Powerful Job Shop Scheduling Solutions for Linear Construction Projects

Summary: This article aims to reveal a strong similarity between a class of construction projects and production systems in a class of manufacturing systems known as job shops and it proposes adoption of powerful job shop scheduling solutions for efficiently generating and optimizing schedules of those projects.

From scheduling point of view, a construction project is viewed as a set of related activities which have certain durations, precedence relations and resource and material requirements. It may also involve some other constraints like material available times and permissions from external agents on the schedules of individual activities. Some uncertain events may occur during its progress and there may be uncontrollable random variation in activity durations. The output of a construction project is a physical entity.

Similarly, manufacturing systems like large fabrication shops make large physical entities by performing a set of operations on shop floor using required materials and resources. An example is custom vehicle production which involves production and assembly of many components and sub-assemblies. Uncertainty and random variation in production operations occur even in manufacturing systems but their magnitude may be less in comparison to those in construction projects. Material available times may have a major impact on production schedules in manufacturing systems.?

Basically, both construction projects and large fabrication shops in manufacturing environment create physical objects by performing a set of activities using required materials and resources. The fabrication shops belong to a class of manufacturing systems known as job shops. In simple words, a job shop is a production system that makes a variety of low-volume products which may have different process requirements. A broad description of job shop is given at a later stage in this article.

Construction of a physical entity can be viewed as manufacturing activity, at least from scheduling point of view.

In fact, a construction project can be easily viewed as production in a job shop. Lean Construction people see a lot of similarity between a construction project and a production line (like a Toyota assembly line) that makes very similar products. For scheduling such projects, they adopt Pull planning and Takt planning inspired by production control based on Takt time and one-piece flow on production lines.

?After realizing a strong similarity between construction projects with repeated activities and job shop production, we can easily adopt powerful, rigorous and efficient scheduling methods for those projects. It is discussed in this article. Those methods will be far more effective than CPM for such projects.?

A Model of Construction Projects with Repeated Activities:

A class of construction projects with repeated activities can be described as follows. All activities of the project can be partitioned into sectors termed as zones, locations or units so that the set of activities is the same to a large extent in every sector, that is, activities are repeated across sectors. Each repeated activity is to be performed in all sectors by a specific trade. Every trade performs its activity in the same sequence of sectors. The sequence in which activities are performed in a sector is the same for all sectors. The sequence in which activities within a sector are to be performed is also the same for every sector.?

For example, activities of a project are partitioned into n sectors S1,S2, ...., Sn and each sector has a set of m activities A1,A2, ...., Am which are to be performed by n distinct trades T1,T2,.....,Tm, respectively. Activities A1,A2,....,Am are to be performed in each sector in the order (A1,A2,....,Am), that is, trades will go through each sector in the order (T1,T2,.....,Tm). Trades have certain productivity rates. The activity durations are computed from productivity rates of trades and the amount of effort involved in each activity in each sector.

?Examples of such projects include (a) construction of a multi-storied building, (b) construction of a set of similar homes in an area and (c) laying a highway, pipeline, long tunnel or railroad.?

In the construction of a multi-storied apartment building, the excavation and foundation are followed by repeated activities like structural framing, welding, concreting, brick masonry, exterior finishes, interior finishes, plumbing, insulation, weather proofing, etc. These activities may be followed later by landscaping, final inspection, testing, etc.?

The construction projects mentioned above are basically linear construction projects with repetition of some activities across the sectors. In such projects, the work progresses through a specific sequence of sectors.?

?Before CPM was developed, construction people were using linear scheduling methods to schedule linear construction projects. Line of Balance method is an example of such methods. These scheduling methods have roots in production lines in manufacturing environment.?

Similarity between Linear Construction Projects and Production in Manufacturing Systems:

From scheduling point of view, we can see a lot of similarity between a linear construction project and a production line that makes similar products one after another. Every product goes through the same sequence of work stations and every work station processes products in the same sequence.?

Sectors in a linear project are like products moving on a production line in a sequence and trades performing repeated activities are like work stations on the line. Some Lean Construction people generate Takt plans of linear construction projects. In manufacturing context, Takt planning amounts to creation of one-piece flow on a balanced production line.?

