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Construction Scheduling Techniques

Monday, November 24, 2014

Construction Scheduling



Common Scheduling techniques

The four scheduling techniques widely used in construction projects are:

. Bar Charts and Linked Bar Charts

. Network Analysis and Critical Path Method

. Line of Balance

. Q Scheduling

each is briefed below with a focus on Q Scheduling being a new technique increasingly being applied in construction management.


1- Bar Charts and Linked Bar Charts;

bar chart schedule
Free On-line Bar Chart Construction Schedule

Bar Charts are the easiest and most widely used form of scheduling in construction management. Even with other scheduling techniques the eventual schedule is presented the form of a bar chart. A typical Bar chart is a list of activities with the start, duration and finish of each activity shown as a bar plotted to a time scale. The level of detail of the activities depends on the intended use of the schedule.

The linked bar chart shows the links between an activity and its preceding activities which have to be complete before this activity can start.

The bar charts are also useful for calculating the resources required for the project. To add the resources to each activity and total them vertically is called a resource aggregation. Bar charts and resource aggregation charts are useful for estimating the work content in terms of man-hours and machine hours.

2- Network Analysis and Critical Path Method

Practically network analysis offers little more than a linked bar chart, though its protagonists claim, with some justification, that the self contained steps of a network are more applicable to complex operations than the bar chart, and that the greater rigor imposed by the logic diagram produces more realistic models of the proposed work. The steps in producing a network are:
- Listing of activities
- Producing a network showing the logical relationship between activities.
- Assessing the duration of each activity, producing a schedule, and determining the start and finish times of each activity and the available float
- Assessing the required resources.

There are now two popular forms of network analysis in construction management practice, activity on the arrow and activity on the node, the latter now usually called a precedence diagram. Each of these approaches offers virtually the same facilities and it seems largely a matter of preference which is used.

3- Line of Balance

The line of Balance is a planning technique for repetitive work. The principles employed are taken from the planning and control of manufacturing processes greatly modified by E.G.Trimple. The basis of the technique is to find the required resources for each stage or operation so that the following stages are not interfered with and the target output can be achieved. The line of balance technique has been applied in construction work mainly to house building and to a lesser extent to jetty work and in conjunction with networks to road works.

4- Q Scheduling

The Q Scheduling is a new technique, though getting rapid popularity among contracting firms. It is the only scheduling technique that reveals a relation between the sequence of doing a job and the cost to be incurred. The Q schedule is similar to the Line of Balance with some modifications made by A. R. A. Z. A in 2004, to allow for a varying volume of repetitive activities at different segments or locations of the construction project, thus the model produced is closer to reality. The following example explains the technique.

Having a project site with three buildings A, B, C with following quantities

ActivityunitAverage daily output Building ABuilding BBuilding C
Excavation m32060 2040
Foundations m31530 1515
Back-filling m33030 6060
Note: Above quantities are chosen for illustration purpose and may not reflect a practical construction case.

There are six possible arrangements for doing the job in respect of location sequence with the constraint of ensuring a continuous flow of work for each activity so that no idle time for employed crews might be encountered, and no allowance for more than one activity to take place simultaneously at one location:

  • A - B - C,
  • A - C - B,
  • B - A - C,
  • B - C - A,
  • C - A - B,
  • C - B - A

Alternative 1 gives a total duration of 10 working days
C 212
B  112  
A32 1  
Days 12 34 56 78 910

 

Alternative 2 gives a total duration of 10 working days
B    1 1 2
C   2 1 2  
A 3 2 1  
Days 12 34 56 78 910

 

Alternative 3 gives a total duration of 9 working days
C       2 1 2 
A   3 21     
B 1    1 2     
Days 12 34 56 78 910

 

Alternative 4 gives a total duration of 10 working days
A     3 2  1
C  2    1  2 
B 1     1 2     
Days 12 34 56 78 910

 

Alternative 5 gives a total duration of 10 working days
B        1  1 2
A    3 2 1   
C 2    1 2     
Days 12 34 56 78 910

 

Alternative 6 gives a total duration of 10 working days
A     3 2 2
B    1    1  1   
C 2    1 2     
Days 12 34 56 78 910

The example shows that alternative 3 can produce a saving of 10% relative to other alternatives, either in output rates if duration is kept same, or in total duration and subsequently in overheads and supervision costs. Actually Q Scheduling software picks up alternative 3 as the most economic sequence and considers other alternatives as having additional cost of 11.11% proportional to the most economic alternative.
However, with a construction project having four locations there would be twenty four possible  alternatives, and for a five location project there would be 120 alternatives. For a project with ten locations there would be three million, six hundred twenty eight thousand, and eight hundred alternatives to consider.



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