LECTURE 2

NETWORK DIAGRAMS

Nodes = Activities Links = Relationships Construction Network Elements of a Construction Network – The Precedence Diagram Method (PDM) Activity 1 Nodes = Activities Links = Relationships Talk about AOA briefly as another type of network representation and mention that we will cover this later. Also mention that there are several types of relationships (list them out) but for now we will only look at F-S. Activity 2 Activity 1 Construction Network Finish-Start i j

Types of Activities on a Network Start Finish Dummy Activities Milestones Activity 1 Tasks

Draw a Network Wall Wall Footing Footing Joint One Set of Wall and Footing Forms are Available

Construction Method Wall 2 Wall 1 Footing 2 Footing 1

WBS Task Description Duration 1 Footing 1 5 Days 2 Footing 2 3 Wall 1 4 Wall 2

The Network Diagram 2 Footing 2 4 Wall 2 Start 1 Footing 1 Finish 3 Stress that knowledge of construction and construction methodology is very important here. There are many ways to draw this network – one is you can start with Footing 2 – two is you can add more detail by scheduling the erection of forms and so on.

Can we improve on the Gantt Chart and the Network? Gantt Chart omits relationships What happens to activity B and C if A is delayed? PD does not have the time element On what day should C start? Is it ok if it starts a day later than planned? Activity A Activity B Activity C Activity B Activity A Activity C

Critical Path Method tries to bridge these two gaps

CRITICAL PATH METHOD - BASICS

Historical Origins Developed by Dupont and Remington Rand in the 1950’s to manage plant maintenance projects Prof. John W. Fondahl, Stanford University adapted CPM for Construction in 1961

Concept 1 : 4 Key Dates Early Start Date (ESD or ES) Early Finish Date (EFD or EF) Late Start Date (LSD or LS) Late Finish Date (LFD or LF)

Total Time Available for Activity A ES EF LS LF Activity A Activity A TA TA Use the Final Project in this class as the example to describe these dates Total Time Available for Activity A

Early Finish Date ES EF Activity A TA EFA = ESA + TA

Early Start Date ESJ = Max EFi V i ESA EFA = 10 ESc ? = 10 ESB EFb = 8 Activity A Activity B Activity C i j i ESc ? = 10 ESB EFb = 8 ESJ = Max EFi V i

Late Finish Date LFi = Min LSj V j LSB =8 LFB LFA? =6 LSC =6 LFC Activity B LFA? =6 j Activity A i Activity C j LSC =6 LFC LFi = Min LSj V j

Late Start Date LS LF Activity A TA LSA = LFA - TA

Concept 2: 12 Relationships 4 categories of relationships Finish-Start Relationships (F-S) Start-Start Relationships (S-S) Finish-Finish Relationships (F-F) Start-Finish Relationships (S-F) Three types for each category No lag Positive lag Negative lag 4 x 3 = 12 relationships Just as for ragas in Carnatic music – all of these 12 relationships do not make sense and so in practice we will have a smaller subset Ask for examples for each type here and on each of the following pages

Finish-Start Relationships +ve lag No lag 1 day 1 day 1 day Place Forms Pour Roof Slab Remove Slab Shoring 14 3 days 2 days Place Forms and Pour slab has no lag. Slab and remove shuttering has 14 day lag. Pour foundations can take place a day before the excavation ends because the site is large and foundations can be poured on the already excavated portion. Excavate Pour Foundations -1 -ve lag

Start-Start Relationships 1 day Place Forms 1 day No lag Prepare Rebar 3 days Excavate 2 days +ve lag 1 day Pour Foundations First has no lag. Second and third have positive lag. FS with lag can be SS with lead, as in the second picture above Start-Start relationship with negative lag can be replaced by S-S with positive lag by alternating the activities. So this representation is not used much. It is useless and confusing. There is a difference between F-S and S-S. For instance, as soon backfill is done and a PCC coating is provided, placing forms as well as creating reinforcing ties can start. However, if the forms are not available, then the reinforcement will lie and rust if it is a F-S relationship. On the other hand if it is S-S, then reinforcement starts later and this avoids rusting. SS therefore facilitates Lean and JIT construction 6 months Design 1 Year +ve lag 3 months Construct

Finish-Finish Relationships 1 day Grade End-Sems 4 months No lag Prepare Final Grades 1 day First has no lag Second has positive lag FF with negative lag is usually not done as the activities can be rearranged and the lag could become positive. Grade End-Sems 1 day 4 months +ve lag Prepare Final Grades

Start-Finish Relationships 1 day Interior Wood Paneling 2 days No lag Interior Carpeting It is probably easier to represent this as a Finish-Start Relationship S-F relationships are rarely used – except in showing combined relationships.

More on Relationships Lags can be represented as percentages Most Relationships are “Can” relationships B CAN finish when A finishes B CANNOT finish before A B can be delayed and finish after A Second activity can usually be delayed 2 days A 2 days B

Concurrent Activities Test Mock-up Wall Build Forms These two activities are completely concurrent as they have SS and FF relationships. Ask the class for some examples here. Get Wall Shop Drawings Approved Assemble Rebar

Representing Activities Early Start Activity Number/Free Float Early Finish Activity Name and Duration Late Start Total Float Late Finish