The Association for the Advancement of the Cost Engineering (AACE International) has recently released Recommended Practice 129R-23, Linear Scheduling Method (LSM). Dr. Amin Terouhid, PE, PMP and Dr. Mirhadi, PMP, Adroit’s principal consultants, are the two primary authors of this recommended practice. This recommended Practice has been peer reviewed and approved by AACE’s planning and scheduling subcommittee members and has now been released for public review.
The AACE International Recommended Practices (RPs) contain valuable reference information and serve as references for project management, cost engineering, and construction claims professionals around the globe. These documents are regularly and carefully updated, go through multiple peer reviews and revisions before publication, and are routinely reviewed by numerous relevant practitioners. The AACE International Recommended Practices (RPs) have been subject to a rigorous peer review process and are intended to be the main technical foundation of AACE’s educational, and certification products and services.
Adroit’s consultants have previously authored other recommended practices too. They received the AACE 2018 Technical Excellence Award because of their role in authoring some of the recommended practices published by AACE international (including Recommended Practice 91R-16 Schedule Development, Recommended Practice 89R-16 Management Summary Schedule, and Recommended Practice 92R-17 Analyzing Near-Critical Paths).
Project planning and scheduling professionals use a variety of project scheduling methods depending on the type, size, and nature of projects. The linear scheduling method (LSM) is typically used on projects wherein the majority of the scope is made up of highly repetitive work elements along a horizontal or vertical alignment. Examples of these projects include pipeline, tunnel, airport runway, highway, transmission line, road resurfacing, railroad, or high-rise construction projects. An LSM schedule (also known as a linear schedule or march chart) is the graphical output of the LSM. Linear schedules use velocity diagrams, which will be described below, to represent each activity and the progress rate to be achieved (or alternatively, the progress actually achieved) over time. The schedule format typically provides planned and/or actual production rates on a time-scaled, linear format.
This recommended practice (RP) is intended to serve as a guideline, not a standard. As a recommended practice of AACE International, the main objectives of this recommended practice (RP) are to increase LSM usage and enhance project management practices by:
Providing an overview of the LSM.
Defining characteristics and applications of the LSM.
Delineating the steps and main considerations in developing, updating, and managing linear schedules.
Highlighting main considerations in interpreting linear schedules
For more information about this recommended practice see:
To find out about the strategies for the effective use of project schedules, including the use of Linear Scheduling Method (LSM), please feel free to contact us.
Please join Adroit’s Amin Terouhid, Ph.D., PE for the following CMAA webinar:
Topic: Typical Deficiencies of Construction Project Schedules: Practices to Avoid
Abstract: This webinar will discuss some of the typical deficiencies of construction project schedules and identifies some of the main considerations necessary to review schedules. The process of reviewing project schedules will also be discussed, and suggestions will be made for preparing complete and reasonable project schedules.
Webinar Date: October 8, 2020 @ 2-3 pm EDT
Sponsored by: Construction Management Association of America (CMAA)
Project schedules are among the key project artifacts that are used as a basis for project control. They are one of the most effective ways that a project team can use to coordinate their activities. Project schedules play a key role in making such coordination and to facilitate achieving a project’s time objectives. However, project schedules can play this role only if they are prepared in a reasonable manner. The reasonableness of project schedules can be evaluated from various perspectives including consistency, clarity, completeness, and feasibility of construction plans.
The
following are some of the main considerations that need to be given to
developing project schedules to ensure they are reasonable:
The schedule is complete and entails all the activities that are needed to successfully implement the scope of work
Proper logical relationships (including finish-to-start, start-to-start-, start-to-finish, or finish-to-finish relationships along with proper lag and lead values) are used in creating the project network
An appropriate combination and choosing of activity relationships (including mandatory, preferential, and scenario-based relationships) are created to define activity dependencies
The schedule accounts for the technical, physical, and technological constraints of performing the work
The schedule meets proper contractual milestones, identifies all interim and ultimate contractual deliverables, and satisfies time and resource constraints outlined in the contract
The schedule is clear, reasonable, and complete
Different sections of the schedule are consistent in terms of the timeline, work priorities, and work sequence
As
noted above, activity dependencies are among the key main considerations in
developing well-prepared schedules. A project network not only contains project
activities but also defines activity dependencies (also known as activity ties
or activity relationships). A variety of activity dependencies exists, and activity
relationships are categorized in different ways.
