
Gantt charts are subjective vs. rule-based, therefore, Gantt charts are not reliable tools for construction scheduling. Planners are compelled not just to scope activities but also to place them on the calendar without any consistent rule to guide the sequencing of activities. The chain of activities controlling completion can only be determined subjectively, commonly by working backward through the Gantt chart.
Logic Gantt charts (with CPM logic ties) are not subjective. For projects involving over one hundred activities, Gantt charts with CPM logic ties fail to convey a sense of logical flow. Combining the time-scaled advantage of Gantt charts with the logic visibility of CPM networks often times yields a convoluted Gantt chart that represents neither well.
What about Excel Gantt charts?
Many project superintendents will use an Excel Gantt chart to create a project schedule. However, Excel is not an effective tool to create a Gantt chart for many reasons. A simple Gantt chart does not have the ability to communicate, track and analyze project data. For example, the project schedule will not automatically update in real-time like it would in other scheduling tools. In fact, it is nearly impossible to track changes and alter timelines within an Excel document. Most importantly there is a breakdown in logic and you cannot involve resources in the project schedule when using an Excel Gantt Chart.
GPM Alternative to the Prior Logic Gantt Charts
There are three networking alternatives: ADM, PDM, & LDM.
Arrow Diagramming Method (ADM)
Original network notation introduced with CPM in 1957; arrows are used to denote activities, and finish-to-start (FS) dependencies are modeled by connecting the finish nodes of activities to their respective successors’ start nodes.
Precedence Diagramming Method (PDM)
Network notation introduced in 1961 and extended in 1963; boxes or scaled task bars (shown) are used to denote activities, and activities are connected with logic ties to model FS dependencies and PDM dependencies known as start-to-start (SS), finish-to-finish (FF) & start-to-finish (SF)
Logic Diagramming Method (LDM)
Time-scaled network notation introduced with GPM in 2008; dimensioned bars with end nodes are used to denote activities, and activities are connected with multiple-arrow links through their end nodes or embedded nodes to model FS dependencies or PDM dependencies, respectively.
A 10-day SS logic tie between Frame Walls and Rough-In MEP is conveyed by connecting an embed offset 10 days after the start (tail) node of Frame Walls with a vertical (V) link (in this case) to the successor’s start (tail) node
10 days remaining in Rough-In MEP after Frame Walls finishes is conveyed by connecting the finish (head) node of Frame Walls with a horizontal-vertical (HV) link (in this case) to an embed offset 10 days before the finish (head) node of Rough-In MEP
LDM Leads in Finish-to-Start Logic
Breakthrough #2:
Planners Can Drive Activity Dates After Early Dates Without Using Start Constraints
Breakthrough #3:
If Planned Start Date > Early Start Date, Activity Has Drift (May Revert to Earlier Dates)
Breakthrough #4:
Drift + Float = Total Float
Breakthrough #5:
Logic Tie Float aka Link Gap
Breakthrough #6:
Total Floats Left of the Data Date
Because GPM calculates total floats as of 31 Jan 13 both before and beyond the data date, GPM algorithmically identifies the critical path left of the data date
Data date (DD), the update closing date, splits the updated schedule between the progressed (as-built) portion left of the DD and the forecast portion right of the DD
Breakthrough #7:
Total Floats Left of the DD May Change as Total Floats Right of the DD Change
As total floats and the critical path change right of the data date, so do correspondingly change forensic total floats, and the as-built critical path left of the data date is the critical path existing for that update
Because GPM upholds the then-existing critical path principle, collapsing the as-built schedule in segments yields a solution that is symmetric with the retrospective time impact analysis counterpart
Breakthrough #8:
Context is the Key to Resource Leveling
Breakthrough #9:
Planners can Pull the Schedule and Build the Network in Reverse Order (Backward from Completion)
Summarizing the GPM Scheme of Thought
Relies on LDM, a visualization of time that combines ADM with PDM, and that has the potential to render Gantt charts with or without logic ties redundant
GPM software architecture relies on objects (i.e., activities, links, embedded nodes, milestones, etc.) that contain embedded rules and GPM algorithms
Activities are scheduled on early start dates or user-selected planned dates without start-no-earlier constraints and without forfeiting drift or reducing total float
Gaps are used to calculate drifts & floats; total floats are less useful than the combination of drifts & floats
In response to manipulation by the planner, GPM software in real time reschedules only activities that are manipulated or affected by the manipulated activities
When the goal is to level resources for the project, GPM software is a computer-aided process that, at each junction, amalgamates schedule context and project managers’ judgment
Offers the flexibility of backward or pull planning, essential on some projects as well as to lean construction and critical chain planners
The Contrast: Summarizing CPM Principles
Predominantly relies on tabular displays and logic Gantt charts developed in microsoft Excel
Activities, logic ties and milestones are housed in an Excel database; the CPM software aka scheduling engine contains the algorithms
Requires drift-choking constraints or drift-choking ‘manual dates’ anytime activities are scheduled after early dates
Calculates early and late start dates, from which total floats are derived
In response to user input (e.g., change a noncritical activity duration by one day), CPM recalculates both the forward pass and backward pass in their entirety (i.e., the CPM engine recalculates all dates, total floats, etc.)
Levels resources through software heuristics that apply rules in a pre-established order/priority regardless of whether or not in context
Hinders use by lean construction practitioners due to lack of backward planning capability
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