In a time of increasing requirements on construction projects, the simulation of construction processes in order to identify optimization potential of the execution becomes more and more important.
Apart from the needed information, the daily accurate acquisition of construction data is one main objective. In order to keep the acquisition effort as low as possible, it has to be clarified which data is really necessary for simulating the construction processes and which is not.
This work shall be a contribution to determine the appropriate level of detail that enables to reconstruct completely the execution of constructing highway bridge curbs.
Therefore different scenarios will be generated in which the availability of the respective data will be modified by generating process delays on purpose. By analyzing the effects of different process delays the importance of the investigated processes shall be determined in order to reduce the acquisition effort by optimizing the execution structure.
Table of contents
1 INTRODUCTION
2 EXECUTION AND SIMULATION MODEL
3 THE SCENARIO ANALYSES
3.1 Scenario 1
3.2 Scenario 2
3.3 Scenario 3
3.4 Scenario 4
3.4.1 Case 1
3.4.2 Case 2
3.4.3 Case 3
3.4.4 Case 4
3.4.5 Case 5
4 OPTIMIZATION OF THE EXECUTION STRUCTURE
4.1 Comparison of the scenario results
4.2 Importance of the examined execution processes
4.3 Appropriate level of execution detail
4.3.1 Combination conditions
4.3.2 Potential process combinations
5 CONCLUSION
5.1 Results
5.2 Discussion
Bibliography
List of Figures
List of Tables
Appendix
1 INTRODUCTION
In a time of increasing requirements on construction projects, the simulation of construction processes in order to identify optimization potential of the execution becomes more and more important.
Apart from the needed information, the daily accurate acquisition of construction data is one main objective. In order to keep the acquisition effort as low as possible, it has to be clarified which data is really necessary for simulating the construction processes and which is not.
This work shall be a contribution to determine the appropriate level of detail that enables to reconstruct completely the execution of constructing highway bridge curbs.
Therefore different scenarios will be generated in which the availability of the respective data will be modified by generating process delays on purpose. By analyzing the effects of different process delays the importance of the investigated processes shall be determined in order to reduce the acquisition effort by optimizing the execution structure.
In the first chapter the basic data of the thesis is mentioned. Thereby the execution structure of constructing bridge curbs and the implementation of the simulation model with its specifics were described.
The following chapter contains of the different scenario analyses. Before simu- lating the scenarios, their characteristics are described. Then the simulation results are explained while focusing on the delay effects of the examined proc- esses.
Chapter 4 focuses on potential possibilities to minimize the execution structure in order to determine the appropriate level of detail. Therefore the scenario re- sults are compared and the importance of the in the scenarios delayed proc- esses will be determined. Finally, some minimization approaches will be pre- sented.
In the last chapter the main results of this thesis will be summarized. Additionally some critical remarks and future prospective were given.
2 EXECUTION AND SIMULATION MODEL
The present chapter shall give an overview over the basic data of the topic.
Basis of the analysis is the already documented process of constructing highway bridge curbs in Germany.
The information used here are the result of documentations of the complete process of constructing bridge curbs on several highway bridges[1],[2]. Apart from information such as the exact number of all the necessary processes, its correspondent duration, predecessor and/or successor, the documentations provide also information that verifies the current state of a process. Further- more the information includes an already drawn Event-driven process chain (EPC) in which all the necessary processes and their sequence are shown.
On the basis of the documented information the execution could be used, amongst others, to create a simulation model. With the model distinct scenarios can be simulated.
The already documented execution of constructing bridge curbs is implemented in Microsoft Office Project.
Before modelling and analyzing several execution scenarios, some basic data concerning the execution, its structure and its conditions have to be made first.
Execution of constructing bridge curbs
The execution of constructing bridge curbs possesses of a lot of different proc- esses. While looking at the structure in a simplified way, it could be described as one path of processes that is later on divided into two parallel paths (see Figure 1). The first path consists of all processes of the preliminary work while the other two paths contain the processes of constructing the inner and the outer cap of the bridge curb. Thereby the execution of each of the two paths consists of identical processes that are arranged in the same order.
illustration not visible in this excerpt
Figure 1: Simplified structure of the execution.
