CE 366 Exam 3

A Practical Guide to the CPM Process

• estimate activity duration

• compute project duration

• establish time intervals for activity execution

• identify activities critical to timely project completion

• shorten project duration on a least cost basis

• use float to make adjustments that will improve resource usage

• place the schedule within appropriate calendar dates

• document planning assumptions and boundary conditions

Estimating durations

duration =quantity / production rate

ex. 200cy of concrete / 100 cy per day = 2 days

What enables accurate time estimation?

company experience

Guidelines for Estimating Durations

• evaluate each activity independently

• assume “normal” crew and equipment

• assume a “normal” time interval (project day)

• focus on estimating the most efficient time within which to complete the activity

• use consistent time units throughout

• assume normal weather/working conditions

Considerations In Time Estimates

• the estimator should be experienced (good team activity)

• production rates provide a good source of information

• basing estimates on personal experience can be:

  • optimistic for one who is no longer in the field

  • generous for one who will run the work in the field

Sources of Info for Time Estimates

• equipment production rates

• labor unit costs and production rates (depending on how the activity is crewed)

• informal determination

Duration Adjustments

• weather days can be inserted after the approx timing of the project is known

• time contingencies are more effective if applied to the project as a whole rather than individual activities

Time Computations

• an ideal computer based analysis, though calcs involve simple addition and subtraction

• a simple manual calc provides great insight into what is done “inside the black box”

• computations are carried out based on expired project working days

Forward pass to calc

• early start: the earliest an activity can begin

• early finish: the earliest an activity can be completed

• project duration is determined by the latest early finish

Backward pass to calc

• late finish: the latest an activity can be completed without delaying the completion of the project

• late start: the latest an activity can begin without delaying the completion of the project

Float

the amount of time an activity can be delayed before bumping up against something

Two passes to determine float

• free float: the amount of time an activity can be delayed before pushing the next activity

• total float: the amount of time an activity can be delayed before delaying the completion of the project

Critical Path

a continuous chain of activities through the project from beginning to end, connecting activities with no float time; defines the length of the project (can be multiple)

Critical Activity

an activity with zero float

If a critical activity is delayed:

• the project completion will be delayed

• the project duration will be extended

Characteristics of Float

• total float is a shared resource

• free float is unique to a specific activity

How is total float a shared resource?

• shared by all of the subsequent activities along the path(s)

• use of a day of total float affects each subsequent activity

• float should be considered a valuable resource

Calendar Dates

project days can be converted to calendar dates

Lag Time

assigns a delay between one activity and another

Why is lag time used?

• a physical requirement (ex. concrete setting)

• a time buffer (ex. delaying the start of electrical rough-in in a partition until X days after start of partition framing)

Hammock Activity

represents a task or series of tasks connecting between 2 activities but having no pre-determined duration

Milestone Activity

has zero duration and identifies an important intermediate point in time for the project

Time-Scaled Network

represents activities as time scaled bars which are interconnected (activity relationships to a traditional bar chart)

CPM Computer Activity Output

• early start

• late start

• early finish

• late finish

• total float

• free float

CPM Computer Project Output

• project duration

• critical path(s)

• calendar dates for each activity

• various graphical and tabular representations of the data

• data sorted appropriately

What does the true worth of a project schedule depend on?

the job logic and the accuracy with which the individual activity times can be estimated

What does the optimum use of CPM information for project time control require?

the user to have a thorough understanding of the computations and the true meaning of the data generated

What does the critical path determine?

the minimum time required to complete the project and also provided the info needed to expedite project completion

What do CPM-based computer programs enable?

extensive sorting of data that, in turn, facilitates more effective project management

Baseline Schedule

represents the initial plan

Updated Schedule

represents the current state of the plan

Short-Term/Look-Ahead Schedules

represents a tactical segment of the plan

Final Schedule

represents how the project was actually built

Precedence Network

provides a concise representation of scheduling logic

Gantt (Bar) Chart

easy to read and understand intuitively

Time Scaled Logic

adds logic to the bar chart

Sorting capability adds…

significant value to the scheduling tool

Activity Number

links the network representation of an activity with the data

Early-Start

most optimistic schedule

Late-Start

indicates when an activity will become critical 

Late-Finish

flags when the project gets behind schedule

Total Float

indicates criticality of each activity

Project Responsibility

a valuable accountability tool

Combined Sorts

provide tactical info for short-term planning

How to Update the Schedule

• collect actual state (AS) and actual finish (AF) dates from daily work log

• for activities in progress, estimate time-to-completion and early finish dates

• enter these dates into CPM schedule, break logics if necessary

• recalculate ES, EF of remaining activities using forward path method; and LS, LF of remaining activities using backward path method

• recalculate total float

What is the Impacted Baseline Schedule used for?

to determine the cost(s) and effect(s) of delays

How to Impacted Baseline Schedule

• begin with the baseline schedule

• insert delay activities into the schedule as predecessors of the activities delayed

• recalculate the schedule

• apportion cost and effect among participants, as appropriate

What is a But-for or Collapsed As-Built Schedule used for?

alternative approach where delays are multiple and more complex

How to But-for or Collapsed As-Built Schedule

• start with the as-built schedule

  • should reflect actual durations, logic and lags

• remove all actual start and finish dates

• recalculate all start and finish dates

• compare the initial and modified as-built schedules and apportion responsibility for delays as appropriate

Legal Aspects of Schedules

• a right to early finish

• owner approval of the contractor’s schedule

• resource loaded schedules

• the legal need for a schedule

A Right To Early Finish

• early finish can be advantageous or detrimental to any stakeholder

• owners can bark at, or try to impede a contractor’s early finish

• the contractor’s right to finish early is generally upheld by the court

• early agreement with the owner and other stakeholders is important

Owner Approval of the Contractor’s Schedule

• owners can protect key dates by specifying them as milestone dates in the contract

• owner requirement to approve of a contractor’s construction schedule can be considered tampering with the contractor’s means and methods, which can trigger owner liability

Resource Loaded Schedules

• craft workers and/or equipment availability often impacts scheduling decisions

• publication of a resource loaded schedule demonstrates the contractor’s intentions

• can be valuable in supporting claims when other stakeholders modify the schedule

The Legal Need for a Schedule

• contracts often require a schedule from the contractor

• supports efficiency and wise management decisions for the contractor

• enables effective communication with other stakeholders

• schedule cuts both ways

Handling Weather and More

• expected to anticipate unknowns

• apply contingencies to estimates and schedules to account

Weather Contingencies

• schedule will indicate when weather sensitive activities occur and time of anticipation

• contingencies for both cost and time can be determined from records

What does the schedule define?

the contractor’s contractual obligations by showing the means, methods, and sequencing that will produce the owner’s product

What happens to scheduling data as the project progresses?

it takes on different forms depending on the state of the project and the intended use

Why does it matter how a schedule is presented?

significantly affects its acceptance and utilization on the project

Presenting A Schedule

• documenting assumptions, concepts, and thoughts underlying the planning basis for a schedule is important

• communication of this info at early stages will enhance overall planning and constrain later disputes

• crew staffing and productivity form a key component of the development of both the estimate and the schedule