Activity
dependencies can be categorized based on the nature of dependencies that exist
between project activities. From this perspective, activity dependencies are
often categorized into the following two types of dependencies:
Mandatory dependency (also known as hard logic): This relationship represents a dependency that is necessary or inherent in the nature of the work.
Discretionary dependency (also known as soft, preferred, or preferential logic): This type of dependency represents preferential logic that is used to establish a desired sequence of work despite alternative sequences that are acceptable.
It is important to note, however,
that mandatory and discretionary relationships are not the only activity relationships
that are used in project schedules. To better identify activity dependencies,
it is suggested that activity dependencies are further categorized as shown in
Figure 1.
Figure 1. Activity relationship
types
As this figure shows, mandatary
relationships can further be broken down into the following three types:
Imposed relationships: Imposed relationships are those relationships that need to be built into a project schedule to satisfy legal, regulatory or contractual requirements. An example includes a contractually-imposed requirement that mandates using a phased approach (where a portion of work has to be implemented after another portion) in completing certain elements of work.
Physical relationships: This relationship represents a dependency that has to be established between two or more activities due to the nature of the work. An example of dependencies that are inherent in the nature of the work is the need to place a foundation first before erecting a column atop the foundation.
Safety relationships: This relationship represents a dependency that has to be established between two or more activities to ensure safety considerations are accounted for in sequencing project activities. An example of a safety relationship is the need to avoid concurrent logic in scheduling two activities that cannot be undertaken simultaneously because of safety concerns (e.g., a crew that cannot work on the second floor of a building because of the ongoing work on the first floor).
Sometimes, project scheduling
professionals use scenario-based relationships to define dependencies between
project activities. The current article uses the term scenario-based to
characterize these relationships because depending on the implementation
strategy chosen to execute a project, scenario-based relationships may or may not
be used in defining work sequences. Resource relationships are examples of
scenario-based relationships. Resource relationships are often added to the
project schedule due to resource management concerns (e.g., resource
constraints).
For example, if a contractor needs
to implement two non-causally-related activities, each of which requiring a
crane, the contractor may decide to add a finish-to-start relationship between
the two activities if the contractor has only one crane in its possession. In
this example, the two activities are not causally related; however, based on
the scenario described, the contractor has established a relationship between
these two activities to satisfy its resource constraint. If the contractor had
two cranes in its possession, defining a dependency between the two activities
was unnecessary because as noted above, the activities are presumably not
causally linked. Therefore, it is reasonable to recognize the above-referenced
activity relationship as a scenario-based relationship because these
relationships may or may not be used depending on the implementation scenario
or strategy used.
Not all scenario-based relationships
are resource relationships; therefore, in Figure 1, scenario-based
relationships are broken down into the two main types of resource relationships
and others. An example of other scenario based dependencies includes a
dependency that is established between two activities based on an assumed
what-if scenario to manage a likely change in the project scope of work. This
relationship may or may not be required to be established depending on whether
the change occurs or not.
The last category of activity
relationships is improper relationships that consist of redundant, incorrect
logic, and logic loops. Incorrect logic relationships can further be
categorized into errors, missing logic, out-of-sequence, and improper use of
lags and leads. These relationships will be described in greater depth in a
future article.
Planning and scheduling
professionals need to make informed decisions in selecting and using the right
relationship type. In general, it is suggested that only mandatory
relationships are used in developing project schedules unless the use of
discretionary or scenario-based relationships is justified. Similarly, the use
of preferential relationships may not be appropriate in demonstrating that a
schedule follows a reasonable logic. It is recommended that, instead of
resource constraints, planning and scheduling professionals use resource
leveling techniques to ensure the schedule is not bounded by too many
dependencies that could have otherwise been accounted for.
Assessing activity relationships is
critical in preparing or investigating time extension requests or delay
assessments because a proper delay analysis has to be based on a reasonable
schedule. A delay analysis based on a project schedule that contains
questionable activity relationships is defective. Project planning and
scheduling, forensic scheduling experts, and claim management professionals
need to ensure project schedules are free of improper relationships. Otherwise,
the schedule will not be reliable or reasonable and it may not serve its
purpose.
Author: Dr. Amin Terouhid, PE, PMP, PSP | Principal
Consultant
If you are interested to find out more about the main
considerations in developing or evaluating project schedules, please contact us.
