CENG106B Network Conventions PDF

## Network Conventions ### The Making and Reading of Network Diagrams The making and reading of a network diagram requires some familiarity of the network conventions. One of the main features of PERT and CPM is their use if a network or precedence diagram which is the key in solving typical probl...

## Network Conventions ### The Making and Reading of Network Diagrams The making and reading of a network diagram requires some familiarity of the network conventions. One of the main features of PERT and CPM is their use if a network or precedence diagram which is the key in solving typical problems. ### Network Diagram Composition The network diagram is composed of a number of **arrows** and **nodes**. ### Network Diagram Conventions There are two slightly different conventions for making a network diagram. They are: - **Activity on Arrow (AOA)** - that is using arrows to designate activities. - **Activity on Node (AON)** - using nodes to designate activities. ### The Elements of Network Diagramming - **Work B cannot start until after work A is completed.** > This can be represented by a diagram where A is a line pointing to B. - **C cannot start until both A and B are completed.** > This can be represented by a diagram where A and B are both pointing to a node, and C is a line pointing away from the node. - **A must be completed before either B or C can start.** > This can be represented by a diagram where A points to a node, which then has separate lines leading to B and C. - **Both A and C must be completed before either B or D can start.** > This can be represented by a diagram where A and C point to a node, which then has separate lines leading to B and D. - **A must be completed before C can start. D depends only on B and A, X is called dummy.** > This can be represented by a diagram where C branches off of A, and D branches off of B. A line labelled "X" connects C to D. - **D depends on A and B. C depends on A only. Y is a dummy.** > This can be represented by a diagram where A and B both point to D, and C branches off of A. A line labelled "Y" connects C to D. ### Principles in Networking or Arrow Diagram #### Principle No. 1 That everything in the network or arrow diagram must have a meaning. - **a) Every arrow represents an item of work and is called Activity** > For example: The arrow labeled "Excavation" represents the activity of excavation. - **b) An Event is the starting point of an Activity represented by a circle, square or any geometrical form.** > The arrow labeled "Excavation" starts at the beginning of a circle. - **c) An Activity is dependent upon and cannot begin until after the completion of all preceding Activities.** > For example, a diagram with "Making forms" pointing to "Pour footing slab" and "Order and deliver cement" pointing to the same "Pour footing slab" means that the "Pour footing slab" activity cannot begin until both "Making forms" and "Order and deliver cement" are complete. - **d) All activities that start with the same event cannot begin until after the completion of all activities that enter that event.** > For example, a diagram with "Pour footing slab" and "Pour pedestal" both pointing to the same event, means that neither can begin until the activities that enter the event are complete. For example, "Making forms" and "Order and deliver cement". #### Principle No. 2 That an activity has a single definite starting point and a single definite ending point. An arrow in the network must satisfy two basic questions: - What activities must be completed before this one can start? - What activities cannot be started if this one is not completed? #### Principle No. 3 The network (arrow diagram) does not describe time relationships, but rather dependency relationships. The length and direction of an activity arrow has no relationship with the amount of time required to accomplish the work presented by it. Similarly, two activities starting with the same event do not necessarily occur at the same time. #### Principle No. 4 That the network (arrow diagram) is hardly ever done by a single person. The accomplishment of the schedule made in the network diagram is influenced by several numbers of people who have something to do with the project and should be consulted when making the network. ### Fundamental Elements of PERT/CPM Networking - **Activity** - is the work operations required to complete a task, or in some cases, a segment of that task. In short, it is the actual work between events. - **Events** - is a point in time signifying the beginning or end of one or more activities. It denotes the completion of all preceding or predecessor activities, and the beginning of one or more succeeding activities. > The event is indicated on the network as a circle, square, ellipse, or any form of geometrical symbols with a number written therein for identification. - **Numbering the events follows the construction of the network.** - **No two events shall have the same identification number in the network.** - **For this reason, one or more dummy activities may be used in a network.** - **Dummy Activity** - is an arrow on a network showing the dependent relationship between two activities. However, dummy activity does not represent actual work efforts and does not consume time. Dummy activity may be used in the network to distinguish between two or more parallel activities. - **Duration of an Activity** - is the time it takes an activity to be finished, or in short, the normal time duration of a task. Duration activity is the span of time from the Early Start to its Early Finish, or from its Late Start to its Late Finish. - **Path** - is a sequence of activities that leads from the starting node to the finishing node. ### Rules for Numbering Events in a Network Diagram 1. All events should be numbered starting from left to right. 2. No event number should be lower than the number of the event preceding it (left to right direction). 3. Event numbers shall increase from left to right. 4. Events should be numbered from top to bottom. Similarly, increasing should be consistent from top to bottom or bottom to top. ### Example From the following table of information: | Activity | Nodes | Duration | |---|---|---| | A | 1-2 | 6 | | B | 1-3 | 5 | | C | 2-4 | 3 | | D | 2-5 | 2 | | E | 3-4 | 4 | | F | 3-5 | 6 | | G | 4-6 | 10 | | H | 5-6 | 7 | > **(a) Draw a precedence diagram** > **(b) Find the critical path** > **(c) Determine the expected duration of the project.** A representation of the precedence diagram is a network diagram that starts at node 1 and ends at node 6. The following paths are possible, with the duration of each activity added to find the total duration of the project: - 1-2-4-6 = A-C-G = 6+3+10 = 19 weeks - 1-2-5-6=A-D-H = 6+2+7 = 15 weeks - 1-3-4-6=B-E-G = 5+4+10 = 19 weeks - 1-3-5-6=B+F+H = 5+6+7 = 18 weeks The **critical path**, also known as the critical chain, is the path of the network that takes the longest amount of time to complete. In this case, the critical paths are 1-2-4-6 and 1-3-4-6, both of which take 19 weeks. Therefore, the expected duration of the project is **19 weeks**. <br>