Line of balance programming

Transcription

1 Line of balance programming 1. INTRODUCTION Most contractors are familiar with programming a single project where a critical path analysis or, more likely, a bar chart is drawn up indicating what activities must occur, when they must occur, and for how long they must occur. With multiple, repetitive projects i.e. the construction of 100 similar houses it is impractical to draw up 100 different bar charts and very cumbersome to show the construction of the 100 houses on a single bar chart. The method of doing the project is also different as the contractor building 100 houses will split his labour force into gangs each of which will specialise on one facet of the house construction, i.e. the superstructure brickwork. What this means is that the contractor will have one or any number of gangs of bricklayers that just build the superstructure for the houses. When they start any particular unit the sub-structure brickwork and floor is already complete and they leave the unit when the superstructure is complete and move onto the next unit. It is this forming of gangs focusing on only one aspect of the job that requires the specialised techniques of balancing the number and productivity of the gangs. This is to ensure a constant flow of work on the construction site. The best way to explain the technique is to work through an example. Lets take a housing project for 100 identical units each as indicated overleaf.

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3 Line of balance programming 2. THE LINE OF BALANCE METHOD The first step is to identify the activities that you determine will be performed by separate gangs. Let us use the following as an example: ACTIVITY EXCAVATE AND CONCRETE TO FOOTINGS SUB-STRUCTURE BRICKWORK FILLING, DPC AND CONCRETE TO FLOOR SUPERSTRUCTURE BRICKWORK ROOF PLASTERING CEILINGS SCREED FINAL FINISHES Each of these activities thus becomes one item on our programme but, more importantly, also represents the work that will be performed by one gang of workers. Thus each housing unit will be built by nine separate crews each handling one of the activities shown above. To keep up with the necessary rate of construction for the overall project, there may well be more than one gang. However only one gang will work on a single unit. Thus there could be three or four units having the superstructure brickwork built at the same time. Step 2 is to determine the number of man hours needed to complete each activity for one house. Example : Superstructure brickwork : Number of man hours = Number of stock bricks divided by the production of one bricklayer per hour. Total number of stock bricks = (rough quantities taken from drawing) Production of bricklayer per hour = 800 per day / 8 hours = 100 per hour. Number of man hours = / 100 = 210 man hours. Use the following as an example : ACTIVITY MAN HOURS PER ACTIVITY EXCAVATE AND CONCRETE TO FOOTINGS 50

4 SUB-STRUCTURE BRICKWORK 70 FILLING, DPC AND CONCRETE TO FLOOR 95 SUPERSTRUCTURE BRICKWORK 210 ROOF 150 PLASTERING 80 CEILING 80 SCREED 90 FINAL FINISHES 150 Step 3 is to decide what the optimum gang size will be per activity per house. Example : Superstructure brickwork : Due to the size of the house you decide that the optimum gang size for the house is 6 bricklayers. The decision as to the gang size is based on experience and the size of the individual unit. The number of men in the gang is the productive workers and does not include assistants or general workers. Use the following as an example: ACTIVITY OPTIMUM GANG SIZEMAN HOURS PER ACTIVITY EXCAVATE AND CONCRETE TO FOOTINGS8 MEN 50 SUB-STRUCTURE BRICKWORK 4 MEN 70 FILLING,DPC AND CONCRETE TO FLOOR 8 MEN 95 SUPERSTRUCTURE BRICKWORK 6 MEN 210 ROOF 3 MEN 150 PLASTERING 3 MEN 80 CEILINGS 4 MEN 80 SCREED 4 MEN 90 FINAL FINISHES 6 MEN 150 Step 4 is to decide what handover rate of completed houses you require per week. This is influenced by the number of houses you are contracted to build and the contract time period stipulated. Let us presume that the contract documents stipulate you must construct 100 houses in a 6 month period i.e. 26 weeks. The first 50% of the contract will be taken up with building the first unit and thereafter the units will be completed at a rate of 100/13 = 7.69 say 8 per week. If once the line of balance calculations are completed you discover that the completion date is not met then the handover rate must be adjusted upwards i.e. to 10 per week. The calculations must then be redone to determine if the new completion date is correct. Step 5 is to determine the total number of men requiredto be working on a specific activity in order to complete exactly the handover rate i.e. 8. This is to adjust the optimum gang size or to determine the number of gangs used on the project.

