Realistic Alternates

I wrote about this small bridge in the Boston Public Garden seven years ago, when I took these pictures, but it turns out I have more to say about it. It’s a wrought-iron chain suspension bridge, constructed in 1867 with a wood deck and altered in 1921 with steel beams an a concrete deck. The modern deck beams, of course, could have easily been made strong enough to carry the load without the chains. And for all I know, they may be. I suspect that the original design had a “live” deck that moved perceptibly under load. The way to make a suspension bridge less live is to increase the mass and stiffness of the deck, and the original wood deck would have had little mass and little stiffness. The bridge has no stiffening trusses unless the handrail serves that purpose – which it might. The handrail is spidery wrought iron, and not what you’d design if you meant it as a truss, but designing wrought iron trusses for a pedestrian bridge this small wouldn’t necessarily lead to the handrail…