Adroit’s consultants have demonstrated their expertise in developing, updating,
constructability review, and forensic evaluation of project schedules and will
be able to assist. You may also be interested to read the following articles:
Schedule constructability review, what does it entail?
Project networks play important roles in carrying out construction activities in a timely manner, and they are among the key means of communication that project teams use to coordinate their efforts throughout the process of construction. Project networks are also among the key project artifacts that are used for preparing or investigating time-related claims and for determining entitlements to time extensions and/or delay damages. Therefore, it is important to have a more in-depth knowledge of activity dependencies and their types.
Activity dependencies are among the key characteristics and building blocks of project schedules. A project network not only contains project activities but also defines activity dependencies (also known as activity ties or activity relationships). A variety of activity dependencies exists, and activity relationships are categorized in different ways.
The four main types of activity dependencies include Finish-to-Start (FS), Start-to-Start (SS), Start-to-Finish (SF), and Finish-to-Finish (FF). The following briefly describes these relationship types:
Finish-to-Start (FS): The successor activity cannot start unless the predecessor activity finishes.
Start-to-Start (SS): The successor activity cannot start unless the predecessor activity starts.
Start-to-Finish (SF): The successor activity cannot finish unless the predecessor activity starts.
Finish-to-Finish (FF): The successor activity cannot finish unless the predecessor activity finishes.
Activity dependencies can also be categorized based on the nature of dependencies that exist between project activities. From this perspective, activity dependencies are often categorized into the following two types of dependencies:
Mandatory dependency (also known as hard logic): This relationship represents a dependency that is necessary or inherent in the nature of the work.
Discretionary dependency (also known as soft, preferred, or preferential logic): This type of dependency represents preferential logic that is used to establish a desired sequence of work despite alternative sequences that are acceptable.
To better identify activity dependencies, it is suggested that activity dependencies are categorized as shown in Figure 1.
Figure 1. Activity relationship types
As this figure shows, mandatary relationships can further be broken down into the following three types:
Imposed relationships: Imposed relationships are those relationships that need to be built into a project schedule to satisfy legal, regulatory or contractual requirements. An example includes a contractually-imposed requirement that mandates using a phased approach (where a portion of work has to be implemented after another portion) in completing certain elements of work.
Physical relationships: This relationship represents a dependency that has to be established between two or more activities due to the nature of the work. An example of dependencies that are inherent in the nature of the work is the need to place a foundation first before erecting a column atop the foundation.
Safety relationships: This relationship represents a dependency that has to be established between two or more activities to ensure safety considerations are accounted for in sequencing project activities. An example of a safety relationship is the need to avoid concurrent logic in scheduling two activities that cannot be undertaken simultaneously because of safety concerns (e.g., a crew that cannot work on the second floor of a building because of the ongoing work on the first floor).
Sometimes, project scheduling professionals use scenario-based relationships to define dependencies between project activities. The current article uses the term scenario-based to characterize these relationships because depending on the implementation strategy chosen to execute a project, scenario-based relationships may or may not be used in defining work sequences. Resource relationships are examples of scenario-based relationships. Resource relationships are often added to the project schedule due to resource management concerns (e.g., resource constraints).
For example, if a contractor needs to implement two non-causally-related activities, each of which requiring a crane, the contractor may decide to add a finish-to-start relationship between the two activities if the contractor has only one crane in its possession. In this example, the two activities are not causally related; however, based on the scenario described, the contractor has established a relationship between these two activities to satisfy its resource constraint. If the contractor had two cranes in its possession, defining a dependency between the two activities was unnecessary because as noted above, the activities are presumably not causally linked. Therefore, it is reasonable to recognize the above-referenced activity relationship as a scenario-based relationship because these relationships may or may not be used depending on the implementation scenario or strategy used.
Not all scenario-based relationships are resource relationships; therefore, in Figure 1, scenario-based relationships are broken down into the two main types of resource relationships and others. An example of other scenario based dependencies includes a dependency that is established between two activities based on an assumed what-if scenario to manage a likely change in the project scope of work. This relationship may or may not be required to be established depending on whether the change occurs or not.
The last category of activity relationships is improper relationships that consist of redundant, incorrect logic, and logic loops. Incorrect logic relationships can further be categorized into errors, missing logic, out-of-sequence, and improper use of lags and leads. These relationships will be described in greater depth in a future article.
