NZ bridges stamps

The first day cover for the Post Office's NZ bridges stamp issue from 1985.  Details of the bridges on the stamps:-

Shotover bridge (35c stamp)
The road bridge across the Shotover River in Central Otago was described at its opening in 1975 as "mathematical precision combined with beautiful form".  The two-lane carriageway is 320 metres long and is made up of 169 pre-cast, pre-tensioned concrete box girder sections.
Alexandra Bridge (40c)
Steel was the chosen material for the bridge built over the Clutha River at Alexandra in 1958 to replace a bridge built in 1882.  Spanning some 158 metres the new tied-steel arch bridge incorporated special design features.  It is anchored to one abutment by means of a hinge and there are bearings, or rollers on the other abutment and all the piers, that allow the bridge to freely expand and contract.
South Rangitikei Rail Bridge (45c)
The South Rangitikei rail bridge forms part of the main trunk railway line north of Mangaweka in the central North Island.  Because of the danger of flash floods and earthquakes the bridge is able to move and in the event of an earthquake the legs of the high slender piers are designed to alternatively lift off their foundations.
Twin Bridges (70c)
An unusual bridge located at the confluence of the Mangkahia and Awarua rivers in Whangarei county, Northland, Twin Bridges takes its name from two bridges built there in the 1920s.  These were replaced in 1970 by a single reinforced concrete structure but the name remains.  A feature of the new bridge is a 19 metre landing which branches off to give access to farm land which would otherwise be isolated by the convolutions of the two rivers.