Cost estimate, schedule and constructability

By the design review, the client needs two numbers it can take to council: what the project will cost and when it will be finished. Both must be honest about uncertainty. This lesson builds a Class 3 estimate from the developed design, attaches a risk-based contingency, schedules construction within one season with a critical path network, and works through how a contractor would actually build the bridge. Constructability is not a separate topic from design; a detail that cannot be built in the field, or only at great cost, is a design error even if every calculation is right.

The estimate

At the design review the drawings are about 60 % complete, which places the estimate in Class 3 under the classification from the construction management course: semi-detailed unit costs from quantities taken off the drawings, with an accuracy range of roughly minus 20 % to plus 30 %. Lucas takes off quantities for each of fourteen items and prices them from three sources: recent city tenders for the path, parking and stormwater work; budget quotations from two prefabricated truss suppliers, who were each sent the performance specification; and first-principles pricing of the piles, abutments and deck from crews and production rates, reviewed by Kestrel Bay's estimator.

# Class 3 estimate with a contingency range, and a critical path schedule for one construction season
import random
UNITS = "SI"
SI = UNITS == "SI"
cur = "CAD" if SI else "USD"
f = 1.0 if SI else 0.82
items = [("Mobilization, bonds, insurance", 140000), ("Erosion and sediment control", 45000),
         ("Steel piles and pile caps", 210000), ("Abutments and wingwalls", 330000),
         ("Prefabricated weathering steel truss, delivered", 690000), ("Erection, crane and splice", 120000),
         ("Concrete deck on steel form", 165000), ("Railings and fencing", 85000),
         ("Riprap at the abutments", 60000), ("Approach paths and retaining edges", 180000),
         ("Trailhead parking lot", 230000), ("Bioretention cell and underdrain", 75000),
         ("Crossing at Ridgeway Road with beacons", 70000), ("Landscaping and restoration", 55000)]
base = sum(v for _, v in items) * f
for n, v in items:
    print(f"{n:48s} {v * f:>10,.0f}")
print(f"{'Construction estimate (' + cur + ')':48s} {base:>10,.0f}")
random.seed(499)
wide = {"Steel piles and pile caps": (0.90, 1.0, 1.60), "Prefabricated weathering steel truss, delivered": (0.95, 1.0, 1.35),
        "Abutments and wingwalls": (0.92, 1.0, 1.30)}            # low, most likely, high factors
runs = []
for _ in range(4000):
    tot = 0.0
    for n, v in items:
        lo, ml, hi = wide.get(n, (0.93, 1.0, 1.15))
        tot += v * f * random.triangular(lo, hi, ml)
    runs.append(tot * random.triangular(0.95, 1.20, 1.0))  # bidding market factor
runs.sort()
p80 = runs[int(0.8 * len(runs)) - 1]
print(f"P50 {runs[len(runs) // 2]:,.0f}  P80 {p80:,.0f}  contingency to P80 {p80 / base - 1:.1%}")
# critical path (working days); predecessors in brackets
acts = {"A": ("Mobilize, erosion control", 5, []), "B": ("Drive abutment piles", 6, ["A"]),
        "C": ("Abutments and wingwalls", 20, ["B"]), "D": ("Truss fabrication (off site)", 55, []),
        "E": ("Erect and splice truss", 4, ["C", "D"]), "F": ("Deck forming and pour", 12, ["E"]),
        "G": ("Railings, fencing", 8, ["F"]), "H": ("Approach paths", 18, ["C"]),
        "I": ("Parking lot and bioretention", 25, ["A"]), "J": ("Ridgeway crossing", 6, ["A"]),
        "K": ("Restoration and closeout", 8, ["G", "H", "I", "J"])}
ES, EF = {}, {}
for k in acts:                                   # activities are listed in a valid order
    ES[k] = max([EF[p] for p in acts[k][2]], default=0); EF[k] = ES[k] + acts[k][1]
end = max(EF.values())
LF, LS = {}, {}
for k in reversed(list(acts)):
    succ = [s for s in acts if k in acts[s][2]]
    LF[k] = min([LS[s] for s in succ], default=end); LS[k] = LF[k] - acts[k][1]
print(f"\nProject duration {end} working days; critical path:", " -> ".join(k for k in acts if LS[k] == ES[k]))
for k, (n, d, _) in acts.items():
    print(f"  {k} {n:30s} ES {ES[k]:3d}  EF {EF[k]:3d}  float {LS[k] - ES[k]:3d}")

Mobilization, bonds, insurance                      140,000
Erosion and sediment control                         45,000
Steel piles and pile caps                           210,000
Abutments and wingwalls                             330,000
Prefabricated weathering steel truss, delivered     690,000
Erection, crane and splice                          120,000
Concrete deck on steel form                         165,000
Railings and fencing                                 85,000
Riprap at the abutments                              60,000
Approach paths and retaining edges                  180,000
Trailhead parking lot                               230,000
Bioretention cell and underdrain                     75,000
Crossing at Ridgeway Road with beacons               70,000
Landscaping and restoration                          55,000
Construction estimate (CAD)                       2,455,000
P50 2,728,141  P80 2,885,372  contingency to P80 17.5%