Planning and scheduling professionals need to make informed decisions in selecting and using the right relationship type. In general, it is suggested that only mandatory relationships are used in developing project schedules unless the use of discretionary or scenario-based relationships is justified. Similarly, the use of preferential relationships may not be appropriate in demonstrating that a schedule follows a reasonable logic. It is recommended that, instead of resource constraints, planning and scheduling professionals use resource leveling techniques to ensure the schedule is not bounded by too many dependencies that could have otherwise been accounted for.
Assessing activity relationships is critical in preparing or investigating time extension requests or delay assessments because a proper delay analysis has to be based on a reasonable schedule. A delay analysis based on a project schedule that contains questionable activity relationships is defective. Project planning and scheduling, forensic scheduling experts, and claim management professionals need to ensure project schedules are free of improper relationships (i.e., redundant, incorrect logic, and logic loops). Otherwise, the schedule will not be reliable or reasonable and it may not serve its purpose.
Author: Dr. Amin Terouhid, PE, PMP, PSP | Principal Consultant
Note:
If you are interested to find out more about the main considerations in developing or evaluating project schedules, please contact us. Adroit’s consultants have demonstrated their expertise in developing, updating, constructability review, and forensic evaluation of project schedules and will be able to assist. You may also be interested to read the following articles:
Project schedules play important roles in coordinating the efforts of project team members and identifying the priorities in performing the work. A project work is decomposed into smaller, more manageable pieces of work once work breakdown structures are prepared. The project schedule is then developed to ensure all responsible parties and team members take each and every step that is needed to achieve time, cost, and scope objectives. But an important question that needs to be answered once a project schedule is prepared is how a project team can ensure the schedule is prepared in an appropriate manner? How can project teams make sure the project schedule is reasonable and contains all necessary elements and logical relationships that the project team needs to successfully implement the project? Schedule constructability reviews are expected to answer such questions. Such reviews aim to build confidence in the project schedule by evaluating it and creating a basis for further improvements. This article aims to define what purposes schedule constructability review intends to serve and how a schedule constructability review is performed.
A schedule constructability review verifies that the schedule under investigation meets and/or exceeds the minimum requirements of preparing project schedules. It aims to assess project schedules to ensure they properly represent the steps that need to be taken in implementing the project and verify the feasibility of the construction plan. Schedule constructability reviews aim to closely examine project schedules and determine if they satisfy the requirements outlined in the project scope of work, and confirm that they meet the needs and expectations of project stakeholders, and satisfy technical and contractual requirements of performing the work.
The Construction Industry Institute (CII) defines constructability as “the optimum use of construction knowledge and experience in planning, design/engineering, procurement, and field operations to achieve overall project objectives.” (Construction Industry Institute, 1986) In a similar way, AACE International defines constructability as a “system (process) for achieving optimum integration of construction knowledge in the construction process, balancing various project and environmental constraints to achieve maximization of project goals and performance.” (AACE International, 2017, p. 23) These definitions can be adopted and be used in the context of assessing project schedules to determine the types of evaluations that need to be performed when a schedule is assessed for constructability. Based on these definitions, it is expected that a schedule constructability review identifies schedule deficiencies such as poor logic, improper duration estimates, omissions, inconsistencies, and conflicts to ensure the schedule is reasonable and sound.
Similar to the way constructability reviews are performed to evaluate construction documents for consistency, clarity, completeness, reasonableness, and feasibility of construction plans, schedule constructability reviews are performed to ensure that a schedule meets the following requirements:
Project work packages and activities are properly identified
The schedule is complete and entails all the activities that are needed to successfully implement the scope of work
Proper logical relationships (including finish-to-start, start-to-start-, start-to-finish, or finish-to-finish relationships along with proper lag and lead values) are used in creating the project network
An appropriate combination and choosing of activity relationships (including physical, preferential, resource, and safety relationships) have been created to define activity dependencies
The schedule accounts for the technical and technological constraints of performing the work
The schedule accounts for site restrictions and physical and space constraints
The schedule meets proper contractual milestones, identifies all interim and ultimate contractual deliverables, and satisfies time and resource constraints outlined in the contract
The schedule is clear, reasonable, and complete
Different sections of the schedule are consistent in terms of the timeline, work priorities, and work sequence
The schedule accounts for preparation times, material and equipment lead times, and preparatory steps that need to be taken or prerequisite work that needs to be completed prior to the succeeding work elements
Since project schedules are prepared at different levels of detail in different stages of progress, schedule reviews need to be performed periodically to ensure project schedules meet the minimum requirements over the course of a project. Project schedules are progressively elaborated over the project lifecycle. In other words, as new information is obtained and scope is further developed, project schedules evolve into more detailed schedules that reflect an appropriate level of detail for that specific planning cycle. Therefore, project teams need to perform schedule constructability reviews periodically to ensure the schedule remains a reliable tool that is reasonable, well-thought-out, and compliant with the technical and contractual requirements.
