The smallest unit of WBS is a project activity, project activity defined as any function or decision in the project that consumes time, resources and cost. project activity is a unique unit with a specified time, project activities can be divided into three groups based on the nature of the activity:
- Production activities: excavation, steel fabrication, formwork, and concrete casting activities are examples of production activities, production activities involve using of resources such as labors, equipment, and materials, production activity will have a specified quantity, resources time and cost. production activities for a project can be determined easily by studying project shop drawing and specifications.
- Procurement activities: this activity involves procuring the required materials and equipment for production activities, procurement activities will have a specific time, delaying of procuring required material will result in delaying the completion of production activities and overall delay of the project. procuring reinforcement steel is an example of procurement activity.
- Management activities: these activities related to a management decision, bad decisions can lead to failure and huge losses of money for the contractor's, on the other hand, good and quick management decisions will contribute to the success of the projects
The detailing of project activities will depend on the project stage and the purpose of preparing the project plan, in the pre-construction stage less detailing is used. during the construction, stage activities should be detailed properly. the level of detailing depends on several factors such as planning stage, size and complexity of the project and works, management expertise. figure 1 shows activities for a double-span bridge.