11-CS-3 Engineering Management · May 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2019 — 11-CS-3 Sustainability, Engineering and the Environment. Closed book; approved Casio or Sharp calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Hazard is the intrinsic potential of a substance or situation to cause harm—its toxicity, energy or danger (e.g. arsenic is carcinogenic; a suspended 23-tonne load can crush). Exposure is the contact between a person and the hazard—how much, how concentrated, how often and for how long. Risk requires both: risk = f(hazard, exposure). A severe hazard poses little risk if exposure is prevented, while a mild hazard can pose real risk under heavy, prolonged exposure.
Rearranging the given ppb = C·R·T/(MW·P) at 25°C and 1 atm (R·T/P = 24.45 L/mol) gives C = ppb × MW ÷ 24.45:
Cancer risk = air concentration × inhalation unit risk:
A lifetime risk of about 3.7 × 10⁻³ (roughly 1 in 270) exceeds the paper's acceptable level of 10⁻⁴ by a factor of about 37, so this is not a safe exposure. The inhalation unit risk already assumes continuous lifetime breathing of the air, so the 20 m³/day and 70 kg are not needed. Even if the result is pro-rated to the residential exposure pattern (350 d/yr for 30 of 70 years), the risk is still about 1.5 × 10⁻³, far above 10⁻⁴.
Average man: BW = 70 kg; residential drinking water IR = 2 L/day, EF = 350 d/yr, ED = 30 yr; C = 6 µg/L = 6×10⁻³ mg/L. Cancer risk (AT = 70-yr lifetime = 25,550 d):
This slightly exceeds the acceptable 10⁻⁴, so the cancer risk is unsafe (marginally). Hazard quotient (non-carcinogenic effects; AT = actual exposure period = 30 yr = 10,950 d):
Since $HQ < 1$, the non-carcinogenic exposure is acceptable. (If "actual exposure" is taken as only the 350 × 30 = 10,500 exposure days, HQ ≈ 0.57—still below 1.) So the cancer risk is marginally unsafe while the non-cancer hazard is safe; the carcinogenic endpoint governs, and the water should be treated or replaced. This matches Canada's drinking-water guideline for arsenic of 10 µg/L, which is set "as low as reasonably achievable" because no concentration carries zero cancer risk.
At the source (remove the hazard): never re-lift a load so that someone can work beneath it—set the culvert down onto blocking or cribbing (timbers or stands) so it is fully supported by solid material before anyone reaches under it, or relocate the date plate (or cast the date into the concrete) so that removing it never requires access to the underside. With the load resting on blocks, a hoist failure cannot drop it on anyone.
Along the path (keep the energy from reaching the worker): if a load must be held up, provide an independent secondary support—mechanical load-holding stands, or a second lifting device or brake that does not share the failed chain drive—and establish a barricaded exclusion zone under suspended loads so no one can enter the fall path; use long-handled tools so the work can be done from outside the load's footprint.
At the worker: train and supervise riggers and operators on the rule "never place any part of the body under a suspended load," with a written safe work procedure for date-plate removal, a pre-lift plan and a stop-work authority; the operator should refuse to hold a load over a person.
Preferred: control at the source. The crane had recently been inspected and found in good working order, yet a single pin failed without warning—so inspection and maintenance alone could not have prevented this. Only removing the worker's exposure to a suspended load (supporting the load on blocking, or eliminating the need to work beneath it) protects against an unforeseeable equipment failure. It does not depend on the equipment or on human vigilance, and it sits at the top of the hierarchy of controls, above engineering controls on the path and far above training and PPE at the worker.
