Engineering and Construction

Constructing a building of any signifi  cant size is a complex and costly endeavor, requiring the combined  efforts of countless participants and the coordination of myriad tasks.

Managing this process requires an in-depth understanding of the work required, of the ways in which differ-
ent aspects of the work depend upon each other, and of the constraints on the sequence in which the work must be performed.

Figure 1.11 captures one moment in the erection of a tall building. The process is led by the construction
of the building’s central, stabilizing core structures (in the photograph, the pair of concrete towerlike structures extending above the highest fl  oor levels). This work is followed by the construction of the surrounding fl  oors, which rely, in part, on the pre-viously completed cores for support.

Figure 1.11 In this photo, the construction sequence of a tall building is readily apparent: A pair of concrete core structures lead the construction, followed by concrete columns and fl oor plates and, fi nally, the enclosing curtain wall.

Attachment of the exterior skin can follow only after the fl oor plates are in place and structurally secure. And as the building skin is installed and fl  oor areas become enclosed and protected from the weather, further operations, such as the roughing in of mechanical and electrical systems, and eventually, the installation of fi  nishes and other elements, can proceed in turn. This simple example illustrates considerations that apply to virtually every aspect of building construction and at every scale from a building’s largest systems to its smallest details: Successful construction requires a detailed understanding of the tasks re-
quired and their interdependencies.

The construction project schedule is used to analyze and represent construction tasks, their relation-
ships, and the sequence in which they must be performed. Development of the schedule is a fundamental part of construction project planning, and regular updating of the schedule throughout the life of the project is essential to its successful management.

In a Gantt chart, a series of horizontal bars represent the duration of various tasks or groups of tasks that make up the project. Gantt charts provide an easy-to-understand representation of construction tasks and their relationships in time. They can be used to provide an overall picture of a project schedule, with only a project’s major phases represented (Figure 1.10), or they can be expanded to represent a larger number of more narrowly de-fi  ned tasks at greater levels of project detail (Figure 1.12).

Figure 1.10 In sequential construction, construction does not begin until design is complete. In phased construction, design and construction activities overlap, with the goal of reducing the overall time required to complete a project.

Figure 1.12 In a Gantt chart, varying levels of detail can be represented. In this example, roughly the top three-quarters of the chart is devoted to a breakdown of preconstruction and procurement activities such as bidding portions of the work to subtrades, preparing cost estimates, and making submittals to the architect (a). Construction activities, represented more broadly, appear in thebottom  portion (b).

The critical path of a project is the sequence of tasks that determines the least amount of time in which a project can be completed. For example, the construction of a building’s primary structural system is commonly on the critical path of a project schedule. If any of the tasks on which the completion of this system depends—such as design, shop drawing production and review, component fabrication, materials de-livery, or erection on site—are delayed, then the fi nal completion date of the project will be extended.


On the other hand, other systems not on the criti-cal path have more fl exibility in their scheduling, and delays (within limits) in their execution will not necessarily impact the overall project schedule.

The  critical path method is a technique for analyzing collections of tasks and optimizing the project schedule to minimize the duration and cost of a project. This requires a detailed breakdown of the work involved in a project and the identifi cation of dependencies between the parts (Figure 1.13). This information is combined with considerations of cost and resources available to perform the work, and then analyzed, usually with the assistance of computer software, to identify optimal scheduling scenarios. Once the critical path of a project has been established, the elements on this path are likely to receive a high degree of scrutiny during the life of the project, since delays in any of these steps will directly impact the overall project schedule.

Figure 1.13 The critical path method depends on the detailed analysis of work tasks and their relationships to generate an optimal construction schedule. Shown here is a schematic network diagram representing task dependencies. For example, task 6 cannot begin until tasks 1, 4, and 5 are completed, and tasks 7 and 9 cannot begin until task 6 is fi nished. The dashed lines on the diagram trace two of many possible paths from the start to the end of the diagram. To determine the critical path for this collection of tasks, all such paths must be identifi ed and the time required to complete each one calculated. The path requiring the most time to complete is the critical path, that is, the sequence of activities that determines the least time in which the collection of tasks as a whole can be completed.