References:
AACE International® (2018). Recommended Practice No. 10S-90 Cost Engineering Terminology. Morgantown, WV: AACE International. Retrieved from http://library.aacei.org/terminology/
Construction Industry Institute (CII). Constructability; A Primer, Publication RS3-1 (July), CM, Austin, Texas, 1986.
Note: Our competent experts have been the primary authors of two industry guidelines, entitled AACE International Recommended Practice 91R-16 Schedule Development and 89R-16 Management Summary Schedule. These industry guidelines are two of the key references used by cost engineers and project management professionals. If your firm is looking for experts who can assist in developing project schedules or performing schedule constructability reviews; and would like your schedules to be prepared using the best practices of project planning and scheduling, please contact us for a free consultation session.
Dangling activities (also known as dangles) are loosely-tied activities in project schedules. They are activities with either open start dates or open end dates. All activities, except the first activity of a network, need to have a predecessor; otherwise, they will have open start dates. Similarly, all activities, except the last activity of a network, need to have a successor; otherwise, they will have open end dates (also known as open-ended or open-end activities).
As noted above, every project activity and milestone except the first and last ones must have at least one predecessor and one successor. An example of the first activity of a network is the notice to proceed milestone and an example of the last activity of a network is the milestone that represents the project completion date. It is recommended that any project schedule starts with a start milestone and finishes with a finish milestone to ensure proper logical ties can be built into the network.
A project schedule that contains dangling activities has deficiencies because its logic is incomplete. This flaw makes the schedule unreliable and inaccurate because the schedule has not fully developed and some activity dependencies (i.e., logical ties) have not been properly identified. The four main types of activity ties include finish-to-finish (FF), finish-to-start (FS), start-to-start (SS) and start-to-finish (SF).
Here are the main issues with dangling activities:
In the event that a schedule contains a dangling activity, one cannot ensure that the projected start or finish dates accurately represent the planned dates because one or more logical ties are missing. For example, an activity with no predecessor (assuming that the activity is not the first activity of the network) has either been forced to be started or completed on particular dates using activity constraints or has been left fully unrestrained. The downside of the former is that instead of a logical tie, one or more constraints have been added to the schedule preventing the schedule from being flexible due to the use of activity constraints in place of activity ties. Schedules need to remain dynamic to ensure the time impact of a delay or a change to an activity’s duration can properly be transmitted to the rest of the project schedule. The disadvantage of the latter is that leaving the activity fully unrestrained allows the activity to move freely every time that the schedule’s as of date (i.e., data date) is changed.
Similarly, an activity with no successor (assuming that the activity is not the last activity of the network) makes the schedule unreliable because one cannot have confidence in the accuracy of the projected start and finish dates (e.g., the finish date of the network). This shortcoming is present because, in the event of a delay that adversely affects the dangling activity, the impact of this delay does not properly transmit to the rest of the schedule and as such, no activity in the network will be delayed as a result because the dangling activity has presumably no successor. In other words, the schedule will not properly be indicative of the impact of a delayed dangling activity because the network’s logic is incomplete. The same issue may become the case when an activity duration is changed because the time impact of a change to an activity’s duration is not properly transmitted to the rest of the project schedule due to the missing ties. A well-prepared project schedule must provide a full network that projects how the project schedule changes in case of a change to activity durations or in the event of delays.
Another issue with dangling activities may surface when delay claims arise. If a dangling activity is negatively affected by one or more delays, the adverse effect of these delays cannot properly be shown by the schedule. In other words, a schedule with an incomplete logic may not be a reliable tool to show the time impact of delays because the schedule logic is flawed. This deficiency results in underestimating the impact of delays. It is generally accepted that networks are reliable when they are fully developed because in this case, the activity ties define the dependencies between activities and accurately determine the projected start and finish dates.
