Go back to the original photos:
Individual truss spans are NOT curved, they are each straight themselves.
The bridges are composed of straight spans, each rotated slightly, to follow the chord across the curve, that give the APPEARANCE of being curved.
The trusses themselves are not curved. They follow the series of chords along the curve. There is a BIG difference in structural analysis–and constructability–although curved girders can be designed and fabricated, the myriad of different members of a truss does NOT lend itself to horizontal curvature within any one span.
Yes, individual spans can taper or widen out as needed to accommodate a special situation like a curve onto a bridge, but the actual truss members themselves are NOT curved.
All truss designs are inherently rigid by nature. Attempting to introduce curvature into a truss would result in additional stresses on the structure that would vastly complicate the design and would lead to structural instability–ie risk of failure.
The bending in more than one direction or about more than one axis is what would lead to problems. Some things can actually be designed, but also not easily fabricated in real life.
Although curved girder bridges are possible, they are practically avoided and are only used in the real world where absolutely necessary. In some photos above, it is clear that straight girders were used, and the width of deck adjusted to allow for curvature. In a couple other situations, it does appear that horizontally curved girders were actually used.
In America, many modelers are modeling some time “many” years ago. Curved girder bridges are built TODAY but would not have been at all common when steam locomotives ran…
John
