A couple of weeks ago, I was visiting California to present and discuss location-based planning at Stanford University and some area companies. In almost all discussions I had, the question about critical path, float and buffers came up. Although I argued that float and criticality do not have much relevance as tools for production control, I will address them in this article because it is a critical issue for contractors working in a CPM-based contractual climate.
Location-Based Planning vs. CPM
Location-based scheduling is a special form of CPM which automatically generates and updates logic based on locations. Therefore, any CPM schedule can be translated to a location-based schedule (with one location), and any location-based schedule can be changed to a CPM schedule (with location names as the activity names). The logic is exactly the same.
The only difference between location-based planning and CPM in float calculations is the powerful continuity constraint. If the continuity constraint is on (and it should be for most trades), the start date of the task is pulled forward, so that all locations of the task happen continuously. This allows the planner to optimize the schedule by finding the optimal production rates for tasks spanning several locations. By doing so, the planner is effectively minimizing the float of the project, but planning work continuity. To preserve the critical path, the float of all locations must be made equal during the backward pass.
Figure 1, below, shows as early as possible schedule of two tasks going through five locations. The successor task is faster than the predecessor, so the task becomes discontinuous. In this case, the total float will be calculated as in normal CPM.
In contrast, the following example delays the start of Task 2 and forces continuity. In this case, the total float of all locations becomes the same (zero) and all locations are critical.
Buffers vs. Lags
Basic CPM has just lags. Location-based scheduling has two similar concepts: the normal CPM lag and the buffer.
Lags are used only when it is technically mandatory to wait a number of days after finishing (starting) a predecessor and starting (finishing) a successor task. A good example of this is concrete curing, or a location lag of two floors between structure and finishes for safety reasons.
Buffers are a way of reducing the schedule risks of a project. Location-based schedules use quantities and productivity rates to define durations, which assume that crews can work with their optimal productivity. In the process of location-based schedule optimization, the total float of a project is decreased. Because of the combination of lesser total float and smaller durations, the total duration of a project is greatly compressed. However, this duration compression is impossible to achieve in practice - unless buffers are planned between the tasks.
Buffers are used to protect the continuous production of a successor task from the possible production deviations of its predecessor. Therefore, they should be owned by the General Contractor who is responsible for production. All the durations of a location-based schedule assume the best possible productivity for the crew in question. In effect, all materials should be available; all design complete and other crews should not be working in the same work area. Location-based planning aims to achieve these optimally productive conditions by isolating the crew using buffers. In the event that something goes wrong, the buffer is absorbed before the next trade suffers.
Visually, buffers and lags look the same in the Flowline. They both force an empty space horizontally between the tasks. Also, float calculations are exactly the same - lags and buffers both give the same critical activities. However, in progress forecasting, buffers are absorbed before the next trade's forecast is changed.
The figure below shows two tasks with a Finish- to- Start relationship in each location, with a buffer of 5 days. The Task 1 start-up delay of up to 5 days will be totally absorbed by the buffer with no consequences to Task 2.
The matter is different if there is any production rate deviation. Any size of buffer will run out if production is going too slowly; as shown in the figure below. The location of the red dot shows that if Task 1 continues at the same rate, Task 2 will run out of space in the second location. In this case, the buffer allows Management to detect the problem and react before the alarm becomes reality.
Criticality and Float as Management Tools
Location-based management puts the greatest emphasis on the management of resource flows and isolating crews from each other using buffers. Any break in work continuity will result in additional costs for the associated subcontractor. Therefore, alarms showing up in a location-based schedule should be the main management tool, because they indicate both time and money will be lost. Buffers allow management to notice the deviation before it happens and gives them time to react.
From a project perspective, production rate and continuity is more important for some tasks. These tasks can be defined by criticality and float. Remember that if any location of the task is critical, the production of all locations becomes critical. This makes controlling easier, because criticality is defined by the type of work - not the location where the work happens. Even if the critical path transfers from one task to another in a later location, it is critical to achieve the correct production rate well before that location. It is crucial to preserve the work continuity of all critical and near-critical tasks, because if a subcontractor has to leave the site because of a lack of work, the task will very likely be delayed by the return delay of the subcontractor. Let the critical subcontractor do his work enjoying optimal conditions!
Location-based management is inherently collaborative in nature. The General Contractor commits to providing optimal work conditions, and the subcontractors commit to providing the required production rate. Subcontractors benefit from higher productivity and cost savings, and the GC benefits from a higher probability of achieving the schedule. Ask your subcontractors - you will find that everyone wants to work this way!
To learn more about location-based management and flowline scheduling theory, please review the archived webinar BIM 401: Model-Based Scheduling. If you're ready to jump to an advanced course, please review Mastering the Schedule in a Hard Bid.