5.4 Construction Engineering
Key Takeaways
- Construction Engineering carries 4-6 FE Civil questions, centered on CPM scheduling, earthwork, and safety.
- Critical Path Method (CPM): total float = LS − ES = LF − EF; activities with zero total float are on the critical path.
- Free float = (earliest ES of following activities) − EF of the current activity.
- Earned value: Cost Variance CV = EV − AC; Schedule Variance SV = EV − PV; SPI = EV/PV; CPI = EV/AC.
- OSHA 1926 Subpart P requires a protective system (sloping, shoring, or shielding) for excavations 5 ft or deeper unless in stable rock.
Scope of Construction Engineering
Construction Engineering is a smaller area (about 4 to 6 questions) but high-yield because the math is fast: schedule networks, earned value, and a few safety rules.
Critical Path Method (CPM)
The Critical Path Method (CPM) finds the longest duration path through an activity network — the critical path — which sets the minimum project duration. Each activity has four computed times:
| Term | Meaning |
|---|---|
| ES | Early start |
| EF | Early finish = ES + duration |
| LS | Late start = LF − duration |
| LF | Late finish |
The forward pass computes ES/EF (take the maximum EF of predecessors). The backward pass computes LS/LF (take the minimum LS of successors).
- Total float = LS − ES = LF − EF (slack before the project end is delayed)
- Free float = (minimum ES of successors) − EF (slack before any successor is delayed)
- Critical activities have zero total float; delaying them delays the whole project.
Earned Value Management
Earned value measures cost and schedule performance with three base quantities:
- PV — Planned Value (budgeted cost of work scheduled)
- EV — Earned Value (budgeted cost of work performed)
- AC — Actual Cost (actual cost of work performed)
| Metric | Formula | Good Result |
|---|---|---|
| Cost Variance | CV = EV − AC | Positive (under budget) |
| Schedule Variance | SV = EV − PV | Positive (ahead) |
| Cost Performance Index | CPI = EV / AC | > 1.0 |
| Schedule Performance Index | SPI = EV / PV | > 1.0 |
Productivity and Estimating
Productivity = output quantity / labor-hours; its inverse is the unit rate used in estimates. A direct-cost estimate sums material, labor, and equipment; adding overhead and profit (markup) yields the bid price. Earthwork estimating relies on shrinkage and swell factors converting between in-place (bank), loose, and compacted volumes — and the recurring conversion 1 yd³ = 27 ft³.
Construction Safety — OSHA
The FE references OSHA 29 CFR 1926 for construction safety. Key recurring rule: under Subpart P (Excavations), any trench or excavation 5 ft (1.5 m) or deeper requires a protective system — sloping, benching, shoring, or a trench shield (box) — unless it is in stable rock. A registered professional engineer must design protective systems for excavations deeper than 20 ft. Spoil piles must be kept at least 2 ft from the trench edge.
Temporary Structures and Project Delivery
Temporary structures — formwork, shoring, falsework, cofferdams, and bracing — are designed for construction loads (fresh concrete pressure, equipment, wind) and removed after the permanent structure is self-supporting. Common project-delivery methods: Design-Bid-Build (sequential, low-bid), Design-Build (single entity for design and construction), and Construction Manager at Risk (CM holds a guaranteed maximum price).
Reference Handbook Tip
CPM float definitions and earned-value formulas are under 'Construction — Project Management / Scheduling.' For OSHA questions, recall the 5 ft trigger and the 20 ft engineer-design threshold; these are concept questions, not lookups.
An activity has early start ES = 8, duration = 5, and late finish LF = 20 (all in days). What is its total float?
A project reports Earned Value EV = $80,000, Planned Value PV = $100,000, and Actual Cost AC = $70,000. Which statement is correct?