The execution consists of a total of 35 superior processes. Most of the 35 su- perior processes are subdivided into subsequent processes1, such as the proc- ess “survey works” that consists of “stake-out capping units”, “record actual carcass height” and “determine concrete cover”. This subdivision results in a total number of 137 processes, which are shown in Table 22. The whole execu- tion starts with the superior process “survey works” and ends with the superior processes “align formwork carriage of outer cap” and “align formwork carriage of inner cap”.
Implementation of information in MS Project:
In order to simulate process delays and show their effects, the execution is modelled in MS Project in form of a Gantt-chart. There, the information of the EPC can be linked with the duration of each process. A detailed list of all processes, their duration and their sub-processes is shown in Appendix 2.
In order to represent the execution model as realistic as possible, the proc- esses in the simulation model have to be linked correctly by different relation- ships. Most of the processes are linked by the so called finish-start relation- ship. This relationship implies that a process cannot start until its predecessor is completed. Some of the execution processes possess of more than one predecessor or successor.
Execution duration
The here investigated execution of constructing highway bridge curbs comprises only the construction of one section of outer and inner cap. Consequently, the duration of a real execution depends on the length of the bridge and its construction sections.
It is assumed that one week consists of five working days, each with eight working hours between 8 a.m. and 5 p.m.. The working time includes automatically a daily pause time between 12 a.m. and 1 p.m
As a result, the duration of preparing the superstructure and constructing one section of outer and inner cap is about 8 weeks. This means 47 days in total, 33 of which are working days (see Table 1). The modelled fictitious execution starts on the first of July and ends on August the 16th 2011.
illustration not visible in this excerpt
Table 1: Project calendar.
While implementing the execution in MS Project, several specifications were necessary.
Details of the implementation
In comparison to the processes of the underlying EPC, in the simulation model some processes were not implemented while another process “slump test” was added.
As the software has its limit, the start date of some processes in the model is as early as possible. These are those processes which do not posses of a pre- decessor and their only condition is to be completed at the latest by the planned start of its successor path. As a result, some processes will theoretical start simultaneously with the start of the whole execution (see Appendix 2). Because of their early start they possess an impractical theoretical buffer time which consists up to four weeks.
This can be found for example in the superior processes “apply primer of outer cap” and “prepare kerb” which start at the first of July, at exactly the same time the execution starts. Those conditions are modelled on the one hand because the underlying information does not give in detail the exact start date of such processes. On the other hand the conditions should be as realistic as possible and should therefore ensure that none of the independently starting processes will delay the planned time schedule.
After describing the basic data of the model, the different scenarios can be created in the following chapter.
3 THE SCENARIO ANALYSES
This chapter aims to show the effects of different process delays on the time schedule of the execution. Based on an ideal execution scenario it should be analyzed, what will happen to the execution if the availability of different data is delayed and not immediately available.
This work should analyze how a delayed delivery of several data can affect the execution in the simulation model. The analyses also seek to examine which processes will be affected by a delayed process and which will not.
Only the completion of some of a total of 64 sub-processes is evidenced by documents (see Appendix 2). For analyzing the delay effects on the time schedule by keeping the analysis as clear as possible, only the processes of the outer cap will be taken into account. That means that 32 documented proc- esses will be modified in the scenarios1 - 3. In addition, in Scenario 4 also processes their completion is not documented by data will be analyzed. There- fore it is assumed, that in the event of a delayed process no acceleration measures will be made up. That enables to show the full extent of any delay.
In the following paragraphs the scenarios are described and will be simulated.
Scenario 1:
Scenario 1 constitutes the ideal case, in which the data accrue directly after the completion of the particular process. Those so called day-to-day data en- able the reproduction of the current project state in the simulation model. Sce- nario 1 stands for the desired situation which, until now, is only rarely found in practice.