The linear project described above in abstract form can be seen as an n-job, m-machine flow shop with a specific sequence of jobs. The duration of the project is the makespan of the flow shop, that is, the total time needed to finish all jobs in the flow shop. Using an Excel worksheet, we can easily compute makespan and find critical tasks for any given sequence of jobs.

Imperfect Linear Construction Projects:

Many construction projects involving repeated activities contain but only repeated activities but also some non-repeated activities. The non-repeated activities may have precedence relations with repeated activities and need some resources.?

For example, in a building construction project, excavation, leveling and foundation precede all the remaining work in the building. Similarly, some activities like landscaping, final inspection, testing, etc. may follow all the other work in the building.? It is also possible that there could be some non-repeated activities in any sector and they may have precedence relations with repeated activities in the sector. Those non-repeated activities may or may not require resources of limited capacity.?

Such imperfect linear construction project with repeated and non-repeated activities can be seen a special case of job shops. In manufacturing environment, job shop is a production system that makes a variety of products which may have different process requirements. A general concept of job shop is a production system in which different products go through different sequences of work stations. This is a narrow concept which does not cover real-life job shops. For the last 20 years, I have been working with more complex job shops in manufacturing world. In a job shop, the set of production operations is not necessarily the same for all products. Similarly, the precedence relations among production operations are not necessarily the same for all products. The imperfect linear construction project is a very special case of job shop because the sequence of sectors through which every trade goes is fixed.

?By considering a linear project as a job shop, we can adopt the powerful, resource-constrained scheduling logic that is available for resource-constrained, complex job shop production. Such scheduling is far more superior to the weak scheduling method CPM which cannot handle resource constraints.

?In this approach, the project schedule can be seen as a function of production rates of trades, that is, a function of crew sizes and calendars. This view enables schedule optimization more effectively than CPM does. Since each repeated activity is performed by a specific trade, for improving the schedule, we can identify bottleneck resources and enhance their capacities in order to improve the schedule. Job shop scheduling will be quite useful to linear projects in this regard.

Regarding the assimilation of the overall project schedule, it is much easier to understand the schedule through a summary Gantt chart than through the standard Gantt chart of CPM schedule. In the summary Gantt chart, we see the schedule of all activities in a sector on a separate line. It is very similar to time-location chart associated with linear scheduling methods.

Some similarities between an imperfect linear project and job shop production:

1. Project and production can both be seen as a set of related activities.

2. In both cases, activities have precedence relations on the basis of technical and engineering aspects.

3. In both cases, activities have material requirements and material supply can cause delays in starting the activities.

4. In both cases, there may be temporal constraints on starting times of activities.

5. Activities are grouped into sectors in the project and grouped into products in production.

6. In terms of scheduling, workload in a sector in the project is similar to the set of operations to be done for a product.

7. Any repeated activity is performed by a specific trade across sectors in the project whereas a resource may perform a specific operation for all or many products in production.

Conclusions:

From scheduling point of view, there is a strong similarity between linear construction projects and making a set of products sequentially in production systems like job shops. A linear construction project can be easily formulated as job shop production for scheduling purpose. Many people in construction projects may not realize this possibility because they may not be aware of the nature of production scheduling in job shops. They may also be unaware of the power of scheduling tools available to job shops.?

Lean Construction sees the similarity between linear construction projects and adopts Takt planning that is very similar to control of one-piece flow on a balanced on a production line. It is not easy to create and implement a Takt plan of a construction project due to random variation in activity durations and uncertain events. A construction project cannot be easily made equivalent to management of a balanced production line with no variation.?

This article suggests formulation of a linear construction project as job shop production for scheduling purpose. This is because the scheduling solutions available to job shops are far more powerful and elegant than CPM for scheduling linear construction projects. It may be noted that linear scheduling methods were successfully implemented for construction projects before CPM was developed. Those methods including Line of Balance method also have roots in manufacturing systems. But job shop scheduling solutions are more powerful and rigorous than the linear scheduling methods for linear construction projects. By creating dispatch lists for resources over short periods, those solutions provide good guidance for workers on site. They also enable fast, reliable and extensive what-if analysis with respect to production rates of crews.

For further information about this topic, contact Dr. Prasad Velaga of Optisol LLC at [email protected] .