Specifying a coastal bus-shelter bench on a fixed budget
A coastal municipality has asked a school design class to specify a replacement bench for a bus shelter 60 m from the high-water mark. Salt spray reaches the site, relative humidity averages 85% and annual rainfall is about 1,100 mm. The bench must be 1.8 m long on three supports, seat three adults (design load 300 kg treated as a uniformly distributed load), and remain serviceable for 15 years with no more than one inspection a year. Three quotations are on the table: A - untreated pine slats, 45 × 70 mm, on a painted mild-steel frame, N$420; B - recycled HDPE slats on a hot-dip galvanised mild-steel frame, N$810; C - 316 stainless steel slats and frame, N$1,030. Zinc-plated fixings are quoted with A and B. The trust can afford B but not C, and its chair has said she 'prefers the look of timber'.
1. Evaluate A, B and C against the three criteria you consider decisive at this site, and state which you would specify. Show how each criterion is applied rather than simply listing it.
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Criterion 1 - resistance to salt and moisture. Untreated pine at 85% RH and 1,100 mm of rain will decay within a few seasons; painted mild steel 60 m from the sea typically needs repainting every 2–3 years, which breaches the 'one inspection a year' constraint. Zinc-plated fixings are anodic to the steel frame and will corrode first, staining the timber. C is immune on all counts. B's HDPE is chemically inert; its galvanised frame survives provided the coating is unbroken and cut ends are sealed.
Criterion 2 - stiffness under the 300 kg design load. HDPE has a modulus of roughly 0.8–1 GPa against about 200 GPa for steel, so a slat of the same depth deflects far more and creeps under sustained load. The 45 × 70 mm section quoted for A cannot be copied into HDPE; the slats need to be about 50 × 100 mm, or the supports moved closer, to stay within a sensible deflection limit.
Criterion 3 - repair and vandalism. One damaged HDPE slat unbolts and is replaced; a bent stainless frame is a workshop job; a decayed pine slat usually means the fixing has rotted too.
Decision: specify B, with deeper slats, 316 fixings and sealed cut ends. C is technically best but outside the budget, and A only looks cheaper until maintenance is counted.
2. For your chosen option, give two specific detailing decisions that answer the conditions at this site, and explain the mechanism each one controls.
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Detail 1 - slotted fixing holes plus load-spreading washers. HDPE expands about 0.2 mm per metre per kelvin, so a 1.8 m slat will grow roughly 11 mm across a 30 K swing between a cold night and a hot afternoon. If it is clamped rigidly at both ends it buckles or splits at the bolt holes. Specify 14 × 9 mm slots for M8 fixings, fixed at the centre support only, giving about ±2.5 mm of movement each side of centre, with 30 mm outside-diameter washers so the bolt load is spread and the plastic does not creep around a small head.
Detail 2 - galvanise after fabrication and seal every breach. Hot-dip galvanising must be done after drilling and welding, because every cut edge and drilled hole is bare steel and is exactly where salt-laden water collects. Any site drilling is touched in with zinc-rich paint. Cap the tube ends and drill a 3 mm drain hole at the lowest point of each leg so condensate cannot stand inside the section. Lay the slats with 6 mm gaps and a 3° fall to the front so rainwater runs off rather than sitting in the joints, and use 316 fixings with nylon isolating washers to break the small galvanic couple between stainless and zinc.
3. The trust argues that A's lower price makes it the better value over 15 years. Construct the argument you would put to them, stating the assumptions you rely on.
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Build the comparison over the whole life, not the invoice.
Assumptions I would state openly: a repaint of A costs about N$180 in materials and labour and is needed every 2 years; A's slats are replaced once at year 7 for about N$150; B needs one slat replacement at N$45; neither includes the cost of closing the shelter during work.
- Option A: 420 + (7 repaints × 180) + 150 = N$1,830 over 15 years.
- Option B: 810 + 45 = N$855 over 15 years.
Sensitivity check. Even if I am pessimistic and A only needs repainting every 5 years, A becomes 420 + (3 × 180) + 150 = N$1,110 - still worse than B and still requiring work between inspections.
The decisive point is not the total. The brief allows one inspection a year. Option A fails that constraint regardless of cost, because a painted steel frame at the coast cannot be left for two years without rust starting under the coating. Cost only ranks options that meet the specification; it cannot rescue one that does not. If the trust genuinely wants timber appearance, the honest route is a timber-look HDPE profile, not untreated pine.