Construction Project Scheduling. Techniques for Scheduling

Transcription

1 Construction Project Scheduling Project scheduling is the process of: Identifying the activities necessary to complete the project Assigning realistic durations to each activity Establishing start and finish dates for each activity Desired outcomes of project planning & scheduling Finish the project on time Continuous work flow Reduced amount of rework Clear understanding of who does what, when and how much Techniques for Scheduling Several techniques and tools can be used for project planning and scheduling Common techniques used for construction scheduling include: Bar Charts Linear Scheduling Critical Path Method (CPM) Tools, techniques and software used for project scheduling depend on: Project Size Project Type Project Complexity Owner Requirements 1

2 Bar Charts A bar chart is a graph showing collection of activities listed in a vertical column with time represented on a horizontal scale Advantages: Disadvantages: Easy to read and understand Does not clearly show the logical relation between Effective communication tool activities between engineer and foreman Ineffective in determining Easy to develop the impact of one activity on project finish time Difficult to update as activities are not linked Linear Scheduling Linear Scheduling Method (LSM) is mainly used for linear repetitive operations such as: Highway Construction Tall Buildings Pipeline Construction Housing Projects A Linear Schedule Diagram is used to plan and record progress on multiple activities On one axis of the diagram time is plotted while on the other axis location or sequence number of repetitive units are plotted Advantages Disadvantages Displays rate of progress for activities Displays activities at a certain location Simple and effective presentation Mainly used for repetitive operations Incapable of handling complex projects 2

3 Linear Schedule for a Pipeline Project CPM Scheduling Network Analysis System A network analysis system is a technique of defining work by a graphical diagram that shows activities and relations between activities Critical Path Method (CPM) is a network analysis system technique commonly used for construction planning and scheduling CPM consist of: 1)Defining activities 2)Assigning durations to activities 3)Defining logical relations between activities 4)Determining activity start dates and floats 3

4 CPM Scheduling-Basic Definitions Activity: A single work step that has a beginning and an end. Examples: design foundations, pour concrete for foundations Network: A diagram that displays relations between activities to complete the project Duration: Estimated time required to perform an activity Early Start (ES): The earliest time an activity can be started Early Finish (EF): The earliest time an activity can be finished and equal to Early Start plus duration: EF = ES + D Late Finish (LF): The latest time an activity can be finished CPM Scheduling-Basic Definitions Late Start: The latest time an activity can be started without delaying completion of the project LS = LF - D Total Float: The amount of time an activity can be delayed without delaying completion date of the project TF= LF EF = LS ES Free Float: The amount of time an activity can be delayed without delaying early start time of the following activity Critical Path: A series of connected activities with each activity having zero float(free, and total) time. 4

5 CPM Example-1 For the following project find the critical path, project duration, ES, EF, LS, LF and TF for all of the activities. A B 2 C 3 D 4 E F 60 CPM Example-1 Critical Path: B-C-D-E, Project Duration: 140 days A B 2 C 3 D 4 E F 60 Activity ES EF LS LF TF A B C D E F

6 CPM Example-2 An activity which is on critical path has a planned duration of 5 days. A) What is the total float for the activity? B) Because of resource limitations the planned duration of this activity is increased to 10 days. What will happen if no changes are made on the schedule logic and on the activity durations? CPM Example-2 Answers: A) Since the activity is on critical path total float = 0 B) There is a 5 days delay on the activity since the activity is on critical path this will delay the project by five days if rescheduling is not performed. 6

7 Steps in CPM-1)Determination of Activities The first step in preparation of any type of schedule is to break the project scope into individual activities Work Breakdown structure should be used for identifying the activities and for organizing the schedule The number of activities included in the schedule depends on the type and complexity of the project If too few activities are used: Difficult to observe progress Problems can not be identified earlier Accurate resource planning could not be made If too many activities are used: Difficult to follow the schedule Difficult to update Too many commitments if the schedule is submitted to the owner WBS Structure for EPC PROJECT EPC PROJECT ENGINEERING PROCUREMENT CONSTRUCTION AREA 50 AREA 70 AREA 72 AREA 83 CIVIL PROCESS MECHANICAL PIPING ELECTRICAL INSTRUMENTATION STRUCTURAL STEEL DESIGN TOWER 1 DESIGN PIPERACKS SECTION 7 DESIGN BUILDING A Design Squad Check Approval Shop Drawings BULK MATERIALS EQUIPMENT ELECTRICAL PACKAGES INSTRUMENTATION PACKAGES MECHANICAL PACKAGES AREA 50 AREA 70 AREA 72 AREA 83 BRIQUETTING PRESS HEAT EXCHANGERS FABRICATED ITEMS BUCKET ELEVATORS Tender Evaluate & Award Manufacture Transportation Customs MOBILIZATION SITE GRADING STORM WATER SANITARY SUBSTATIONS AREA 50 AREA 70 AREA 72 AREA 83 STRUCTURAL STEEL MECHANICAL PIPING ELECTRICAL INSTRUMENTATION CIVIL TOWER 1 PIPERACKS SECTION 7 BUILDING A FOUNDATIONS ELEVATED SLABS SLAB AT EL SLAB AT EL SLAB AT EL Form Rebar Pour 7