5 A simple formula exists, as follows : Example : Superstructure Brickwork : This implies that you have 7 gangs of 6 men working on 7 houses at any one time. The optimum gang size was decided as 6 men so there will be seven groups of such men. The following sheet has calculated the total number of men up to roof. As an exercise it would be good to calculate the total number of men requiredfor the other activities. Step 6 is to adjust this total number of men into multiples of the optimum gang size. The total number of men is not always a multiple of what you decided to use as the optimum gang size. For example the optimum gang size for Sub-Structure brickwork is 4. The total number of men requiredis 14 which is not a multiple of 4. You must therefore choose how many gangs you would

6 prefer to use. You can either use 3 gangs of 4 men giving 3 x 4 = 12 men in total or 4 gangs of 4 men giving 4 x 4 = 16 men in total. Or you might re-examine the choice you made of optimum gang size. Your choice of number of gangs has a big influence on the contract completion date as we shall soon discover. Step 7 is to review the rate of production per activity. To meet the contract period we calculated 8 units per week to be our handover rate. That was based on using exactly the right number of men to achieve that rate e.g. for sub-structure brickwork the number of men was 14. In step 6 we had to choose a multiple of 4 which resulted in the total number of men changing to 16. This results in the handover rate for that activity increasing in relation to the number of extra men used. The revised rate of production is thus calculated as being: In the case of sub-structure: Let us call this revised rate of production RN. Thus we see that the increased number of men has resulted in the handover rate for the activity sub-structure brickwork increasing from 8 to NOTE: If less men had been used than the original exact gang size then the revised handover rate would be less than 8. The following sheet has calculated the revised rate of production up to roof. It would be good to complete the rest of the table for yourself.

7 Step 8 is to calculate the time period between the start time of an activity on the first house and the start time for the same activity on the last house. In other words if the activity "Excavate and concrete to foundation" of house number 1, starts on day 1, then when does the, "Excavate and concrete to foundations" activity start on house No 100. The formula for this calculation is to take the total number of units (100) and deduct 1 (i.e. 99). Multiply this figure by 5 (the number of working days in a week). This figure then has to be divided by the rate of production, as revised. So if activity "Excavate and concrete to foundations" for house no 1 starts on day 1 then "Excavate and concrete to foundations" for house no 100 will start on day 78. The following sheet has calculated the total time lapse up to roof level and the actual time calculated rounded off to complete days.

8 Step 9 is the final step before drawing the programme. Thus now we must calculate how long it takes to complete each activity of the house. For example how long does it take to complete the roof on one of the units. This is easily calculated by taking the man hours per activity and dividing it by the gang size that is working on the activity and multiply it by the number of hours in the working day. Let s do the calculation for roof: In other words it takes the roof gang 7 days to erect the roof on each house. We are now ready to draw the Line of Balance Chart, based on the Figures in the following table:

9 Step 10 is to draw the line of balance chart. Take a piece of graph paper and indicate the number of days on the horizontal scale and the number of units on the vertical scale: See Diagram A. DIAGRAM A The diagram above shows the units along the vertical axis. Each division represents one unit with the total being 100. As these are the units to be built the bottom line is not zero, as often happens with graphs, but one - that is the first Unit. The basis for drawing up the Line of Balance diagram is contained in the figures in the table. The figures that were calculated in the early part of this module. The contract will start with the first activity on the first unit. That is "Excavation & Concrete" on Unit No 1. DIAGRAM B

