cpm-scheduling

name	cpm-scheduling
description	Build and maintain a CPM schedule: activity list, durations, logic ties (FS/SS/FF), critical-path calculation, float analysis, baseline establishment, weekly updates, look-ahead schedules, delay analysis (as-planned vs. as-built, TIA), and recovery scheduling. Tool-agnostic; notes P6 and MS Project specifics.

CPM Scheduling

Purpose: produce a CPM schedule that is the project's primary contract management tool — not a Gantt chart decoration — and maintain it as a living document from baseline to closeout.

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Step 1 — Build the Work Breakdown Structure (WBS)

1. Organize by area (building/floor/zone) + phase (sitework, foundations, structure, enclosure, MEP rough-in, finishes, commissioning) OR by CSI division — whichever matches the contract milestones.
2. Keep activities at the right level: too coarse (one activity per trade) → can't manage it; too fine (every bolt torqued) → can't maintain it. Rule of thumb: activity duration 1–15 working days.

Step 2 — Activity list and durations

For each activity:

• Description: clear, starts with a verb (Install, Pour, Frame, Erect).
• Duration: computed from quantity ÷ productivity (e.g., 200 CY concrete ÷ 50 CY/crew-day = 4 days). Document the productivity assumption. Don't guess.
• Calendar: standard 5-day, 6-day, or project calendar with holidays and weather days.
• Resources: crew type and size (if resource-loading).

Step 3 — Logic ties

Every activity must have a predecessor AND a successor, except the project start and project finish milestones.

Dependency type	When to use
Finish-to-Start (FS)	Default: B cannot start until A finishes
Start-to-Start (SS) with lag	B can start X days after A starts (concurrent work)
Finish-to-Finish (FF) with lag	B must finish within X days of A finishing

• Document why each non-FS relationship exists.
• Use lags sparingly and only for real constraints (cure time, permit lead time). Hidden float disguised as a lag is a CPM anti-pattern.

Step 4 — Critical-path calculation

The CPM algorithm (forward pass / backward pass):

• Forward pass: Early Start (ES) and Early Finish (EF) for each activity.
• Backward pass: Late Start (LS) and Late Finish (LF).
• Total Float (TF) = LS − ES = LF − EF. Activities with TF = 0 are on the critical path.
• Free Float (FF) = ES(successor) − EF(activity). Float that belongs to this activity.

Near-critical path: activities with TF ≤ 5 days. Flag these — they become critical with any small disruption.

Step 5 — Baseline establishment

1. Get the baseline approved (owner, GC PM, major subs) before work starts.
2. Save the baseline as a locked copy. Never overwrite the baseline.
3. Record the data date (status date) of the baseline.
4. Update against the baseline — the delta between baseline and current is the schedule variance.

Step 6 — Weekly schedule updates

1. Enter actual start and finish dates for completed activities.
2. For in-progress activities: enter actual start + remaining duration (or % complete).
3. Revise logic ties only where field conditions have genuinely changed (document the reason).
4. Run the CPM recalculation. Identify the new critical path.
5. Report: SPI (Schedule Performance Index = earned value ÷ planned value if cost-loaded), critical-path change from last update, float trend.

Step 7 — Look-ahead schedule

Extract from the CPM: all activities scheduled to start or continue in the next 3 (or 6) weeks. Field format: date rows, trade columns, activity descriptions. Add:

• Predecessor completions required this week
• Material deliveries expected
• Submittals whose approval is needed before work starts
• Open RFIs blocking this scope

Step 8 — Delay analysis

When a delay occurs, document it real-time (don't reconstruct later):

Method	When to use
As-planned vs. as-built	Simple projects; compare baseline dates to actual dates by activity
Windows (contemporaneous path)	Complex projects; divide the project into analysis windows, analyze each window separately
Time Impact Analysis (TIA)	For prospective or concurrent delay; model the impacted baseline with a fragnet

For each delay:

• Excusable / non-compensable — force majeure, owner-caused weather (owner grants time only)
• Excusable / compensable — owner-caused delay (owner grants time + costs)
• Non-excusable — contractor-caused (no time extension; may owe LDs)
• Concurrent delay — both parties contributed; entitlement is jurisdiction-specific

Submittal integration in CPM

Add submittal lead times as a chain of activities: Sub prepares submittal → GC review → Transmit to A/E → A/E review (contract days) → Approval → Procurement → Fabrication → Delivery → Installation

These chains often run 8–20 weeks for MEP equipment. If they're not in the schedule, you won't see the procurement squeeze coming.

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Anti-patterns

• Activities with no predecessor or no successor (except start/finish).
• Float disguised as inflated durations or hidden lags.
• A "schedule update" that only extends the finish date without a recovery narrative.
• Delay analysis done from memory months after the fact — contemporaneous documentation is the only defensible record.

Output

A CPM schedule in P6 (.xer) or MS Project (.mpp), plus a schedule narrative: critical path, float summary, key milestones, top-3 schedule risks.