Scenario 2:
Scenario 2 is created to show the effects of a slightly delayed arrival of the data on the reproduced project state. For this purpose it is suggested that the data which documents the completion of the particular process arrives not directly after its completion, but in the evening of the same day.
Scenario 3:
Scenario 3 is supposed to show what will happen to the execution in the simulation model when the required data is separately collected and is not immediately available but like it is found in practise.
Scenario 4:
In Scenario 4 the effects of delayed interference-prone processes will be investigated. Therefore delays of varying durations will be simulated.
In the following paragraphs the four scenarios will be described and analyzed in detail.
3.1 Scenario 1
Scenario 1 shows the ideal execution of constructing highway bridge curbs.
“Ideal” means on the one hand that the execution runs as planned; without delays or interruptions. It also means, on the other hand, that the execution is regularly and completely documented. This implies that the completion of several processes is documented and the appropriate data with its information is available directly after the process completion.
Moreover, it is assumed in this scenario that no interferences and no delays do occur during the execution. Consequently it is suggested that the necessary capacities (such as material, machinery and devices) and resources (personal) are available at the very moment they are needed. The ideal scenario implies furthermore that all the necessary determination processes will deliver optimal results and additional processes are not required.
Moreover it is suggested that several processes could be split over more than one day.
As a result of the above mentioned conditions all necessary processes start and end as planned. Consequently, the execution starts and ends in time and has with a total of 47 days exactly the same duration that is mentioned in chapter 2. The complete execution is shown in Appendix 2.
With exception of some independent starting processes the execution proc- esses do not possess of buffer time (see Table 11). As a result the majority of the processes is on the so called critical path that is marked in red (see Appendix 2).
3.2 Scenario 2
In Scenario 2 it is suggested that the data of the documented processes do not accrue directly after the process completion, but at 5 p.m. of the completion day. According to this, the delay represents the difference between the process completion and the moment when the data is available that confirm the comple- tion.
It is assumed that the subsequent processes cannot begin until the data, which documents the process completion, is available. Therefore, the subsequent processes may not begin until that document is received which verifies their completion.
During the analysis of the scenario two different types of delays will be exam- ined. To analyze the impact of a delay of each documented process, in a first experiment the delays will be generated separately for each (subsequent) doc- umented process. In the next step the documented processes in the model will be purposely delayed to 5 p.m. of its completion day. Processes which are al- ready completed by the end of one working day will accordingly not be delayed by purpose.
Results of the separately delayed processes
The simulation of a delay of separate delayed processes provides different re- sults.
As can be seen in Figure 2, different delay durations have been simulated. The delay durations varies from 0,5 working hours to 8,5 working hours.
Most of the delayed processes have a negative effect on the execution time- schedule, whereby some delayed processes causes more effects than others. As shown in Figure 2, some delays affect exclusively itself, a delay of process 48 causes also a delay of its superior process and most of the delayed processes exceed the execution.
illustration not visible in this excerpt
Figure 2: Scenario 2 - effects of separately delayed processes.
The first listed process of Figure 2 has no delay duration. The process was not delayed because it ends already at the end of the day. Consequently this does not provide information about its effect on the execution.
Processes that do not affect the execution are, with the excepting of the proc- ess “install built-in components”, transport processes. While looking at the pro- ject structure in Appendix 3 it is obvious that the transport processes contain of a buffer time about minimum 24 days (see Table 11). Consequently, a process delay about maximum 8,5 hours will neither affect the superior process nor the execution.
The process 57 “install built-in components” does not affect the execution be- cause of its relationship with process 56. Both processes are parallel while process 56 has a 5 five hours longer duration than process 57. The here gen- erated delay of 0,5 hours will not cause any affect on the execution because of the 5 hours buffer time, the process possess of. Looking at the project struc- ture in Appendix 3, it is obvious that process 57 may affect the execution only if its exceeded duration is longer than the duration of process 56.
