Determination Of Duration

a) Logical Critical Path:

On most projects, there are two types of critical paths. The first, and most generally recognized form, is a logical sequence of activities which, when the durations are added up, will give the longest time or "path" on the project. The critical path method (CPM) was developed to provide analysis of logical sequences of work so as to determine the longest chain or critical path of a schedule. A CPM schedule is usually referred to as a network.

b) Manning and Peak Labor:

The second type of critical path is the dependency of construction manpower. Very often, the overall construction duration of a very tight project schedule is determined by the ability to place a maximum number of men on the work. Historical manpower density data can clearly show appropriate or possible manpower levels. The data will also show impossible manpower levels.

c) Working Hours:

When an "impossible" level of manpower is required for the construction schedule, then the only alternative is to increase the number of hours work through an extended workweek or additional work shifts. Both of these alternatives may add increased costs. Refer to Cost Estimating, Chapter 6, and Keeping a Project on Time and Within Budget, Chapter 7 for information on extended workweek/overtime productivity losses. Figure 3-25 illustrates the effect of manpower requirements based on schedule.

Figure 3-25. Determination of Duration.

CRITICAL PATH - LOGICAL SEQUENCE OF ACTIVITIES

Figure 3-25. Determination of Duration.

CRITICAL PATH - LOGICAL SEQUENCE OF ACTIVITIES

CONSTRUCTION BULK PROGRESS

PRACTICAL SCHEDULING - BASIC SCOPE APPRECIATION

The use of historical data, typical relationships, statistical correlations and practical "rules of thumb" can greatly add to the effectiveness of a project scheduling program. Such information can provide guidance in:

• developing/evaluating schedules

• assessing manpower requirements

• determining appropriate productivity levels

• improving cost/schedule assumptions

• carrying out trend analysis

• establishing the cost of the project

• evaluating the status and performance of the work

• recognize the scope of work at all times

It is this last item that really highlights the key to effective project scheduling; SCOPE RECOGNITION. This equates to the ability to properly establish the scope in the first place, through a good estimate and, thereafter, to constantly recognize the true scope of the work as the project develops and is executed. The "testing" and measuring of actual performance against past experience can be a valuable source of verifying status, determining trends and making predictions. Naturally, the application of historical data to a specific project must always be carefully assessed.

The following figures represent historical and typical schedule "rules of thumb" that can assist in the establishment and development of schedules during the execution phase of a project. This information is especially useful at the front end of a project when engineering is at a low percentage of completion, resulting in a preliminary and overall schedule.

"FAST TRACK" SCHEDULING RELATIONSHIP

Figure 3-26 illustrates the scheduling relationship of engineering and construction for a project on a fast track program, with the "full scope" of engineering, procurement and construction (EPC). Such a project is often referred to as an EPC Project. The schedule only covers the execution phase. This phase

Figure 3-26. Typical Overall Scheduling Relationship

Full Scope(EPC) Projects FAST TRACK APPROACH - EXECUTION PHASE

PROJECT EXBCUTIC« PHASE

study Process release Case selection 1(20%)

Ratio/Parts of Overall Duration

BUILDUP

BUILDUP

RUN com

Note: The engineering sdhadule is the sane schedule as the overall project duration.

ENGINEERING(and PROCUREMENT)

ENGINEERING SCHEDULE

20% i_£ major site prep-30% if minimal site prep.

MECHANICAL

CCMPI£IICN

20% i_£ major site prep-30% if minimal site prep.

MECHANICAL

CCMPI£IICN

BUILD UP

Ratio/Parts of Construction Duration CCNSirFUCTICH SCHEEWi

BUILD UP

Ratio/Parts of Construction Duration CCNSirFUCTICH SCHEEWi

■Rie above schedule ratio/parts are "typical", but based an historical experience (see historical charts for specific nimtiers). Each project schedule should be carefully evaluated for it's specific application to the standard, historical data.

is sometimes referred to as the Phase 2, where the earlier Phase 1 covers the conceptual design studies of process/utility alternatives and case selection.

It should also be noted that the "schedules" are illustrated as "trapezoids." This concept is extremely important as it shows that all "complex work" is executed with:

This concept, this fact, then forms the basis of quality scheduling and manpower assessment. It is only "simple" work, where the task is performed by a single crew or squad that has no build-up or rundown.

As indicated on Figure 3-26 the ratio breakdowns of the buildup, peak and rundown, have been rounded off to whole numbers as the historical numbers are slightly different. But, at this overall level of scheduling, such minor numerical differences are of little consequence. Based on the author's altered historical data base of several projects, there is only a slight difference with the "idealized" trape-zoids. For example, the engineering build-up, shown as 20%, is 22%, and the construction build-up, shown as 50%, is 57%.

CONSTRUCTION DURATION - TRAPEZOIDAL TECHNIQUE

Figure 3-27 shows the trapezoidal technique for the construction phase. The calculation procedure shows two formulas.

Formula 1 is used to determine the peak duration, shown as X, on the basis that the following information is known, or assumed:

• effective monthly hours, per man

• build-up, usually developed from standard or actual schedules

Formula 2 covers the calculation of peak men, if the battery limits area (plot plan) is known. By evaluating a labor density level

(usually in the range of 150-300 sq. ft,/man), the peak number of men can then be determined.

This schedule/manpower evaluation technique is a very powerful program for developing or checking an overall construction duration. The key assumption requiring good judgment is the assessment of the labor density level. If this assessment is good, then the resulting scheduling evaluation is of a high quality.

Figure 3-27. Construction Duration (Trapezoidal Technique)

Figure 3-27. Construction Duration (Trapezoidal Technique)

CALCULATION PROCEDURE.

Scape in ni«nhOurs

Eftective Monthly f-tannour

(2) Peak manpower » Buccery iimics area Peak DensiCy LeveL

SOLVE FOR PEAK DURATI01

It is emphasized that the Peak Manpower/Density Level application can only be used on single process units or complete process areas/buildings. The calculation process does not always work for individual categories of work.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Responses

  • sante
    Is it possible to find secondary critical path using trapezoidal numbers?
    3 years ago

Post a comment