Project duration 87 working days; critical path: D -> E -> F -> G -> K
  A Mobilize, erosion control      ES   0  EF   5  float  24
  B Drive abutment piles           ES   5  EF  11  float  24
  C Abutments and wingwalls        ES  11  EF  31  float  24
  D Truss fabrication (off site)   ES   0  EF  55  float   0
  E Erect and splice truss         ES  55  EF  59  float   0
  F Deck forming and pour          ES  59  EF  71  float   0
  G Railings, fencing              ES  71  EF  79  float   0
  H Approach paths                 ES  31  EF  49  float  30
  I Parking lot and bioretention   ES   5  EF  30  float  49
  J Ridgeway crossing              ES   5  EF  11  float  68
  K Restoration and closeout       ES  79  EF  87  float   0

The point estimate for construction is CAD 2.46 million. The prefabricated truss, delivered to site, is the largest single item at CAD 690,000, followed by the abutments, the trailhead parking lot and the steel piles. The Monte Carlo analysis puts the 80th percentile at CAD 2.89 million, a contingency of 17.5 %. That is larger than the 11 % on the Ridgeway road contract, as it should be: the design is less complete, the steel market has been volatile, and the piles carry real ground risk.

The city's capital budget for construction is CAD 3.0 million, so the P80 cost of CAD 2.89 million fits with a small margin. Engineering, permits, the early boreholes and the city's own costs are carried in a separate part of the capital budget, as the construction management course recommended.

The schedule

The construction season in the region runs from late April, when frost leaves the ground and the spring flood passes, to late October. The schedule must fit the field work into that window. The critical path network has eleven activities, with the truss fabricated off site in parallel with the foundations. Lucas's script computes early and late dates and float, as in the scheduling lesson of the construction management course.

The critical path runs through truss fabrication, erection, the deck, the railings and closeout, for a total of 87 working days from the start of fabrication. The foundations and abutments have 24 days of float, and the parking lot and the Ridgeway crossing have much more. The truss, not the site work, controls the schedule. Fabrication takes about 55 working days after the supplier's shop drawings are approved, and shop drawings take four to six weeks to prepare and review. So the truss must be ordered as soon as the contract is awarded, which means awarding in February or early March.

The team adds two time constraints that the network alone does not show. The work near the creek, including the riprap and the outfall, is timed to avoid the fish timing window in the permits. And the deck pour must happen before the end of September, so that the concrete gains strength before cold weather; a late pour would need heating and blankets and would add cost and risk. With award in early March, shop drawings by mid-April and fabrication complete by the end of June, the truss is erected in early July and the deck is poured in mid-July, leaving a comfortable margin.

Constructability

Priya arranges a constructability review with a local bridge contractor's superintendent, a common practice that the city supports. He looks at the drawings for an afternoon and asks the questions that design teams forget. Where will the crane stand, and on what? How will the truss halves reach the north abutment down a narrow access road? Where will the concrete trucks park for the deck pour, and how far is the pump boom reach? How does the pile-driving rig get onto the south abutment without crossing the floodplain? What happens if a pile refuses on a boulder at half depth?

His comments lead to changes. The crane pad is placed on the plateau beside the south abutment, on the sand, with a gravel working platform designed by Kenji for the outrigger loads, rather than in the floodplain as the first sketch showed. The truss halves are delivered from the south, along the Ridgeway Road path corridor before it is paved, spliced on blocking beside the south abutment, and lifted across the creek in one pick. The deck is poured from a concrete pump on the same pad. The pile-driving rig works from the abutment areas without entering the floodplain. And the specification includes a procedure for piles that refuse early: a redrive after a waiting period, then an added pile if needed, with the cost covered by a unit price for additional piles in the bid form.

The crane pick is the single most hazardous operation on the job. The lift is a little over 20 tonnes including rigging, at a radius of about 25 m to reach the far bearing, which calls for a large mobile crane. The overhead distribution line on the south side of Ridgeway Road is within the crane's working area. The team cannot design the lift, which is the contractor's responsibility under the contract, but it can design out the hazard: the specification requires the line to be de-energized or relocated by the utility for the day of the lift, with the utility's confirmation in writing, and lesson 10 records this as a safety-in-design decision.

How the project will be delivered

The team recommends design-bid-build with a unit-price contract, matching the city's usual practice for park and road projects. The prefabricated truss is specified by performance, with the supplier's engineer designing the connections and the erection scheme, and the general contractor engaging the supplier. A pre-qualification step for truss suppliers, based on experience with similar spans and a quality certification for steel fabrication, protects the city against an inexperienced fabricator. The team also suggests that the city consider issuing the truss as a separate early procurement if the main tender is delayed, because the truss is on the critical path and the supplier's lead time is the main schedule risk.

Practical points

• State the estimate class and attach a contingency from a risk analysis, not a rule of thumb.
• Get budget quotations for specialist items, such as prefabricated trusses.
• Identify the critical path early; off-site fabrication often controls.
• Fit the field work into the construction season and the permit timing windows.
• Hold a constructability review with an experienced contractor before the design is final.