Understanding Scheduling in Construction Projects

1. Design Stage 1 Preconstruction Stage 2: Procurement Conceptual Planning Stage3: Construction Stage 4: Project Close-out Chapters8, 9, and 10

2. Scheduling in Construction Projects • The time to construct a project is limited and specified in the contract. • The owner will expect to be reimbursed for delays. Liquidated damages provision may be included in the contract. • The contractor will save overhead costs and will be able to reassign some of the resources if the project is not delayed.

3. a large number of different parties is involved in most construction projects. Schedules are essential to provide a common means of communication between them. • Scheduling is important in estimating and leveling resources. • Scheduling is done at several stages: • Design in final stages to produce a general or master schedule • Procurement: contractors need to submit a schedule with their offer and check that they are committed to a possible schedule • Construction: detailed scheduling, updated as appropriate

4. Bar Charts( Gantt Charts)

5. Bar Charts( Gantt Charts) • Advantages: • easy to interpret. • show clearly the general progress of a project by showing the scheduled versus the actual progress. • effective means of communicating to upper management about the overall progress. • Disadvantages: • Hard to show complicated interdependencies between activities. • Bar Charts oversimplify the activities and represent activities that are broad in scope. What will take place on any given day? • Without a software, bar charts are not easily updated. • Time-Scaled bar charts, combined with networks,avoid many of the disadvantages of bar charts, see schedule of oceanography building.

6. Network Analysis • Notations: arrow and precedence, example. • We will cover arrow diagrams manually and precedence diagrams through SureTrack. • Advantages: • Networks show the interdependencies of all activities. • They can easily be updated with a computer software. which can determine the impact of any change on the other activities. • Permit the determination of critical activities.

7. Term DefinitionsArrow diagrams • Activity: Performance of an operation that consumes time. denoted by arrows in arrow diagrams. • Event: a point in time such as the beginning or the end of an activity. Can be presented by a node. • “i” and “j” nodes: start and end of an activity. • Duration: the estimated time to perform and activity. • Dummy: an activity that uses no resources and consumes no time. Used, when necessary, to provide a unique notation for each activity and to show the proper logic in a network.

8. Activity Relationship in a network • Activities • Are represented by arrows. • Must have a definite starting or ending point or node. • Referenced by their node designations. The beginning is described as the “i” node, the end is described as “j” node. For example, activity (3-4). • Nodes • Points in time • Consume no resources • They are the beginning and ending points of an arrow(activity). 3 4

9. An activity can be followed by more than one activity (burst). • More than one activity may be the immediate predecessors of one activity (merge). • Dummies: superficial activities inserted to accurately portray the proper relationship in a complex network. Also, activities must have unique IDs. Examples: • Activities A and B are followed by C but activity D is preceded by B only? • Activities A and B start and end at the same node. More network logic examples.

10. Arrow Networks Rules • A network has a single starting node and a single end node. • Networks are continuos. • No activity can start until all preceding activities have been completed. • Arrows are not to scale • Each arrow indicates a single activity

11. Calculating Network Information • Each network, that we will consider, starts and ends with a single node. • The first node is assigned the start of the first day. • All values computed describe node occurrence times in terms of the start of a work day. • What is the meaning of each number in the network? Remember that there are different ways to organize this • Numbers inside the nodes are node designations (i or j). • Numbers on the nodes are the early point in time which is represented by the node (EET). • Numbers on or under the middle of the arrows are the duration of the activities.

12. Numbers at the head of the arrows are the finish time of the activities • Numbers at the beginning of the arrows are the beginning time of the activities. • Numbers underneath the arrow heads, tails, or the nodes (LET) are equivalent to the numbers on top of them computed by performing a “backward pass”. They represent “late times” 27 39 12 27 39 3 4 28 40 28 40 28

13. More Definitions • Early Start (ES): the earliest time that an activity can start. Determined by the latest early completion of all its preceding activities. • Early Finish (EF): the earliest time that an activity can finish. Determined by adding the duration of the activity to the early start of that activity. 13 ? 17

14. Late Finish (LF): the latest time that an activity can finish without delaying the entire project completion. Determined by the earliest late start of all its succeeding activities. • Late Start (LS): the latest time that an activity can be started without delaying the project completion. Determined by subtracting the duration of an activity from its late finish. • Critical: a term that implies that an activity cannot be delayed without extending the project duration 24 ? 26

15. 14 7 14 8 1 6 21 4 5 16 6 4 12 Forward Path • Forward Path: • used to determine the early times • starts with a node at day 1, which is the early start of all succeeding activities. Add the duration of each activity to its early start to get the early finish. • In case of a merge the early event time of a node is the latest early finish of merging activities. • The early event time of a node is the early start of all succeeding activities. ? ? ? 12

16. 14 7 14 8 1 6 21 4 14 21 25 5 16 6 4 12 25 17 17 Backward Path • To compute the late times • Starts at the last node. The late time of the last node is the early time. • Subtract the duration of each activity from the late date. • In case of a burst the late finish of the node is the smallest finish time of the succeeding activities 25 21 ? ?

17. We will discuss total float and free float. • Total float of an activity is the amount of time that the duration of the activity can be extended without increasing the duration of the project. What happens when a total float = zero ? • An activity of zero total float is a critical activity and lies on the critical path. • The critical path consists of a series of connected activities between start and end nodes. Activities on the critical path have zero total float. • There should be at least one critical path in each network.

18. 34 28 13 7 13 8 0 5 20 4 13 20 24 5 15 6 4 11 24 16 16 TF = 6 Or simply compare: 34 40 Total float = TFact = LSact -ESact = LFact-EFact TF = 0 TF = 0 TF = 0 13 TF = 0 24 5 0 20 5 11 TF =? TF =?

19. Free Float • The number of days that an activity duration can be increased before the start date of any activity in the network is effected. Free float = early event time of node j - EF of activity i-j or, simply subtract:EET - EF EET EF

20. Summary of network computations 9 ? ? • Node times: • Forward (EET): • merge: take the largest EF • burst: same as EF • Backward (LET): • merge: same as LS: • burst: take smallest LF 5 9 ? 9 ? 34 34 34 29 29

21. Float: • total float = c - a • free float = b - a • All you really need to compute are the EET and LET at the nodes and the EF of the activities. • Example b a c