When selecting outdoor furniture for urban environments—parks, plazas, sidewalks, and transit stops—weight is a critical factor. It affects installation cost, structural anchoring, theft resistance, and daily usability. Cast aluminum occupies a unique middle ground among common materials. Here is a detailed comparison of its weight profile against steel, wrought iron, aluminum extrusion, and plastic.
Cast aluminum is produced by pouring molten aluminum into a mold, which creates a dense, slightly porous structure. Its density is approximately 2.7 g/cm³, which is one-third the density of steel (7.85 g/cm³). However, because casting requires thicker walls for equivalent strength, a cast aluminum bench typically weighs 30–50% less than a comparable steel bench. For example, a standard 6-foot urban bench in cast aluminum weighs around 45–60 kg (100–132 lbs), while the same design in steel would weigh 60–90 kg (132–198 lbs).
Compared to wrought iron (density ~7.7 g/cm³), cast aluminum is dramatically lighter—often half the weight or less. Wrought iron furniture is prized for its classic look but is notoriously heavy and prone to rust without constant maintenance. A single wrought iron armchair can weigh 25–30 kg, while a cast aluminum armchair of similar style weighs just 8–12 kg. This 60% weight reduction significantly cuts shipping and handling costs and reduces the risk of back injuries for maintenance crews.
In contrast, extruded aluminum (used in tubular frames) is even lighter than cast aluminum because it uses thin-walled profiles. Extruded aluminum furniture may weigh only 30–50% of cast aluminum’s weight. However, extruded aluminum bends more easily under vandalism or heavy loads. Cast aluminum’s thicker walls and complex geometry offer superior impact resistance, making it the better choice for high-traffic urban areas where an accidental forklift bump or deliberate abuse is likely.
Plastic (HDPE or recycled polymer) is the lightest option, typically weighing 15–30 kg for a full bench. But plastic’s low weight creates a major urban problem: wind uplift and theft. A plastic bench can be carried away by strong gusts or moved by two people without tools. Cast aluminum’s moderate weight (40–60 kg) strikes the perfect balance—heavy enough to stay put in 40 mph winds, yet light enough for two workers to lift without a crane. This reduces anchor bolt requirements compared to steel, which often needs concrete-filled bases.
From a structural engineering perspective, the weight-to-strength ratio of cast aluminum is excellent. Its yield strength (around 150–200 MPa) is lower than steel’s, but its thicker casting compensates. For urban furniture, the limiting factor is rarely static load—it is deflection and fatigue. Cast aluminum’s weight keeps it stable, while its corrosion resistance eliminates the need for heavy galvanized coatings that add extra pounds to steel.
In summary, here is a comparative weight ranking (for a typical 6-foot bench):
1. Plastic – 15–25 kg (lightest)
2. Extruded aluminum – 25–35 kg
3. Cast aluminum – 45–60 kg (ideal middle range)
4. Steel (hollow tube) – 60–90 kg
5. Wrought iron – 80–110 kg (heaviest)
The practical takeaway for urban planners is this: cast aluminum offers the gravity of steel without the installation burden. It avoids the wind-fragility of plastic and the rust-driven weight penalty of iron. Its weight also deters casual theft while remaining manageable for forklift-free repositioning. For cities that need furniture to survive decades of abuse without a crane on standby, cast aluminum is the optimal weight class.