As furthermore shown in Figure 2, most of the process delays cause an execution exceedance that is about the same duration while some process delays can partly be compensated or reduced during the execution.
The reduction is on the one hand to cause with the buffer time, some of the processes on the critical path contain of (see Table 11). On the other hand the in MS Project generated relationships between several processes lead to this reduction. Consequently the temporal effects on the execution are lower than the delay duration itself. Such processes are processes 63, 64 and 68. The maximum delay reduction is just about one hour.
Moreover, Figure 2 shows that a process delay of less than one hour has only a minimal impact on the execution completion date. This is to cause with the planned date on which the execution shall be completed which is on Tuesday, the 16th of August at 4 p.m That implies that a delay about more than 1,5 hours will automatically cause a postponement of the completion to the next day
All at once delay of the documented processes
After simulating the effects of separately delayed processes an analysis must follow as to what will happen to the execution if all the documented processes will be delayed until the end of the working day. Therefore some of the rela- tions between several processes have to be modified in the model by changing some finish-to-start relationships to finish-to-finish relationships. That means that those processes do end at the same time. The changes are necessary in order to enable that those processes will end in time and to guarantee the planned start of the processes that are on the critical path as it is in reality.
The purposely delay of all documented processes causes a significant ex- tended execution. As shown in Appendix in this scenario the simulated execu- tion will be completed at the 25th of August at 5 p.m. Compared with the planned end date, the 16th of August at 4 p.m., the execution will be completed 9 days (7 working days) later.
It is obvious that a delayed start of one process that is on the critical path causes a delayed start of the subsequent process(es) and so forth. That means that such a delay may affect nearly the whole execution.
Only those processes that belong to a superior process which does not have a predecessor and does start independently, such as process 63 “concrete supply” or process 33 “transport rails”, will start on time. In contrast, processes of an independently starting superior process that are directly related to the criti- cal path, such as process 65 “placing concrete” or process 13 “apply priming”, are affected by the delay of the predecessors and a delay will automatically af- fect the subsequent processes.
3.3 Scenario 3
In Scenario 3 the simulated delay of several processes is based on the availability of its data that has been documented on several construction sites [1, p. 49], [3, p. 84].
Like done in Scenario 2, the documented processes will be delayed until the moment the documents, that contents of the completion information, are avail- able.
As shown in Table 2, during an execution some documents are immediately available while others are not available prior to one or more days after the completion of the process.
illustration not visible in this excerpt
Table 2: Several documents and their date of availability. [1, p. 49], [3, p. 84].
Transferring the information of Table 2 to the simulation model, 28 of 32 docu- mented processes will be purposely delayed in this scenario. This is due to the fact that processes, which data is available directly after the process comple- tion, have not to be delayed. Consequently, their delay is numbered with a “0”.
In contrast to the simulation in Scenario 2, the here generated delays are expressed in days and not in working hours.
Considering all the data in Table 2, the execution will be completed by the 9th of November (see Appendix 4). By comparison with the planned execution completion date on the 16th of August that implies an enormous execution de- lay of more than 13 weeks what is 132 days in total instead of 47 days.
To show the delays and its effects more clearly, according to the time schedule in Appendix 4, in Figure 3 the respective processes, its delays and the delay effects are demonstrated.
illustration not visible in this excerpt
Figure 3: Scenario 3 - Effects of process delays in days.
As it is shown in Figure 3 already the survey report, which is available only 30 days after the start of the survey works, causes a month-long execution delay.
It is also important to note, that a delay in the transport processes does not cause any effect on the execution, excepting the processes “transport rails (72)” and “erect formwork (59)”.
These two processes are part of an independently starting superior process but in contrast to the other transport processes, both processes are directly linked to the critical path. Due to this, a process delay will automatically have an impact on the execution.