In sum, every project activity and milestone except the first and last ones must have at least one predecessor and one successor. It is recommended that any project schedule starts with a start milestone and finishes with a finish milestone to ensure proper logical ties can be built into the network. The schedule model must identify all logical relationships to generate a full network. Schedules need to remain dynamic to ensure the time impact of a delay or a change to an activity’s duration can properly be transmitted to the rest of the project schedule. Loosely-tied activities are examples of schedule deficiencies that prevent schedules from properly showing the impact of delays or changes to the schedule on the rest of the network.
If you are in need of schedule development services or would like to monitor and control your schedules in an effective manner, Adroit will be able to assist. For more information, please contact us.
In preparing project time schedules in Oracle’s Primavera P6, project planning and scheduling professionals need to properly select duration types. Primavera P6 uses the following two formulae to determine units of work:
Resource Units = Resource Units per Time Unit * Duration
Remaining Resource Units = Resource Units per Time Unit * Remaining Duration
Based on these two formulae, the user is able to make one or two element(s) of the equation fixed, and input or change the other element(s). That way, Primavera P6 will calculate the remaining elements of the equation. To determine which element(s) of the formula to solidify, the users need to take the nature of the work or information at-hand into account and make an informed decision concerning the elements that need to be solidified. This decision then helps the user to choose among the four main types of activity duration types.
Four types of activity duration types can be defined in Primavera P6. They include 1) Fixed Duration & Units, 2) Fixed Duration & Units/Time, 3) Fixed Units, and 4) Fixed Units/Time. The following discusses each of these activity duration types in more depth:
1- Fixed Duration & Units: This types of activity duration is used in Primavera P6 when the duration and the amount of the resources are known and supposed to remain fixed in the schedule. It is recommended that project planning and scheduling professionals use this duration type for time- and budget- constrained projects prior to making schedule updates. The two possibilities include the following:
Option 1: Duration does not change when resources are added or removed, or if the user changes Units/Time.
Option 2: A change to the Duration will change the Units/Time; however, Units remains unchanged.
2- Fixed Duration & Units/Time: This type of activity duration is used in Primavera P6 when the duration and resource performance are known and are supposed to remain fixed (i.e., unchanged) in the schedule. In other words, activity durations remains unchanged in the schedule; however, the remaining units change. If an activity is supposed to be completed within a certain, fixed time frame irrespective of the number or amount of resources being assigned to the activity, this activity duration type is the right choice that needs to be used for that activity. This activity duration type is most often used if the user uses task dependent activities (not resource dependent activities). The two possibilities include the following:
Option 1: Duration does not change when resources are added or removed, or when Units/Time changes.
Option 2: A change to the Duration will change the Units; however, Units/Time remains unchanged.
The use of this activity duration type locks the duration, and the default Units/Time (productivity) values for each resource added. Nevertheless, this activity duration types allows the overall Unit cost to increase when resources are assigned to the activity. It is recommended that project planning and scheduling professionals use this duration type during the planning phase because doing so will force Primavera P6 to honor activity duration estimates and increase the work (Units) and, therefore, the budget, based on additional quantities of work performed (Units/Time).
It is important to note that this duration type disables the User Preferences, Calculations tab option Recalculate the Units, Duration, and Units/Time for existing assignments based on the activity types.
3- Fixed Units: Primavera P6 users need to use this type of activity duration if the amount of work needed to complete an activity (e.g., 8,000 bricks to be laid) is fixed. If this type of activity duration is used, decreasing units per time causes the activity duration to increase; however, if the user updates the duration or units per time, the Units remain unchanged. Increasing the resources allocated to an activity whose duration types is Fixed Unit, decreases the activity duration. It is best to use this activity duration type where the duration is “resource dependent” (and not “task dependent”). If in a project, the budget is set and it is difficult to get additional cost increases approved, the Fixed Units activity duration type is the right choice assuming the other above-mentioned requirements are also satisfied.
