5.1 Construction Sequencing

Construction Sequencing Fundamentals

Why this topic matters for the PE Construction exam: Project sequencing forms the basis for all scheduling questions. You must understand how activities interlock to form a complete network, and be able to interpret leads, lags, and relationship types properly to accurately evaluate a given schedule.

At the heart of any effective construction schedule is the proper sequencing of activities. Sequencing dictates the order in which work proceeds, ensuring that physical, safety, and resource constraints are met. In a complex civil engineering project, no activity exists in a vacuum. The interdependencies between tasks determine the overall flow of the project and ultimately dictate the project's completion date.

Logical Relationships

To build a project schedule, we use logical relationships to link predecessor activities (the tasks that must occur first) to successor activities (the tasks that follow). The PE exam tests your ability to map real-world construction conditions to these four distinct relationship types.

Finish-to-Start (FS)

This is the most common and intuitive relationship. The successor activity cannot start until the predecessor activity has completely finished.

• Example: Concrete curing (Predecessor) must finish before Formwork stripping (Successor) can start.
• Formula Notation: $Start_B \ge Finish_A$

Start-to-Start (SS)

The successor activity cannot start until the predecessor activity has started. They do not have to start at exactly the same time, but the initiation of the first allows the second to begin. This is often used for overlapping trades.

• Example: Excavation for a trench (Predecessor) starts, and shortly after, pipe laying (Successor) can start in the excavated portion.
• Formula Notation: $Start_B \ge Start_A$

Finish-to-Finish (FF)

The successor activity cannot finish until the predecessor activity has finished. The two activities can run concurrently, but the completion of the successor depends on the completion of the predecessor.

• Example: Landscaping installation (Predecessor) must finish before the final site cleanup (Successor) can finish.
• Formula Notation: $Finish_B \ge Finish_A$

Start-to-Finish (SF)

The successor activity cannot finish until the predecessor activity has started. This is the rarest relationship in construction scheduling, often used for just-in-time deliveries or phase-out operations.

• Example: A new temporary power generator must start operating (Predecessor) before the old power system decommissioning can finish (Successor).
• Formula Notation: $Finish_B \ge Start_A$

Leads and Lags

To refine logical relationships, schedulers apply leads and lags. These are time modifiers that allow for more realistic modeling of construction processes.

• Lag: A directed delay or waiting period inserted between two linked activities. A positive lag pushes the successor further into the future.
  • Example: Pour Concrete (FS + 7 days lag) $\rightarrow$ Strip Forms. The 7 days represent the required curing time.
• Lead: An overlap between two activities. It allows a successor activity to start before its predecessor has finished. In modern scheduling software, a lead is often expressed as a negative lag.
  • Example: Frame Walls (FS - 2 days) $\rightarrow$ Install Electrical Rough-in. The electricians can start 2 days before the framers are completely finished.

[!WARNING] Exam Trap: Be careful when interpreting lags on non-FS relationships. For instance, an SS + 3 lag means the successor can start 3 days after the predecessor starts, not after it finishes.

Schedule Constraints

While logical relationships are driven by the physical nature of the work, constraints are imposed dates that force an activity to start or finish on, before, or after a specific point in time. Constraints override the natural flow of the schedule and reduce flexibility.

Common constraint types include:

• Start No Earlier Than (SNET): Used when materials or site access will not be available until a certain date.
• Finish No Later Than (FNLT): Imposed by contract deadlines or regulatory requirements (e.g., environmental restrictions on in-water work).
• Mandatory Start/Finish: Rigid constraints that fix an activity to an exact date, completely removing any float. These should be used sparingly.

Milestone Tracking

A milestone is an activity with zero duration ($d=0$). It represents a significant event, goal, or decision point in the project rather than actual work being performed.

Milestones are crucial for high-level tracking and reporting.

• Start Milestones: E.g., "Notice to Proceed (NTP) Issued", "Site Access Granted".
• Finish Milestones: E.g., "Building Enclosed (Dry-in)", "Substantial Completion", "Certificate of Occupancy Received".

Because milestones have no duration, they do not consume resources or time, but they can be constrained and linked to other activities to track whether critical phases are completed on schedule.

Trade Coordination: Civil, Structural, and MEP

Proper sequencing is not just about drawing boxes and arrows; it requires a deep understanding of trade interactions. The sequencing must prevent the "stacking of trades"—having too many workers in the same space, which plummets productivity and increases safety risks.

1. Civil/Site Work: This usually dictates the early project phases. Earthwork, deep foundations, and site utilities must be sequenced to establish a stable pad and clear access.
2. Structural Work: This follows the civil phase. Steel erection or cast-in-place concrete forms the skeleton of the project. Structural activities often drive the critical path through the middle of the schedule.
3. MEP (Mechanical, Electrical, and Plumbing): MEP systems are woven through the structure. Coordination here is notoriously difficult. A standard sequence is: Plumbing underground $\rightarrow$ MEP overhead rough-in $\rightarrow$ In-wall rough-in $\rightarrow$ Finishes $\rightarrow$ MEP trim-out.

Failure to coordinate these trades leads to out-of-sequence work, where a successor starts before its predecessor is ready, causing rework and severe delays. On the exam, you may be asked to identify logical errors in a proposed sequence based on standard construction practices.