February 21, 2008

The basic assumption of location-based planning is that working continuously results in higher productivity. Multiple crews working in the same location also result in decreased productivity and increased risk because resources might leave the site and return late.

In my PhD research I collected progress data from three projects weekly. All the projects showed the same result. If multiple crews are working in the same location, work either slows down or stops altogether. Slowdowns and stoppages happened even if the tasks did not have technical logic link with each other. Many tasks can be done in free sequence but they can not be done at the same time in the same location. For example floor covering work or plasterwork interferes with any other trade working in the same area.

Figure below shows a typical example from one of the case projects showing interference. Notice that the actual slopes of all of the tasks are very close to planned slope but only if the tasks can keep the complete location to themselves. Immediately when tasks happen at the same time in the same location, work either stops or slows down considerably. Average productivity loss is 20 to 50 % depending on the particular mix of tasks. In a project of 15 000 m2, I identified over 350 issues where productivity was lost because of these conflicts. It was also scary to notice from meeting memos that subcontractors had the same crew on site but achieved 50 % of production rate when there were conflicts. Think about the effects on subcontractor bottom line...

What are the implications for planning?

• CPM is not a valid planning technique because slowdowns and stoppages happening in locations even without technical precedence have larger effect than a delays on critical path (will be shown in my PhD)
• Buffers are necessary to keep trades far enough from each other
• Weekly location-based controlling is required to achieve productivity benefits and to prevent trades from clashing

For subcontractors, this will affect the bottom line directly. For General Contractors, project contractual milestones become easier to achieve because each task can be produced faster. Schedule risk of the project will decrease because slowdowns and stoppages are by nature unpredictable. Some of the time saving is required for buffers but at least 10 % duration reduction is possible without increases in risk level. Alternatively the same duration can be achieved with much lower risk.

November 26, 2007 

A couple of weeks ago, I was visiting California to present and discuss location-based planning at Stanford University and some 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 if they realize both time and money will be lost. Buffers allow management to notice the deviation before it happens and give 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!

I will write more on the topic of criticality, and how location-based management should be used for delay analysis and in the support of site meetings in later articles. Next time, I will return to the MEP (Mechanical, Electrical, and Plumbing) theme and describe the location-based planning process which includes MEP contractors. MEP is somewhat of a special case in location-based management, because their resources can work in multiple tasks. When one task runs out of work, there is usually something else that can be done to balance the resources. This will be addressed in later articles.

October 27, 2007 

Planning and production phase problems:

Problem 1: Location Breakdown Structure

Fig 1: Planned vs. actual status of MEP ducts task in shopping hall

Problem 2: Insufficient Quantities and Consumptions Information

The level of detail was usually insufficient, and many mistakes were made in the scheduling process. For example, the work of multiple subcontractors was lumped to the same task. The worst example was the task “MEP overhead”, which included the work of mechanical, electrical, plumbing and fire protection subcontractors and was planned with an estimated duration.

Much of the logic between the MEP and construction tasks is soft by nature. It can often be changed or reversed, but there may be cost consequences in doing so. For example, in spaces without suspended ceilings, the roof can be painted before or after MEP has been installed. If painting is done before or at the same time as MEP, the ducts and cables need to be painted with the same color prior to installation. Another common example is MEP and slab-on-grade: It is more productive to install MEP from the top of each floor, but it is possible, with additional cost, to install from the top of the gravel. 

Problem 5: Resource Constraints

Magnitude of Problems

Interviews of GCs and MEP contractors

GC Point of View

•  MEP tasks are long, dummy lines in master schedules

•  We do not have information about work quantities or required resources 

•  MEP contractors actually tend to have a balanced amount of resources in projects. This results in production problems, because we plan unbalanced resource use for them (because of a lack of knowledge)

•  Location breakdown structures do not take into account the requirements of MEP

•  We do not have enough contractual methods of controlling MEP production

•  Our own schedule performance is weak, so MEP contractors can always say: “work is progressing in parallel with the construction work” – because there are no resources in the schedule, it is impossible to determine the underlying reasons for any delays 
•  

MEP contractors agreed immediately that there is a huge problem with production control, and it is costing them lots of money. Electrical subcontractors in particular, suffer, because the bulk of their work is done in the final stage of the project. Production problems pile up and the electrical contractor is the poor guy who has to make up the delays of all the other contractors by adding huge amounts of resources in the final weeks of the project. These catch-up costs eat into their calculated margin. All MEP contractors say that productivity losses and uncertainty really hit their margins, because much of the work is done directly. The MEP contractor issue list is below:

• Resource needs are not balanced Resource needs cannot be predicted 

• It is difficult to control employees who work on a piece-rate basis (they maximize their own return)

• The Location Breakdown Structure way of thinking is not widely understood

• Production control happens only when the GC requires it

• MEP contractor cannot trust the GCs’ schedules

• Tasks cannot start on time because of the preceding tasks, and cannot be finished  

Trying to Find a Solution

• Location Breakdown Structures
• Task lists and logic
• Quantities in locations
• Task criticality and prioritization
• Risks
• Production control measures
• Contractor meeting procedures

• Resource and production rate commitments