10 The table shows that it takes one day to complete the excavation and concrete to each unit. So the start of our first "bar" will be at Day One and Unit One. This puts it in the bottom left corner of the diagram. From that point we now need to plot the "Time Lapse" - that is the time that will pass before Activity One is done on Unit 100. Thus the excavation on Unit 100 will be done on Day , this being the Time Lapse shown in the table for this activity. We can thus plot an inclined line, or bar form Day 1 to Day 78. The duration of the activity, in this case one day, is shown by the width of the bar. We will add these bars to our diagram as we go along until all nine activities have been added. DIAGRAM C The following activity is "Sub-structure Brickwork". Here the duration of the activity is 3 days and the Time Lapse between Unit 1 and Unit 100 is 54 days. Since this lapse of time is shorter than that of Activity One this will mean that it will be bar at a steeper angle than the first one. This is because the progress from first Unit to last Unit is only 54 days, as opposed to 77 days for Activity One. This will mean that we need to start drawing our bar from the top down. Obviously the gang doing this activity will be working as a faster pace than the previous gang. They cannot do their work until the previous activity is complete. So we need to programme them to follow on at Unit 100 immediately after the gang doing Activity One. Thus they can start work on the sub-structure brickwork on Unit 100 on day 79, the previous gang finished their work on Day 78. Working back from there we find that this activity can start on Unit 1 on Day 25, immediately after Day 24 on the horizontal scale. The duration of the activity is 3 days so the width of the bar will indicate this. This bar then will run from Day 25 to Day 27 for Unit 1 and from Day 79 to Day 81 for Unit 100. We see here one of the strengths of this form of programming. The inclination might be to start this gang on Unit 1 immediately after the first gang had finished their work. However because they move at a faster pace than the previous activity, they would soon run out of work and be waiting

11 for the previous gang to complete. Now we know that we can bring them in more than three weeks afterwards and still keep our project on programme for final completion. DIAGRAM D The next activity "Fill & Concrete" is the casting of the floor slab, which is programmed to be done before the superstructure brickwork is started. The duration is two days and the elapsed time is 74 days. This is considerably longer than the previous activity so it can start immediately after the substructure brickwork on Unit 1 is done. As this gang is slower they will progressively fall further and further behind the gang working on Activity Two. So on Day 28 the gang can start backfilling on Unit 1. The concrete will be cast the following day, Day 29 and then they will proceed to Unit 2. With the time lapse being 74 days work will start on Unit 100 on Day 102. The bar itself is narrower since the duration is only two days. DIAGRAM E

12 . Now comes a major activity, that of the "Superstructure Brickwork". This activity take five days to complete on each unit. It has a time lapse period of 62 days, so will catch up on the previous gang. So once again we will start calculating from Unit 100 and work backwards. The previous activity ends on Unit 100 on Day 104. Working back from there that gives us Day 42 (104-62) as the first day of Activity Four on Unit 1. The activity takes five days so work will end on Unit 1 on Day 46. Again we have a case here of a gang that need not be brought onto site until the previous activity is well advanced. In fact some twenty units will have had the concrete floor cast before they need to start work. Then they will be right behind on Unit 100, if all goes according to the programme. DIAGRAM F

13 Looking at the figures for Activity Five we see that although the duration is longer, 7 days, the time lapse works out to the same 62 days. Thus the bar will run exactly parallel to the previous one, though it will be a thicker bar. Theoretically work could start immediately on the following day, Day 47. However it is often advisable to leave a day or so between such activities for any delays that may occur on the previous activity. This is known as a "buffer" and makes some allowance for things that may go wrong. This is particularly important on activities that run parallel, such as these two. Any delay with the bricklaying gang will immediately affect the roofing team. The buffer does make some allowance for such things. So the bar for Activity Five runs next to the previous one with just a one day allowance between them. DIAGRAM G

14 This brings us to "Plaster", Activity Six. The duration is four days and the elapsed time is 66 days. This makes it a slower activity than the previous one. Thus we can start immediately after Activity Five is completed on Unit 1 and expect to see the gap between the two bars widening as the job progresses. Work will start on Day 55 and Unit 1 will be completed by this team on Day 58. With a time lapse of 66 days, work will begin on Unit 100 on Day 121 with completion on Day 124. DIAGRAM H

15 The "Ceilings" are the next activity in the programme. This again moves at a faster rate than the plaster with an elapsed time of 57 days. Working from Unit 100, work can start on that unit straight after the plaster is completed. That means work will start on Day 125. Taking away the elapsed time of 57 days we get to Day 68 as the time the gang will start work on site. They will begin Unit 1 on that day, completing it after four days of work, on Day 71. DIAGRAM I The floor screeds will now be done. This is Activity Eight and it has a duration of three days. The elapsed time is 69 days so that progress will be slower than the previous activity. Thus we be able to set the gang to work on Unit 1 as soon as it is available. They will begin work on Day 72, finishing on Day 74. The gang will complete work on Unit 100 over the period of Day 141 to Day 143. DIAGRAM J

