Scheduling means the process of arranging, controlling and optimizing work and workloads in construction, production or manufacturing process to ensure optimal and economical use of resources. In short form, it means putting the plan on calendar basis. There are basically two types of scheduling that can be found in construction process viz: resource scheduling and work scheduling. The aim of resource scheduling is to use optimum resources or work within resource constraints. The resources in construction sites are basically three: materials, equipment and human resources (skilled and unskilled labour). For instance, in construction work, casting of concrete would require materials such as cement, sand, gravel and water; equipment such as shovels, head pans, concrete mixer etc and human resources such as foreman, mason and labourers.
Factors considered in resource scheduling
- Client requirement
- Construction method
- Site constraints
- Availability of manpower in the location
- Availability of appropriate plant and equipment
- Output per gang
- Plant output
Factors considered in work scheduling
- Quantity as specified in BEME or BOQ
- Construction methodology
- Site constraints
- Number of gang available
- Available plants and equipment
- Output per gang
- Plant output
The need for resource scheduling arose because the demand for certain type of resource may fluctuate from very high at one time to very low at another time. If it is material or unskilled labour which has to be procured from time to time, the fluctuation in demand will not affect much the cost of the project. But if it is some personnel who cannot be recruited and retrenched during the project or equipment which are to be hired for the total project duration, the fluctuation in their demand will affect the total cost due to high idle times. The two main objectives of resource scheduling are:
- To reduce costs of the project and
- Reduce pressure on limited resources in conflicting demands.
Resource scheduling can be divided into two viz: equipment scheduling and labour scheduling. In this post I would show practical examples of labour scheduling.
Two types of labour force usually found in the construction site are skilled labour and unskilled labour. Skilled labour refers to highly trained, educated, or experienced segments of the workforce that can complete more complex mental or physical tasks on the job. Example of skilled labour are Engineers, Surveyors, Project Managers, and Laboratory Technicians etc. Unskilled labor refers to workers who have no special training or experience. It is a part of the workforce with a very limited skill set. Examples of unskilled labour are Bricklayers, Carpenters, Tilers, Iron fabricators etc.
Labour scheduling can involve determining the number of days required to carry out a project when the quantity of work, output per gang or number of gang is known or determining the gang when the quantity of work, output per gang and duration of the work is known (a gang is a combination of skilled and unskilled workers). The later is normally used for projects that have limited time. This time usually vary with the project consultant involved and circumstances surrounding the project. In consideration of the method statement and consequently the contract programme, the list of activities (elements) with dates of commencement can be obtained.
The process for labour scheduling for the later option can be itemized as follow:
- Determine the quantity of each item of work or get the quantity from the quantity Surveyor. The quantity would include site clearance, excavation, block laying, concrete works etc.
- Assign possible duration to each item of work based on experience or interviewing people who have done similar projects. The duration would also consider the quantity of the item.
- Determine the output of each worker whether skilled or unskilled.
- Assign the duration of work per day and days per week. It is usually eight (8) hours per day and five (5) days per week.
- Determine the gang time. Gang are used for items of work that involves combination of skilled and unskilled workers. A 40:60 skilled and unskilled workers input ratio are normally used.
- Based on the gang time, assign the number of skilled and unskilled workers required for the work.
How to calculate hour of work per day
This is applicable to the former option. The work shall base its calculations on a five (5) days working week and eight (8) working hours per day.
Work hours per day can be summarized as follows (though this may vary according to the organization):
8am – 11am (morning work hours)
11am – 12pm (1 hour lunch break)
12pm – 5pm (afternoon work hours)
In one (1) day, 8 hours are used for work. The 8 hours are assumed based on 100% efficiency but this is often not the case. It is more ideal to assume like 80% efficiency. 80% of 8 hours = 6 hours and 40 minutes. This shortage could be due to unforeseen circumstances like machine breakdown, the event of rain or community disturbances etc.
METHOD ONE
The daily average manpower (MP) required for each scheduled activity can be assessed as follows:
MP required = (quantity of work / output per skilled worker) x duration in days
Example 1
Determine the number of days required to produce 300m2 of block wall if the output per gang-hour is 2.5m2/hr and the number of gang is 2
Solution
Duration (hr) = quantity of production/output of gang per hour = 300/2.5 = 120 hours
Number of days = duration (hr)/hour of works per day = 120/6.666667 = 18 days
Number of days to complete task = Total number of days/number of gang = 18/2 = 9 days
Example 2
Determine the time required to produce 180 m3 of concrete in oversite concrete if the gang is 3 and output per gang hour is 6 m3/hr
Solution
Duration (hr) = quantity of production/output of gang per hour = 180/6 = 30 hours
Number of days = duration (hr)/hour of works per day = 30/6.666667 = 4.5 days
Number of days to complete task = Total number of days/number of gang = 4.5/3 = 1.5 days, say 2 days
METHOD TWO
The ratio of the skilled and unskilled workers in construction gang depends on the characteristics and nature of work. The aggregation of manpower required for the scheduled activities on the chart indicates the daily strength of workforce, category-wise required to accomplish the scheduled work according to the project plan. Daily workforce required for scheduled work = sum of construction manpower for the scheduled activity on data.
Example 1
If it is required to clear a site of area 1520 m2 in 4 weeks, determine the gang required to carry out the task?
Solution
Assuming output per gang is 12.96 m2
1520 m2 will take 1520/12.96 = 117.28 days to complete the task
Time period allowed = 4 weeks = 4 x 5 days = 20 days
No. of workers required = time to complete work/time allowed = 117.28/ 20 = 5.86
Since clearing requires only unskilled workers, it would be say 6 unskilled workers to do the worker.
Example 2
Assuming the blockwork of 2500 m2 is required to be completed in 3 weeks, determine the gang required for the work?
Solution
If output is 140 blocks per gang day and since there are 9 blocks per square metre of the wall, then, 2500 m2 would contain, 2500 x 9 = 22,500 blocks.
This implies that 2500 m2 would require 22500/140 = 161 gang days
Since time allowed for the project = 3 weeks = 3 x 5 days = 15 days
No of gang required = 161/15 = 10.7, say 11 gangs
This would comprise 11 skilled workers and 17 unskilled workers
Example 3
We want to produce 250 m3 of concrete. If output is 1.25 m3 gang hours, determine the gang to do the work?
Solution
For 250 m3, gang hours = 250/1.25 = 200 gang hours
If time allowed for the project is 2 weeks = 2 x 5 days = 10 days = 10 x 8 hours = 80 hours
No of gang required = 200/80 = 3 gangs
The number of workers would be 3 skilled workers plus 5 unskilled workers
Points to note:
- The usual ratio of skilled to unskilled workers used here = 40:60 = 1.5
- The number of skilled workers must be same with the number of gang calculated while the number of unskilled workers is got by multiplying the number of skilled workers by the ratio (=1.5).
- This process is followed for all the entire project based on the BOQ or BEME until the total required human resources to carry out the project within the time allowed is got and assigned properly during the preparation of the project schedule.
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