The Coastal Courtyard Bench Brief
A school wants three benches for a courtyard 200 m from the sea. Each bench is 1800 mm long, seat 450 mm high and 400 mm deep, seating four students (about 320 kg total), and students will stand on them. The materials budget is €200 per bench and the benches must last five years with one maintenance visit a year in salt-laden wind and 1100 mm of annual rain. Priced options are 45 × 95 mm tanalised pine at €4.20/m, composite decking board 140 × 25 mm at €9.50/m, and 50 × 50 × 3 mm galvanised steel angle at €11/m. The workshop has a mitre saw, a drill and hand tools but no welder, so the steel must be drilled and bolted. Fixings available are A2 and A4 stainless bolts at €1.10 each. The technician wants fixings that are hidden and a bench that can be dismantled for maintenance. The sustainability policy bans unverified tropical hardwood and PVC.
1. Choose the frame and the slat material and justify the choice with at least three properties and a rough cost per bench.
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Frame: 50 × 50 × 3 mm galvanised angle, bolted. Slats: composite decking, 140 × 25 mm.
Cost per bench (approximate).
- Frame: three A-frames or leg sets, each about 1.3 m of angle (two 450 mm legs plus a 400 mm cross member) = 3.9 m × €11 = €43.
- Slats: seat 400 mm deep needs three 140 mm boards with 10 mm gaps = 3 × 1.8 m = 5.4 m; add a backrest or a fourth board and it is about 7.2 m × €9.50 = €68.
- Fixings: 20 × A4 stainless bolts at €1.10 = €22.
- Total ≈ €133, inside the €200 budget.
Justification.
- Moisture and salt. Galvanised steel at 85 µm coating gives a 20+ year life in a coastal atmosphere; tanalised pine at 45 × 95 is only treated to Use Class 3 and will check, warp and rot at the end grain within five years unless it is oiled yearly.
- Maintenance. Composite needs no annual finish; five years with one visit a year suits inspection only, not re-coating.
- Vandalism and standing. Composite is dense and does not splinter when stood on; a 140 mm board is far more stable underfoot than a 95 mm pine slat, which tips.
- Sustainability. Composite uses recycled polymer and wood flour, and the policy only bans PVC and unverified tropical hardwood, so it is compliant.
Risks to manage. Composite expands about 3–4 mm per metre thermally, so use slotted or oversized holes and leave 5 mm end gaps; it is heavy, so the frame needs rigid cross members.
2. Specify the fixings and the finish schedule, explaining the corrosion reasoning behind each choice.
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Fixings.
- Use A4 (316) stainless bolts, not A2 (304). A2 pits in chloride-rich coastal air; 316 contains molybdenum and is the grade for marine exposure.
- M8 × 80 mm countersunk bolts with cup washers, through-bolted rather than screwed. Screw withdrawal from softwood is only around 50 N per mm of penetration, which is marginal for students standing on a slat; a through-bolt with a 25 mm washer spreads the load.
- Put a nylon or EPDM washer between the stainless bolt and the galvanised frame. Stainless is cathodic to zinc, so in wet conditions the zinc becomes the anode and the coating is eaten locally around each hole. Isolating the metals stops that small-area galvanic cell.
- Use the same washer under the slat to allow movement and to stop the composite crushing.
- Seal every cut end of galvanised angle with a cold galvanising zinc-rich paint; a cut edge has no coating and is where rust starts.
Finish schedule.
- Steel: inspect annually, touch up any white rust or scratches with zinc-rich paint.
- Composite: wash once a year with water and a stiff brush; never use a wire brush, which opens the surface.
- If pine were used anyway: two coats of penetrating oil with a UV absorber on all faces, including end grain, and re-coat annually - that annual re-coat is the reason pine was rejected.
Hidden fixings. Counterbore the slat, drive the bolt from below through the angle, and plug the hole with a composite pellet so nothing protrudes to be unscrewed or to catch clothing.
3. Check whether a 45 × 95 mm pine slat spanning 900 mm between supports would be strong enough for four students. What does the answer tell you about the real design driver?
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Load per slat. Four students at 320 kg over three slats is about 107 kg per slat as a worst case; assume a uniformly distributed load. w = 107 × 9.81 / 0.9 m = 1166 N/m = 1.166 N/mm over L = 900 mm.
Bending moment. M = wL²/8 = 1.166 × 900² / 8 = 118 000 Nmm.
Section modulus. For b = 45 mm, d = 95 mm: Z = bd²/6 = 45 × 9025 / 6 = 67 700 mm³.
Bending stress. σ = M / Z = 118 000 / 67 700 = 1.74 N/mm². Allowable stress for softwood in wet conditions is around 5–8 N/mm², so the slat is roughly four times stronger than needed.
Deflection. I = bd³/12 = 45 × 95³/12 = 3.21 × 10⁶ mm⁴. With E = 10 000 N/mm²: δ = 5wL⁴/(384EI) = 5 × 1.166 × 900⁴ / (384 × 10 000 × 3.21 × 10⁶) = 0.31 mm. Negligible.
Point-load check. A student standing mid-span with 800 N: M = PL/4 = 800 × 900/4 = 180 000 Nmm, σ = 2.7 N/mm². Still acceptable.
Conclusion. Strength is not the driver - a 45 × 95 pine slat is comfortably adequate. The design is decided by durability, fixings and stability underfoot: rot at the end grain and at bolt holes, slat tipping on a 95 mm width, and galvanic corrosion. That is why the money should go into the galvanised frame, stainless fixings and wider boards rather than into a heavier section.