Material Choice for a Coastal Bike Rack
A council wants 40 bike racks for a foreshore promenade 30 m from the sea. The brief requires a 10-year service life, minimal maintenance, and a budget of AUD 320 per unit installed. The design team shortlists three options. Option A: hot-dip galvanised mild steel tube, 3 mm wall, coated with a two-pack epoxy topcoat. Option B: 6061-T6 aluminium tube, 3 mm wall, mill finish. Option C: 316 stainless steel tube, 2 mm wall, brushed finish. Salt-spray testing suggests galvanised steel with coating will show first red rust at roughly 6-8 years in this exposure, aluminium will pit cosmetically within 3 years but retain structural capacity, and 316 stainless will remain sound beyond 15 years. Installed costs per unit are AUD 210, AUD 260 and AUD 470 respectively. The client also wants a consistent appearance across the promenade.
1. Evaluate the three options against the brief and recommend one, justifying any compromise you accept.
Read the worked answer
Option A (galvanised steel + epoxy): meets cost (AUD 210 < 320) and appearance (coating can be colour-matched consistently), but the predicted 6-8 years to red rust falls short of the 10-year life. Once the coating is damaged, corrosion concentrates at breaks, so maintenance is not truly 'minimal'. It fails the life criterion unless recoated, which adds cost.
Option B (aluminium, mill finish): meets cost and is maintenance-light structurally, but cosmetic pitting within 3 years conflicts with the client's demand for consistent appearance and would look degraded across 40 units. It survives structurally but fails the appearance criterion.
Option C (316 stainless): clearly meets life (>15 years) and appearance (uniform brushed finish, no coating to fail), but at AUD 470 it exceeds the per-unit budget by AUD 150, i.e. AUD 6000 over 40 units.
Recommendation: Option C, with a negotiated budget increase or value engineering elsewhere (e.g. reducing count to 30 units, or a standard catalogue profile to cut fabrication cost). The reasoning: the brief's 10-year life and minimal-maintenance requirements are hard constraints tied to whole-of-life cost, whereas the AUD 320 figure is a target. Whole-of-life, Option A likely costs more once recoating at year 6-8 is priced in across 40 units. Option C is the only one satisfying all performance criteria; the cost gap should be resolved by scope, not by accepting a design that fails in service.
2. The client insists the budget cannot move. Explain what you would change in the design or specification to deliver a compliant outcome, and state the risk you are accepting.
Read the worked answer
With the budget fixed at AUD 320/unit, Option C at 470 is not viable as specified. Feasible moves:
- Reduce the stainless content. Use 316 stainless for the visible wear surfaces (the cradle and top rail where bikes and users contact) and galvanised steel for the buried base and lower legs, which are less visible but more corrosion-prone. This is a risk: the buried section is where corrosion is hardest to inspect, so it should be the more protected part, not less. A better split is stainless for buried and contact zones and coated steel for the mid-span.
- Change the profile. A 2 mm wall 316 tube in a simpler bent form reduces fabrication and welding cost; welded joints in 316 need passivation, which is a hidden cost. Fewer welds lowers price.
- Reduce unit count to 30 and place them at higher-traffic nodes, keeping total spend within budget while every installed unit is compliant.
- Increase coating specification on Option A (e.g. duplex: galvanising plus a thicker epoxy) to push first rust beyond 10 years, accepting that this narrows the cost gap and still requires inspection.
Risk accepted: any hybrid introduces a galvanic couple between stainless and galvanised steel. In a wet, salty environment this accelerates corrosion of the galvanised (less noble) component unless the joint is electrically insulated or the steel is well coated. This must be detailed and inspected, and the maintenance schedule must include checking those interfaces. I would document this as a residual risk and recommend annual inspection for the first three years.