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.
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
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. Bar Charts( Gantt Charts)
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.
Term Definitions Arrow 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.
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). 34
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.
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
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.
–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” 34 12 2739 27 2840 39 40 28
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. More Definitions 13 17 ?
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 ?
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 5 8 6 4 7 416 14 12 16 21 ? ? ?
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 5 8 6 4 7 416 14 12 16 21 25 17 14 ??
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.
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 EF EET
Summary of network computations Node times: –Forward (EET): merge: take the largest EF burst: same as EF –Backward (LET): merge: same as LS: burst: take smallest LF ? 9 5 ? 9 9 ? ? 34 29
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 a b c