In Engineering, What is sucker deck principle for variable depth bridge decks?

A sucker deck bridge is a multispan bridge which features a continuous deck of variable construction depth. The term 'sucker' is appropriate because such decks possess the unique structural property of' sucking' bending moment away from the minimum depth regions towards the maximum depth regions, or haunches. The Paper shows how sucker deck haunches may be so arranged as to locate the minimum deck depth over the highest point of the headroom envelope of the obstacle crossed by the bridge. As this minimum deck depth is less than the depth of a conventional uniform depth deck crossing the same obstacle, the sucker deck bridge gains through reduced rise and length, with attendant economies. After the advantages of bridge rise reduction have been demonstrated, the Paper describes the moment and stress savings associated with sucker decks. It then shows how appropriate forms of sucker decks can be fashioned for various headroom envelopes to effect savings in bridge rises to advantage. The further benefits of weight savings associated with sucker decks, which are usually more efficient and easier to construct than voided decks, together with the aesthetic gains possible, are also described.

Both answers are excellent

For a variable depth bridge deck, the depth of continuous multi-span bridge deck is increased in pier supports and this absorbs sagging moments in the mid-span with the consequent increase in hogging moments in pier supports. As a result, the mid-span depth can be significantly reduced due to the reduction in sagging moment. In essence, this sucker deck principle is applied in locations where headroom requirement is of great concern.

Moreover, in terms of structural performance, sucker decks are effective in reducing dead loads than voided slab of equivalent uniform depth for span length between20-40m. In terms of aesthetics point of view, the public tends to appreciate the structural form of arches and curved soffit rather than boring uniform deck alignment.