4- Fixed Units/Time: Primavera P6 users need to use this type of activity duration if the activity has fixed productivity output per time period (regardless of activity duration). In other words, this duration type is supposed to be used when the user would like the resource units per time to remain unchanged while the activity duration or units change. For example, if a piece of equipment requires two workers to operate, the Fixed Units/Time duration type might be the right choice. When the duration of an activity whose duration type is Fixed Units/Time increases, the amount of budgeted labor units also increases while resource Units/Time remains unchanged. This activity duration type is most often used if the user uses resource-dependent activities.
In addition, users have the choice to choose to preserve “the Units, Duration, and Units/Time for existing assignments” or recalculate “the Units, Duration, and Units/Time for existing assignments” in the User Preferences, Calculations tab of Primavera P6. This choice, as well as the choice of activity duration types, determines what element(s) remain(s) unchanged and what element(s) change(s). These scenarios are outlined in the following two tables:
The User Preferences, Calculations tab option “Preserve the Units, Duration, and Units/Time for existing assignments” is chosen when the user adds or removes multiple resource assignments on activities but would like Units, Units/Time, and Durations to remain unchanged when additional resources are assigned to an activity. When the User Preferences, Calculations tab option “Preserve the Units, Duration, and Units/Time for existing assignments” is selected, here are the various scenarios that are encountered:
The User Preferences, Calculations tab option “Recalculate the Units, Duration, and Units/Time for existing assignments” is chosen when the user adds or removes multiple resource assignments on activities and would like to determine a resource assignment’s remaining values based on the activity’s duration type. When the User Preferences, Calculations tab option “Recalculate the Units, Duration, and Units/Time for existing assignments” is selected, here are the various scenarios that are encountered:
As discussed, four types of activity duration types can be defined in Primavera P6. They include 1) Fixed Duration & Units, 2) Fixed Duration & Units/Time, 3) Fixed Units, and 4) Fixed Units/Time. It is important to pay attention to the nature of work that is being performed to select the right type of activity duration types. This article defined each of this activity duration types and explained where each option needs to be used and what the implications of using these duration types are from the scheduling perspective.
References:
Harris, Paul E (2017). Planning and Control Using Oracle Primavera P6 Versions 8 to 17 PPM Professional. Eastwood Harris Pty Ltd.
This article is part of a technical article, entitled Planning and scheduling requirements of subway station rehabilitation/renovation projects, that Adroit’s competent professionals have authored and presented in AACE International’s 2016 Annual Meeting in Toronto.
Subway station rehabilitation / renovation projects (hereinafter referred to as subway rehabilitation projects) are among the projects with special needs [1] in which a variety of stakeholders including the Department of Transportation (DOT), Department of Environmental Protection (DEP), Department of Historic Resources (DHR), utility companies, regulatory organizations, municipalities, and community organizations are usually involved. Some rehabilitation projects interfere with daily operations at stations; therefore, specific execution strategies such as diverting trains and working in confined spaces are required for successful completion of these projects. Execution of these strategies usually requires proper management of project-specific constraints, assumptions, and objectives and being equipped with a right set of skills and knowledge.
Subway rehabilitation projects have special characteristics which differentiate them from other types of construction projects. The most important characteristics of subway rehabilitation projects from a project planning perspective are shown in the figure below.
Key Planning and Scheduling Considerations for Subway Rehabilitation Projects
Maintaining the reliability of the existing railway transit system while following safety and punctuality guidelines is an important factor that needs to be accounted for. According to the study performed by Collins and Rowe [2], some of the unique considerations of transit projects are as follows:
Acquisition of right-of-way
Acquisition of special services
Relocation of utilities
Relocation of construction infrastructure.
These requirements are all applicable to subway rehabilitation projects; hence, they should be addressed in such a way that the typically large number of private and public stakeholders remain satisfied with the project. Based on the foregoing, it is important to investigate planning and scheduling requirements of subway rehabilitation projects to ensure these projects are executed efficiently and delivered per expectation. The following section describes special characteristics of subway rehabilitation projects in more depth:
Special services are among the distinct aspects of subway rehabilitation projects. It is important to identify different types of special services that are commonly used in these projects. The following provides a brief explanation of some of the special services frequently encountered on subway rehabilitation projects:
Diversions
Subway projects often require temporary service diversions due to interference with normal train operations. A diversion occurs when a train changes its track in a station (e.g. from a local track to an express track); whereas, a by-passing occurs when a train passes a station through its normal tack of service and without any stop. Some tasks can be performed during a by-passing, however, some tasks such as working on tracks need diversion.