8 Steps in CPM-2)Assigning Durations The durations assigned to activities are important because the critical path, timing of activities, distribution of costs all depend on activity durations The durations of activities will depend on mainly: Quantity of work and quality requirements Size of work crew assigned to activity Availability of equipment and material Labor productivity The durations of activities should be reasonable. Allowances should be made for interruptions and other events impacting time Usually there are three methods for assigning durations: Historical records of previous projects Data obtained from publish manuals Experience and judgment of the the scheduler Feedback from project team (supervisors, foreman) should also be included during assigning durations Steps in CPM-3) Definition of Logic The order in which activities will be performed is called logic How one activity relates to another activity is called logical relation Three logical relations types are generally used for construction scheduling: Finish to Start Finish to Finish Start to Start Defining proper logic requires understanding of the project A scheduler must interact with all disciplines and trades in order to understand how activities relate to each other There is always more than once correct logic for completion of a project. The idea is to define a reasonable and possible logic 8

9 Steps in CPM-3) Logical Relations Finish to Start Relation A-PLACE REBAR FS B-POUR Activity B starts after CONCRETE activity A is completed Finish to Finish Relation A-FORM FOUNDATION FF + 5 B-PLACE REBAR Activity B finishes 5 days after activity A is completed Start to Start Relation SS + 15 A-MOBILIZATION B-SITE GRADING Activity B starts 15 days after activity A starts Steps in CPM-3) Logic Diagrams Logic diagrams are used to define/trace logic There are two methods for drawing CPM diagrams: Activity on arrow diagram Activity on node diagram Activity on arrow diagram allows only finish to start relation, and may require dummy activities to define certain relations Activity on node diagram is usually preferred D B D F B F A B' H A C G E C E G H Activitity on Arrow Diagram Activitity on Node Diagram 9

10 Steps in CPM-4) Start Dates, Floats A software is used to determine start and finish dates, critical path and activity floats. Common software used for construction project scheduling are: Microsoft Project Primavera Project Planner Several reports can be obtained from the software including: Classical Schedule Report (Early Start/Finish, Late Start/Finish, activity floats) Bar Charts, Logic Diagrams Cost Curves Resource Histograms Updating the Schedule Only in very rare cases the planned schedule and the actual schedule are same Marking the progress on the schedule is called updating. After the schedule is updated it needs to be checked against the target schedule for deviations Performing major changes on schedule logic, durations and resources are called rescheduling or revision Schedule updating is very important for identifying problems early and taking the necessary corrective action 10

11 Completing the Project On Time What happens if the project is not completed on time: Delay penalty applies (Usually 5% - 15% depending on contract conditions) Increased overhead costs (office, management, insurance etc.) and possibly increased direct labor cost Problems in the cash flow Dissatisfied owner EPC Scheduling Lime Slaking Unit 11

12 EPC Scheduling Lime Slaking Unit LIME SLAKING UNIT ENGINEERING PROCUREMENT CONSTRUCTION TESTING PROCESS MECHANICAL/PIPING CIVIL/STRUCTURAL ELECTRICAL/INSTRUMT. EQUIPMENT SLAKING EQUIPMENT TANKS BULK MATERIAL PIPING STEEL ELECTRICAL/INSTRUMT. CIVIL Mobilization-Camp Facilities Excavation-Foundations Form/Rebar/Pour-Foundations Structural Steel MECHANICAL / PIPING Lime Tank Slaking Tank Slaking Equipment Piping ELECTRICAL / INSTRUMENTATION TANKS PIPING ELECTRICAL / INST. Electrical / Instr. Installation EPC Schedule Determination of Logic Activity ID Activity description Predecessors Duration Area Total Manhours 100 Contract Award - 0 PMIL 110 Process Design 100/FS 60 DES 120 Mechanical/Piping Design 110/SS+30,110/FF DESN 130 Civil/Structural Design 120/SS+30,120/FF DESN 140 Electrical/Instrumentation Design 120/FS 60 DESN 150 Slaking Equipment--Tender/Award 110/SS PROC 160 Slaking Equipment-- Manufacture/Deliver 150/FS 120 PROC 170 Tanks-Tender/Award 120/SS PROC 180 Tanks - Fabricate/Deliver 170/FS 60 PROC 190 Structural Steel -Take-Off/Tender/Award 130/SS PROC 200 Structural Steel - Fabricate/Deliver 190/FS 45 PROC 210 Piping Material-Take-Off/Tender/Award 120/FS 40 PROC 220 Piping Material - Deliver 210/FS 40 PROC 230 Electrical Material/Instr.-Take-Off/Tender/Award 140/FS 40 PROC 240 Electrical Material/Instr. - Deliver 230/FS 50 PROC 250 Mobilization - Establish Camp Facilities 130/FS 30 CONS 260 Excavation - Foundations 130/SS+50,250/FS 5 CONS Form/Rebar/Pour - Foundations 260/FS 15 CONS Install Structural Steel 200/FS,270/FS 25 CONS Install Lime Tank 180/FS,270/FS 20 CONS Install Slaking Tanks 180/FS,280/FS 10 CONS Install Slaking Equipment 160/FS,280/FS 5 CONS Piping Installation 220/FS,290/FS,300/FS 20 CONS Electrical / Instrumentation Installation 240/FS,310/FS 20 CONS Testing-Tanks 290/FS,300/FS 10 TEST 350 Testing-Piping 320/FS 10 TEST 360 Testing-Electrical / Instrumentation 330/FS 10 TEST 370 Mechanical Completion 340/FS,350/FS,360/FS 0 PMIL 12

13 EPC Schedule-Logic Diagram EPC Schedule-Primavera Output 13

14 Primavera Optain a copy of P6 (R.8.4- student version) Saman AminBakhsh K