16 Then we come to the final activity, that of "Final Finishes". It has a duration of four days and an elapsed time of 62 days. This will require us to work backwards from Unit 100. Thus the finishings to Unit 100 can take place over the time period of Days 144 to 147. Going backwards we then find out this team will be able to start on Unit 1 on Day 82. They will complete on Day 85. If one looks at the final programme most of the activities are fairly well balanced, i.e. without too much idle time between the finish of one activity and the start of the next activity. Remember the idle time is caused by the time lapses for activities being different. There is idle time caused by the fact that the "sub-structure brickwork" activity has a time lapse of 54 days comparedto excavations before it with a 77 day time lapse and "fill and concrete" after it with a 74 day time lapse. If we could get sub-structure brickworks time lapse to be closer to days then the situation could be improved. This can be achieved in a number of ways: 1. When revising our total number of men to be used in sub-structure brickwork we could round the total down to the lesser multiple of the gang size per house. This would slow down the revised rate of build and therefore increase the time lapse. 2. We could take another look at our optimum size and change the number of men per activity. As we based our original calculations on what we consideredto be the optimum gang size this form of balancing should be the last to be undertaken. 3. If the handover rate is too slow an extra gang could be introduced for the whole duration of the activity or only a portion of the activity. For example in order to increase the handover rate of screed for a portion of the contract one could increase the number of gangs from 5 to 6 from midway through the activity. This would mean that for the first 50 units the handover rate would be 7.2 and the time lapse 69. For the second 50 units the handover rate would be units and the time lapse 46. This again would have the effect of reducing the idle time between activities.

17 Line of balance programming 3. MONITORING PROGRESS The target progress on site is easily read from the graph by drawing a vertical line from the date on the horizontal axis that coincides with the date on which the progress report is being done on. This is shown on Diagram K. Where the vertical line drawn intersects each activity a horizontal line is drawn to the vertical axis. From this the number of units which should have been completed can be determined. The actual progress must be arrived at by walking around the site and noting what activities for what houses are actually complete. DIAGRAM K: From this diagram we can see the following: it is Day 60 when the progress is being checked Activity 1 - the excavation and concrete to the footings of Unit No 82 should be underway Activity 2 - the substructure brickwork to Unit 75 should be complete Activity 3 - fill and concrete to Unit 43 should have been done Activity 4 - superstructure brickwork should be completed as far as Unit 24 Activity 5 - Unit 11 should have its roof completed Activity 6 - the plastering team will have just started on site and should have completed the first three Units

18 If a check on site reveals progress on site to be different to that indicated above then an investigation must take place. A gang working at a slower pace will sooner or later interfere with the progress of the following teams. A gang working at a faster pace will catch up the one in front of them and run out of work.

19 Line of balance programming 4. CONCLUSION The Line of Balance System of programming was developed for housing schemes with multiple units. It is ideal where the units are identical or very similar in size as the amount of work in each unit will be substantially the same. The system has also been used for programming the tower sections of high-rise buildings with multiple floors that are basically similar. Once the principles are understood it is a simple system, though the calculations involved can be onerous. Obviously the secret for a smooth flow of work is to balance the work teams so that they produce completed units at approximately the same rate. For activities with a large work content it may require having more than one gang. Thus several units may be under construction at the same time but the overall rate of production will be approximately the same as the other activities. Thus this activity is kept in balance with the others. For activities where the work content is low, for example glazing on a small housing unit, there may be no alternative but that the gang is called in when required. Thus the glazier will start work and, because he progresses at above the normal rate, he will catch up to the team in front. He will then leave the site and return once there were enough units ready for him to make it worthwhile. Line of Balance is a powerful programming tool for multi-unit contracts. It is easy to understand and provides vital information on the progress requiredon every unit to be able to meet the contract period.