Public transit systems constantly try to maximize the productivity of tracks and minimize track outages and passenger inconvenience; therefore, service diversions for maintenance, emergencies, and special events are costly and not easily coordinated. Before approving any diversion requests, all impacts (i.e., definitive and potential impacts) on the affected tracks need to be closely analyzed by the planning and operations departments; therefore, detailed planning for each diversion is of great importance in advance of subway construction, rehabilitation, or renovation projects.
Depending on the project type, location of the station, and daily average subway ridership in the station, the following considerations need to be taken into account for diversion planning:
Permissible days and hours of work during each service diversion
Number of diversions provided within a month and during the construction phase
Timeframes during which no diversion is available (e.g., holiday seasons)
Work restrictions during diversions
A so-called piggy-back diversion is scheduled when a project utilizes the available diversion opportunities of another project or a maintenance task. Piggy-backing has several benefits including:
Utilization of available diversions
Maximizing track access productivity
Minimizing track access schedule conflicts
Reducing customer inconvenience
Reducing the project expenditure
For planning of subway projects with multiple diversions, the during-diversion work should be broken down into scopes of work scheduled for each diversion based on each diversion’s calendar, sequence of activities, and open (i.e., available) work-fronts. With this method of planning, contractors and owners will be able to analyze the number of diversions required to complete the project and assess the feasibility of the tasks scheduled for each diversion. This method of planning also helps contractors make timely requests for adequate work-trains and services they need.
Efficient utilization of diversions is a crucial factor to consider in time and cost management of subway rehabilitation projects. To reduce costs and inconvenience of passengers, the activities that require a diversion should be scheduled together to the extent possible [3]. That being said, safety should not be compromised by stacking several trades in limited areas. In addition, it should be noted that inefficient use of diversion opportunities usually result in deviations from plans and cause disputes and unexpected losses.
Flagging
Some tasks that are performed on or under the tracks or in close proximity with the platforms (i.e., close to train tracks, or in certain heights above the platform) may require flaggers. Construction flaggers signal approaching trains to slow down and warn project personnel of oncoming trains. Some activities such as working on elevators, mezzanine areas, or street entrances are executed without a need for diversion and flaggers; however, some activities are planned either with service diversions or under traffic with flagging protection (i.e., without suspension of train services). Examples of activities that require flagging include surveying operations, inspections, and activities that require work- or test-trains. Operations departments usually prefer activities to be executed under traffic with flagging rather than having activities performed with diversions. Nonetheless, flagging is costly and clients and contractors typically try to minimize the use of flagging. The following steps should be performed before requesting flagging:
Identify the tasks that require flagging and determine the timeframes during which flagging is required;
Calculate the numbers of required flaggers for each task.
Determine the total number of flaggers required for each event.
Specify the flaggers’ working hours.
Consider alternative methods for minimizing the number of required flaggers. Some alternative methods include but are not limited to barricades, safety railings, and barricaded scaffolds.
Analyze the sequence of the tasks that require flagging to check if they can be done sequentially – doing so may decrease the total number of flaggers required for each event.
Work-Trains
In subway rehabilitation projects, work-trains with special equipment (such as crane and scaffold) are sometimes required. Since work-trains of a company are usually enterprise-wide resources that are shared among multiple projects, issuing timely requests and accounting for a reasonable lead-time to acquire work-trains is paramount. Some companies have guidelines for requesting and proper scheduling of work-trains as part of their enterprise resource planning system. Project planners should familiarize themselves with their company’s equipment request procedures. The following considerations need to be taken into account for requesting work-trains:
The number of required work-trains during each planning timeframe
Safety considerations in scheduling activities that involve the use of work-trains
Required modifications/additions to work-trains (e.g., mounting work platforms or cable reels)
The amount of material to be transported by each work-train
The number of required round trips
The required equipment for and the party responsible for loading/unloading work-trains
The allowable hours and locations for loading/unloading operations
The possibility of using other projects’ idle work-trains.
Conclusion
Subway rehabilitation projects have unique characteristics; and various uncertainties exist in these types of projects. Some special characteristics of subway rehabilitation projects include the need for work-trains, diversions, flaggers, and utility relocations. Keeping various stakeholders (e.g., local communities, operation and maintenance departments, utility agencies, and environmental agencies) satisfied over the course of the project is also extremely important.
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