Please give me a source.
I'm having trouble figuring out the scientific explanation of this
Please & thank you.
The super structure of a bridge would extended to a higher position
if the bridge span was longer between peers that hold it up.
That same principal works with Arch Bridges that have all superstructure under the Deck. Example are the Golden Gate Bridge
W.V.New River Gorge Bridge. Mackinac Bridge in Michigan.
The height of the towers of these truss bridges allow the distributed
weight to be even between spans.
The curved top chord of a truss bridge is a means to have the top chord match the moment curve. At the ends of a simply supported bridge, the moments are zero. So technically the curve of the top chord can start at some distance above the deck of the bridge. The initial truss height will be governed by the end shears which are higher than anywhere else along the bridge span.
As the moment increases along the span, the depth of the bridge truss must increase to resist the moment. The compressive force in the top chord of the bridge truss is simply the moment divided by the distance between the centroids of the top and bottom chords. The tension force in the bottom chord is the same as the compressive force in the top chord. This is why the top chords of some bridges have a curved top chord. These types of trusses are called curvilinear trusses or bowstring trusses. They are difficult to fabricate since all the connections between the top chord and diagonals and verticals will all be different. Also, what may appear to be a curved top chord are actually many straight arcs between the upper truss nodes.
I hope this helps.