Apart from the transport processes also the delayed process 57 “install built-in components” does not affect the execution. This is caused by the fact that the processes 57 and 56 run parallel to each other. Because of this relationship and because of the longer duration of process 56, the significant delay of proc- ess 57, that is 5 days, does not affect the execution. The scenario does not answer the question what would happen if process 57 instead of process 56 is delayed for about 5 days.
While transferring the dates of availability in the simulation model, some of the documented processes will not be delayed because their respective data is immediately available. This leads to the fact that in total six of the 32 docu- mented processes have not been delayed (see Figure 3). To generate appro- priate results, additional analyses of the processes 4, 5, 9, 10, 21, and 65 are required.
Moreover, the three documented processes “concrete supply”, “slump test” and “placing of concrete” are a special characteristic of Scenario 3. They are, in real life, inseparably related and therefore had to be executed on the same day. The completion of these three processes is documented by different data that has different dates of availability (see Table 2). While the documents “con- crete test report” and “concrete delivery note” are available four days after the process completion, the “site daily report” that documents the completion of the concrete placing is available only five days after the completed process.
By taking into account in which sequence the three processes are arranged, one processes cannot start until the data of the successor process is available. This leads to the fact, that in the model the processes were split over 12 days in total what causes an enormous delay. As a result in the simulation model the process “placing of concrete” starts four days after the slump test and 8 days after the concrete supply. This process separation is purely theoretical. It is furthermore not practical, not only because the concrete has to be unloaded at latest 90 minutes after having being load [4, p. 5].
Many of the in Figure 3 listed delays seem to extend during the execution. The completion of process “apply priming (13)” for example was purposely delayed about one day whereas they cause an execution delay that is two days longer. The extension is caused by the fact that caused by the delayed start of some processes they will end on a Friday. By delaying this process about one day, automatically all subsequent processes will start one day later. Because of the intervening weekend it will end on a Monday. This is a delay of a total of three days, instead of one day. After deducting the non-working days of the week- ends, each process delay causes an execution delay of the same duration. This reasoning explains also the extended delay of the processes 56, 60, 68, 74 and 76.
Because of its enormous delays in relation to the process durations none of the delays can be significantly minimized. The in this scenario chosen time unit (days) implies that a buffer of maximum five hours will not cause a visible delay minimization.
The scenario showed that the in practice extended documentation does not enable an evaluation of the day-to-day status of the construction progress. Reconstructing the current on-site status based on the documentation means an execution duration that is almost twice as long as the ideal execution in Scenario 1.
3.4 Scenario 4
The aim of this scenario is to analyze the effects of delayed interference-prone processes. To design the scenario the basic execution of Scenario 1 has to be used once more.
In contrast to the three above mentioned scenarios, in scenario 4 all interference-prone processes will be taken into account, regardless of whether their completion is documented by data or not.
To enable analyzing the effects of delayed interference-prone processes additional information is necessary. Such information was already generated in various analyzes before. Inter alia, interference-prone processes are characterized by the following criteria [5, p. 63]:
− weather dependent,
− no existence of buffer time,
− necessity of special qualifications, − high number of workers,
− high number of successors, − etc.
The execution of highway bridge curbs contains of three interference-prone processes of the preliminary work and six interference-prone processes per execution of outer and inner cap (see Figure 4) [5, p. 70].
illustration not visible in this excerpt
Figure 4: The interference-prone processes of the execution.
As can be seen in the figure above, the execution paths of outer and inner cap contain of the same interference-prone processes. Because of this, the execution possesses of 15 interference-prone processes.
To achieve the required results of this scenario, on the one hand, it is to simulate what will happen to the execution if only one of these processes is delayed. Therefore those nine processes will be separately delayed to the end of their completion day.
On the other hand the effects of a delay of all interference-prone processes will be analyzed. Moreover several delayed interference-prone processes with different delay durations will be combined.
[...]
1 Those processes are in MS Project labelled as so called “project summary tasks” .
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